Document API change for GPGME_EVENT_DONE from 2009.

[gpgme.git] / doc / gpgme.texi

\input texinfo                   @c -*- mode: texinfo; coding: latin-1; -*-
@documentencoding ISO-8859-1
@setfilename gpgme.info
@settitle The `GnuPG Made Easy' Reference Manual

@dircategory GNU Libraries
@direntry
* @acronym{GPGME}: (gpgme).          Adding support for cryptography to your program.
@end direntry

@c Unify some of the indices.
@syncodeindex tp fn
@syncodeindex pg fn

@copying
Copyright @copyright{} 2002, 2003, 2004, 2005, 2006, 2007,
2008, 2010, 2012, 2013 g10 Code GmbH.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 3 of the License, or (at your
option) any later version. The text of the license can be found in the
section entitled ``Copying''.
@end quotation

This document is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.
@end copying

@include version.texi

@c Macros used by the description of the UI server protocol
@macro clnt
  @sc{c:} @c
@end macro
@macro srvr
  @sc{s:} @c
@end macro


@c
@c  T I T L E  P A G E
@c
@ifinfo
This file documents the @acronym{GPGME} library.

This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION}.

@c NOTE: Don't forget to update the year for the TeX version, too.
@insertcopying

@end ifinfo

@c We do not want that bastard short titlepage.
@c @iftex
@c @shorttitlepage The `GnuPG Made Easy' Reference Manual
@c @end iftex
@titlepage
@center @titlefont{The `GnuPG Made Easy'}
@sp 1
@center @titlefont{Reference Manual}
@sp 6
@center Edition @value{EDITION}
@sp 1
@center last updated @value{UPDATED}
@sp 1
@center for version @value{VERSION}
@page
@vskip 0pt plus 1filll
Published by g10 Code GmbH@* Hüttenstr. 61@* 40699 Erkrath, Germany

@insertcopying
@end titlepage
@page

@summarycontents
@contents

@ifnottex
@node Top
@top Main Menu
This is Edition @value{EDITION}, last updated @value{UPDATED}, of
@cite{The `GnuPG Made Easy' Reference Manual}, for Version
@value{VERSION} of the @acronym{GPGME} library.
@end ifnottex

@menu
* Introduction::                  How to use this manual.
* Preparation::                   What you should do before using the library.
* Protocols and Engines::         Supported crypto protocols.
* Algorithms::                    Supported algorithms.
* Error Handling::                Error numbers and their meanings.
* Exchanging Data::               Passing data to and from @acronym{GPGME}.
* Contexts::                      Handling @acronym{GPGME} contexts.

Appendices

* UI Server Protocol::            The GnuPG UI Server Protocol.
* Debugging::                     How to solve problems.

* Library Copying::               The GNU Lesser General Public License says
                                  how you can copy and share `GnuPG Made Easy'.
* Copying::                       The GNU General Public License says how you
                                  can copy and share this manual.

Indices

* Concept Index::                 Index of concepts and programs.
* Function and Data Index::       Index of functions, variables and data types.


@detailmenu
 --- The Detailed Node Listing ---

Introduction

* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.

Preparation

* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Largefile Support (LFS)::       How to use @acronym{GPGME} with LFS.
* Using Automake::                Compiler options to be used the easy way.
* Using Libtool::                 Avoiding compiler options entirely.
* Library Version Check::         Getting and verifying the library version.
* Signal Handling::               How @acronym{GPGME} affects signal handling.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.

Protocols and Engines

* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* Engine Configuration::          Changing the engine configuration.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.

Algorithms

* Public Key Algorithms::         A list of all public key algorithms.
* Hash Algorithms::               A list of all hash algorithms.

Error Handling

* Error Values::                  The error value and what it means.
* Error Codes::                   A list of important error codes.
* Error Sources::                 A list of important error sources.
* Error Strings::                 How to get a descriptive string from a value.

Exchanging Data

* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.

Creating Data Buffers

* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.

Manipulating Data Buffers

* Data Buffer I/O Operations::    I/O operations on data buffers.
* Data Buffer Meta-Data::         Meta-data manipulation of data buffers.

Contexts

* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Result Management::             Managing the result of crypto operations.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.

Context Attributes

* Protocol Selection::            Selecting the protocol used by a context.
* Crypto Engine::                 Configuring the crypto engine.
* ASCII Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::         Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.
* Locale::                        Setting the locale of a context.

Key Management

* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.
* Advanced Key Editing::          Advanced key edit operation.

Trust Item Management

* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting information about trust items.
* Manipulating Trust Items::      Operations on trust items.

Crypto Operations

* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.

Sign

* Selecting Signers::             How to choose the keys to sign with.
* Creating a Signature::          How to create a signature.
* Signature Notation Data::       How to add notation data to a signature.

Encrypt

* Encrypting a Plaintext::        How to encrypt a plaintext.

Run Control

* Waiting For Completion::        Waiting until an operation is completed.
* Using External Event Loops::    Advanced control over what happens when.
* Cancellation::                  How to end pending operations prematurely.

Using External Event Loops

* I/O Callback Interface::        How I/O callbacks are registered.
* Registering I/O Callbacks::     How to use I/O callbacks for a context.
* I/O Callback Example::          An example how to use I/O callbacks.
* I/O Callback Example GTK+::     How to integrate @acronym{GPGME} in GTK+.
* I/O Callback Example GDK::      How to integrate @acronym{GPGME} in GDK.
* I/O Callback Example Qt::       How to integrate @acronym{GPGME} in Qt.

@end detailmenu
@end menu

@node Introduction
@chapter Introduction

`GnuPG Made Easy' (@acronym{GPGME}) is a C language library that
allows to add support for cryptography to a program.  It is designed
to make access to public key crypto engines like GnuPG or GpgSM easier
for applications.  @acronym{GPGME} provides a high-level crypto API
for encryption, decryption, signing, signature verification and key
management.

@acronym{GPGME} uses GnuPG and GpgSM as its backends to support
OpenPGP and the Cryptographic Message Syntax (CMS).

@menu
* Getting Started::               Purpose of the manual, and how to use it.
* Features::                      Reasons to install and use @acronym{GPGME}.
* Overview::                      Basic architecture of the @acronym{GPGME} library.
@end menu


@node Getting Started
@section Getting Started

This manual documents the @acronym{GPGME} library programming
interface.  All functions and data types provided by the library are
explained.

The reader is assumed to possess basic knowledge about cryptography in
general, and public key cryptography in particular.  The underlying
cryptographic engines that are used by the library are not explained,
but where necessary, special features or requirements by an engine are
mentioned as far as they are relevant to @acronym{GPGME} or its users.

This manual can be used in several ways.  If read from the beginning
to the end, it gives a good introduction into the library and how it
can be used in an application.  Forward references are included where
necessary.  Later on, the manual can be used as a reference manual to
get just the information needed about any particular interface of the
library.  Experienced programmers might want to start looking at the
examples at the end of the manual, and then only read up those parts
of the interface which are unclear.


@node Features
@section Features

@acronym{GPGME} has a couple of advantages over other libraries doing
a similar job, and over implementing support for GnuPG or other crypto
engines into your application directly.

@table @asis
@item it's free software
Anybody can use, modify, and redistribute it under the terms of the GNU
Lesser General Public License (@pxref{Library Copying}).

@item it's flexible
@acronym{GPGME} provides transparent support for several cryptographic
protocols by different engines.  Currently, @acronym{GPGME} supports
the OpenPGP protocol using GnuPG as the backend, and the Cryptographic
Message Syntax using GpgSM as the backend.

@item it's easy
@acronym{GPGME} hides the differences between the protocols and
engines from the programmer behind an easy-to-use interface.  This way
the programmer can focus on the other parts of the program, and still
integrate strong cryptography in his application.  Once support for
@acronym{GPGME} has been added to a program, it is easy to add support
for other crypto protocols once @acronym{GPGME} backends provide them.
@end table


@node Overview
@section Overview

@acronym{GPGME} provides a data abstraction that is used to pass data
to the crypto engine, and receive returned data from it.  Data can be
read from memory or from files, but it can also be provided by a
callback function.

The actual cryptographic operations are always set within a context.
A context provides configuration parameters that define the behaviour
of all operations performed within it.  Only one operation per context
is allowed at any time, but when one operation is finished, you can
run the next operation in the same context.  There can be more than
one context, and all can run different operations at the same time.

Furthermore, @acronym{GPGME} has rich key management facilities
including listing keys, querying their attributes, generating,
importing, exporting and deleting keys, and acquiring information
about the trust path.

With some precautions, @acronym{GPGME} can be used in a multi-threaded
environment, although it is not completely thread safe and thus needs
the support of the application.


@node Preparation
@chapter Preparation

To use @acronym{GPGME}, you have to perform some changes to your
sources and the build system.  The necessary changes are small and
explained in the following sections.  At the end of this chapter, it
is described how the library is initialized, and how the requirements
of the library are verified.

@menu
* Header::                        What header file you need to include.
* Building the Source::           Compiler options to be used.
* Largefile Support (LFS)::       How to use @acronym{GPGME} with LFS.
* Using Automake::                Compiler options to be used the easy way.
* Using Libtool::                 Avoiding compiler options entirely.
* Library Version Check::         Getting and verifying the library version.
* Signal Handling::               How @acronym{GPGME} affects signal handling.
* Multi Threading::               How @acronym{GPGME} can be used in an MT environment.
@end menu


@node Header
@section Header
@cindex header file
@cindex include file

All interfaces (data types and functions) of the library are defined
in the header file `gpgme.h'.  You must include this in all programs
using the library, either directly or through some other header file,
like this:

@example
#include <gpgme.h>
@end example

The name space of @acronym{GPGME} is @code{gpgme_*} for function names
and data types and @code{GPGME_*} for other symbols.  Symbols internal
to @acronym{GPGME} take the form @code{_gpgme_*} and @code{_GPGME_*}.

Because @acronym{GPGME} makes use of the GPG Error library, using
@acronym{GPGME} will also use the @code{GPG_ERR_*} name space
directly, and the @code{gpg_err*} and @code{gpg_str*} name space
indirectly.


@node Building the Source
@section Building the Source
@cindex compiler options
@cindex compiler flags

If you want to compile a source file including the `gpgme.h' header
file, you must make sure that the compiler can find it in the
directory hierarchy.  This is accomplished by adding the path to the
directory in which the header file is located to the compilers include
file search path (via the @option{-I} option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, gpgme ships with a small
helper program @command{gpgme-config} that knows about the path to the
include file and other configuration options.  The options that need
to be added to the compiler invocation at compile time are output by
the @option{--cflags} option to @command{gpgme-config}.  The following
example shows how it can be used at the command line:

@example
gcc -c foo.c `gpgme-config --cflags`
@end example

Adding the output of @samp{gpgme-config --cflags} to the compiler
command line will ensure that the compiler can find the
@acronym{GPGME} header file.

A similar problem occurs when linking the program with the library.
Again, the compiler has to find the library files.  For this to work,
the path to the library files has to be added to the library search
path (via the @option{-L} option).  For this, the option
@option{--libs} to @command{gpgme-config} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with @acronym{GPGME} (in particular, the
@samp{-lgpgme} option).  The example shows how to link @file{foo.o}
with the @acronym{GPGME} library to a program @command{foo}.

@example
gcc -o foo foo.o `gpgme-config --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{gpgme-config}:

@example
gcc -o foo foo.c `gpgme-config --cflags --libs`
@end example

If you want to link to one of the thread-safe versions of
@acronym{GPGME}, you must specify the @option{--thread} option before
any other option to select the thread package you want to link with.
Supported thread packages are @option{--thread=pth} and
@option{--thread=pthread}.


@node Largefile Support (LFS)
@section Largefile Support (LFS)
@cindex largefile support
@cindex LFS

@acronym{GPGME} is compiled with largefile support by default, if it
is available on the system.  This means that GPGME supports files
larger than two gigabyte in size, if the underlying operating system
can.  On some systems, largefile support is already the default.  On
such systems, nothing special is required.  However, some systems
provide only support for files up to two gigabyte in size by default.
Support for larger file sizes has to be specifically enabled.

To make a difficult situation even more complex, such systems provide
two different types of largefile support.  You can either get all
relevant functions replaced with alternatives that are largefile
capable, or you can get new functions and data types for largefile
support added.  Those new functions have the same name as their
smallfile counterparts, but with a suffix of 64.

An example: The data type @code{off_t} is 32 bit wide on GNU/Linux PC
systems.  To address offsets in large files, you can either enable
largefile support add-on.  Then a new data type @code{off64_t} is
provided, which is 64 bit wide.  Or you can replace the existing
@code{off_t} data type with its 64 bit wide counterpart.  All
occurences of @code{off_t} are then automagically replaced.

As if matters were not complex enough, there are also two different
types of file descriptors in such systems.  This is important because
if file descriptors are exchanged between programs that use a
different maximum file size, certain errors must be produced on some
file descriptors to prevent subtle overflow bugs from occuring.

As you can see, supporting two different maximum file sizes at the
same time is not at all an easy task.  However, the maximum file size
does matter for @acronym{GPGME}, because some data types it uses in
its interfaces are affected by that.  For example, the @code{off_t}
data type is used in the @code{gpgme_data_seek} function, to match its
@acronym{POSIX} counterpart.  This affects the call-frame of the
function, and thus the ABI of the library.  Furthermore, file
descriptors can be exchanged between GPGME and the application.

For you as the user of the library, this means that your program must
be compiled in the same file size mode as the library.  Luckily, there
is absolutely no valid reason for new programs to not enable largefile
support by default and just use that.  The compatibility modes (small
file sizes or dual mode) can be considered an historic artefact, only
useful to allow for a transitional period.

On POSIX platforms @acronym{GPGME} is compiled using largefile support
by default.  This means that your application must do the same, at
least as far as it is relevant for using the @file{gpgme.h} header
file.  All types in this header files refer to their largefile
counterparts, if they are different from any default types on the
system.

On 32 and 64 bit Windows platforms @code{off_t} is declared as 32 bit
signed integer.  There is no specific support for LFS in the C
library.  The recommendation from Microsoft is to use the native
interface (@code{CreateFile} et al.) for large files.  Released binary
versions of @acronym{GPGME} (libgpgme-11.dll) have always been build
with a 32 bit @code{off_t}.  To avoid an ABI break we stick to this
convention for 32 bit Windows by using @code{long} there.
@acronym{GPGME} versions for 64 bit Windows have never been released
and thus we are able to use @code{int64_t} instead of @code{off_t}
there.  For easier migration the typedef @code{gpgme_off_t} has been
defined.  The reason we cannot use @code{off_t} directly is that some
toolchains (e.g. mingw64) introduce a POSIX compatible hack for
@code{off_t}.  Some widely used toolkits make use of this hack and in
turn @acronym{GPGME} would need to use it also.  However, this would
introduce an ABI break and existing software making use of libgpgme
might suffer from a severe break.  Thus with version 1.4.2 we
redefined all functions using @code{off_t} to use @code{gpgme_off_t}
which is defined as explained above.  This way we keep the ABI well
defined and independent of any toolchain hacks.  The bottom line is
that LFS support in @acronym{GPGME} is only available on 64 bit
versions of Windows.

On POSIX platforms you can enable largefile support, if it is
different from the default on the system the application is compiled
on, by using the Autoconf macro @code{AC_SYS_LARGEFILE}.  If you do
this, then you don't need to worry about anything else: It will just
work.  In this case you might also want to use @code{AC_FUNC_FSEEKO}
to take advantage of some new interfaces, and @code{AC_TYPE_OFF_T}
(just in case).

If you do not use Autoconf, you can define the preprocessor symbol
@code{_FILE_OFFSET_BITS} to 64 @emph{before} including any header
files, for example by specifying the option
@code{-D_FILE_OFFSET_BITS=64} on the compiler command line.  You will
also want to define the preprocessor symbol @code{LARGEFILE_SOURCE} to
1 in this case, to take advantage of some new interfaces.

If you do not want to do either of the above, you probably know enough
about the issue to invent your own solution.  Just keep in mind that
the @acronym{GPGME} header file expects that largefile support is
enabled, if it is available.  In particular, we do not support dual
mode (@code{_LARGEFILE64_SOURCE}).


@node Using Automake
@section Using Automake
@cindex automake
@cindex autoconf

It is much easier if you use GNU Automake instead of writing your own
Makefiles.  If you do that you do not have to worry about finding and
invoking the @command{gpgme-config} script at all.  @acronym{GPGME}
provides an extension to Automake that does all the work for you.

@c A simple macro for optional variables.
@macro ovar{varname}
@r{[}@var{\varname\}@r{]}
@end macro
@defmac AM_PATH_GPGME (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
@defmacx AM_PATH_GPGME_PTH (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
@defmacx AM_PATH_GPGME_PTHREAD (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
Check whether @acronym{GPGME} (at least version @var{minimum-version},
if given) exists on the host system.  If it is found, execute
@var{action-if-found}, otherwise do @var{action-if-not-found}, if
given.

Additionally, the function defines @code{GPGME_CFLAGS} to the flags
needed for compilation of the program to find the @file{gpgme.h}
header file, and @code{GPGME_LIBS} to the linker flags needed to link
the program to the @acronym{GPGME} library.

@code{AM_PATH_GPGME_PTH} checks for the version of @acronym{GPGME}
that can be used with GNU Pth, and defines @code{GPGME_PTH_CFLAGS} and
@code{GPGME_PTH_LIBS}.

