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	\input texinfo @c -- Texinfo --
	@c %**start of header
	@setfilename gcrypt.info
	@include version.texi
	@settitle The Libgcrypt Reference Manual
	@c Unify some of the indices.
	@syncodeindex tp fn
	@syncodeindex pg fn
	@c %**end of header
	@copying
	This manual is for Libgcrypt
	(version @value{VERSION}, @value{UPDATED}),
	which is GNU's library of cryptographic building blocks.

	Copyright @copyright{} 2000, 2002, 2003, 2004 Free Software Foundation, Inc.

	@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 2 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
	@end copying

	@dircategory GNU Libraries
	@direntry
	* libgcrypt: (gcrypt). Cryptographic function library.
	@end direntry



	@c
	@c Titlepage
	@c
	@setchapternewpage odd
	@titlepage
	@title The Libgcrypt Reference Manual
	@subtitle Version @value{VERSION}
	@subtitle @value{UPDATED}
	@author Werner Koch (@email{wk@@gnupg.org})
	@author Moritz Schulte (@email{mo@@g10code.com})

	@page
	@vskip 0pt plus 1filll
	@insertcopying
	@end titlepage


	@summarycontents
	@contents
	@page


	@ifnottex
	@node Top
	@top The Libgcrypt Library
	@insertcopying
	@end ifnottex


	@menu
	* Introduction:: What is @acronym{Libgcrypt}.
	* Preparation:: What you should do before using the library.
	* Generalities:: General library functions and data types.
	* Handler Functions:: Working with handler functions.
	* Symmetric cryptography:: How to use symmetric cryptography.
	* Hashing:: How to use hashing.
	* Public Key cryptography (I):: How to use public key cryptography.
	* Public Key cryptography (II):: How to use public key cryptography, alternatively.
	* Random Numbers:: How to work with random numbers.
	* S-expressions:: How to manage S-expressions.
	* MPI library:: How to work with multi-precision-integers.
	* Utilities:: Utility functions.

	Appendices

	* Library Copying:: The GNU Lesser General Public License
	says how you can copy and share `Libgcrypt'.
	* Copying:: The GNU General Public License says how you
	can copy and share some parts of `Libgcrypt'.

	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:: How to use this manual.
	* Features:: A glance at @acronym{Libgcrypt}'s features.
	* Overview:: Overview about the library.

	Preparation
	* Header:: What header file you need to include.
	* Building sources:: How to build sources using the library.
	* Building sources using Automake:: How to build sources with the help of Automake.
	* Initializing the library:: How to initialize the library.
	* Multi Threading:: How @acronym{Libgcrypt} can be used in a MT environment.

	Generalities
	* Controlling the library:: Controlling @acronym{Libgcrypt}'s behavior.
	* Modules:: Description of extension modules.
	* Error Handling:: Error codes and such.

	Handler Functions
	* Progress handler:: Using a progress handler function.
	* Allocation handler:: Using special memory allocation functions.
	* Error handler:: Using error handler functions.
	* Logging handler:: Using a special logging function.

	Symmetric cryptography
	* Available ciphers:: List of ciphers supported by the library.
	* Cipher modules:: How to work with cipher modules.
	* Available cipher modes:: List of cipher modes supported by the library.
	* Working with cipher handles:: How to perform operations related to cipher handles.
	* General cipher functions:: General cipher functions independent of cipher handles.

	Hashing
	* Available hash algorithms:: List of hash algorithms supported by the library.
	* Hash algorithm modules:: How to work with hash algorithm modules.
	* Working with hash algorithms:: List of functions related to hashing.

	Public Key cryptography (I)
	* Used S-expressions:: Introduction into the used S-expression.
	* Available algorithms:: Algorithms supported by the library.
	* Public key modules:: How to work with public key modules.
	* Cryptographic Functions:: Functions for performing the cryptographic actions.
	* General public-key related Functions:: General functions, not implementing any cryptography.

	Public Key cryptography (II)
	* Available asymmetric algorithms:: List of algorithms supported by the library.
	* Working with sets of data:: How to work with sets of data.
	* Working with handles:: How to use handles.
	* Working with keys:: How to work with keys.
	* Using cryptographic functions:: How to perform cryptographic operations.
	* Handle-independent functions:: General functions independent of handles.

	Random Numbers
	* Quality of random numbers:: @acronym{Libgcrypt} uses different quality levels.
	* Retrieving random numbers:: How to retrieve random numbers.

	S-expressions
	* Data types for S-expressions:: Data types related with S-expressions.
	* Working with S-expressions:: How to work with S-expressions.

	MPI library
	* Data types:: MPI related data types.
	* Basic functions:: First steps with MPI numbers.
	* MPI formats:: External representation of MPIs.
	* Calculations:: Performing MPI calculations.
	* Comparisons:: How to compare MPI values.
	* Bit manipulations:: How to access single bits of MPI values.
	* Miscellaneous:: Miscellaneous MPI functions.

	Utilities
	* Memory allocation:: Functions related with memory allocation.

	@end detailmenu

	@end menu



	@c **********************************************************
	@c ***************** Introduction *********************
	@c **********************************************************
	@node Introduction
	@chapter Introduction
	`@acronym{Libgcrypt}' is a library providing cryptographic building blocks.

	@menu
	* Getting Started:: How to use this manual.
	* Features:: A glance at @acronym{Libgcrypt}'s features.
	* Overview:: Overview about the library.
	@end menu

	@node Getting Started
	@section Getting Started

	This manual documents the `@acronym{Libgcrypt}' library application programming
	interface (API). All functions and data types provided by the library
	are explained.

	@noindent
	The reader is assumed to possess basic knowledge about applied
	cryptography.

	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

	`Libgcrypt' might have a couple of advantages over other libraries doing
	a similar job.

	@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}). Note, that
	some parts (which are not needed on a GNU or GNU/Linux system) are
	subject to the terms of the GNU General Public License
	(@pxref{Copying}); please see the README file of the distribution for of
	list of these parts.

	@item It encapsulates the low level cryptography
	`@acronym{Libgcrypt}' provides a high level interface to cryptographic building
	blocks using an extendable and flexible API.

	@end table

	@node Overview
	@section Overview

	@noindent
	The `@acronym{Libgcrypt}' library is fully thread-safe, where it makes
	sense to be thread-safe. An exception for thread-safety are some
	cryptographic functions that modify a certain context stored in
	handles. If the user really intents to use such functions from
	different threads on the same handle, he has to take care of the
	serialization of such functions himself. If not described otherwise,
	every function is thread-safe.

	@acronym{Libgcrypt} depends on the library `libgpg-error', which
	contains common error handling related code for GnuPG components.

	@c **********************************************************
	@c ***************** Preparation **********************
	@c **********************************************************
	@node Preparation
	@chapter Preparation

	To use `@acronym{Libgcrypt}', 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 sources:: How to build sources using the library.
	* Building sources using Automake:: How to build sources with the help of Automake.
	* Initializing the library:: How to initialize the library.
	* Multi Threading:: How @acronym{Libgcrypt} can be used in a MT environment.
	@end menu


	@node Header
	@section Header

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

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

	The name space of `@acronym{Libgcrypt}' is @code{gcry_*} for function
	and type names and @code{GCRY*} for other symbols. In addition the
	same name prefixes with one prepended underscore are reserved for
	internal use and should never be used by an application. Furthermore
	`libgpg-error' defines functions prefixed with `gpg_' and preprocessor
	symbols prefixed with `GPG_'. Note that @acronym{Libgcrypt} uses
	libgpg-error, which uses @code{gpg_err_*} as name space for function
	and type names and @code{GPG_ERR_*} for other symbols, including all
	the error codes.

	@node Building sources
	@section Building sources

	If you want to compile a source file including the `gcrypt.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, `@acronym{Libgcrypt}' ships with a small
	helper program @command{libgcrypt-config} that knows 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{libgcrypt-config}. The following
	example shows how it can be used at the command line:

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

	Adding the output of @samp{libgcrypt-config --cflags} to the compilers
	command line will ensure that the compiler can find the `@acronym{Libgcrypt}' 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{libgcrypt-config} can be used. For convenience, this option
	also outputs all other options that are required to link the program
	with the `@acronym{Libgcrypt}' libraries (in particular, the @samp{-lgcrypt}
	option). The example shows how to link @file{foo.o} with the `@acronym{Libgcrypt}'
	library to a program @command{foo}.

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

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

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

	@node Building sources using Automake
	@section Building sources using Automake

	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{libgcrypt-config} script at all.
	@acronym{Libgcrypt} 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_LIBGCRYPT (@ovar{minimum-version}, @ovar{action-if-found}, @ovar{action-if-not-found})
	Check whether @acronym{Libgcrypt} (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{LIBGCRYPT_CFLAGS} to the
	flags needed for compilation of the program to find the
	@file{gcrypt.h} header file, and @code{LIBGCRYPT_LIBS} to the linker
	flags needed to link the program to the @acronym{Libgcrypt} library.
	@end defmac

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

	@example
	AM_CPPFLAGS = $(LIBGCRYPT_CFLAGS)
	LDADD = $(LIBGCRYPT_LIBS)
	@end example

	@node Initializing the library
	@section Initializing the library

	It is often desirable to check that the version of `@acronym{Libgcrypt}' used is
	indeed one which fits all requirements. Even with binary compatibility
	new features may have been introduced but due to problem with the
	dynamic linker an old version is actually used. So you may want to
	check that the version is okay right after program startup.

	@deftypefun const char gcry_check_version (const char @var{req_version})

	The function @code{gcry_check_version} has three 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 of @acronym{Libgcrypt}.
	@end deftypefun

	@node Multi Threading
	@section Multi Threading

	As mentioned earlier, the `@acronym{Libgcrypt}' library is
	thread-safe if you adhere to the following requirements:

	@itemize @bullet
	@item
	If your application is multi-threaded, you must set the thread support
	callbacks with the @code{GCRYCTL_SET_THREAD_CBS} command
	@strong{before} any other function in the library.

	This is easy enough if you are indeed writing an application using
	Libgcrypt. It is rather problematic if you are writing a library
	instead. Here are some tips what to do if you are writing a library:

	If your library requires a certain thread package, just initialize
	Libgcrypt to use this thread package. If your library supports multiple
	thread packages, but needs to be configured, you will have to
	implement a way to determine which thread package the application
	wants to use with your library anyway. Then configure Libgcrypt to use
	this thread package.

	If your library is fully reentrant without any special support by a
	thread package, then you are lucky indeed. Unfortunately, this does
	not relieve you from doing either of the two above, or use a third
	option. The third option is to let the application initialize Libgcrypt
	for you. Then you are not using Libgcrypt transparently, though.

	As if this was not difficult enough, a conflict may arise if two
	libraries try to initialize Libgcrypt independently of each others, and
	both such libraries are then linked into the same application. To
	make it a bit simpler for you, this will probably work, but only if
	both libraries have the same requirement for the thread package. This
	is currently only supported for the non-threaded case, GNU Pth and
	pthread. Support for more thread packages is easy to add, so contact
	us if you require it.

	@item
	The function @code{gcry_check_version} must be called before any other
	function in the library, except the @code{GCRYCTL_SET_THREAD_CBS}
	command (called via the @code{gcry_control} function), because it
	initializes the thread support subsystem in @acronym{Libgcrypt}. To
	achieve this in multi-threaded programs, you must synchronize the
	memory with respect to other threads that also want to use
	@acronym{Libgcrypt}. For this, it is sufficient to call
	@code{gcry_check_version} before creating the other threads using
	@acronym{Libgcrypt}@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, more relaxed or more
	strict rules may apply.}.

	@item

	As with the function @code{gpg_strerror}, @code{gcry_strerror} is not
	thread safe. You have to use @code{gpg_strerror_r} instead.
	@end itemize


	@acronym{Libgcrypt} contains convenient macros, which define the
	necessary thread callbacks for PThread and for GNU Pth:

	@table @code
	@item GCRY_THREAD_OPTION_PTH_IMPL

	This macro defines the following (static) symbols: gcry_pth_init,
	gcry_pth_mutex_init, gcry_pth_mutex_destroy, gcry_pth_mutex_lock,
	gcry_pth_mutex_unlock, gcry_pth_read, gcry_pth_write, gcry_pth_select,
	gcry_pth_waitpid, gcry_pth_accept, gcry_pth_connect, gcry_threads_pth.

	After including this macro, gcry_control() shall be used with a
	command of GCRYCTL_SET_THREAD_CBS in order to register the thread
	callback structure named ``gcry_threads_pth''.

	@item GCRY_THREAD_OPTION_PTHREAD_IMPL

	This macro defines the following (static) symbols:
	gcry_pthread_mutex_init, gcry_pthread_mutex_destroy, gcry_mutex_lock,
	gcry_mutex_unlock, gcry_threads_pthread.