@code{AM_PATH_GPGME_PTHREAD} checks for the version of @acronym{GPGME}
that can be used with the native pthread implementation, and defines
@code{GPGME_PTHREAD_CFLAGS} and @code{GPGME_PTHREAD_LIBS}.
@end defmac

You can use the defined Autoconf variables like this in your
@file{Makefile.am}:

@example
AM_CPPFLAGS = $(GPGME_CFLAGS)
LDADD = $(GPGME_LIBS)
@end example


@node Using Libtool
@section Using Libtool
@cindex libtool

The easiest way is to just use GNU Libtool.  If you use libtool, and
link to @code{libgpgme.la}, @code{libgpgme-pth.la} or
@code{libgpgme-pthread.la} respectively, everything will be done
automatically by Libtool.


@node Library Version Check
@section Library Version Check
@cindex version check, of the library

@deftypefun {const char *} gpgme_check_version (@w{const char *@var{required_version}})
The function @code{gpgme_check_version} has four purposes.  It can be
used to retrieve the version number of the library.  In addition it
can verify that the version number is higher than a certain required
version number.  In either case, the function initializes some
sub-systems, and for this reason alone it must be invoked early in
your program, before you make use of the other functions in
@acronym{GPGME}.  The last purpose is to run selftests.

As a side effect for W32 based systems, the socket layer will get
initialized.


If @var{required_version} is @code{NULL}, the function returns a
pointer to a statically allocated string containing the version number
of the library.

If @var{required_version} is not @code{NULL}, it should point to a
string containing a version number, and the function checks that the
version of the library is at least as high as the version number
provided.  In this case, the function returns a pointer to a
statically allocated string containing the version number of the
library.  If @var{REQUIRED_VERSION} is not a valid version number, or
if the version requirement is not met, the function returns
@code{NULL}.

If you use a version of a library that is backwards compatible with
older releases, but contains additional interfaces which your program
uses, this function provides a run-time check if the necessary
features are provided by the installed version of the library.

If a selftest fails, the function may still succeed.  Selftest errors
are returned later when invoking @code{gpgme_new}, so that a detailed
error code can be returned (historically, @code{gpgme_check_version}
does not return a detailed error code).
@end deftypefun


@deftypefun {int} gpgme_set_global_flag  @
            (@w{const char *@var{name}}, @
            @w{const char *@var{value}})

On some systems it is not easy to set environment variables and thus
hard to use @acronym{GPGME}'s internal trace facility for debugging.
This function has been introduced as an alternative way to enable
debugging.  It is important to assure that only one thread accesses
@acronym{GPGME} functions between a call to this function and after
the return from the call to @code{gpgme_check_version}.

To enable debugging, you need to call this function as early as
possible --- even before @code{gpgme_check_version} --- with the
string ``debug'' for @var{name} and @var{value} identical to the value
used with the environment variable @code{GPGME_DEBUG}.

This function returns @code{0} on success.  In contrast to other
functions the non-zero return value on failure does not convey any
error code.  For setting ``debug'' the only possible error cause is an
out of memory condition; which would exhibit itself later anyway.
Thus the return value may be ignored.
@end deftypefun


After initializing @acronym{GPGME}, you should set the locale
information to the locale required for your output terminal.  This
locale information is needed for example for the curses and Gtk
pinentry.  Here is an example of a complete initialization:

@example
#include <locale.h>
#include <gpgme.h>

void
init_gpgme (void)
@{
  /* Initialize the locale environment.  */
  setlocale (LC_ALL, "");
  gpgme_check_version (NULL);
  gpgme_set_locale (NULL, LC_CTYPE, setlocale (LC_CTYPE, NULL));
#ifdef LC_MESSAGES
  gpgme_set_locale (NULL, LC_MESSAGES, setlocale (LC_MESSAGES, NULL));
#endif
@}
@end example

Note that you are highly recommended to initialize the locale settings
like this.  @acronym{GPGME} can not do this for you because it would
not be thread safe.  The conditional on LC_MESSAGES is only necessary
for portability to W32 systems.


@node Signal Handling
@section Signal Handling
@cindex signals
@cindex signal handling

The @acronym{GPGME} library communicates with child processes (the
crypto engines).  If a child process dies unexpectedly, for example
due to a bug, or system problem, a @code{SIGPIPE} signal will be
delivered to the application.  The default action is to abort the
program.  To protect against this, @code{gpgme_check_version} sets the
@code{SIGPIPE} signal action to @code{SIG_IGN}, which means that the
signal will be ignored.

@acronym{GPGME} will only do that if the signal action for
@code{SIGPIPE} is @code{SIG_DEF} at the time
@code{gpgme_check_version} is called.  If it is something different,
@code{GPGME} will take no action.

This means that if your application does not install any signal
handler for @code{SIGPIPE}, you don't need to take any precautions.
If you do install a signal handler for @code{SIGPIPE}, you must be
prepared to handle any @code{SIGPIPE} events that occur due to
@acronym{GPGME} writing to a defunct pipe.  Furthermore, if your
application is multi-threaded, and you install a signal action for
@code{SIGPIPE}, you must make sure you do this either before
@code{gpgme_check_version} is called or afterwards.


@node Multi Threading
@section Multi Threading
@cindex thread-safeness
@cindex multi-threading

The @acronym{GPGME} library is not entirely thread-safe, but it can
still be used in a multi-threaded environment if some care is taken.
If the following requirements are met, there should be no race
conditions to worry about:

@itemize @bullet
@item
@acronym{GPGME} supports the thread libraries pthread and GNU Pth.
The support for this has to be enabled at compile time.
@acronym{GPGME} will automatically detect the location in which the
thread libraries are installed and activate the support for them at
build time.

Support for other thread libraries is very easy to add.  Please
contact us if you have the need.

@item
If you want to use @acronym{GPGME} with threads, you must link to the
right version of the library.  The name of the right library is
@code{libgpgme-} followed by the name of the thread package you use.
For example, if you use GNU Pth, the right name is
@code{libgpgme-pth}.  Use the Automake macros or
@command{gpgme-config} program for simplicity.


@item
The function @code{gpgme_check_version} must be called before any
other function in the library, because it initializes the thread
support subsystem in @acronym{GPGME}.  To achieve this in
multi-threaded programs, you must synchronize the memory with respect
to other threads that also want to use @acronym{GPGME}.  For this, it
is sufficient to call @code{gpgme_check_version} before creating the
other threads using @acronym{GPGME}@footnote{At least this is true for
POSIX threads, as @code{pthread_create} is a function that
synchronizes memory with respects to other threads.  There are many
functions which have this property, a complete list can be found in
POSIX, IEEE Std 1003.1-2003, Base Definitions, Issue 6, in the
definition of the term ``Memory Synchronization''.  For other thread
packages other, more relaxed or more strict rules may apply.}.

@item
Any @code{gpgme_data_t} and @code{gpgme_ctx_t} object must only be
accessed by one thread at a time.  If multiple threads want to deal
with the same object, the caller has to make sure that operations on
that object are fully synchronized.

@item
Only one thread at any time is allowed to call @code{gpgme_wait}.  If
multiple threads call this function, the caller must make sure that
all invocations are fully synchronized.  It is safe to start
asynchronous operations while a thread is running in gpgme_wait.

@item
The function @code{gpgme_strerror} is not thread safe.  You have to
use @code{gpgme_strerror_r} instead.
@end itemize


@node Protocols and Engines
@chapter Protocols and Engines
@cindex protocol
@cindex engine
@cindex crypto engine
@cindex backend
@cindex crypto backend

@acronym{GPGME} supports several cryptographic protocols, however, it
does not implement them.  Rather it uses backends (also called
engines) which implement the protocol.  @acronym{GPGME} uses
inter-process communication to pass data back and forth between the
application and the backend, but the details of the communication
protocol and invocation of the backend is completely hidden by the
interface.  All complexity is handled by @acronym{GPGME}.  Where an
exchange of information between the application and the backend is
necessary, @acronym{GPGME} provides the necessary callback function
hooks and further interfaces.

@deftp {Data type} {enum gpgme_protocol_t}
@tindex gpgme_protocol_t
The @code{gpgme_protocol_t} type specifies the set of possible protocol
values that are supported by @acronym{GPGME}.  The following protocols
are supported:

@table @code
@item GPGME_PROTOCOL_OpenPGP
This specifies the OpenPGP protocol.

@item GPGME_PROTOCOL_CMS
This specifies the Cryptographic Message Syntax.

@item GPGME_PROTOCOL_ASSUAN
Under development.  Please ask on @email{gnupg-devel@@gnupg.org} for help.

@item GPGME_PROTOCOL_G13
Under development.  Please ask on @email{gnupg-devel@@gnupg.org} for help.

@item GPGME_PROTOCOL_UISERVER
Under development.  Please ask on @email{gnupg-devel@@gnupg.org} for help.

@item GPGME_PROTOCOL_UNKNOWN
Reserved for future extension.  You may use this to indicate that the
used protocol is not known to the application.  Currently,
@acronym{GPGME} does not accept this value in any operation, though,
except for @code{gpgme_get_protocol_name}.
@end table
@end deftp


@deftypefun {const char *} gpgme_get_protocol_name (@w{gpgme_protocol_t @var{protocol}})
The function @code{gpgme_get_protocol_name} returns a statically
allocated string describing the protocol @var{protocol}, or
@code{NULL} if the protocol number is not valid.
@end deftypefun

@menu
* Engine Version Check::          Verifying the engine version.
* Engine Information::            Obtaining more information about the engines.
* Engine Configuration::          Changing the engine configuration.
* OpenPGP::                       Support for the OpenPGP protocol.
* Cryptographic Message Syntax::  Support for the CMS.
@end menu


@node Engine Version Check
@section Engine Version Check
@cindex version check, of the engines

@deftypefun gpgme_error_t gpgme_engine_check_version (@w{gpgme_protocol_t @var{protocol}})
The function @code{gpgme_engine_check_version} verifies that the
engine implementing the protocol @var{PROTOCOL} is installed in the
expected path and meets the version requirement of @acronym{GPGME}.

This function returns the error code @code{GPG_ERR_NO_ERROR} if the
engine is available and @code{GPG_ERR_INV_ENGINE} if it is not.
@end deftypefun


@node Engine Information
@section Engine Information
@cindex engine, information about

@deftp {Data type} {gpgme_engine_info_t}
@tindex gpgme_protocol_t
The @code{gpgme_engine_info_t} type specifies a pointer to a structure
describing a crypto engine.  The structure contains the following
elements:

@table @code
@item gpgme_engine_info_t next
This is a pointer to the next engine info structure in the linked
list, or @code{NULL} if this is the last element.

@item gpgme_protocol_t protocol
This is the protocol for which the crypto engine is used.  You can
convert this to a string with @code{gpgme_get_protocol_name} for
printing.

@item const char *file_name
This is a string holding the file name of the executable of the crypto
engine.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.

@item const char *home_dir
This is a string holding the directory name of the crypto engine's
configuration directory.  If it is @code{NULL}, then the default
directory is used.

@item const char *version
This is a string containing the version number of the crypto engine.
It might be @code{NULL} if the version number can not be determined,
for example because the executable doesn't exist or is invalid.

@item const char *req_version
This is a string containing the minimum required version number of the
crypto engine for @acronym{GPGME} to work correctly.  This is the
version number that @code{gpgme_engine_check_version} verifies
against.  Currently, it is never @code{NULL}, but using @code{NULL} is
reserved for future use, so always check before you use it.
@end table
@end deftp

@deftypefun gpgme_error_t gpgme_get_engine_info (@w{gpgme_engine_info_t *@var{info}})
The function @code{gpgme_get_engine_info} returns a linked list of
engine info structures in @var{info}.  Each info structure describes
the defaults of one configured backend.

The memory for the info structures is allocated the first time this
function is invoked, and must not be freed by the caller.

This function returns the error code @code{GPG_ERR_NO_ERROR} if
successful, and a system error if the memory could not be allocated.
@end deftypefun

Here is an example how you can provide more diagnostics if you receive
an error message which indicates that the crypto engine is invalid.

@example
gpgme_ctx_t ctx;
gpgme_error_t err;

[...]

if (gpgme_err_code (err) == GPG_ERR_INV_ENGINE)
  @{
    gpgme_engine_info_t info;
    err = gpgme_get_engine_info (&info);
    if (!err)
      @{
        while (info && info->protocol != gpgme_get_protocol (ctx))
          info = info->next;
        if (!info)
          fprintf (stderr, "GPGME compiled without support for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
        else if (info->file_name && !info->version)
          fprintf (stderr, "Engine %s not installed properly",
                   info->file_name);
        else if (info->file_name && info->version && info->req_version)
          fprintf (stderr, "Engine %s version %s installed, "
                   "but at least version %s required", info->file_name,
                   info->version, info->req_version);
        else
          fprintf (stderr, "Unknown problem with engine for protocol %s",
                   gpgme_get_protocol_name (info->protocol));
      @}
  @}
@end example


@node Engine Configuration
@section Engine Configuration
@cindex engine, configuration of
@cindex configuration of crypto backend

You can change the configuration of a backend engine, and thus change
the executable program and configuration directory to be used.  You
can make these changes the default or set them for some contexts
individually.

@deftypefun gpgme_error_t gpgme_set_engine_info (@w{gpgme_protocol_t @var{proto}}, @w{const char *@var{file_name}}, @w{const char *@var{home_dir}})
The function @code{gpgme_set_engine_info} changes the default
configuration of the crypto engine implementing the protocol
@var{proto}.

@var{file_name} is the file name of the executable program
implementing this protocol, and @var{home_dir} is the directory name
of the configuration directory for this crypto engine.  If
@var{home_dir} is @code{NULL}, the engine's default will be used.

The new defaults are not applied to already created GPGME contexts.

This function returns the error code @code{GPG_ERR_NO_ERROR} if
successful, or an eror code on failure.
@end deftypefun

The functions @code{gpgme_ctx_get_engine_info} and
@code{gpgme_ctx_set_engine_info} can be used to change the engine
configuration per context.  @xref{Crypto Engine}.


@node OpenPGP
@section OpenPGP
@cindex OpenPGP
@cindex GnuPG
@cindex protocol, GnuPG
@cindex engine, GnuPG

OpenPGP is implemented by GnuPG, the @acronym{GNU} Privacy Guard.
This is the first protocol that was supported by @acronym{GPGME}.

The OpenPGP protocol is specified by @code{GPGME_PROTOCOL_OpenPGP}.


@node Cryptographic Message Syntax
@section Cryptographic Message Syntax
@cindex CMS
@cindex cryptographic message syntax
@cindex GpgSM
@cindex protocol, CMS
@cindex engine, GpgSM
@cindex S/MIME
@cindex protocol, S/MIME

@acronym{CMS} is implemented by GpgSM, the S/MIME implementation for
GnuPG.

The @acronym{CMS} protocol is specified by @code{GPGME_PROTOCOL_CMS}.


@node Algorithms
@chapter Algorithms
@cindex algorithms

The crypto backends support a variety of algorithms used in public key
cryptography.@footnote{Some engines also provide symmetric only
encryption; see the description of the encryption function on how to use
this.}  The following sections list the identifiers used to denote such
an algorithm.

@menu
* Public Key Algorithms::         A list of all public key algorithms.
* Hash Algorithms::               A list of all hash algorithms.
@end menu


@node Public Key Algorithms
@section Public Key Algorithms
@cindex algorithms, public key
@cindex public key algorithms

Public key algorithms are used for encryption, decryption, signing and
verification of signatures.

@deftp {Data type} {enum gpgme_pubkey_algo_t}
@tindex gpgme_pubkey_algo_t
The @code{gpgme_pubkey_algo_t} type specifies the set of all public key
algorithms that are supported by @acronym{GPGME}.  Possible values
are:

@table @code
@item GPGME_PK_RSA
This value indicates the RSA (Rivest, Shamir, Adleman) algorithm.

@item GPGME_PK_RSA_E
Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman)
algorithm for encryption and decryption only.

@item GPGME_PK_RSA_S
Deprecated.  This value indicates the RSA (Rivest, Shamir, Adleman)
algorithm for signing and verification only.

@item GPGME_PK_DSA
This value indicates DSA, the Digital Signature Algorithm.

@item GPGME_PK_ELG
This value indicates ElGamal.

@item GPGME_PK_ELG_E
This value also indicates ElGamal and is used specifically in GnuPG.

@item GPGME_PK_ELG_E
This value also indicates ElGamal and is used specifically in GnuPG.

@item GPGME_PK_ECDSA
This value indicates ECDSA, the Elliptic Curve Digital Signature
Algorithm as defined by FIPS 186-2.

@item GPGME_PK_ECDH
This value indicates ECDH, the Eliptic Curve Diffie-Hellmann encryption
algorithm as defined by the ECC in OpenPGP draft.

@end table
@end deftp

@deftypefun {const char *} gpgme_pubkey_algo_name (@w{gpgme_pubkey_algo_t @var{algo}})
The function @code{gpgme_pubkey_algo_name} returns a pointer to a
statically allocated string containing a description of the public key
algorithm @var{algo}.  This string can be used to output the name of
the public key algorithm to the user.