	After including this macro, gcry_control() shall be used with a
	command of GCRYCTL_SET_THREAD_CBS in order to register the thread
	callback structure named ``gcry_threads_pthread''.
	@end table

	Note that these macros need to be terminated with a semicolon. Keep
	in mind that these are convenient macros for C programmers; C++
	programmers might have to wrap these macros in an ``extern C'' body.

	@c **********************************************************
	@c ***************** General **************************
	@c **********************************************************
	@node Generalities
	@chapter Generalities

	@menu
	* Controlling the library:: Controlling @acronym{Libgcrypt}'s behavior.
	* Modules:: Description of extension modules.
	* Error Handling:: Error codes and such.
	@end menu

	@node Controlling the library
	@section Controlling the library

	@deftypefun gcry_error_t gcry_control (enum gcry_ctl_cmds @var{cmd}, ...)

	This function can be used to influence the general behavior of
	@acronym{Libgcrypt} in several ways. Depending on @var{cmd}, more
	arguments can or have to be provided.

	@end deftypefun

	@node Modules
	@section Modules

	@acronym{Libgcrypt} supports the use of `extension modules', which
	implement algorithms in addition to those already built into the
	library directly.

	@deftp {Data type} gcry_module_t
	This data type represents a `module'.
	@end deftp

	Functions registering modules provided by the user take a `module
	specification structure' as input and return a value of
	@code{gcry_module_t} and an ID that is unique in the modules'
	category. This ID can be used to reference the newly registered
	module. After registering a module successfully, the new functionality
	should be able to be used through the normal functions provided by
	@acronym{Libgcrypt} until it is unregistered again.

	@c **********************************************************
	@c ***************** Errors **************************
	@c **********************************************************
	@node Error Handling
	@section Error Handling

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


	Most functions in @acronym{Libgcrypt} return an error code in the case
	of failure. 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 canceling 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.

	GnuPG components, including Libgcrypt, use an extra library named
	libgpg-error to provide a common error handling scheme. For more
	information on libgpg-error, see the according manual.

	@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
	@subsection Error Values
	@cindex error values
	@cindex error codes
	@cindex error sources

	@deftp {Data type} {gcry_err_code_t}
	The @code{gcry_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} {gcry_err_source_t}
	The @code{gcry_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} {gcry_error_t}
	The @code{gcry_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{Libgcrypt}, the error source is used purely for
	diagnostic 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 {gcry_err_code_t} gcry_err_code (@w{gcry_error_t @var{err}})
	The static inline function @code{gcry_err_code} returns the
	@code{gcry_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 {gcry_err_source_t} gcry_err_source (@w{gcry_error_t @var{err}})
	The static inline function @code{gcry_err_source} returns the
	@code{gcry_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 {gcry_error_t} gcry_err_make (@w{gcry_err_source_t @var{source}}, @w{gcry_err_code_t @var{code}})
	The static inline function @code{gcry_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 {gcry_error_t} gcry_error (@w{gcry_err_code_t @var{code}})
	The static inline function @code{gcry_error} returns the error value
	consisting of the default error source and the error code @var{code}.

	For @acronym{GCRY} applications, the default error source is
	@code{GPG_ERR_SOURCE_USER_1}. You can define
	@code{GCRY_ERR_SOURCE_DEFAULT} before including @file{gcrypt.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
	errno numbers.

	@deftypefun {gcry_error_t} gcry_err_make_from_errno (@w{gcry_err_source_t @var{source}}, @w{int @var{err}})
	The function @code{gcry_err_make_from_errno} is like
	@code{gcry_err_make}, but it takes a system error like @code{errno}
	instead of a @code{gcry_err_code_t} error code.
	@end deftypefun

	@deftypefun {gcry_error_t} gcry_error_from_errno (@w{int @var{err}})
	The function @code{gcry_error_from_errno} is like @code{gcry_error},
	but it takes a system error like @code{errno} instead of a
	@code{gcry_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 {gcry_err_code_t} gcry_err_code_from_errno (@w{int @var{err}})
	The function @code{gcry_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} gcry_err_code_to_errno (@w{gcry_err_code_t @var{err}})
	The function @code{gcry_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
	@subsection 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{Libgctypt} 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.

	@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
	OpenPGP 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{Libgcrypt} 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{gcry_error} and @code{gcry_error_from_errno},
	unless you define @code{GCRY_ERR_SOURCE_DEFAULT} before including
	@file{gcrypt.h}.
	@end table


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

	The library @code{libgpg-error} defines many error values. The
	following list includes the most important error codes.

	@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_EFOO 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.

	@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 data was expected where no data was found.

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

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

	@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{Libgcrypt}
	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
	@subsection Error Strings
	@cindex error values, printing of
	@cindex error codes, printing of
	@cindex error sources, printing of
	@cindex error strings

	@deftypefun {const char *} gcry_strerror (@w{gcry_error_t @var{err}})
	The function @code{gcry_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.
	@end deftypefun


	@deftypefun {const char *} gcry_strsource (@w{gcry_error_t @var{err}})
	The function @code{gcry_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 the functions described
	above:

	@example
	@{
	gcry_cipher_hd_t handle;
	gcry_error_t err = 0;

	err = gcry_cipher_open (&handle, GCRY_CIPHER_AES, GCRY_CIPHER_MODE_CBC, 0);
	if (err)
	@{
	fprintf (stderr, "Failure: %s/%s\n",
	gcry_strsource (err),
	gcry_strerror (err));
	@}
	@}
	@end example

	@c **********************************************************
	@c ***************** General **************************
	@c **********************************************************
	@node Handler Functions
	@chapter Handler Functions

	@acronym{Libgcrypt} makes it possible to install so called `handler functions',
	which get called by @acronym{Libgcrypt} in case of certain events.

	@menu
	* Progress handler:: Using a progress handler function.
	* Allocation handler:: Using special memory allocation functions.
	* Error handler:: Using error handler functions.
	* Logging handler:: Using a special logging function.
	@end menu

	@node Progress handler
	@section Progress handler

	It is often useful to retrieve some feedback while long running
	operations are performed.

	@deftp {Data type} gcry_handler_progress_t
	Progress handler functions have to be of the type
	@code{gcry_handler_progress_t}, which is defined as:

	@code{void (gcry_handler_progress_t) (void , const char *, int, int, int)}
	@end deftp

	The following function may be used to register a handler function for
	this purpose.

	@deftypefun void gcry_set_progress_handler (gcry_handler_progress_t @var{cb}, void *@var{cb_data})

	This function installs @var{cb} as the `Progress handler' function.
	@var{cb} must be defined as follows:

	@example
	void
	my_progress_handler (void @var{cb_data}, const char @var{what},
	int @var{printchar}, int @var{current}, int @var{total})
	@{
	/* Do something. */
	@}
	@end example

	A description of the arguments of the progress handler function follows.

	@table @var
	@item cb_data
	The argument provided in the call to @code{gcry_set_progress_handler}.
	@item what
	A string identifying the type of the progress output. The following
	values for @var{what} are defined:

	@table @code
	@item need_entropy
	Not enough entropy is available. @var{total} holds the number of
	required bytes.

	@item primegen
	Values for @var{printchar}:
	@table @code
	@item \n
	Prime generated.
	@item !
	Need to refresh the pool of prime numbers.
	@item <, >
	Number of bits adjusted.
	@item ^
	Searching for a generator.
	@item .
	Fermat test on 10 candidates failed.
	@item :
	Restart with a new random value.
	@item +
	Rabin Miller test passed.
	@end table

	@end table

	@end table
	@end deftypefun

	@node Allocation handler
	@section Allocation handler

	It is possible to make @acronym{Libgcrypt} use special memory
	allocation functions instead of the built-in ones.

	Memory allocation functions are of the following types:
	@deftp {Data type} gcry_handler_alloc_t
	This type is defined as: @code{void (gcry_handler_alloc_t) (size_t n)}.
	@end deftp
	@deftp {Data type} gcry_handler_secure_check_t
	This type is defined as: @code{int (gcry_handler_secure_check_t) (const void *)}.
	@end deftp
	@deftp {Data type} gcry_handler_realloc_t
	This type is defined as: @code{void (gcry_handler_realloc_t) (void *p, size_t n)}.
	@end deftp
	@deftp {Data type} gcry_handler_free_t
	This type is defined as: @code{void (gcry_handler_free_t) (void *)}.
	@end deftp

	Special memory allocation functions can be installed with the
	following function:

	@deftypefun void gcry_set_allocation_handler (gcry_handler_alloc_t @var{func_alloc}, gcry_handler_alloc_t @var{func_alloc_secure}, gcry_handler_secure_check_t @var{func_secure_check}, gcry_handler_realloc_t @var{func_realloc}, gcry_handler_free_t @var{func_free})
	Install the provided functions and use them instead of the built-in
	functions for doing memory allocation.
	@end deftypefun

	@node Error handler
	@section Error handler

	The following functions may be used to register handler functions that
	are called by @acronym{Libgcrypt} in case certain error conditions
	occur.

	@deftp {Data type} gcry_handler_no_mem_t
	This type is defined as: @code{void (gcry_handler_no_mem_t) (void , size_t, unsigned int)}
	@end deftp
	@deftypefun void gcry_set_outofcore_handler (gcry_handler_no_mem_t @var{func_no_mem}, void *@var{cb_data})
	This function registers @var{func_no_mem} as `out-of-core handler',
	which means that it will be called in the case of not having enough
	memory available.
	@end deftypefun

	@deftp {Data type} gcry_handler_error_t
	This type is defined as: @code{void (gcry_handler_error_t) (void , int, const char *)}
	@end deftp

	@deftypefun void gcry_set_fatalerror_handler (gcry_handler_error_t @var{func_error}, void *@var{cb_data})
	This function registers @var{func_error} as `error handler',
	which means that it will be called in error conditions.
	@end deftypefun

	@node Logging handler
	@section Logging handler

	@deftp {Data type} gcry_handler_log_t
	This type is defined as: @code{void (gcry_handler_log_t) (void , int, const char *, va_list)}
	@end deftp

	@deftypefun void gcry_set_log_handler (gcry_handler_log_t @var{func_log}, void *@var{cb_data})
	This function registers @var{func_log} as `logging handler', which
	means that it will be called in case @acronym{Libgcrypt} wants to log
	a message.
	@end deftypefun

	@c **********************************************************
	@c ***************** Ciphers **************************
	@c **********************************************************
	@c @include cipher-ref.texi
	@node Symmetric cryptography
	@chapter Symmetric cryptography

	The cipher functions are used for symmetrical cryptography,
	i.e. cryptography using a shared key. The programming model follows
	an open/process/close paradigm and is in that similar to other
	building blocks provided by @acronym{Libgcrypt}.

	@menu
	* Available ciphers:: List of ciphers supported by the library.
	* Cipher modules:: How to work with cipher modules.
	* Available cipher modes:: List of cipher modes supported by the library.
	* Working with cipher handles:: How to perform operations related to cipher handles.
	* General cipher functions:: General cipher functions independent of cipher handles.
	@end menu

	@node Available ciphers
	@section Available ciphers

	@table @code
	@item GCRY_CIPHER_NONE
	This is not a real algorithm but used by some functions as error return.
	The value always evaluates to false.

	@item GCRY_CIPHER_IDEA
	This is the IDEA algorithm. The constant is provided but there is
	currently no implementation for it because the algorithm is patented.

	@item GCRY_CIPHER_3DES
	Triple-DES with 3 Keys as EDE. The key size of this algorithm is 168 but
	you have to pass 192 bits because the most significant bits of each byte
	are ignored.

	@item GCRY_CIPHER_CAST5
	CAST128-5 block cipher algorithm. The key size is 128 bits.

	@item GCRY_CIPHER_BLOWFISH
	The blowfish algorithm. The current implementation allows only for a key
	size of 128 bits.

	@item GCRY_CIPHER_SAFER_SK128
	Reserved and not currently implemented.

	@item GCRY_CIPHER_DES_SK
	Reserved and not currently implemented.

	@item GCRY_CIPHER_AES
	@itemx GCRY_CIPHER_AES128
	@itemx GCRY_CIPHER_RIJNDAEL
	@itemx GCRY_CIPHER_RIJNDAEL128
	AES (Rijndael) with a 128 bit key.

	@item GCRY_CIPHER_AES192
	@itemx GCRY_CIPHER_RIJNDAEL192
	AES (Rijndael) with a 192 bit key.

	@item GCRY_CIPHER_AES256
	@itemx GCRY_CIPHER_RIJNDAEL256
	AES (Rijndael) with a 256 bit key.

	@item GCRY_CIPHER_TWOFISH
	The Twofish algorithm with a 256 bit key.

	@item GCRY_CIPHER_TWOFISH128
	The Twofish algorithm with a 128 bit key.

	@item GCRY_CIPHER_ARCFOUR
	An algorithm which is 100% compatible with RSA Inc.'s RC4 algorithm.
	Note that this is a stream cipher and must be used very carefully to
	avoid a couple of weaknesses.