If @var{algo} is not a valid public key algorithm, @code{NULL} is
returned.
@end deftypefun


@node Hash Algorithms
@section Hash Algorithms
@cindex algorithms, hash
@cindex algorithms, message digest
@cindex hash algorithms
@cindex message digest algorithms

Hash (message digest) algorithms are used to compress a long message
to make it suitable for public key cryptography.

@deftp {Data type} {enum gpgme_hash_algo_t}
@tindex gpgme_hash_algo_t
The @code{gpgme_hash_algo_t} type specifies the set of all hash algorithms
that are supported by @acronym{GPGME}.  Possible values are:

@table @code
@item GPGME_MD_MD5
@item GPGME_MD_SHA1
@item GPGME_MD_RMD160
@item GPGME_MD_MD2
@item GPGME_MD_TIGER
@item GPGME_MD_HAVAL
@item GPGME_MD_SHA256
@item GPGME_MD_SHA384
@item GPGME_MD_SHA512
@item GPGME_MD_MD4
@item GPGME_MD_CRC32
@item GPGME_MD_CRC32_RFC1510
@item GPGME_MD_CRC24_RFC2440
@end table
@end deftp

@deftypefun {const char *} gpgme_hash_algo_name (@w{gpgme_hash_algo_t @var{algo}})
The function @code{gpgme_hash_algo_name} returns a pointer to a
statically allocated string containing a description of the hash
algorithm @var{algo}.  This string can be used to output the name of
the hash algorithm to the user.

If @var{algo} is not a valid hash algorithm, @code{NULL} is returned.
@end deftypefun


@node Error Handling
@chapter Error Handling
@cindex error handling

Many functions in @acronym{GPGME} can return an error if they fail.
For this reason, the application should always catch the error
condition and take appropriate measures, for example by releasing the
resources and passing the error up to the caller, or by displaying a
descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the
operation, but the result of an operation that failed properly.  For
example, if you try to decrypt a tempered message, the decryption will
fail.  Another error value actually means that the end of a data
buffer or list has been reached.  The following descriptions explain
for many error codes what they mean usually.  Some error values have
specific meanings if returned by a certain functions.  Such cases are
described in the documentation of those functions.

@acronym{GPGME} uses the @code{libgpg-error} library.  This allows to
share the error codes with other components of the GnuPG system, and
thus pass error values transparently from the crypto engine, or some
helper application of the crypto engine, to the user.  This way no
information is lost.  As a consequence, @acronym{GPGME} does not use
its own identifiers for error codes, but uses those provided by
@code{libgpg-error}.  They usually start with @code{GPG_ERR_}.

However, @acronym{GPGME} does provide aliases for the functions
defined in libgpg-error, which might be preferred for name space
consistency.

@menu
* Error Values::                  The error value and what it means.
* Error Sources::                 A list of important error sources.
* Error Codes::                   A list of important error codes.
* Error Strings::                 How to get a descriptive string from a value.
@end menu


@node Error Values
@section Error Values
@cindex error values
@cindex error codes
@cindex error sources

@deftp {Data type} {gpgme_err_code_t}
The @code{gpgme_err_code_t} type is an alias for the @code{libgpg-error}
type @code{gpg_err_code_t}.  The error code indicates the type of an
error, or the reason why an operation failed.

A list of important error codes can be found in the next section.
@end deftp

@deftp {Data type} {gpgme_err_source_t}
The @code{gpgme_err_source_t} type is an alias for the
@code{libgpg-error} type @code{gpg_err_source_t}.  The error source
has not a precisely defined meaning.  Sometimes it is the place where
the error happened, sometimes it is the place where an error was
encoded into an error value.  Usually the error source will give an
indication to where to look for the problem.  This is not always true,
but it is attempted to achieve this goal.

A list of important error sources can be found in the next section.
@end deftp

@deftp {Data type} {gpgme_error_t}
The @code{gpgme_error_t} type is an alias for the @code{libgpg-error}
type @code{gpg_error_t}.  An error value like this has always two
components, an error code and an error source.  Both together form the
error value.

Thus, the error value can not be directly compared against an error
code, but the accessor functions described below must be used.
However, it is guaranteed that only 0 is used to indicate success
(@code{GPG_ERR_NO_ERROR}), and that in this case all other parts of
the error value are set to 0, too.

Note that in @acronym{GPGME}, the error source is used purely for
diagnostical purposes.  Only the error code should be checked to test
for a certain outcome of a function.  The manual only documents the
error code part of an error value.  The error source is left
unspecified and might be anything.
@end deftp

@deftypefun {static inline gpgme_err_code_t} gpgme_err_code (@w{gpgme_error_t @var{err}})
The static inline function @code{gpgme_err_code} returns the
@code{gpgme_err_code_t} component of the error value @var{err}.  This
function must be used to extract the error code from an error value in
order to compare it with the @code{GPG_ERR_*} error code macros.
@end deftypefun

@deftypefun {static inline gpgme_err_source_t} gpgme_err_source (@w{gpgme_error_t @var{err}})
The static inline function @code{gpgme_err_source} returns the
@code{gpgme_err_source_t} component of the error value @var{err}.  This
function must be used to extract the error source from an error value in
order to compare it with the @code{GPG_ERR_SOURCE_*} error source macros.
@end deftypefun

@deftypefun {static inline gpgme_error_t} gpgme_err_make (@w{gpgme_err_source_t @var{source}}, @w{gpgme_err_code_t @var{code}})
The static inline function @code{gpgme_err_make} returns the error
value consisting of the error source @var{source} and the error code
@var{code}.

This function can be used in callback functions to construct an error
value to return it to the library.
@end deftypefun

@deftypefun {static inline gpgme_error_t} gpgme_error (@w{gpgme_err_code_t @var{code}})
The static inline function @code{gpgme_error} returns the error value
consisting of the default error source and the error code @var{code}.

For @acronym{GPGME} applications, the default error source is
@code{GPG_ERR_SOURCE_USER_1}.  You can define
@code{GPGME_ERR_SOURCE_DEFAULT} before including @file{gpgme.h} to
change this default.

This function can be used in callback functions to construct an error
value to return it to the library.
@end deftypefun

The @code{libgpg-error} library provides error codes for all system
error numbers it knows about.  If @var{err} is an unknown error
number, the error code @code{GPG_ERR_UNKNOWN_ERRNO} is used.  The
following functions can be used to construct error values from system
errnor numbers.

@deftypefun {gpgme_error_t} gpgme_err_make_from_errno (@w{gpgme_err_source_t @var{source}}, @w{int @var{err}})
The function @code{gpgme_err_make_from_errno} is like
@code{gpgme_err_make}, but it takes a system error like @code{errno}
instead of a @code{gpgme_err_code_t} error code.
@end deftypefun

@deftypefun {gpgme_error_t} gpgme_error_from_errno (@w{int @var{err}})
The function @code{gpgme_error_from_errno} is like @code{gpgme_error},
but it takes a system error like @code{errno} instead of a
@code{gpgme_err_code_t} error code.
@end deftypefun

Sometimes you might want to map system error numbers to error codes
directly, or map an error code representing a system error back to the
system error number.  The following functions can be used to do that.

@deftypefun {gpgme_err_code_t} gpgme_err_code_from_errno (@w{int @var{err}})
The function @code{gpgme_err_code_from_errno} returns the error code
for the system error @var{err}.  If @var{err} is not a known system
error, the function returns @code{GPG_ERR_UNKNOWN_ERRNO}.
@end deftypefun

@deftypefun {int} gpgme_err_code_to_errno (@w{gpgme_err_code_t @var{err}})
The function @code{gpgme_err_code_to_errno} returns the system error
for the error code @var{err}.  If @var{err} is not an error code
representing a system error, or if this system error is not defined on
this system, the function returns @code{0}.
@end deftypefun


@node Error Sources
@section Error Sources
@cindex error codes, list of

The library @code{libgpg-error} defines an error source for every
component of the GnuPG system.  The error source part of an error
value is not well defined.  As such it is mainly useful to improve the
diagnostic error message for the user.

If the error code part of an error value is @code{0}, the whole error
value will be @code{0}.  In this case the error source part is of
course @code{GPG_ERR_SOURCE_UNKNOWN}.

The list of error sources that might occur in applications using
@acronym{GPGME} is:

@table @code
@item GPG_ERR_SOURCE_UNKNOWN
The error source is not known.  The value of this error source is
@code{0}.

@item GPG_ERR_SOURCE_GPGME
The error source is @acronym{GPGME} itself.  This is the default for
errors that occur in the @acronym{GPGME} library.

@item GPG_ERR_SOURCE_GPG
The error source is GnuPG, which is the crypto engine used for the
OpenPGP protocol.

@item GPG_ERR_SOURCE_GPGSM
The error source is GPGSM, which is the crypto engine used for the
CMS protocol.

@item GPG_ERR_SOURCE_GCRYPT
The error source is @code{libgcrypt}, which is used by crypto engines
to perform cryptographic operations.

@item GPG_ERR_SOURCE_GPGAGENT
The error source is @command{gpg-agent}, which is used by crypto
engines to perform operations with the secret key.

@item GPG_ERR_SOURCE_PINENTRY
The error source is @command{pinentry}, which is used by
@command{gpg-agent} to query the passphrase to unlock a secret key.

@item GPG_ERR_SOURCE_SCD
The error source is the SmartCard Daemon, which is used by
@command{gpg-agent} to delegate operations with the secret key to a
SmartCard.

@item GPG_ERR_SOURCE_KEYBOX
The error source is @code{libkbx}, a library used by the crypto
engines to manage local keyrings.

@item GPG_ERR_SOURCE_USER_1
@item GPG_ERR_SOURCE_USER_2
@item GPG_ERR_SOURCE_USER_3
@item GPG_ERR_SOURCE_USER_4
These error sources are not used by any GnuPG component and can be
used by other software.  For example, applications using
@acronym{GPGME} can use them to mark error values coming from callback
handlers.  Thus @code{GPG_ERR_SOURCE_USER_1} is the default for errors
created with @code{gpgme_error} and @code{gpgme_error_from_errno},
unless you define @code{GPGME_ERR_SOURCE_DEFAULT} before including
@file{gpgme.h}.
@end table


@node Error Codes
@section Error Codes
@cindex error codes, list of

The library @code{libgpg-error} defines many error values.  Most of
them are not used by @code{GPGME} directly, but might be returned by
@acronym{GPGME} because it received them from the crypto engine.  The
below list only includes such error codes that have a specific meaning
in @code{GPGME}, or which are so common that you should know about
them.

@table @code
@item GPG_ERR_EOF
This value indicates the end of a list, buffer or file.

@item GPG_ERR_NO_ERROR
This value indicates success.  The value of this error code is
@code{0}.  Also, it is guaranteed that an error value made from the
error code @code{0} will be @code{0} itself (as a whole).  This means
that the error source information is lost for this error code,
however, as this error code indicates that no error occured, this is
generally not a problem.

@item GPG_ERR_GENERAL
This value means that something went wrong, but either there is not
enough information about the problem to return a more useful error
value, or there is no separate error value for this type of problem.

@item GPG_ERR_ENOMEM
This value means that an out-of-memory condition occurred.

@item GPG_ERR_E...
System errors are mapped to GPG_ERR_FOO where FOO is the symbol for
the system error.

@item GPG_ERR_INV_VALUE
This value means that some user provided data was out of range.  This
can also refer to objects.  For example, if an empty
@code{gpgme_data_t} object was expected, but one containing data was
provided, this error value is returned.

@item GPG_ERR_UNUSABLE_PUBKEY
This value means that some recipients for a message were invalid.

@item GPG_ERR_UNUSABLE_SECKEY
This value means that some signers were invalid.

@item GPG_ERR_NO_DATA
This value means that a @code{gpgme_data_t} object which was expected
to have content was found empty.

@item GPG_ERR_CONFLICT
This value means that a conflict of some sort occurred.

@item GPG_ERR_NOT_IMPLEMENTED
This value indicates that the specific function (or operation) is not
implemented.  This error should never happen.  It can only occur if
you use certain values or configuration options which do not work,
but for which we think that they should work at some later time.

@item GPG_ERR_DECRYPT_FAILED
This value indicates that a decryption operation was unsuccessful.

@item GPG_ERR_BAD_PASSPHRASE
This value means that the user did not provide a correct passphrase
when requested.

@item GPG_ERR_CANCELED
This value means that the operation was canceled.

@item GPG_ERR_INV_ENGINE
This value means that the engine that implements the desired protocol
is currently not available.  This can either be because the sources
were configured to exclude support for this engine, or because the
engine is not installed properly.

@item GPG_ERR_AMBIGUOUS_NAME
This value indicates that a user ID or other specifier did not specify
a unique key.

@item GPG_ERR_WRONG_KEY_USAGE
This value indicates that a key is not used appropriately.

@item GPG_ERR_CERT_REVOKED
This value indicates that a key signature was revoced.

@item GPG_ERR_CERT_EXPIRED
This value indicates that a key signature expired.

@item GPG_ERR_NO_CRL_KNOWN
This value indicates that no certificate revocation list is known for
the certificate.

@item GPG_ERR_NO_POLICY_MATCH
This value indicates that a policy issue occured.

@item GPG_ERR_NO_SECKEY
This value indicates that no secret key for the user ID is available.

@item GPG_ERR_MISSING_CERT
This value indicates that a key could not be imported because the
issuer certificate is missing.

@item GPG_ERR_BAD_CERT_CHAIN
This value indicates that a key could not be imported because its
certificate chain is not good, for example it could be too long.

@item GPG_ERR_UNSUPPORTED_ALGORITHM
This value means a verification failed because the cryptographic
algorithm is not supported by the crypto backend.

@item GPG_ERR_BAD_SIGNATURE
This value means a verification failed because the signature is bad.

@item GPG_ERR_NO_PUBKEY
This value means a verification failed because the public key is not
available.

@item GPG_ERR_USER_1
@item GPG_ERR_USER_2
@item ...
@item GPG_ERR_USER_16
These error codes are not used by any GnuPG component and can be
freely used by other software.  Applications using @acronym{GPGME}
might use them to mark specific errors returned by callback handlers
if no suitable error codes (including the system errors) for
these errors exist already.
@end table


@node Error Strings
@section Error Strings
@cindex error values, printing of
@cindex error codes, printing of
@cindex error sources, printing of
@cindex error strings

@deftypefun {const char *} gpgme_strerror (@w{gpgme_error_t @var{err}})
The function @code{gpgme_strerror} returns a pointer to a statically
allocated string containing a description of the error code contained
in the error value @var{err}.  This string can be used to output a
diagnostic message to the user.

This function is not thread safe.  Use @code{gpgme_strerror_r} in
multi-threaded programs.
@end deftypefun


@deftypefun {int} gpgme_strerror_r (@w{gpgme_error_t @var{err}}, @w{char *@var{buf}}, @w{size_t @var{buflen}})
The function @code{gpgme_strerror_r} returns the error string for
@var{err} in the user-supplied buffer @var{buf} of size @var{buflen}.
This function is, in contrast to @code{gpgme_strerror}, thread-safe if
a thread-safe @code{strerror_r} function is provided by the system.
If the function succeeds, 0 is returned and @var{buf} contains the
string describing the error.  If the buffer was not large enough,
ERANGE is returned and @var{buf} contains as much of the beginning of
the error string as fits into the buffer.
@end deftypefun


@deftypefun {const char *} gpgme_strsource (@w{gpgme_error_t @var{err}})
The function @code{gpgme_strerror} returns a pointer to a statically
allocated string containing a description of the error source
contained in the error value @var{err}.  This string can be used to
output a diagnostic message to the user.
@end deftypefun

The following example illustrates the use of @code{gpgme_strerror}:

@example
gpgme_ctx_t ctx;
gpgme_error_t err = gpgme_new (&ctx);
if (err)
  @{
    fprintf (stderr, "%s: creating GpgME context failed: %s: %s\n",
             argv[0], gpgme_strsource (err), gpgme_strerror (err));
    exit (1);
  @}
@end example


@node Exchanging Data
@chapter Exchanging Data
@cindex data, exchanging

A lot of data has to be exchanged between the user and the crypto
engine, like plaintext messages, ciphertext, signatures and
information about the keys.  The technical details about exchanging
the data information are completely abstracted by @acronym{GPGME}.
The user provides and receives the data via @code{gpgme_data_t} objects,
regardless of the communication protocol between @acronym{GPGME} and
the crypto engine in use.

@deftp {Data type} {gpgme_data_t}
The @code{gpgme_data_t} type is a handle for a container for generic
data, which is used by @acronym{GPGME} to exchange data with the user.
@end deftp

@code{gpgme_data_t} objects do not provide notifications on events.
It is assumed that read and write operations are blocking until data
is available.  If this is undesirable, the application must ensure
that all GPGME data operations always have data available, for example
by using memory buffers or files rather than pipes or sockets.  This
might be relevant, for example, if the external event loop mechanism
is used.

@deftp {Data type} {gpgme_off_t}
On POSIX platforms the @code{gpgme_off_t} type is an alias for
@code{off_t}; it may be used interchangeable.  On Windows platforms
@code{gpgme_off_t} is defined as a long (i.e. 32 bit) for 32 bit
Windows and as a 64 bit signed integer for 64 bit Windows.
@end deftp

@deftp {Data type} {gpgme_ssize_t}
The @code{gpgme_ssize_t} type is an alias for @code{ssize_t}.  It has
only been introduced to overcome portability problems pertaining to
the declaration of @code{ssize_t} by different toolchains.
@end deftp


@menu
* Creating Data Buffers::         Creating new data buffers.
* Destroying Data Buffers::       Releasing data buffers.
* Manipulating Data Buffers::     Operations on data buffers.
@end menu


@node Creating Data Buffers
@section Creating Data Buffers
@cindex data buffer, creation

Data objects can be based on memory, files, or callback functions
provided by the user.  Not all operations are supported by all
objects.


@menu
* Memory Based Data Buffers::     Creating memory based data buffers.
* File Based Data Buffers::       Creating file based data buffers.
* Callback Based Data Buffers::   Creating callback based data buffers.
@end menu


@node Memory Based Data Buffers
@subsection Memory Based Data Buffers

Memory based data objects store all data in allocated memory.  This is
convenient, but only practical for an amount of data that is a
fraction of the available physical memory.  The data has to be copied
from its source and to its destination, which can often be avoided by
using one of the other data object

@deftypefun gpgme_error_t gpgme_data_new (@w{gpgme_data_t *@var{dh}})
The function @code{gpgme_data_new} creates a new @code{gpgme_data_t}
object and returns a handle for it in @var{dh}.  The data object is
memory based and initially empty.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_mem (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{buffer}}, @w{size_t @var{size}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_mem} creates a new
@code{gpgme_data_t} object and fills it with @var{size} bytes starting
from @var{buffer}.

If @var{copy} is not zero, a private copy of the data is made.  If
@var{copy} is zero, the data is taken from the specified buffer as
needed, and the user has to ensure that the buffer remains valid for
the whole life span of the data object.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{buffer} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_file (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{filename}}, @w{int @var{copy}})
The function @code{gpgme_data_new_from_file} creates a new
@code{gpgme_data_t} object and fills it with the content of the file
@var{filename}.

If @var{copy} is not zero, the whole file is read in at initialization
time and the file is not used anymore after that.  This is the only
mode supported currently.  Later, a value of zero for @var{copy} might
cause all reads to be delayed until the data is needed, but this is
not yet implemented.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{filename} is not a valid pointer,
@code{GPG_ERR_NOT_IMPLEMENTED} if @var{code} is zero, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_filepart (@w{gpgme_data_t *@var{dh}}, @w{const char *@var{filename}}, @w{FILE *@var{fp}}, @w{off_t @var{offset}}, @w{size_t @var{length}})
The function @code{gpgme_data_new_from_filepart} creates a new
@code{gpgme_data_t} object and fills it with a part of the file specified
by @var{filename} or @var{fp}.

Exactly one of @var{filename} and @var{fp} must be non-zero, the other
must be zero.  The argument that is not zero specifies the file from
which @var{length} bytes are read into the data object, starting from
@var{offset}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} and exactly one of @var{filename} and @var{fp} is not a valid
pointer, and @code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun


@node File Based Data Buffers
@subsection File Based Data Buffers

File based data objects operate directly on file descriptors or
streams.  Only a small amount of data is stored in core at any time,
so the size of the data objects is not limited by @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_new_from_fd (@w{gpgme_data_t *@var{dh}}, @w{int @var{fd}})
The function @code{gpgme_data_new_from_fd} creates a new
@code{gpgme_data_t} object and uses the file descriptor @var{fd} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the file descriptor than is actually needed by the
crypto engine in the desired operation because of internal buffering.

Note that GPGME assumes that the file descriptor is set to blocking
mode.  Errors during I/O operations, except for EINTR, are usually
fatal for crypto operations.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_new_from_stream (@w{gpgme_data_t *@var{dh}}, @w{FILE *@var{stream}})
The function @code{gpgme_data_new_from_stream} creates a new
@code{gpgme_data_t} object and uses the I/O stream @var{stream} to read
from (if used as an input data object) and write to (if used as an
output data object).

When using the data object as an input buffer, the function might read
a bit more from the stream than is actually needed by the crypto
engine in the desired operation because of internal buffering.

Note that GPGME assumes that the stream is in blocking mode.  Errors
during I/O operations, except for EINTR, are usually fatal for crypto
operations.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun


@node Callback Based Data Buffers
@subsection Callback Based Data Buffers

If neither memory nor file based data objects are a good fit for your
application, you can implement the functions a data object provides
yourself and create a data object from these callback functions.

@deftp {Data type} {ssize_t (*gpgme_data_read_cb_t) (@w{void *@var{handle}}, @w{void @var{*buffer}}, @w{size_t @var{size}})}
@tindex gpgme_data_read_cb_t
The @code{gpgme_data_read_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to read data from a user-implemented
data object.  The function should read up to @var{size} bytes from the
current read position into the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

Note that GPGME assumes that the read blocks until data is available.
Errors during I/O operations, except for EINTR, are usually fatal for
crypto operations.

The function should return the number of bytes read, 0 on EOF, and -1
on error.  If an error occurs, @var{errno} should be set to describe
the type of the error.
@end deftp

@deftp {Data type} {ssize_t (*gpgme_data_write_cb_t) (@w{void *@var{handle}}, @w{const void @var{*buffer}}, @w{size_t @var{size}})}
@tindex gpgme_data_write_cb_t
The @code{gpgme_data_write_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to write data to a user-implemented
data object.  The function should write up to @var{size} bytes to the
current write position from the space starting at @var{buffer}.  The
@var{handle} is provided by the user at data object creation time.

Note that GPGME assumes that the write blocks until data is available.
Errors during I/O operations, except for EINTR, are usually fatal for
crypto operations.

The function should return the number of bytes written, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {off_t (*gpgme_data_seek_cb_t) (@w{void *@var{handle}}, @w{off_t @var{offset}}, @w{int @var{whence}})}
@tindex gpgme_data_seek_cb_t
The @code{gpgme_data_seek_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to change the current read/write
position in a user-implemented data object, just like the @code{lseek}
function.

The function should return the new read/write position, and -1 on
error.  If an error occurs, @var{errno} should be set to describe the
type of the error.
@end deftp

@deftp {Data type} {void (*gpgme_data_release_cb_t) (@w{void *@var{handle}})}
@tindex gpgme_data_release_cb_t
The @code{gpgme_data_release_cb_t} type is the type of functions which
@acronym{GPGME} calls if it wants to destroy a user-implemented data
object.  The @var{handle} is provided by the user at data object
creation time.
@end deftp

@deftp {Data type} {struct gpgme_data_cbs}
This structure is used to store the data callback interface functions
described above.  It has the following members:

@table @code
@item gpgme_data_read_cb_t read
This is the function called by @acronym{GPGME} to read data from the
data object.  It is only required for input data object.

@item gpgme_data_write_cb_t write
This is the function called by @acronym{GPGME} to write data to the
data object.  It is only required for output data object.

@item gpgme_data_seek_cb_t seek
This is the function called by @acronym{GPGME} to change the current
read/write pointer in the data object (if available).  It is optional.

@item gpgme_data_release_cb_t release
This is the function called by @acronym{GPGME} to release a data
object.  It is optional.
@end table
@end deftp

@deftypefun gpgme_error_t gpgme_data_new_from_cbs (@w{gpgme_data_t *@var{dh}}, @w{gpgme_data_cbs_t @var{cbs}}, @w{void *@var{handle}})
The function @code{gpgme_data_new_from_cbs} creates a new
@code{gpgme_data_t} object and uses the user-provided callback functions
to operate on the data object.

The handle @var{handle} is passed as first argument to the callback
functions.  This can be used to identify this data object.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun

The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_new_with_read_cb (@w{gpgme_data_t *@var{dh}}, @w{int (*@var{readfunc})} (@w{void *@var{hook}}, @w{char *@var{buffer}}, @w{size_t @var{count}}, @w{size_t *@var{nread}}), @w{void *@var{hook_value}})
The function @code{gpgme_data_new_with_read_cb} creates a new
@code{gpgme_data_t} object and uses the callback function @var{readfunc}
to retrieve the data on demand.  As the callback function can supply
the data in any way it wants, this is the most flexible data type
@acronym{GPGME} provides.  However, it can not be used to write data.

The callback function receives @var{hook_value} as its first argument
whenever it is invoked.  It should return up to @var{count} bytes in
@var{buffer}, and return the number of bytes actually read in
@var{nread}.  It may return @code{0} in @var{nread} if no data is
currently available.  To indicate @code{EOF} the function should
return with an error code of @code{-1} and set @var{nread} to
@code{0}.  The callback function may support to reset its internal
read pointer if it is invoked with @var{buffer} and @var{nread} being
@code{NULL} and @var{count} being @code{0}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
data object was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{dh} or @var{readfunc} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if not enough memory is available.
@end deftypefun


@node Destroying Data Buffers
@section Destroying Data Buffers
@cindex data buffer, destruction

@deftypefun void gpgme_data_release (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_release} destroys the data object with
the handle @var{dh}.  It releases all associated resources that were
not provided by the user in the first place.
@end deftypefun

@deftypefun {char *} gpgme_data_release_and_get_mem (@w{gpgme_data_t @var{dh}}, @w{size_t *@var{length}})
The function @code{gpgme_data_release_and_get_mem} is like
@code{gpgme_data_release}, except that it returns the data buffer and
its length that was provided by the object.

The user has to release the buffer with @code{gpgme_free}.  In case
the user provided the data buffer in non-copy mode, a copy will be
made for this purpose.

In case an error returns, or there is no suitable data buffer that can
be returned to the user, the function will return @code{NULL}.  In any
case, the data object @var{dh} is destroyed.
@end deftypefun


@deftypefun void gpgme_free (@w{void *@var{buffer}})
The function @code{gpgme_free} releases the memory returned by
@code{gpgme_data_release_and_get_mem}.  It should be used instead of
the system libraries @code{free} function in case different allocators
are used in a single program.
@end deftypefun


@node Manipulating Data Buffers
@section Manipulating Data Buffers
@cindex data buffer, manipulation

Data buffers contain data and meta-data.  The following operations can
be used to manipulate both.


@menu
* Data Buffer I/O Operations::    I/O operations on data buffers.
* Data Buffer Meta-Data::         Meta-data manipulation of data buffers.
* Data Buffer Convenience::       Convenience fucntion for data buffers.
@end menu


@node Data Buffer I/O Operations
@subsection Data Buffer I/O Operations
@cindex data buffer, I/O operations
@cindex data buffer, read
@cindex data buffer, write
@cindex data buffer, seek

@deftypefun ssize_t gpgme_data_read (@w{gpgme_data_t @var{dh}}, @w{void *@var{buffer}}, @w{size_t @var{length}})
The function @code{gpgme_data_read} reads up to @var{length} bytes
from the data object with the handle @var{dh} into the space starting
at @var{buffer}.

If no error occurs, the actual amount read is returned.  If the end of
the data object is reached, the function returns 0.

In all other cases, the function returns -1 and sets @var{errno}.
@end deftypefun

@deftypefun ssize_t gpgme_data_write (@w{gpgme_data_t @var{dh}}, @w{const void *@var{buffer}}, @w{size_t @var{size}})
The function @code{gpgme_data_write} writes up to @var{size} bytes
starting from @var{buffer} into the data object with the handle
@var{dh} at the current write position.

The function returns the number of bytes actually written, or -1 if an
error occurs.  If an error occurs, @var{errno} is set.
@end deftypefun

@deftypefun off_t gpgme_data_seek (@w{gpgme_data_t @var{dh}}, @w{off_t @var{offset}}, @w{int @var{whence}})
The function @code{gpgme_data_seek} changes the current read/write
position.

The @var{whence} argument specifies how the @var{offset} should be
interpreted.  It must be one of the following symbolic constants:

@table @code
@item SEEK_SET
Specifies that @var{offset} is a count of characters from the
beginning of the data object.

@item SEEK_CUR
Specifies that @var{offset} is a count of characters from the current
file position.  This count may be positive or negative.

@item SEEK_END
Specifies that @var{offset} is a count of characters from the end of
the data object.  A negative count specifies a position within the
current extent of the data object; a positive count specifies a
position past the current end.  If you set the position past the
current end, and actually write data, you will extend the data object
with zeros up to that position.
@end table

If successful, the function returns the resulting file position,
measured in bytes from the beginning of the data object.  You can use
this feature together with @code{SEEK_CUR} to read the current
read/write position.

If the function fails, -1 is returned and @var{errno} is set.
@end deftypefun

The following function is deprecated and should not be used.  It will
be removed in a future version of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_data_rewind (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_rewind} is equivalent to:

@example
  return (gpgme_data_seek (dh, 0, SEEK_SET) == -1)
    ? gpgme_error_from_errno (errno) : 0;
@end example
@end deftypefun




@node Data Buffer Meta-Data
@subsection Data Buffer Meta-Data
@cindex data buffer, meta-data
@cindex data buffer, file name
@cindex data buffer, encoding

@deftypefun {char *} gpgme_data_get_file_name (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_get_file_name} returns a pointer to a
string containing the file name associated with the data object.  The
file name will be stored in the output when encrypting or signing the
data and will be returned to the user when decrypting or verifying the
output data.

If no error occurs, the string containing the file name is returned.
Otherwise, @code{NULL} will be returned.
@end deftypefun


@deftypefun gpgme_error_t gpgme_data_set_file_name (@w{gpgme_data_t @var{dh}}, @w{const char *@var{file_name}})
The function @code{gpgme_data_set_file_name} sets the file name
associated with the data object.  The file name will be stored in the
output when encrypting or signing the data and will be returned to the
user when decrypting or verifying the output data.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{dh} is not a valid pointer and @code{GPG_ERR_ENOMEM} if not
enough memory is available.
@end deftypefun


@deftp {Data type} {enum gpgme_data_encoding_t}
@tindex gpgme_data_encoding_t
The @code{gpgme_data_encoding_t} type specifies the encoding of a
@code{gpgme_data_t} object.  For input data objects, the encoding is
useful to give the backend a hint on the type of data.  For output
data objects, the encoding can specify the output data format on
certain operations.  Please note that not all backends support all
encodings on all operations.  The following data types are available:

@table @code
@item GPGME_DATA_ENCODING_NONE
This specifies that the encoding is not known.  This is the default
for a new data object.  The backend will try its best to detect the
encoding automatically.

@item GPGME_DATA_ENCODING_BINARY
This specifies that the data is encoding in binary form; i.e. there is
no special encoding.

@item GPGME_DATA_ENCODING_BASE64
This specifies that the data is encoded using the Base-64 encoding
scheme as used by @acronym{MIME} and other protocols.

@item GPGME_DATA_ENCODING_ARMOR
This specifies that the data is encoded in an armored form as used by
OpenPGP and PEM.

@item GPGME_DATA_ENCODING_URL
The data is a list of linefeed delimited URLs.  This is only useful with
@code{gpgme_op_import}.

@item GPGME_DATA_ENCODING_URL0
The data is a list of binary zero delimited URLs.  This is only useful
with @code{gpgme_op_import}.

@item GPGME_DATA_ENCODING_URLESC
The data is a list of linefeed delimited URLs with all control and space
characters percent escaped.  This mode is is not yet implemented.

@end table
@end deftp

@deftypefun gpgme_data_encoding_t gpgme_data_get_encoding (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_get_encoding} returns the encoding of
the data object with the handle @var{dh}.  If @var{dh} is not a valid
pointer (e.g. @code{NULL}) @code{GPGME_DATA_ENCODING_NONE} is
returned.
@end deftypefun

@deftypefun gpgme_error_t gpgme_data_set_encoding (@w{gpgme_data_t @var{dh}, gpgme_data_encoding_t @var{enc}})
The function @code{gpgme_data_set_encoding} changes the encoding of
the data object with the handle @var{dh} to @var{enc}.
@end deftypefun

@node Data Buffer Convenience
@subsection Data Buffer Convenience Functions
@cindex data buffer, convenience
@cindex type of data
@cindex identify

@deftp {Data type} {enum gpgme_data_type_t}
@tindex gpgme_data_type_t
The @code{gpgme_data_type_t} type is used to return the detected type
of the content of a data buffer.
@end deftp

@table @code
@item GPGME_DATA_TYPE_INVALID
This is returned by @code{gpgme_data_identify} if it was not possible
to identify the data.  Reasons for this might be a non-seekable stream
or a memory problem.  The value is 0.
@item GPGME_DATA_TYPE_UNKNOWN
The type of the data is not known.
@item GPGME_DATA_TYPE_PGP_SIGNED
The data is an OpenPGP signed message.  This may be a binary
signature, a detached one or a cleartext signature.
@item GPGME_DATA_TYPE_PGP_OTHER
This is a generic OpenPGP message.  In most cases this will be
encrypted data.
@item GPGME_DATA_TYPE_PGP_KEY
This is an OpenPGP key (private or public).
@item GPGME_DATA_TYPE_CMS_SIGNED
This is a CMS signed message.
@item GPGME_DATA_TYPE_CMS_ENCRYPTED
This is a CMS encrypted (enveloped data) message.
@item GPGME_DATA_TYPE_CMS_OTHER
This is used for other CMS message types.
@item GPGME_DATA_TYPE_X509_CERT
The data is a X.509 certificate
@item GPGME_DATA_TYPE_PKCS12
The data is a PKCS#12 message.  This is commonly used to exchange
private keys for X.509.
@end table

@deftypefun gpgme_data_type_t gpgme_data_identify (@w{gpgme_data_t @var{dh}})
The function @code{gpgme_data_identify} returns the type of the data
with the handle @var{dh}.  If it is not possible to perform the
identification, the function returns zero
(@code{GPGME_DATA_TYPE_INVALID}).  Note that depending on how the data
object has been created the identification may not be possible or the
data object may change its internal state (file pointer moved).  For
file or memory based data object, the state should not change.
@end deftypefun


@c
@c    Chapter Contexts
@c
@node Contexts
@chapter Contexts
@cindex context

All cryptographic operations in @acronym{GPGME} are performed within a
context, which contains the internal state of the operation as well as
configuration parameters.  By using several contexts you can run
several cryptographic operations in parallel, with different
configuration.