	@item GCRY_CIPHER_DES
	Standard DES with a 56 bit key. You need to pass 64 bit but the high
	bits of each byte are ignored. Note, that this is a weak algorithm
	which can be broken in reasonable time using a brute force approach.

	@end table

	@node Cipher modules
	@section Cipher modules

	@acronym{Libgcrypt} makes it possible to load additional `cipher
	modules'; these cipher can be used just like the cipher algorithms
	that are built into the library directly. For an introduction into
	extension modules, see @xref{Modules}.

	@deftp {Data type} gcry_cipher_spec_t
	This is the `module specification structure' needed for registering
	cipher modules, which has to be filled in by the user before it can be
	used to register a module. It contains the following members:

	@table @code
	@item const char *name
	The primary name of the algorithm.
	@item const char **aliases
	A list of strings that are `aliases' for the algorithm. The list must
	be terminated with a NULL element.
	@item gcry_cipher_oid_spec_t *oids
	A list of OIDs that are to be associated with the algorithm. The
	list's last element must have it's `oid' member set to NULL. See
	below for an explanation of this type.
	@item size_t blocksize
	The block size of the algorithm, in bytes.
	@item size_t keylen
	The length of the key, in bits.
	@item size_t contextsize
	The size of the algorithm-specific `context', that should be allocated
	for each handle.
	@item gcry_cipher_setkey_t setkey
	The function responsible for initializing a handle with a provided
	key. See below for a description of this type.
	@item gcry_cipher_encrypt_t encrypt
	The function responsible for encrypting a single block. See below for
	a description of this type.
	@item gcry_cipher_decrypt_t decrypt
	The function responsible for decrypting a single block. See below for
	a description of this type.
	@item gcry_cipher_stencrypt_t stencrypt
	Like `encrypt', for stream ciphers. See below for a description of
	this type.
	@item gcry_cipher_stdecrypt_t stdecrypt
	Like `decrypt', for stream ciphers. See below for a description of
	this type.
	@end table
	@end deftp

	@deftp {Data type} gcry_cipher_oid_spec_t
	This type is used for associating a user-provided algorithm
	implementation with certain OIDs. It contains the following members:
	@table @code
	@item const char *oid
	Textual representation of the OID.
	@item int mode
	Cipher mode for which this OID is valid.
	@end table
	@end deftp

	@deftp {Data type} gcry_cipher_setkey_t
	Type for the `setkey' function, defined as: gcry_err_code_t
	(gcry_cipher_setkey_t) (void c, const unsigned char *key, unsigned
	keylen)
	@end deftp

	@deftp {Data type} gcry_cipher_encrypt_t
	Type for the `encrypt' function, defined as: gcry_err_code_t
	(gcry_cipher_encrypt_t) (void c, const unsigned char *outbuf, const
	unsigned char *inbuf)
	@end deftp

	@deftp {Data type} gcry_cipher_decrypt_t
	Type for the `decrypt' function, defined as: gcry_err_code_t
	(gcry_cipher_decrypt_t) (void c, const unsigned char *outbuf, const
	unsigned char *inbuf)
	@end deftp

	@deftp {Data type} gcry_cipher_stencrypt_t
	Type for the `stencrypt' function, defined as: gcry_err_code_t
	(gcry_cipher_stencrypt_t) (void c, const unsigned char *outbuf, const
	unsigned char *, unsigned int n)
	@end deftp

	@deftp {Data type} gcry_cipher_stdecrypt_t
	Type for the `stdecrypt' function, defined as: gcry_err_code_t
	(gcry_cipher_stdecrypt_t) (void c, const unsigned char *outbuf, const
	unsigned char *, unsigned int n)
	@end deftp

	@deftypefun gcry_error_t gcry_cipher_register (gcry_cipher_spec_t @var{cipher}, unsigned int algorithm_id, gcry_module_t *@var{module})

	Register a new cipher module whose specification can be found in
	@var{cipher}. On success, a new algorithm ID is stored in
	@var{algorithm_id} and a pointer representing this module is stored
	in @var{module}.
	@end deftypefun

	@deftypefun void gcry_cipher_unregister (gcry_module_t @var{module})
	Unregister the cipher identified by @var{module}, which must have been
	registered with gcry_cipher_register.
	@end deftypefun

	@deftypefun gcry_error_t gcry_cipher_list (int @var{list}, int @var{list_length})
	Get a list consisting of the IDs of the loaded cipher modules. If
	@var{list} is zero, write the number of loaded cipher modules to
	@var{list_length} and return. If @var{list} is non-zero, the first
	*@var{list_length} algorithm IDs are stored in @var{list}, which must
	be of according size. In case there are less cipher modules than
	@var{list_length}, @var{list_length} is updated to the correct
	number.
	@end deftypefun

	@node Available cipher modes
	@section Available cipher modes

	@table @code
	@item GCRY_CIPHER_MODE_NONE
	No mode specified, may be set later using other functions. The value
	of this constant is always 0.

	@item GCRY_CIPHER_MODE_ECB
	Electronic Codebook mode.

	@item GCRY_CIPHER_MODE_CFB
	Cipher Feedback mode.

	@item GCRY_CIPHER_MODE_CBC
	Cipher Block Chaining mode.

	@item GCRY_CIPHER_MODE_STREAM
	Stream mode, only to be used with stream cipher algorithms.

	@item GCRY_CIPHER_MODE_OFB
	Outer Feedback mode.

	@item GCRY_CIPHER_MODE_CTR
	Counter mode.

	@end table

	@node Working with cipher handles
	@section Working with cipher handles

	To use a cipher algorithm, you must first allocate an according
	handle. This is to be done using the open function:

	@deftypefun gcry_error_t gcry_cipher_open (gcry_cipher_hd_t *@var{hd}, int @var{algo}, int @var{mode}, unsigned int @var{flags})

	This function creates the context handle required for most of the
	other cipher functions and returns a handle to it in `hd'. In case of
	an error, an according error code is returned.

	The ID of algorithm to use must be specified via @var{algo}. See
	@xref{Available ciphers}, for a list of supported ciphers and the
	according constants.

	Besides using the constants directly, the function
	@code{gcry_cipher_map_name} may be used to convert the textual name of
	an algorithm into the according numeric ID.

	The cipher mode to use must be specified via @var{mode}. See
	@xref{Available cipher modes}, for a list of supported cipher modes
	and the according constants. Note, that some modes do not work
	together with all algorithms.

	The third argument @var{flags} can either be passed as @code{0} or as
	the bit-wise OR of the following constants.