@deftp {Data type} {gpgme_ctx_t}
The @code{gpgme_ctx_t} type is a handle for a @acronym{GPGME} context,
which is used to hold the configuration, status and result of
cryptographic operations.
@end deftp

@menu
* Creating Contexts::             Creating new @acronym{GPGME} contexts.
* Destroying Contexts::           Releasing @acronym{GPGME} contexts.
* Result Management::             Managing the result of crypto operations.
* Context Attributes::            Setting properties of a context.
* Key Management::                Managing keys with @acronym{GPGME}.
* Trust Item Management::         Managing trust items with @acronym{GPGME}.
* Crypto Operations::             Using a context for cryptography.
* Run Control::                   Controlling how operations are run.
@end menu


@node Creating Contexts
@section Creating Contexts
@cindex context, creation

@deftypefun gpgme_error_t gpgme_new (@w{gpgme_ctx_t *@var{ctx}})
The function @code{gpgme_new} creates a new @code{gpgme_ctx_t} object
and returns a handle for it in @var{ctx}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
context was successfully created, @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if not
enough memory is available.  Also, it returns
@code{GPG_ERR_NOT_OPERATIONAL} if @code{gpgme_check_version} was not
called to initialize GPGME, and @code{GPG_ERR_SELFTEST_FAILED} if a
selftest failed.  Currently, the only selftest is for Windows MingW32
targets to see if @code{-mms-bitfields} was used (as required).
@end deftypefun


@node Destroying Contexts
@section Destroying Contexts
@cindex context, destruction

@deftypefun void gpgme_release (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_release} destroys the context with the handle
@var{ctx} and releases all associated resources.
@end deftypefun


@node Result Management
@section Result Management
@cindex context, result of operation

The detailed result of an operation is returned in operation-specific
structures such as @code{gpgme_decrypt_result_t}.  The corresponding
retrieval functions such as @code{gpgme_op_decrypt_result} provide
static access to the results after an operation completes.  The
following interfaces make it possible to detach a result structure
from its associated context and give it a lifetime beyond that of the
current operation or context.

@deftypefun void gpgme_result_ref (@w{void *@var{result}})
The function @code{gpgme_result_ref} acquires an additional reference
for the result @var{result}, which may be of any type
@code{gpgme_*_result_t}.  As long as the user holds a reference, the
result structure is guaranteed to be valid and unmodified.
@end deftypefun

@deftypefun void gpgme_result_unref (@w{void *@var{result}})
The function @code{gpgme_result_unref} releases a reference for the
result @var{result}.  If this was the last reference, the result
structure will be destroyed and all resources associated to it will be
released.
@end deftypefun

Note that a context may hold its own references to result structures,
typically until the context is destroyed or the next operation is
started.  In fact, these references are accessed through the
@code{gpgme_op_*_result} functions.


@node Context Attributes
@section Context Attributes
@cindex context, attributes

@menu
* Protocol Selection::            Selecting the protocol used by a context.
* Crypto Engine::                 Configuring the crypto engine.
* ASCII Armor::                   Requesting @acronym{ASCII} armored output.
* Text Mode::                     Choosing canonical text mode.
* Included Certificates::       Including a number of certificates.
* Key Listing Mode::              Selecting key listing mode.
* Passphrase Callback::           Getting the passphrase from the user.
* Progress Meter Callback::       Being informed about the progress.
* Locale::                        Setting the locale of a context.
@end menu


@node Protocol Selection
@subsection Protocol Selection
@cindex context, selecting protocol
@cindex protocol, selecting

@deftypefun gpgme_error_t gpgme_set_protocol (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_protocol_t @var{proto}})
The function @code{gpgme_set_protocol} sets the protocol used within
the context @var{ctx} to @var{proto}.  All crypto operations will be
performed by the crypto engine configured for that protocol.
@xref{Protocols and Engines}.

Setting the protocol with @code{gpgme_set_protocol} does not check if
the crypto engine for that protocol is available and installed
correctly.  @xref{Engine Version Check}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
protocol could be set successfully, and @code{GPG_ERR_INV_VALUE} if
@var{protocol} is not a valid protocol.
@end deftypefun

@deftypefun gpgme_protocol_t gpgme_get_protocol (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_protocol} retrieves the protocol currently
use with the context @var{ctx}.
@end deftypefun


@node Crypto Engine
@subsection Crypto Engine
@cindex context, configuring engine
@cindex engine, configuration per context

The following functions can be used to set and retrieve the
configuration of the crypto engines of a specific context.  The
default can also be retrieved without any particular context.
@xref{Engine Information}.  The default can also be changed globally.
@xref{Engine Configuration}.

@deftypefun gpgme_engine_info_t gpgme_ctx_get_engine_info (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_ctx_get_engine_info} returns a linked list of
engine info structures.  Each info structure describes the
configuration of one configured backend, as used by the context
@var{ctx}.

The result is valid until the next invocation of
@code{gpgme_ctx_set_engine_info} for this particular context.

This function can not fail.
@end deftypefun

@deftypefun gpgme_error_t gpgme_ctx_set_engine_info (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_protocol_t @var{proto}}, @w{const char *@var{file_name}}, @w{const char *@var{home_dir}})
The function @code{gpgme_ctx_set_engine_info} changes the
configuration of the crypto engine implementing the protocol
@var{proto} for the context @var{ctx}.

@var{file_name} is the file name of the executable program
implementing this protocol, and @var{home_dir} is the directory name
of the configuration directory for this crypto engine.  If
@var{home_dir} is @code{NULL}, the engine's default will be used.

Currently this function must be used before starting the first crypto
operation.  It is unspecified if and when the changes will take effect
if the function is called after starting the first operation on the
context @var{ctx}.

This function returns the error code @code{GPG_ERR_NO_ERROR} if
successful, or an eror code on failure.
@end deftypefun


@c FIXME: Unfortunately, using @acronym here breaks texi2dvi.
@node ASCII Armor
@subsection @acronym{ASCII} Armor
@cindex context, armor mode
@cindex @acronym{ASCII} armor
@cindex armor mode

@deftypefun void gpgme_set_armor (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_armor} specifies if the output should be
@acronym{ASCII} armored.  By default, output is not @acronym{ASCII}
armored.

@acronym{ASCII} armored output is disabled if @var{yes} is zero, and
enabled otherwise.
@end deftypefun

@deftypefun int gpgme_get_armor (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_armor} returns 1 if the output is
@acronym{ASCII} armored, and @code{0} if it is not, or if @var{ctx} is
not a valid pointer.
@end deftypefun


@node Text Mode
@subsection Text Mode
@cindex context, text mode
@cindex text mode
@cindex canonical text mode

@deftypefun void gpgme_set_textmode (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{yes}})
The function @code{gpgme_set_textmode} specifies if canonical text mode
should be used.  By default, text mode is not used.

Text mode is for example used for the RFC2015 signatures; note that
the updated RFC 3156 mandates that the mail user agent does some
preparations so that text mode is not needed anymore.

This option is only relevant to the OpenPGP crypto engine, and ignored
by all other engines.

Canonical text mode is disabled if @var{yes} is zero, and enabled
otherwise.
@end deftypefun

@deftypefun int gpgme_get_textmode (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_textmode} returns 1 if canonical text
mode is enabled, and @code{0} if it is not, or if @var{ctx} is not a
valid pointer.
@end deftypefun


@node Included Certificates
@subsection Included Certificates
@cindex certificates, included

@deftypefun void gpgme_set_include_certs (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{nr_of_certs}})
The function @code{gpgme_set_include_certs} specifies how many
certificates should be included in an S/MIME signed message.  By
default, only the sender's certificate is included.  The possible
values of @var{nr_of_certs} are:

@table @code
@item GPGME_INCLUDE_CERTS_DEFAULT
Fall back to the default of the crypto backend.  This is the default
for GPGME.
@item -2
Include all certificates except the root certificate.
@item -1
Include all certificates.
@item 0
Include no certificates.
@item 1
Include the sender's certificate only.
@item n
Include the first n certificates of the certificates path, starting
from the sender's certificate.  The number @code{n} must be positive.
@end table

Values of @var{nr_of_certs} smaller than -2 are undefined.

This option is only relevant to the CMS crypto engine, and ignored by
all other engines.
@end deftypefun

@deftypefun int gpgme_get_include_certs (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_include_certs} returns the number of
certificates to include into an S/MIME signed message.
@end deftypefun


@node Key Listing Mode
@subsection Key Listing Mode
@cindex key listing mode
@cindex key listing, mode of

@deftypefun gpgme_error_t gpgme_set_keylist_mode (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_keylist_mode_t @var{mode}})
The function @code{gpgme_set_keylist_mode} changes the default
behaviour of the key listing functions.  The value in @var{mode} is a
bitwise-or combination of one or multiple of the following bit values:

@table @code
@item GPGME_KEYLIST_MODE_LOCAL
The @code{GPGME_KEYLIST_MODE_LOCAL} symbol specifies that the local
keyring should be searched for keys in the keylisting operation.  This
is the default.

@item GPGME_KEYLIST_MODE_EXTERN
The @code{GPGME_KEYLIST_MODE_EXTERN} symbol specifies that an external
source should be searched for keys in the keylisting operation.  The
type of external source is dependant on the crypto engine used and
whether it is combined with @code{GPGME_KEYLIST_MODE_LOCAL}.  For
example, it can be a remote keyserver or LDAP certificate server.

@item GPGME_KEYLIST_MODE_SIGS
The @code{GPGME_KEYLIST_MODE_SIGS} symbol specifies that the key
signatures should be included in the listed keys.

@item GPGME_KEYLIST_MODE_SIG_NOTATIONS
The @code{GPGME_KEYLIST_MODE_SIG_NOTATIONS} symbol specifies that the
signature notations on key signatures should be included in the listed
keys.  This only works if @code{GPGME_KEYLIST_MODE_SIGS} is also
enabled.

@item GPGME_KEYLIST_MODE_EPHEMERAL
The @code{GPGME_KEYLIST_MODE_EPHEMERAL} symbol specifies that keys
flagged as ephemeral are included in the listing.

@item GPGME_KEYLIST_MODE_VALIDATE
The @code{GPGME_KEYLIST_MODE_VALIDATE} symbol specifies that the
backend should do key or certificate validation and not just get the
validity information from an internal cache.  This might be an
expensive operation and is in general not useful.  Currently only
implemented for the S/MIME backend and ignored for other backends.

@end table

At least one of @code{GPGME_KEYLIST_MODE_LOCAL} and
@code{GPGME_KEYLIST_MODE_EXTERN} must be specified.  For future binary
compatibility, you should get the current mode with
@code{gpgme_get_keylist_mode} and modify it by setting or clearing the
appropriate bits, and then using that calculated value in the
@code{gpgme_set_keylisting_mode} operation.  This will leave all other
bits in the mode value intact (in particular those that are not used
in the current version of the library).

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
mode could be set correctly, and @code{GPG_ERR_INV_VALUE} if @var{ctx}
is not a valid pointer or @var{mode} is not a valid mode.
@end deftypefun


@deftypefun gpgme_keylist_mode_t gpgme_get_keylist_mode (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_get_keylist_mode} returns the current key
listing mode of the context @var{ctx}.  This value can then be
modified and used in a subsequent @code{gpgme_set_keylist_mode}
operation to only affect the desired bits (and leave all others
intact).

The function returns 0 if @var{ctx} is not a valid pointer, and the
current mode otherwise.  Note that 0 is not a valid mode value.
@end deftypefun


@node Passphrase Callback
@subsection Passphrase Callback
@cindex callback, passphrase
@cindex passphrase callback

@deftp {Data type} {gpgme_error_t (*gpgme_passphrase_cb_t)(void *@var{hook}, const char *@var{uid_hint}, const char *@var{passphrase_info}, @w{int @var{prev_was_bad}}, @w{int @var{fd}})}
@tindex gpgme_passphrase_cb_t
The @code{gpgme_passphrase_cb_t} type is the type of functions usable as
passphrase callback function.

The argument @var{uid_hint} might contain a string that gives an
indication for which user ID the passphrase is required.  If this is
not available, or not applicable (in the case of symmetric encryption,
for example), @var{uid_hint} will be @code{NULL}.

The argument @var{passphrase_info}, if not @code{NULL}, will give
further information about the context in which the passphrase is
required.  This information is engine and operation specific.

If this is the repeated attempt to get the passphrase, because
previous attempts failed, then @var{prev_was_bad} is 1, otherwise it
will be 0.

The user must write the passphrase, followed by a newline character,
to the file descriptor @var{fd}.  The function @code{gpgme_io_writen}
should be used for the write operation.  Note that if the user returns
0 to indicate success, the user must at least write a newline
character before returning from the callback.

If an error occurs, return the corresponding @code{gpgme_error_t}
value.  You can use the error code @code{GPG_ERR_CANCELED} to abort
the operation.  Otherwise, return @code{0}.
@end deftp

@deftypefun void gpgme_set_passphrase_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_passphrase_cb_t @var{passfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_passphrase_cb} sets the function that is
used when a passphrase needs to be provided by the user to
@var{passfunc}.  The function @var{passfunc} needs to implemented by
the user, and whenever it is called, it is called with its first
argument being @var{hook_value}.  By default, no passphrase callback
function is set.

Not all crypto engines require this callback to retrieve the
passphrase.  It is better if the engine retrieves the passphrase from
a trusted agent (a daemon process), rather than having each user to
implement their own passphrase query.  Some engines do not even
support an external passphrase callback at all, in this case the error
code @code{GPG_ERR_NOT_SUPPORTED} is returned.

The user can disable the use of a passphrase callback function by
calling @code{gpgme_set_passphrase_cb} with @var{passfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_passphrase_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_passphrase_cb_t *@var{passfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_passphrase_cb} returns the function that
is used when a passphrase needs to be provided by the user in
@var{*passfunc}, and the first argument for this function in
@var{*hook_value}.  If no passphrase callback is set, or @var{ctx} is
not a valid pointer, @code{NULL} is returned in both variables.

@var{passfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Progress Meter Callback
@subsection Progress Meter Callback
@cindex callback, progress meter
@cindex progress meter callback

@deftp {Data type} {void (*gpgme_progress_cb_t)(void *@var{hook}, const char *@var{what}, int @var{type}, int @var{current}, int @var{total})}
@tindex gpgme_progress_cb_t
The @code{gpgme_progress_cb_t} type is the type of functions usable as
progress callback function.

The arguments are specific to the crypto engine.  More information
about the progress information returned from the GnuPG engine can be
found in the GnuPG source code in the file @file{doc/DETAILS} in the
section PROGRESS.
@end deftp

@deftypefun void gpgme_set_progress_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_progress_cb_t @var{progfunc}}, @w{void *@var{hook_value}})
The function @code{gpgme_set_progress_cb} sets the function that is
used when progress information about a cryptographic operation is
available.  The function @var{progfunc} needs to implemented by the
user, and whenever it is called, it is called with its first argument
being @var{hook_value}.  By default, no progress callback function
is set.

Setting a callback function allows an interactive program to display
progress information about a long operation to the user.

The user can disable the use of a progress callback function by
calling @code{gpgme_set_progress_cb} with @var{progfunc} being
@code{NULL}.
@end deftypefun

@deftypefun void gpgme_get_progress_cb (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_progress_cb_t *@var{progfunc}}, @w{void **@var{hook_value}})
The function @code{gpgme_get_progress_cb} returns the function that is
used to inform the user about the progress made in @var{*progfunc},
and the first argument for this function in @var{*hook_value}.  If no
progress callback is set, or @var{ctx} is not a valid pointer,
@code{NULL} is returned in both variables.

@var{progfunc} or @var{hook_value} can be @code{NULL}.  In this case,
the corresponding value will not be returned.
@end deftypefun


@node Locale
@subsection Locale
@cindex locale, default
@cindex locale, of a context

A locale setting can be associated with a context.  This locale is
passed to the crypto engine, and used for applications like the PIN
entry, which is displayed to the user when entering a passphrase is
required.

The default locale is used to initialize the locale setting of all
contexts created afterwards.

@deftypefun gpgme_error_t gpgme_set_locale (@w{gpgme_ctx_t @var{ctx}}, @w{int @var{category}}, @w{const char *@var{value}})
The function @code{gpgme_set_locale} sets the locale of the context
@var{ctx}, or the default locale if @var{ctx} is a null pointer.

The locale settings that should be changed are specified by
@var{category}.  Supported categories are @code{LC_CTYPE},
@code{LC_MESSAGES}, and @code{LC_ALL}, which is a wildcard you can use
if you want to change all the categories at once.

The value to be used for the locale setting is @var{value}, which will
be copied to @acronym{GPGME}'s internal data structures.  @var{value}
can be a null pointer, which disables setting the locale, and will
make PIN entry and other applications use their default setting, which
is usually not what you want.

Note that the settings are only used if the application runs on a text
terminal, and that the settings should fit the configuration of the
output terminal.  Normally, it is sufficient to initialize the default
value at startup.

The function returns an error if not enough memory is available.
@end deftypefun


@node Key Management
@section Key Management
@cindex key management

Some of the cryptographic operations require that recipients or
signers are specified.  This is always done by specifying the
respective keys that should be used for the operation.  The following
section describes how such keys can be selected and manipulated.

@deftp {Data type} gpgme_subkey_t
The @code{gpgme_subkey_t} type is a pointer to a subkey structure.
Sub keys are one component of a @code{gpgme_key_t} object.  In fact,
subkeys are those parts that contains the real information about the
individual cryptographic keys that belong to the same key object.  One
@code{gpgme_key_t} can contain several subkeys.  The first subkey in
the linked list is also called the primary key.

The subkey structure has the following members:

@table @code
@item gpgme_subkey_t next
This is a pointer to the next subkey structure in the linked list, or
@code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the subkey is revoked.

@item unsigned int expired : 1
This is true if the subkey is expired.

@item unsigned int disabled : 1
This is true if the subkey is disabled.

@item unsigned int invalid : 1
This is true if the subkey is invalid.

@item unsigned int can_encrypt : 1
This is true if the subkey can be used for encryption.

@item unsigned int can_sign : 1
This is true if the subkey can be used to create data signatures.

@item unsigned int can_certify : 1
This is true if the subkey can be used to create key certificates.

@item unsigned int can_authenticate : 1
This is true if the subkey can be used for authentication.

@item unsigned int is_qualified : 1
This is true if the subkey can be used for qualified signatures
according to local government regulations.

@item unsigned int secret : 1
This is true if the subkey is a secret key.  Note that it will be false
if the key is actually a stub key; i.e. a secret key operation is
currently not possible (offline-key).

@item gpgme_pubkey_algo_t pubkey_algo
This is the public key algorithm supported by this subkey.

@item unsigned int length
This is the length of the subkey (in bits).

@item char *keyid
This is the key ID of the subkey in hexadecimal digits.

@item char *fpr
This is the fingerprint of the subkey in hexadecimal digits, if
available.

@item long int timestamp
This is the creation timestamp of the subkey.  This is -1 if the
timestamp is invalid, and 0 if it is not available.

@item long int expires
This is the expiration timestamp of the subkey, or 0 if the subkey
does not expire.
@end table
@end deftp

@deftp {Data type} gpgme_key_sig_t
The @code{gpgme_key_sig_t} type is a pointer to a key signature structure.
Key signatures are one component of a @code{gpgme_key_t} object, and
validate user IDs on the key.

The signatures on a key are only available if the key was retrieved
via a listing operation with the @code{GPGME_KEYLIST_MODE_SIGS} mode
enabled, because it can be expensive to retrieve all signatures of a
key.

The signature notations on a key signature are only available if the
key was retrieved via a listing operation with the
@code{GPGME_KEYLIST_MODE_SIG_NOTATIONS} mode enabled, because it can
be expensive to retrieve all signature notations.

The key signature structure has the following members:

@table @code
@item gpgme_key_sig_t next
This is a pointer to the next key signature structure in the linked
list, or @code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the key signature is a revocation signature.

@item unsigned int expired : 1
This is true if the key signature is expired.

@item unsigned int invalid : 1
This is true if the key signature is invalid.

@item unsigned int exportable : 1
This is true if the key signature is exportable.

@item gpgme_pubkey_algo_t pubkey_algo
This is the public key algorithm used to create the signature.

@item char *keyid
This is the key ID of the key (in hexadecimal digits) used to create
the signature.

@item long int timestamp
This is the creation timestamp of the key signature.  This is -1 if
the timestamp is invalid, and 0 if it is not available.

@item long int expires
This is the expiration timestamp of the key signature, or 0 if the key
signature does not expire.

@item gpgme_error_t status
This is the status of the signature and has the same meaning as the
member of the same name in a @code{gpgme_signature_t} object.

@item unsigned int sig_class
This specifies the signature class of the key signature.  The meaning
is specific to the crypto engine.

@item char *uid
This is the main user ID of the key used to create the signature.

@item char *name
This is the name component of @code{uid}, if available.

@item char *comment
This is the comment component of @code{uid}, if available.

@item char *email
This is the email component of @code{uid}, if available.

@item gpgme_sig_notation_t notations
This is a linked list with the notation data and policy URLs.
@end table
@end deftp

@deftp {Data type} gpgme_user_id_t
A user ID is a component of a @code{gpgme_key_t} object.  One key can
have many user IDs.  The first one in the list is the main (or
primary) user ID.

The user ID structure has the following members.

@table @code
@item gpgme_user_id_t next
This is a pointer to the next user ID structure in the linked list, or
@code{NULL} if this is the last element.

@item unsigned int revoked : 1
This is true if the user ID is revoked.

@item unsigned int invalid : 1
This is true if the user ID is invalid.

@item gpgme_validity_t validity
This specifies the validity of the user ID.

@item char *uid
This is the user ID string.

@item char *name
This is the name component of @code{uid}, if available.

@item char *comment
This is the comment component of @code{uid}, if available.

@item char *email
This is the email component of @code{uid}, if available.

@item gpgme_key_sig_t signatures
This is a linked list with the signatures on this user ID.
@end table
@end deftp

@deftp {Data type} gpgme_key_t
The @code{gpgme_key_t} type is a pointer to a key object.  It has the
following members:

@table @code
@item gpgme_keylist_mode_t keylist_mode
The keylist mode that was active when the key was retrieved.

@item unsigned int revoked : 1
This is true if the key is revoked.

@item unsigned int expired : 1
This is true if the key is expired.

@item unsigned int disabled : 1
This is true if the key is disabled.

@item unsigned int invalid : 1
This is true if the key is invalid. This might have several reasons,
for a example for the S/MIME backend, it will be set in during key
listsing if the key could not be validated due to a missing
certificates or unmatched policies.

@item unsigned int can_encrypt : 1
This is true if the key (ie one of its subkeys) can be used for
encryption.

@item unsigned int can_sign : 1
This is true if the key (ie one of its subkeys) can be used to create
data signatures.

@item unsigned int can_certify : 1
This is true if the key (ie one of its subkeys) can be used to create
key certificates.

@item unsigned int can_authenticate : 1
This is true if the key (ie one of its subkeys) can be used for
authentication.

@item unsigned int is_qualified : 1
This is true if the key can be used for qualified signatures according
to local government regulations.

@item unsigned int secret : 1
This is true if the key is a secret key.  Note, that this will always be
true even if the corresponding subkey flag may be false (offline/stub
keys).

@item gpgme_protocol_t protocol
This is the protocol supported by this key.

@item char *issuer_serial
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
issuer serial.

@item char *issuer_name
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
issuer name.

@item char *chain_id
If @code{protocol} is @code{GPGME_PROTOCOL_CMS}, then this is the
chain ID, which can be used to built the certificate chain.

@item gpgme_validity_t owner_trust
If @code{protocol} is @code{GPGME_PROTOCOL_OpenPGP}, then this is the
owner trust.

@item gpgme_subkey_t subkeys
This is a linked list with the subkeys of the key.  The first subkey
in the list is the primary key and usually available.

@item gpgme_user_id_t uids
This is a linked list with the user IDs of the key.  The first user ID
in the list is the main (or primary) user ID.
@end table
@end deftp

@menu
* Listing Keys::                  Browsing the list of available keys.
* Information About Keys::        Requesting detailed information about keys.
* Key Signatures::                Listing the signatures on a key.
* Manipulating Keys::             Operations on keys.
* Generating Keys::               Creating new key pairs.
* Exporting Keys::                Retrieving key data from the key ring.
* Importing Keys::                Adding keys to the key ring.
* Deleting Keys::                 Removing keys from the key ring.
* Changing Passphrases::          Change the passphrase of a key.
* Advanced Key Editing::          Advanced key edit operation.
@end menu


@node Listing Keys
@subsection Listing Keys
@cindex listing keys
@cindex key listing
@cindex key listing, start
@cindex key ring, list
@cindex key ring, search

@deftypefun gpgme_error_t gpgme_op_keylist_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{secret_only}})
The function @code{gpgme_op_keylist_start} initiates a key listing
operation inside the context @var{ctx}.  It sets everything up so that
subsequent invocations of @code{gpgme_op_keylist_next} return the keys
in the list.

If @var{pattern} is @code{NULL}, all available keys are returned.
Otherwise, @var{pattern} contains an engine specific expression that
is used to limit the list to all keys matching the pattern.  Note that
the total length of the pattern is restricted to an engine-specific
maximum (a couple of hundred characters are usually accepted).  The
pattern should be used to restrict the search to a certain common name
or user, not to list many specific keys at once by listing their
fingerprints or key IDs.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_ext_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{int @var{secret_only}}, @w{int @var{reserved}})
The function @code{gpgme_op_keylist_ext_start} initiates an extended
key listing operation inside the context @var{ctx}.  It sets
everything up so that subsequent invocations of
@code{gpgme_op_keylist_next} return the keys in the list.

If @var{pattern} or @var{*pattern} is @code{NULL}, all available keys
are returned.  Otherwise, @var{pattern} is a @code{NULL} terminated
array of strings that are used to limit the list to all keys matching
at least one of the patterns verbatim.  Note that the total length of
all patterns is restricted to an engine-specific maximum (the exact
limit also depends on the number of patterns and amount of quoting
required, but a couple of hundred characters are usually accepted).
Patterns should be used to restrict the search to a certain common
name or user, not to list many specific keys at once by listing their
fingerprints or key IDs.

If @var{secret_only} is not @code{0}, the list is restricted to secret
keys only.

The value of @var{reserved} must be @code{0}.

The context will be busy until either all keys are received (and
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_keylist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_next (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t *@var{r_key}})
The function @code{gpgme_op_keylist_next} returns the next key in the
list created by a previous @code{gpgme_op_keylist_start} operation in
the context @var{ctx}.  The key will have one reference for the user.
@xref{Manipulating Keys}.

This is the only way to get at @code{gpgme_key_t} objects in
@acronym{GPGME}.

If the last key in the list has already been returned,
@code{gpgme_op_keylist_next} returns @code{GPG_ERR_EOF}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{r_key} is not a valid pointer, and
@code{GPG_ERR_ENOMEM} if there is not enough memory for the operation.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_keylist_end (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_keylist_end} ends a pending key list
operation in the context @var{ctx}.

After the operation completed successfully, the result of the key
listing operation can be retrieved with
@code{gpgme_op_keylist_result}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun

The following example illustrates how all keys containing a certain
string (@code{g10code}) can be listed with their key ID and the name
and e-mail address of the main user ID:

@example
gpgme_ctx_t ctx;
gpgme_key_t key;
gpgme_error_t err = gpgme_new (&ctx);

if (!err)
  @{
    err = gpgme_op_keylist_start (ctx, "g10code", 0);
    while (!err)
      @{
        err = gpgme_op_keylist_next (ctx, &key);
        if (err)
          break;
        printf ("%s:", key->subkeys->keyid);
        if (key->uids && key->uids->name)
          printf (" %s", key->uids->name);
        if (key->uids && key->uids->email)
          printf (" <%s>", key->uids->email);
        putchar ('\n');
        gpgme_key_release (key);
      @}
    gpgme_release (ctx);
  @}
if (gpg_err_code (err) != GPG_ERR_EOF)
  @{
    fprintf (stderr, "can not list keys: %s\n", gpgme_strerror (err));
    exit (1);
  @}
@end example

@deftp {Data type} {gpgme_keylist_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_keylist_*} operation.  After successfully ending a key
listing operation, you can retrieve the pointer to the result with
@code{gpgme_op_keylist_result}.  The structure contains the following
member:

@table @code
@item unsigned int truncated : 1
This is true if the crypto backend had to truncate the result, and
less than the desired keys could be listed.
@end table
@end deftp

@deftypefun gpgme_keylist_result_t gpgme_op_keylist_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_keylist_result} returns a
@code{gpgme_keylist_result_t} pointer to a structure holding the
result of a @code{gpgme_op_keylist_*} operation.  The pointer is only
valid if the last operation on the context was a key listing
operation, and if this operation finished successfully.  The returned
pointer is only valid until the next operation is started on the
context.
@end deftypefun

In a simple program, for which a blocking operation is acceptable, the
following function can be used to retrieve a single key.

@deftypefun gpgme_error_t gpgme_get_key (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{fpr}}, @w{gpgme_key_t *@var{r_key}}, @w{int @var{secret}})
The function @code{gpgme_get_key} gets the key with the fingerprint
(or key ID) @var{fpr} from the crypto backend and return it in
@var{r_key}.  If @var{secret} is true, get the secret key.  The
currently active keylist mode is used to retrieve the key.  The key
will have one reference for the user.

If the key is not found in the keyring, @code{gpgme_get_key} returns
the error code @code{GPG_ERR_EOF} and *@var{r_key} will be set to
@code{NULL}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{r_key} is not a valid pointer or @var{fpr} is not a
fingerprint or key ID, @code{GPG_ERR_AMBIGUOUS_NAME} if the key ID was
not a unique specifier for a key, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun


@node Information About Keys
@subsection Information About Keys
@cindex key, information about
@cindex key, attributes
@cindex attributes, of a key

Please see the beginning of this section for more information about
@code{gpgme_key_t} objects.

@deftp {Data type} gpgme_validity_t
The @code{gpgme_validity_t} type is used to specify the validity of a user ID
in a key.  The following validities are defined:

@table @code
@item GPGME_VALIDITY_UNKNOWN
The user ID is of unknown validity.  The string representation of this
validity is ``?''.

@item GPGME_VALIDITY_UNDEFINED
The validity of the user ID is undefined.  The string representation of this
validity is ``q''.

@item GPGME_VALIDITY_NEVER
The user ID is never valid.  The string representation of this
validity is ``n''.

@item GPGME_VALIDITY_MARGINAL
The user ID is marginally valid.  The string representation of this
validity is ``m''.

@item GPGME_VALIDITY_FULL
The user ID is fully valid.  The string representation of this
validity is ``f''.

@item GPGME_VALIDITY_ULTIMATE
The user ID is ultimately valid.  The string representation of this
validity is ``u''.
@end table
@end deftp


The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

@deftp {Data type} gpgme_attr_t
The @code{gpgme_attr_t} type is used to specify a key or trust item
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of a sub key.  It is representable as a string.

For trust items, the trust item refers to the key with this ID.

@item GPGME_ATTR_FPR
This is the fingerprint of a sub key.  It is representable as a
string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm for which the sub key can be used.  It
is representable as a string and as a number.  The numbers correspond
to the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_LEN
This is the key length of a sub key.  It is representable as a
number.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of a sub key.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of a sub key.  It is representable as a
number.

@item GPGME_ATTR_OTRUST
XXX FIXME  (also for trust items)

@item GPGME_ATTR_USERID
This is a user ID.  There can be more than one user IDs in a
@var{gpgme_key_t} object.  The first one (with index 0) is the primary
user ID.  The user ID is representable as a number.

For trust items, this is the user ID associated with this trust item.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_VALIDITY
This is the validity belonging to a user ID.  It is representable as a
string and as a number.  See below for a list of available validities.

For trust items, this is the validity that is associated with this
trust item.

@item GPGME_ATTR_UID_REVOKED
This specifies if a user ID is revoked.  It is representable as a
number, and is @code{1} if the user ID is revoked, and @code{0}
otherwise.

@item GPGME_ATTR_UID_INVALID
This specifies if a user ID is invalid.  It is representable as a
number, and is @code{1} if the user ID is invalid, and @code{0}
otherwise.

@item GPGME_ATTR_LEVEL
This is the trust level of a trust item.

@item GPGME_ATTR_TYPE
This returns information about the type of key.  For the string function
this will eother be "PGP" or "X.509".  The integer function returns 0
for PGP and 1 for X.509.  It is also used for the type of a trust item.

@item GPGME_ATTR_IS_SECRET
This specifies if the key is a secret key.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a sub key is revoked.  It is representable as a
number, and is @code{1} if the key is revoked, and @code{0} otherwise.

@item GPGME_ATTR_KEY_INVALID
This specifies if a sub key is invalid.  It is representable as a
number, and is @code{1} if the key is invalid, and @code{0} otherwise.

@item GPGME_ATTR_KEY_EXPIRED
This specifies if a sub key is expired.  It is representable as a
number, and is @code{1} if the key is expired, and @code{0} otherwise.

@item GPGME_ATTR_KEY_DISABLED
This specifies if a sub key is disabled.  It is representable as a
number, and is @code{1} if the key is disabled, and @code{0} otherwise.

@item GPGME_ATTR_KEY_CAPS
This is a description of the capabilities of a sub key.  It is
representable as a string.  The string contains the letter ``e'' if
the key can be used for encryption, ``s'' if the key can be used for
signatures, and ``c'' if the key can be used for certifications.

@item GPGME_ATTR_CAN_ENCRYPT
This specifies if a sub key can be used for encryption.  It is
representable as a number, and is @code{1} if the sub key can be used
for encryption, and @code{0} otherwise.

@item GPGME_ATTR_CAN_SIGN
This specifies if a sub key can be used to create data signatures.  It
is representable as a number, and is @code{1} if the sub key can be
used for signatures, and @code{0} otherwise.