	@table @code
	@item GCRY_CIPHER_SECURE
	Make sure that all operations are allocated in secure memory. This is
	useful, when the key material is highly confidential.
	@item GCRY_CIPHER_ENABLE_SYNC
	This flag enables the CFB sync mode, which is a special feature of
	@acronym{Libgcrypt}'s CFB mode implementation to allow for OpenPGP's CFB variant.
	See @code{gcry_cipher_sync}.
	@item GCRY_CIPHER_CBC_CTS
	Enable cipher text stealing (CTS) for the CBC mode. Cannot be used
	simultaneous as GCRY_CIPHER_CBC_MAC
	@item GCRY_CIPHER_CBC_MAC
	Compute CBC-MAC keyed checksums. This is the same as CBC mode, but
	only output the last block. Cannot be used simultaneous as
	GCRY_CIPHER_CBC_CTS.
	@end table
	@end deftypefun

	Use the following function to release an existing handle:

	@deftypefun void gcry_cipher_close (gcry_cipher_hd_t @var{h})

	This function releases the context created by @code{gcry_cipher_open}.
	@end deftypefun

	In order to use a handle for performing cryptographic operations, a
	`key' has to be set first:

	@deftypefun gcry_error_t gcry_cipher_setkey (gcry_cipher_hd_t @var{h}, void *@var{k}, size_t @var{l})

	Set the key @var{k} used for encryption or decryption in the context
	denoted by the handle @var{h}. The length @var{l} of the key @var{k}
	must match the required length of the algorithm set for this context or
	be in the allowed range for algorithms with variable key size. The
	function checks this and returns an error if there is a problem. A
	caller should always check for an error.

	Note, this is currently implemented as a macro but may be changed to a
	function in the future.
	@end deftypefun

	Most crypto modes requires an initialization vector (IV), which
	usually is a non-secret random string acting as a kind of salt value.
	The CTR mode requires a counter, which is also similar to a salt
	value. To set the IV or CTR, use these functions:

	@deftypefun gcry_error_t gcry_cipher_setiv (gcry_cipher_hd_t @var{h}, void *@var{k}, size_t @var{l})

	Set the initialization vector used for encryption or decryption. The
	vector is passed as the buffer @var{K} of length @var{l} and copied to
	internal data structures. The function checks that the IV matches the
	requirement of the selected algorithm and mode. Note, that this is
	implemented as a macro.
	@end deftypefun

	@deftypefun gcry_error_t gcry_cipher_setctr (gcry_cipher_hd_t @var{h}, void *@var{c}, size_t @var{l})

	Set the counter vector used for encryption or decryption. The counter
	is passed as the buffer @var{c} of length @var{l} and copied to
	internal data structures. The function checks that the counter
	matches the requirement of the selected algorithm (i.e., it must be
	the same size as the block size). Note, that this is implemented as a
	macro.
	@end deftypefun

	@deftypefun gcry_error_t gcry_cipher_reset (gcry_cipher_hd_t @var{h})

	Set the given handle's context back to the state it had after the last
	call to gcry_cipher_setkey and clear the initialization vector.

	Note, that gcry_cipher_reset is implemented as a macro.
	@end deftypefun

	The actual encryption and decryption is done by using one of the
	following functions. They may be used as often as required to process
	all the data.

	@deftypefun gcry_error_t gcry_cipher_encrypt (gcry_cipher_hd_t @var{h}, void {out}, size_t @var{outsize}, const void @var{in}, size_t @var{inlen})

	@code{gcry_cipher_encrypt} is used to encrypt the data. This function
	can either work in place or with two buffers. It uses the cipher
	context already setup and described by the handle @var{h}. There are 2
	ways to use the function: If @var{in} is passed as @code{NULL} and
	@var{inlen} is @code{0}, in-place encryption of the data in @var{out} or
	length @var{outsize} takes place. With @var{in} being not @code{NULL},
	@var{inlen} bytes are encrypted to the buffer @var{out} which must have
	at least a size of @var{inlen}. @var{outlen} must be set to the
	allocated size of @var{out}, so that the function can check that there
	is sufficient space. Note, that overlapping buffers are not allowed.

	Depending on the selected algorithms and encryption mode, the length of
	the buffers must be a multiple of the block size.

	The function returns @code{0} on success or an error code.
	@end deftypefun


	@deftypefun gcry_error_t gcry_cipher_decrypt (gcry_cipher_hd_t @var{h}, void {out}, size_t @var{outsize}, const void @var{in}, size_t @var{inlen})

	@code{gcry_cipher_decrypt} is used to decrypt the data. This function
	can either work in place or with two buffers. It uses the cipher
	context already setup and described by the handle @var{h}. There are 2
	ways to use the function: If @var{in} is passed as @code{NULL} and
	@var{inlen} is @code{0}, in-place decryption of the data in @var{out} or
	length @var{outsize} takes place. With @var{in} being not @code{NULL},
	@var{inlen} bytes are decrypted to the buffer @var{out} which must have
	at least a size of @var{inlen}. @var{outlen} must be set to the
	allocated size of @var{out}, so that the function can check that there
	is sufficient space. Note, that overlapping buffers are not allowed.

	Depending on the selected algorithms and encryption mode, the length of
	the buffers must be a multiple of the block size.

	The function returns @code{0} on success or an error code.
	@end deftypefun


	OpenPGP (as defined in RFC-2440) requires a special sync operation in
	some places, the following function is used for this:

	@deftypefun gcry_error_t gcry_cipher_sync (gcry_cipher_hd_t @var{h})

	Perform the OpenPGP sync operation on context @var{h}. Note, that this
	is a no-op unless the context was created with the flag
	@code{GCRY_CIPHER_ENABLE_SYNC}
	@end deftypefun

	Some of the described functions are implemented as macros utilizing a
	catch-all control function. This control function is rarely used
	directly but there is nothing which would inhibit it:

	@deftypefun gcry_error_t gcry_cipher_ctl (gcry_cipher_hd_t @var{h}, int @var{cmd}, void *@var{buffer}, size_t @var{buflen})

	@code{gcry_cipher_ctl} controls various aspects of the cipher module and
	specific cipher contexts. Usually some more specialized functions or
	macros are used for this purpose. The semantics of the function and its
	parameters depends on the the command @var{cmd} and the passed context
	handle @var{h}. Please see the comments in the source code
	(@code{src/global.c}) for details.
	@end deftypefun

	@deftypefun gcry_error_t gcry_cipher_info (gcry_cipher_hd_t @var{h}, int @var{what}, void @var{buffer}, size_t @var{nbytes})

	@code{gcry_cipher_info} is used to retrieve various
	information about a cipher context or the cipher module in general.