@item GPGME_ATTR_CAN_CERTIFY
This specifies if a sub key can be used to create key certificates.
It is representable as a number, and is @code{1} if the sub key can be
used for certifications, and @code{0} otherwise.

@item GPGME_ATTR_SERIAL
The X.509 issuer serial attribute of the key.  It is representable as
a string.

@item GPGME_ATTR_ISSUE
The X.509 issuer name attribute of the key.  It is representable as a
string.

@item GPGME_ATTR_CHAINID
The X.509 chain ID can be used to build the certification chain.  It
is representable as a string.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_get_string_attr (@w{gpgme_key_t @var{key}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_string_attr} returns the value of the
string-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_get_ulong_attr (@w{gpgme_key_t @var{key}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_get_ulong_attr} returns the value of the
number-representable attribute @var{what} of key @var{key}.  If the
attribute is an attribute of a sub key or an user ID, @var{idx}
specifies the sub key or user ID of which the attribute value is
returned.  The argument @var{reserved} is reserved for later use and
should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range, or
@var{reserved} not @code{NULL}.
@end deftypefun


@node Key Signatures
@subsection Key Signatures
@cindex key, signatures
@cindex signatures, on a key

The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

The signatures on a key are only available if the key was retrieved
via a listing operation with the @code{GPGME_KEYLIST_MODE_SIGS} mode
enabled, because it is expensive to retrieve all signatures of a key.

So, before using the below interfaces to retrieve the signatures on a
key, you have to make sure that the key was listed with signatures
enabled.  One convenient, but blocking, way to do this is to use the
function @code{gpgme_get_key}.

@deftp {Data type} gpgme_attr_t
The @code{gpgme_attr_t} type is used to specify a key signature
attribute.  The following attributes are defined:

@table @code
@item GPGME_ATTR_KEYID
This is the key ID of the key which was used for the signature.  It is
representable as a string.

@item GPGME_ATTR_ALGO
This is the crypto algorithm used to create the signature.  It is
representable as a string and as a number.  The numbers correspond to
the @code{enum gcry_pk_algos} values in the gcrypt library.

@item GPGME_ATTR_CREATED
This is the timestamp at creation time of the signature.  It is
representable as a number.

@item GPGME_ATTR_EXPIRE
This is the expiration time of the signature.  It is representable as
a number.

@item GPGME_ATTR_USERID
This is the user ID associated with the signing key.  The user ID is
representable as a number.

@item GPGME_ATTR_NAME
This is the name belonging to a user ID.  It is representable as a string.

@item GPGME_ATTR_EMAIL
This is the email address belonging to a user ID.  It is representable
as a string.

@item GPGME_ATTR_COMMENT
This is the comment belonging to a user ID.  It is representable as a
string.

@item GPGME_ATTR_KEY_REVOKED
This specifies if a key signature is a revocation signature.  It is
representable as a number, and is @code{1} if the key is revoked, and
@code{0} otherwise.

@c @item GPGME_ATTR_KEY_EXPIRED
@c This specifies if a key signature is expired.  It is representable as
@c a number, and is @code{1} if the key is revoked, and @code{0}
@c otherwise.
@c
@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_CLASS
This specifies the signature class of a key signature.  It is
representable as a number.  The meaning is specific to the crypto
engine.

@item GPGME_ATTR_SIG_STATUS
This is the same value as returned by @code{gpgme_get_sig_status}.
@end table
@end deftp

@deftypefun {const char *} gpgme_key_sig_get_string_attr (@w{gpgme_key_t @var{key}}, @w{int @var{uid_idx}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_string_attr} returns the value of
the string-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun {unsigned long} gpgme_key_sig_get_ulong_attr (@w{gpgme_key_t @var{key}}, @w{int @var{uid_idx}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_key_sig_get_ulong_attr} returns the value of
the number-representable attribute @var{what} of the signature
@var{idx} on the user ID @var{uid_idx} in the key @var{key}.  The
argument @var{reserved} is reserved for later use and should be
@code{NULL}.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{uid_idx} or @var{idx}
out of range, or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Keys
@subsection Manipulating Keys
@cindex key, manipulation

@deftypefun void gpgme_key_ref (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_ref} acquires an additional reference for
the key @var{key}.
@end deftypefun

@deftypefun void gpgme_key_unref (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_unref} releases a reference for the key
@var{key}.  If this was the last reference, the key will be destroyed
and all resources associated to it will be released.
@end deftypefun


The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun void gpgme_key_release (@w{gpgme_key_t @var{key}})
The function @code{gpgme_key_release} is equivalent to
@code{gpgme_key_unref}.
@end deftypefun


@node Generating Keys
@subsection Generating Keys
@cindex key, creation
@cindex key ring, add

@deftypefun gpgme_error_t gpgme_op_genkey (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{parms}}, @w{gpgme_data_t @var{public}}, @w{gpgme_data_t @var{secret}})
The function @code{gpgme_op_genkey} generates a new key pair in the
context @var{ctx}.  The meaning of @var{public} and @var{secret}
depends on the crypto backend.

GnuPG does not support @var{public} and @var{secret}, they should be
@code{NULL}.  GnuPG will generate a key pair and add it to the
standard key ring.  The fingerprint of the generated key is available
with @code{gpgme_op_genkey_result}.

GpgSM requires @var{public} to be a writable data object.  GpgSM will
generate a secret key (which will be stored by @command{gpg-agent},
and return a certificate request in @var{public}, which then needs to
be signed by the certification authority and imported before it can be
used.  GpgSM does not make the fingerprint available.

The argument @var{parms} specifies parameters for the key in an XML
string.  The details about the format of @var{parms} are specific to
the crypto engine used by @var{ctx}.  Here is an example for GnuPG as
the crypto engine (all parameters of OpenPGP key generation are
documented in the GPG manual):

@example
<GnupgKeyParms format="internal">
Key-Type: default
Subkey-Type: default
Name-Real: Joe Tester
Name-Comment: with stupid passphrase
Name-Email: joe@@foo.bar
Expire-Date: 0
Passphrase: abc
</GnupgKeyParms>
@end example

Here is an example for GpgSM as the crypto engine (all parameters of
OpenPGP key generation are documented in the GPGSM manual):

@example
<GnupgKeyParms format="internal">
Key-Type: RSA
Key-Length: 1024
Name-DN: C=de,O=g10 code,OU=Testlab,CN=Joe 2 Tester
Name-Email: joe@@foo.bar
</GnupgKeyParms>
@end example

Strings should be given in UTF-8 encoding.  The only format supported
for now is ``internal''.  The content of the @code{GnupgKeyParms}
container is passed verbatim to the crypto backend.  Control
statements are not allowed.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_genkey_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{parms} is not a valid XML string, @code{GPG_ERR_NOT_SUPPORTED} if
@var{public} or @var{secret} is not valid, and @code{GPG_ERR_GENERAL}
if no key was created by the backend.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_genkey_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{parms}}, @w{gpgme_data_t @var{public}}, @w{gpgme_data_t @var{secret}})
The function @code{gpgme_op_genkey_start} initiates a
@code{gpgme_op_genkey} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{parms} is not a valid XML string, and
@code{GPG_ERR_NOT_SUPPORTED} if @var{public} or @var{secret} is not
@code{NULL}.
@end deftypefun

@deftp {Data type} {gpgme_genkey_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_genkey} operation.  After successfully generating a
key, you can retrieve the pointer to the result with
@code{gpgme_op_genkey_result}.  The structure contains the following
members:

@table @code
@item unsigned int primary : 1
This is a flag that is set to 1 if a primary key was created and to 0
if not.

@item unsigned int sub : 1
This is a flag that is set to 1 if a subkey was created and to 0
if not.

@item char *fpr
This is the fingerprint of the key that was created.  If both a
primary and a sub key were generated, the fingerprint of the primary
key will be returned.  If the crypto engine does not provide the
fingerprint, @code{fpr} will be a null pointer.
@end table
@end deftp

@deftypefun gpgme_genkey_result_t gpgme_op_genkey_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_genkey_result} returns a
@code{gpgme_genkey_result_t} pointer to a structure holding the result of
a @code{gpgme_op_genkey} operation.  The pointer is only valid if the
last operation on the context was a @code{gpgme_op_genkey} or
@code{gpgme_op_genkey_start} operation, and if this operation finished
successfully.  The returned pointer is only valid until the next
operation is started on the context.
@end deftypefun


@node Exporting Keys
@subsection Exporting Keys
@cindex key, export
@cindex key ring, export from

Exporting keys means the same as running @command{gpg} with the command
@option{--export}.  However, a mode flag can be used to change the way
the export works.  The available mode flags are described below, they
may be or-ed together.

@table @code

@item GPGME_EXPORT_MODE_EXTERN
If this bit is set, the output is send directly to the default
keyserver. This is currently only allowed for OpenPGP keys.  It is good
practise to not send more than a few dozens key to a keyserver at one
time.  Using this flag requires that the @var{keydata} argument of the
export function is set to @code{NULL}.

@item GPGME_EXPORT_MODE_MINIMAL
If this bit is set, the smallest possible key is exported.  For OpenPGP
keys it removes all signatures except for the latest self-signatures.
For X.509 keys it has no effect.


@end table



@deftypefun gpgme_error_t gpgme_op_export (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export} extracts public keys and returns
them in the data buffer @var{keydata}.  The output format of the key
data returned is determined by the @acronym{ASCII} armor attribute set
for the context @var{ctx}, or, if that is not set, by the encoding
specified for @var{keydata}.

If @var{pattern} is @code{NULL}, all available keys are returned.
Otherwise, @var{pattern} contains an engine specific expression that
is used to limit the list to all keys matching the pattern.

@var{mode} is usually 0; other values are described above.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} is not a valid empty data buffer, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_start} initiates a
@code{gpgme_op_export} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{keydata} is not a valid empty data buffer.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_ext (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export} extracts public keys and returns
them in the data buffer @var{keydata}.  The output format of the key
data returned is determined by the @acronym{ASCII} armor attribute set
for the context @var{ctx}, or, if that is not set, by the encoding
specified for @var{keydata}.

If @var{pattern} or @var{*pattern} is @code{NULL}, all available keys
are returned.  Otherwise, @var{pattern} is a @code{NULL} terminated
array of strings that are used to limit the list to all keys matching
at least one of the patterns verbatim.

@var{mode} is usually 0; other values are described above.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} is not a valid empty data buffer, and passes through any
errors that are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_ext_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}[]}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_ext_start} initiates a
@code{gpgme_op_export_ext} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{keydata} is not a valid empty data buffer.
@end deftypefun


@deftypefun gpgme_error_t gpgme_op_export_keys (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t keys[]}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_keys} extracts public keys and returns
them in the data buffer @var{keydata}.  The output format of the key
data returned is determined by the @acronym{ASCII} armor attribute set
for the context @var{ctx}, or, if that is not set, by the encoding
specified for @var{keydata}.

The keys to export are taken form the @code{NULL} terminated array
@var{keys}.  Only keys of the the currently selected protocol of
@var{ctx} which do have a fingerprint set are considered for export.
Other keys specified by the @var{keys} are ignored.  In particular
OpenPGP keys retrieved via an external key listing are not included.

@var{mode} is usually 0; other values are described above.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} is not a valid empty data buffer, @code{GPG_ERR_NO_DATA}
if no useful keys are in @var{keys} and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_export_keys_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{keys}[]}, @w{gpgme_export_mode_t @var{mode}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_export_keys_start} initiates a
@code{gpgme_op_export_ext} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{keydata} is not a valid empty data buffer, @code{GPG_ERR_NO_DATA}
if no useful keys are in @var{keys} and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun


@node Importing Keys
@subsection Importing Keys
@cindex key, import
@cindex key ring, import to

Importing keys means the same as running @command{gpg} with the command
@option{--import}.


@deftypefun gpgme_error_t gpgme_op_import (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_import} adds the keys in the data buffer
@var{keydata} to the key ring of the crypto engine used by @var{ctx}.
The format of @var{keydata} can be @acronym{ASCII} armored, for example,
but the details are specific to the crypto engine.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_import_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import was completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
and @code{GPG_ERR_NO_DATA} if @var{keydata} is an empty data buffer.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_import_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}})
The function @code{gpgme_op_import_start} initiates a
@code{gpgme_op_import} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
and @code{GPG_ERR_NO_DATA} if @var{keydata} is an empty data buffer.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_import_keys (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t *@var{keys}})
The function @code{gpgme_op_import_keys} adds the keys described by the
@code{NULL} terminated array @var{keys} to the key ring of the crypto
engine used by @var{ctx}.  This function is the general interface to
move a key from one crypto engine to another as long as they are
compatible.  In particular it is used to actually import and make keys
permanent which have been retrieved from an external source (i.e. using
@code{GPGME_KEYLIST_MODE_EXTERN}).  @footnote{Thus it is a replacement
for the usual workaround of exporting and then importing a key to make
an X.509 key permanent.}

Only keys of the the currently selected protocol of @var{ctx} are
considered for import.  Other keys specified by the @var{keys} are
ignored.  As of now all considered keys must have been retrieved using
the same method, that is the used key listing mode must be identical.

After the operation completed successfully, the result can be
retrieved with @code{gpgme_op_import_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import was completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
@code{GPG_ERR_CONFLICT} if the key listing mode does not match, and
@code{GPG_ERR_NO_DATA} if no keys are considered for export.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_import_keys_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t *@var{keys}})
The function @code{gpgme_op_import_keys_start} initiates a
@code{gpgme_op_import_keys} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
import was completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{keydata} if @var{ctx} or @var{keydata} is not a valid pointer,
@code{GPG_ERR_CONFLICT} if the key listing mode does not match, and
@code{GPG_ERR_NO_DATA} if no keys are considered for export.
@end deftypefun

@deftp {Data type} {gpgme_import_status_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_import} operation.  For each considered key one
status is added that contains information about the result of the
import.  The structure contains the following members:

@table @code
@item gpgme_import_status_t next
This is a pointer to the next status structure in the linked list, or
@code{NULL} if this is the last element.

@item char *fpr
This is the fingerprint of the key that was considered.

@item gpgme_error_t result
If the import was not successful, this is the error value that caused
the import to fail.  Otherwise the error code is
@code{GPG_ERR_NO_ERROR}.

@item unsigned int status
This is a bit-wise OR of the following flags that give more
information about what part of the key was imported.  If the key was
already known, this might be 0.

@table @code
@item GPGME_IMPORT_NEW
The key was new.

@item GPGME_IMPORT_UID
The key contained new user IDs.

@item GPGME_IMPORT_SIG
The key contained new signatures.

@item GPGME_IMPORT_SUBKEY
The key contained new sub keys.

@item GPGME_IMPORT_SECRET
The key contained a secret key.
@end table
@end table
@end deftp

@deftp {Data type} {gpgme_import_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_import} operation.  After a successful import
operation, you can retrieve the pointer to the result with
@code{gpgme_op_import_result}.  The structure contains the following
members:

@table @code
@item int considered
The total number of considered keys.

@item int no_user_id
The number of keys without user ID.

@item int imported
The total number of imported keys.

@item imported_rsa
The number of imported RSA keys.

@item unchanged
The number of unchanged keys.

@item new_user_ids
The number of new user IDs.

@item new_sub_keys
The number of new sub keys.

@item new_signatures
The number of new signatures.

@item new_revocations
The number of new revocations.

@item secret_read
The total number of secret keys read.

@item secret_imported
The number of imported secret keys.

@item secret_unchanged
The number of unchanged secret keys.

@item not_imported
The number of keys not imported.

@item gpgme_import_status_t imports
A list of gpgme_import_status_t objects which contain more information
about the keys for which an import was attempted.
@end table
@end deftp

@deftypefun gpgme_import_result_t gpgme_op_import_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_import_result} returns a
@code{gpgme_import_result_t} pointer to a structure holding the result
of a @code{gpgme_op_import} operation.  The pointer is only valid if
the last operation on the context was a @code{gpgme_op_import} or
@code{gpgme_op_import_start} operation, and if this operation finished
successfully.  The returned pointer is only valid until the next
operation is started on the context.
@end deftypefun

The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun gpgme_error_t gpgme_op_import_ext (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{keydata}}, @w{int *@var{nr}})
The function @code{gpgme_op_import_ext} is equivalent to:

@example
  gpgme_error_t err = gpgme_op_import (ctx, keydata);
  if (!err)
    @{
      gpgme_import_result_t result = gpgme_op_import_result (ctx);
      *nr = result->considered;
    @}
@end example
@end deftypefun


@node Deleting Keys
@subsection Deleting Keys
@cindex key, delete
@cindex key ring, delete from

@deftypefun gpgme_error_t gpgme_op_delete (@w{gpgme_ctx_t @var{ctx}}, @w{const gpgme_key_t @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete} deletes the key @var{key} from the
key ring of the crypto engine used by @var{ctx}.  If
@var{allow_secret} is @code{0}, only public keys are deleted,
otherwise secret keys are deleted as well, if that is supported.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the key
was deleted successfully, @code{GPG_ERR_INV_VALUE} if @var{ctx} or
@var{key} is not a valid pointer, @code{GPG_ERR_NO_PUBKEY} if
@var{key} could not be found in the keyring,
@code{GPG_ERR_AMBIGUOUS_NAME} if the key was not specified
unambiguously, and @code{GPG_ERR_CONFLICT} if the secret key for
@var{key} is available, but @var{allow_secret} is zero.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_delete_start (@w{gpgme_ctx_t @var{ctx}}, @w{const gpgme_key_t @var{key}}, @w{int @var{allow_secret}})
The function @code{gpgme_op_delete_start} initiates a
@code{gpgme_op_delete} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation was started successfully, and @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{key} is not a valid pointer.
@end deftypefun


@node Changing Passphrases
@subsection  Changing Passphrases
@cindex passphrase, change

@deftypefun gpgme_error_t gpgme_op_passwd      @
             (@w{gpgme_ctx_t @var{ctx}},       @
              @w{const gpgme_key_t @var{key}}, @
              @w{unsigned int @var{flags}})

The function @code{gpgme_op_passwd} changes the passphrase of the
private key associated with @var{key}.  The only allowed value for
@var{flags} is @code{0}.  The backend engine will usually popup a window
to ask for the old and the new passphrase.  Thus this function is not
useful in a server application (where passphrases are not required
anyway).