	Currently no information is available.
	@end deftypefun

	@node General cipher functions
	@section General cipher functions

	To work with the algorithms, several functions are available to map
	algorithm names to the internal identifiers, as well as ways to
	retrieve information about an algorithm or the current cipher context.

	@deftypefun gcry_error_t gcry_cipher_algo_info (int @var{algo}, int @var{what}, void @var{buffer}, size_t @var{nbytes})

	This function is used to retrieve information on a specific algorithm.
	You pass the cipher algorithm ID as @var{algo} and the type of
	information requested as @var{what}. The result is either returned as
	the return code of the function or copied to the provided @var{buffer}
	whose allocated length must be available in an integer variable with the
	address passed in @var{nbytes}. This variable will also receive the
	actual used length of the buffer.

	Here is a list of supported codes for @var{what}:

	@c begin constants for gcry_cipher_algo_info
	@table @code
	@item GCRYCTL_GET_KEYLEN:
	Return the length of the key. If the algorithm supports multiple key
	lengths, the maximum supported value is returned. The length is
	returned as number of octets (bytes) and not as number of bits in
	@var{nbytes}; @var{buffer} must be zero.

	@item GCRYCTL_GET_BLKLEN:
	Return the block length of the algorithm. The length is returned as a
	number of octets in @var{nbytes}; @var{buffer} must be zero.

	@item GCRYCTL_TEST_ALGO:
	Returns @code{0} when the specified algorithm is available for use.
	@var{buffer} and @var{nbytes} must be zero.

	@end table
	@c end constants for gcry_cipher_algo_info

	@end deftypefun
	@c end gcry_cipher_algo_info

	@deftypefun const char *gcry_cipher_algo_name (int @var{algo})

	@code{gcry_cipher_algo_name} returns a string with the name of the
	cipher algorithm @var{algo}. If the algorithm is not known or another
	error occurred, an empty string is returned. This function will never
	return @code{NULL}.
	@end deftypefun

	@deftypefun int gcry_cipher_map_name (const char *@var{name})

	@code{gcry_cipher_map_name} returns the algorithm identifier for the
	cipher algorithm described by the string @var{name}. If this algorithm
	is not available @code{0} is returned.
	@end deftypefun

	@deftypefun int gcry_cipher_mode_from_oid (const char *@var{string})

	Return the cipher mode associated with an @acronym{ASN.1} object
	identifier. The object identifier is expected to be in the
	@acronym{IETF}-style dotted decimal notation. The function returns
	@code{0} for an unknown object identifier or when no mode is associated
	with it.
	@end deftypefun


	@c **********************************************************
	@c ***************** Hash Functions *******************
	@c **********************************************************
	@node Hashing
	@chapter Hashing

	@acronym{Libgcrypt} provides an easy and consistent to use interface
	for hashing. Hashing is buffered and several hash algorithms can be
	updated at once. It is possible to calculate a MAC using the same
	routines. The programming model follows an open/process/close
	paradigm and is in that similar to other building blocks provided by
	@acronym{Libgcrypt}.

	For convenience reasons, a few cyclic redundancy check value operations
	are also supported.

	@menu
	* Available hash algorithms:: List of hash algorithms supported by the library.
	* Hash algorithm modules:: How to work with hash algorithm modules.
	* Working with hash algorithms:: List of functions related to hashing.
	@end menu

	@node Available hash algorithms
	@section Available hash algorithms

	@c begin table of hash algorithms
	@table @code
	@item GCRY_MD_NONE
	This is not a real algorithm but used by some functions as an error
	return value. This constant is guaranteed to have the value @code{0}.

	@item GCRY_MD_SHA1
	This is the SHA-1 algorithm which yields a message digest of 20 bytes.

	@item GCRY_MD_RMD160
	This is the 160 bit version of the RIPE message digest (RIPE-MD-160).
	Like SHA-1 it also yields a digest of 20 bytes.

	@item GCRY_MD_MD5
	This is the well known MD5 algorithm, which yields a message digest of
	16 bytes.

	@item GCRY_MD_MD4
	This is the MD4 algorithm, which yields a message digest of 16 bytes.

	@item GCRY_MD_MD2
	This is an reserved identifier for MD-2; there is no implementation yet.

	@item GCRY_MD_TIGER
	This is the TIGER/192 algorithm which yields a message digest of 24 bytes.

	@item GCRY_MD_HAVAL
	This is an reserved for the HAVAL algorithm with 5 passes and 160
	bit. It yields a message digest of 20 bytes. Note that there is no
	implementation yet available.

	@item GCRY_MD_SHA256
	This is the SHA-256 algorithm which yields a message digest of 32 bytes.
	See FIPS 180-2 for the specification.

	@item GCRY_MD_SHA384
	This is reserved for SHA-2 with 384 bits. It yields a message digest of
	48 bytes. Note that there is no implementation yet available.

	@item GCRY_MD_SHA512
	This is reserved for SHA-2 with 512 bits. It yields a message digest of
	64 bytes. Note that there is no implementation yet available.

	@item GCRY_MD_CRC32
	This is the ISO 3309 and ITU-T V.42 cyclic redundancy check. It
	yields an output of 4 bytes.

	@item GCRY_MD_CRC32_RFC1510
	This is the above cyclic redundancy check function, as modified by RFC
	1510. It yields an output of 4 bytes.

	@item GCRY_MD_CRC24_RFC2440
	This is the OpenPGP cyclic redundancy check function. It yields an
	output of 3 bytes.

	@end table
	@c end table of hash algorithms

	@node Hash algorithm modules
	@section Hash algorithm modules

	@acronym{Libgcrypt} makes it possible to load additional `message
	digest modules'; these cipher can be used just like the message digest
	algorithms that are built into the library directly. For an
	introduction into extension modules, see @xref{Modules}.