Note that old @code{gpg} engines (before version 2.0.15) do not support
this command and will silently ignore it.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_passwd_start      @
             (@w{gpgme_ctx_t @var{ctx}},       @
              @w{const gpgme_key_t @var{key}}, @
              @w{unsigned int @var{flags}})

The function @code{gpgme_op_passwd_start} initiates a
@code{gpgme_op_passwd} operation.    It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns @code{0} if the operation was started successfully,
and an error code if one of the arguments is not valid or the oepration
could not be started.
@end deftypefun


@node Advanced Key Editing
@subsection Advanced Key Editing
@cindex key, edit

@deftp {Data type} {gpgme_error_t (*gpgme_edit_cb_t) (@w{void *@var{handle}}, @w{gpgme_status_code_t @var{status}}, @w{const char *@var{args}}, @w{int @var{fd}})}
@tindex gpgme_edit_cb_t
The @code{gpgme_edit_cb_t} type is the type of functions which
@acronym{GPGME} calls if it a key edit operation is on-going.  The
status code @var{status} and the argument line @var{args} are passed
through by @acronym{GPGME} from the crypto engine.  The file
descriptor @var{fd} is -1 for normal status messages.  If @var{status}
indicates a command rather than a status message, the response to the
command should be written to @var{fd}.  The @var{handle} is provided
by the user at start of operation.

The function should return @code{GPG_ERR_NO_ERROR} or an error value.
@end deftp

@deftypefun gpgme_error_t gpgme_op_edit (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{key}}, @w{gpgme_edit_cb_t @var{fnc}}, @w{void *@var{handle}}, @w{gpgme_data_t @var{out}})
The function @code{gpgme_op_edit} processes the key @var{KEY}
interactively, using the edit callback function @var{FNC} with the
handle @var{HANDLE}.  The callback is invoked for every status and
command request from the crypto engine.  The output of the crypto
engine is written to the data object @var{out}.

Note that the protocol between the callback function and the crypto
engine is specific to the crypto engine and no further support in
implementing this protocol correctly is provided by @acronym{GPGME}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
edit operation completes successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{key} is not a valid pointer, and any error returned
by the crypto engine or the edit callback handler.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_edit_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{key}}, @w{gpgme_edit_cb_t @var{fnc}}, @w{void *@var{handle}}, @w{gpgme_data_t @var{out}})
The function @code{gpgme_op_edit_start} initiates a
@code{gpgme_op_edit} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation was started successfully, and @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{key} is not a valid pointer.
@end deftypefun


@deftypefun gpgme_error_t gpgme_op_card_edit (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{key}}, @w{gpgme_edit_cb_t @var{fnc}}, @w{void *@var{handle}}, @w{gpgme_data_t @var{out}})
The function @code{gpgme_op_card_edit} is analogous to
@code{gpgme_op_edit}, but should be used to process the smart card corresponding to the key @var{key}.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_card_edit_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_key_t @var{key}}, @w{gpgme_edit_cb_t @var{fnc}}, @w{void *@var{handle}}, @w{gpgme_data_t @var{out}})
The function @code{gpgme_op_card_edit_start} initiates a
@code{gpgme_op_card_edit} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation was started successfully, and @code{GPG_ERR_INV_VALUE} if
@var{ctx} or @var{key} is not a valid pointer.
@end deftypefun


@node Trust Item Management
@section Trust Item Management
@cindex trust item

@strong{Caution:} The trust items interface is experimental.

@deftp {Data type} gpgme_trust_item_t
The @code{gpgme_trust_item_t} type is a pointer to a trust item object.
It has the following members:

@table @code
@item char *keyid
This is a string describing the key to which this trust items belongs.

@item int type
This is the type of the trust item.  A value of 1 refers to a key, a
value of 2 refers to a user ID.

@item int level
This is the trust level.

@item char *owner_trust
The owner trust if @code{type} is 1.

@item char *validity
The calculated validity.

@item char *name
The user name if @code{type} is 2.
@end table
@end deftp

@menu
* Listing Trust Items::           Browsing the list of available trust items.
* Information About Trust Items:: Requesting information about trust items.
* Manipulating Trust Items::      Operations on trust items.
@end menu


@node Listing Trust Items
@subsection Listing Trust Items
@cindex trust item list

@deftypefun gpgme_error_t gpgme_op_trustlist_start (@w{gpgme_ctx_t @var{ctx}}, @w{const char *@var{pattern}}, @w{int @var{max_level}})
The function @code{gpgme_op_trustlist_start} initiates a trust item
listing operation inside the context @var{ctx}.  It sets everything up
so that subsequent invocations of @code{gpgme_op_trustlist_next} return
the trust items in the list.

The string @var{pattern} contains an engine specific expression that
is used to limit the list to all trust items matching the pattern.  It
can not be the empty string.

The argument @var{max_level} is currently ignored.

The context will be busy until either all trust items are received
(and @code{gpgme_op_trustlist_next} returns @code{GPG_ERR_EOF}), or
@code{gpgme_op_trustlist_end} is called to finish the operation.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_trustlist_next (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_trust_item_t *@var{r_item}})
The function @code{gpgme_op_trustlist_next} returns the next trust
item in the list created by a previous @code{gpgme_op_trustlist_start}
operation in the context @var{ctx}.  The trust item can be destroyed
with @code{gpgme_trust_item_release}.  @xref{Manipulating Trust Items}.

This is the only way to get at @code{gpgme_trust_item_t} objects in
@acronym{GPGME}.

If the last trust item in the list has already been returned,
@code{gpgme_op_trustlist_next} returns @code{GPG_ERR_EOF}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if @var{ctx} or
@var{r_item} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if
there is not enough memory for the operation.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_trustlist_end (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_trustlist_end} ends a pending trust list
operation in the context @var{ctx}.

The function returns the error code @code{GPG_ERR_INV_VALUE} if
@var{ctx} is not a valid pointer, and @code{GPG_ERR_ENOMEM} if at some
time during the operation there was not enough memory available.
@end deftypefun


@node Information About Trust Items
@subsection Information About Trust Items
@cindex trust item, information about
@cindex trust item, attributes
@cindex attributes, of a trust item

The following interfaces are deprecated and only provided for backward
compatibility.  Don't use them.  They will be removed in a future
version of @acronym{GPGME}.

Trust items have attributes which can be queried using the interfaces
below.  The attribute identifiers are shared with those for key
attributes.  @xref{Information About Keys}.

@deftypefun {const char *} gpgme_trust_item_get_string_attr (@w{gpgme_trust_item_t @var{item}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_string_attr} returns the value
of the string-representable attribute @var{what} of trust item
@var{item}.  The arguments @var{idx} and @var{reserved} are reserved
for later use and should be @code{0} and @code{NULL} respectively.

The string returned is only valid as long as the key is valid.

The function returns @code{0} if an attribute can't be returned as a
string, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun

@deftypefun int gpgme_trust_item_get_int_attr (@w{gpgme_trust_item_t @var{item}}, @w{gpgme_attr_t @var{what}}, @w{const void *@var{reserved}}, @w{int @var{idx}})
The function @code{gpgme_trust_item_get_int_attr} returns the value of
the number-representable attribute @var{what} of trust item
@var{item}.  If the attribute occurs more than once in the trust item,
the index is specified by @var{idx}.  However, currently no such
attribute exists, so @var{idx} should be @code{0}.  The argument
@var{reserved} is reserved for later use and should be @code{NULL}.

The function returns @code{0} if the attribute can't be returned as a
number, @var{key} is not a valid pointer, @var{idx} out of range,
or @var{reserved} not @code{NULL}.
@end deftypefun


@node Manipulating Trust Items
@subsection Manipulating Trust Items
@cindex trust item, manipulation

@deftypefun void gpgme_trust_item_ref (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_ref} acquires an additional
reference for the trust item @var{item}.
@end deftypefun

@deftypefun void gpgme_trust_item_unref (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_unref} releases a reference for
the trust item @var{item}.  If this was the last reference, the trust
item will be destroyed and all resources associated to it will be
released.
@end deftypefun


The following interface is deprecated and only provided for backward
compatibility.  Don't use it.  It will be removed in a future version
of @acronym{GPGME}.

@deftypefun void gpgme_trust_item_release (@w{gpgme_trust_item_t @var{item}})
The function @code{gpgme_trust_item_release} is an alias for
@code{gpgme_trust_item_unref}.
@end deftypefun


@node Crypto Operations
@section Crypto Operations
@cindex cryptographic operation

Sometimes, the result of a crypto operation returns a list of invalid
keys encountered in processing the request.  The following structure
is used to hold information about such a key.

@deftp {Data type} {gpgme_invalid_key_t}
This is a pointer to a structure used to store a part of the result of
a crypto operation which takes user IDs as one input parameter.  The
structure contains the following members:

@table @code
@item gpgme_invalid_key_t next
This is a pointer to the next invalid key structure in the linked
list, or @code{NULL} if this is the last element.

@item char *fpr
The fingerprint or key ID of the invalid key encountered.

@item gpgme_error_t reason
An error code describing the reason why the key was found invalid.
@end table
@end deftp


@menu
* Decrypt::                       Decrypting a ciphertext.
* Verify::                        Verifying a signature.
* Decrypt and Verify::            Decrypting a signed ciphertext.
* Sign::                          Creating a signature.
* Encrypt::                       Encrypting a plaintext.
@end menu


@node Decrypt
@subsection Decrypt
@cindex decryption
@cindex cryptographic operation, decryption

@deftypefun gpgme_error_t gpgme_op_decrypt (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt} decrypts the ciphertext in the
data object @var{cipher} and stores it into the data object
@var{plain}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
ciphertext could be decrypted successfully, @code{GPG_ERR_INV_VALUE}
if @var{ctx}, @var{cipher} or @var{plain} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if @var{cipher} does not contain any data to
decrypt, @code{GPG_ERR_DECRYPT_FAILED} if @var{cipher} is not a valid
cipher text, @code{GPG_ERR_BAD_PASSPHRASE} if the passphrase for the
secret key could not be retrieved, and passes through any errors that
are reported by the crypto engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_decrypt_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{cipher}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_decrypt_start} initiates a
@code{gpgme_op_decrypt} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, and @code{GPG_ERR_INV_VALUE}
if @var{cipher} or @var{plain} is not a valid pointer.
@end deftypefun

@deftp {Data type} {gpgme_recipient_t}
This is a pointer to a structure used to store information about the
recipient of an encrypted text which is decrypted in a
@code{gpgme_op_decrypt} operation.  This information (except for the
status field) is even available before the operation finished
successfully, for example in a passphrase callback.  The structure
contains the following members:

@table @code
@item gpgme_recipient_t next
This is a pointer to the next recipient structure in the linked list,
or @code{NULL} if this is the last element.

@item gpgme_pubkey_algo_t
The public key algorithm used in the encryption.

@item unsigned int wrong_key_usage : 1
This is true if the key was not used according to its policy.

@item char *keyid
This is the key ID of the key (in hexadecimal digits) used as
recipient.

@item gpgme_error_t status
This is an error number with the error code GPG_ERR_NO_SECKEY if the
secret key for this recipient is not available, and 0 otherwise.
@end table
@end deftp

@deftp {Data type} {gpgme_decrypt_result_t}
This is a pointer to a structure used to store the result of a
@code{gpgme_op_decrypt} operation.  After successfully decrypting
data, you can retrieve the pointer to the result with
@code{gpgme_op_decrypt_result}.  The structure contains the following
members:

@table @code
@item char *unsupported_algorithm
If an unsupported algorithm was encountered, this string describes the
algorithm that is not supported.

@item unsigned int wrong_key_usage : 1
This is true if the key was not used according to its policy.

@item gpgme_recipient_t recipients
This is a linked list of recipients to which this message was encrypted.

@item char *file_name
This is the filename of the original plaintext message file if it is
known, otherwise this is a null pointer.
@end table
@end deftp

@deftypefun gpgme_decrypt_result_t gpgme_op_decrypt_result (@w{gpgme_ctx_t @var{ctx}})
The function @code{gpgme_op_decrypt_result} returns a
@code{gpgme_decrypt_result_t} pointer to a structure holding the
result of a @code{gpgme_op_decrypt} operation.  The pointer is only
valid if the last operation on the context was a
@code{gpgme_op_decrypt} or @code{gpgme_op_decrypt_start} operation.
If the operation failed this might be a @code{NULL} pointer.  The
returned pointer is only valid until the next operation is started on
the context.
@end deftypefun


@node Verify
@subsection Verify
@cindex verification
@cindex signature, verification
@cindex cryptographic operation, verification
@cindex cryptographic operation, signature check
@cindex signature notation data
@cindex notation data

@deftypefun gpgme_error_t gpgme_op_verify (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_data_t @var{signed_text}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_verify} verifies that the signature in the
data object @var{sig} is a valid signature.  If @var{sig} is a
detached signature, then the signed text should be provided in
@var{signed_text} and @var{plain} should be a null pointer.
Otherwise, if @var{sig} is a normal (or cleartext) signature,
@var{signed_text} should be a null pointer and @var{plain} should be a
writable data object that will contain the plaintext after successful
verification.

The results of the individual signature verifications can be retrieved
with @code{gpgme_op_verify_result}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be completed successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{sig} or @var{plain} is not a valid pointer,
@code{GPG_ERR_NO_DATA} if @var{sig} does not contain any data to
verify, and passes through any errors that are reported by the crypto
engine support routines.
@end deftypefun

@deftypefun gpgme_error_t gpgme_op_verify_start (@w{gpgme_ctx_t @var{ctx}}, @w{gpgme_data_t @var{sig}}, @w{gpgme_data_t @var{signed_text}}, @w{gpgme_data_t @var{plain}})
The function @code{gpgme_op_verify_start} initiates a
@code{gpgme_op_verify} operation.  It can be completed by calling
@code{gpgme_wait} on the context.  @xref{Waiting For Completion}.

The function returns the error code @code{GPG_ERR_NO_ERROR} if the
operation could be started successfully, @code{GPG_ERR_INV_VALUE} if
@var{ctx}, @var{sig} or @var{plain} is not a valid pointer, and
@code{GPG_ERR_NO_DATA} if @var{sig} or @var{plain} does not contain
any data to verify.
@end deftypefun

@deftp {Data type} {gpgme_sig_notation_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_verify} operation.  The structure contains the
following members:

@table @code
@item gpgme_sig_notation_t next
This is a pointer to the next new signature notation structure in the
linked list, or @code{NULL} if this is the last element.

@item char *name
The name of the notation field.  If this is @code{NULL}, then the
member @code{value} will contain a policy URL.

@item int name_len
The length of the @code{name} field.  For strings the length is
counted without the trailing binary zero.

@item char *value
The value of the notation field.  If @code{name} is @code{NULL}, then
this is a policy URL.

@item int value_len
The length of the @code{value} field.  For strings the length is
counted without the trailing binary zero.

@item gpgme_sig_notation_flags_t flags
The accumulated flags field.  This field contains the flags associated
with the notation data in an accumulated form which can be used as an
argument to the function @code{gpgme_sig_notation_add}.  The value
@code{flags} is a bitwise-or combination of one or multiple of the
following bit values:

@table @code
@item GPGME_SIG_NOTATION_HUMAN_READABLE
The @code{GPGME_SIG_NOTATION_HUMAN_READABLE} symbol specifies that the
notation data is in human readable form

@item GPGME_SIG_NOTATION_CRITICAL
The @code{GPGME_SIG_NOTATION_CRITICAL} symbol specifies that the
notation data is critical.

@end table

@item unsigned int human_readable : 1
This is true if the @code{GPGME_SIG_NOTATION_HUMAN_READABLE} flag is
set and false otherwise.  This flag is only valid for notation data,
not for policy URLs.

@item unsigned int critical : 1
This is true if the @code{GPGME_SIG_NOTATION_CRITICAL} flag is set and
false otherwise.  This flag is valid for notation data and policy URLs.

@end table
@end deftp

@deftp {Data type} {gpgme_signature_t}
This is a pointer to a structure used to store a part of the result of
a @code{gpgme_op_verify} operation.  The structure contains the
following members:

@table @code
@item gpgme_signature_t next
This is a pointer to the next new signature structure in the linked
list, or @code{NULL} if this is the last element.

@item gpgme_sigsum_t summary
This is a bit vector giving a summary of the signature status.  It
provides an easy interface to a defined semantic of the signature
status.  Checking just one bit is sufficient to see whether a
signature is valid without any restrictions.

The defined bits are:
  @table @code