	@deftp {Data type} gcry_md_spec_t
	This is the `module specification structure' needed for registering
	message digest modules, which has to be filled in by the user before
	it can be used to register a module. It contains the following
	members:

	@table @code
	@item const char *name
	The primary name of this algorithm.
	@item unsigned char *asnoid
	Array of bytes that form the ASN OID.
	@item int asnlen
	Length of bytes in `asnoid'.
	@item gcry_md_oid_spec_t *oids
	A list of OIDs that are to be associated with the algorithm. The
	list's last element must have it's `oid' member set to NULL. See
	below for an explanation of this type. See below for an explanation
	of this type.
	@item int mdlen
	Length of the message digest algorithm. See below for an explanation
	of this type.
	@item gcry_md_init_t init
	The function responsible for initializing a handle. See below for an
	explanation of this type.
	@item gcry_md_write_t write
	The function responsible for writing data into a message digest
	context. See below for an explanation of this type.
	@item gcry_md_final_t final
	The function responsible for `finalizing' a message digest context.
	See below for an explanation of this type.
	@item gcry_md_read_t read
	The function responsible for reading out a message digest result. See
	below for an explanation of this type.
	@item size_t contextsize
	The size of the algorithm-specific `context', that should be
	allocated for each handle.
	@end table
	@end deftp

	@deftp {Data type} gcry_md_oid_spec_t
	This type is used for associating a user-provided algorithm
	implementation with certain OIDs. It contains the following members:

	@table @code
	@item const char *oidstring
	Textual representation of the OID.
	@end table
	@end deftp

	@deftp {Data type} gcry_md_init_t
	Type for the `init' function, defined as: void (*gcry_md_init_t) (void
	*c)
	@end deftp

	@deftp {Data type} gcry_md_write_t
	Type for the `write' function, defined as: void (*gcry_md_write_t)
	(void c, unsigned char buf, size_t nbytes)
	@end deftp

	@deftp {Data type} gcry_md_final_t
	Type for the `final' function, defined as: void (*gcry_md_final_t)
	(void *c)
	@end deftp

	@deftp {Data type} gcry_md_read_t
	Type for the `read' function, defined as: unsigned char
	(gcry_md_read_t) (void *c)
	@end deftp

	@deftypefun gcry_error_t gcry_md_register (gcry_md_spec_t @var{digest}, unsigned int algorithm_id, gcry_module_t *@var{module})

	Register a new digest module whose specification can be found in
	@var{digest}. On success, a new algorithm ID is stored in
	@var{algorithm_id} and a pointer representing this module is stored
	in @var{module}.
	@end deftypefun

	@deftypefun void gcry_md_unregister (gcry_module_t @var{module})
	Unregister the digest identified by @var{module}, which must have been
	registered with gcry_md_register.
	@end deftypefun

	@deftypefun gcry_error_t gcry_md_list (int @var{list}, int @var{list_length})
	Get a list consisting of the IDs of the loaded message digest modules.
	If @var{list} is zero, write the number of loaded message digest
	modules to @var{list_length} and return. If @var{list} is non-zero,
	the first *@var{list_length} algorithm IDs are stored in @var{list},
	which must be of according size. In case there are less message
	digests modules than @var{list_length}, @var{list_length} is updated
	to the correct number.
	@end deftypefun

	@node Working with hash algorithms
	@section Working with hash algorithms

	To use most of these function it is necessary to create a context;
	this is done using:

	@deftypefun gcry_error_t gcry_md_open (gcry_md_hd_t *@var{hd}, int @var{algo}, unsigned int @var{flags})

	Create a message digest object for algorithm @var{algo}. @var{flags}
	may be given as an bitwise OR of constants described below. @var{algo}
	may be given as @code{0} if the algorithms to use are later set using
	@code{gcry_md_enable}. @var{hd} is guaranteed to either receive a valid
	handle or NULL.

	For a list of supported algorithms, see @xref{Available hash
	algorithms}.

	The flags allowed for @var{mode} are:

	@c begin table of hash flags
	@table @code
	@item GCRY_MD_FLAG_SECURE
	Allocate all buffers and the resulting digest in "secure memory". Use
	this is the hashed data is highly confidential.

	@item GCRY_MD_FLAG_HMAC
	Turn the algorithm into a HMAC message authentication algorithm. This
	does only work if just one algorithm is enabled for the handle and
	SHA-384 and SHA512 is not used. Note that the function
	@code{gcry_md_setkey} must be used set the MAC key. If you want CBC
	message authentication codes based on a cipher, see @xref{Working with
	cipher handles}.

	@end table
	@c begin table of hash flags

	You may use the function @code{gcry_md_is_enabled} to later check
	whether an algorithm has been enabled.

	@end deftypefun
	@c end function gcry_md_open

	If you want to calculate several hash algorithms at the same time, you
	have to use the following function right after the @code{gcry_md_open}:

	@deftypefun gcry_error_t gcry_md_enable (gcry_md_hd_t @var{h}, int @var{algo})

	Add the message digest algorithm @var{algo} to the digest object
	described by handle @var{h}. Duplicated enabling of algorithms is
	detected and ignored.
	@end deftypefun

	If the flag @code{GCRY_MD_FLAG_HMAC} was used, the key for the MAC must
	be set using the function:

	@deftypefun gcry_error_t gcry_md_setkey (gcry_md_hd_t @var{h}, const void *@var{key}, size_t @var{keylen})

	For use with the HMAC feature, set the MAC key to the value of @var{key}
	of length @var{keylen}.
	@end deftypefun


	After you are done with the hash calculation, you should release the
	resources by using:

	@deftypefun void gcry_md_close (gcry_md_hd_t @var{h})

	Release all resources of hash context @var{h}. @var{h} should not be
	used after a call to this function. A @code{NULL} passed as @var{h} is
	ignored.

	@end deftypefun

	Often you have to do several hash operations using the same algorithm.
	To avoid the overhead of creating and releasing context, a reset function
	is provided:

	@deftypefun void gcry_md_reset (gcry_md_hd_t @var{h})

	Reset the current context to its initial state. This is effectively
	identical to a close followed by an open and enabling all currently
	active algorithms.
	@end deftypefun


	Often it is necessary to start hashing some data and than continue to
	hash different data. To avoid hashing the same data several times (which
	might not even be possible if the data is received from a pipe), a
	snapshot of the current hash context can be taken and turned into a new
	context:

	@deftypefun gcry_error_t gcry_md_copy (gcry_md_hd_t *@var{handle_dst}, gcry_md_hd_t @var{handle_src})

	Create a new digest object as an exact copy of the object described by
	handle @var{handle_src} and store it in @var{handle_dst}. The context
	is not reset and you can continue to hash data using this context and
	independently using the original context.
	@end deftypefun


	Now that we have prepared everything to calculate hashes, its time to
	see how it is actually done. There are 2 ways for this, one to
	update the hash with a block of memory and one macro to update the hash
	by just one character. Both may be used intermixed.

	@deftypefun void gcry_md_write (gcry_md_hd_t @var{h}, const void *@var{buffer}, size_t @var{length})

	Pass @var{length} bytes of the data in @var{buffer} to the digest object
	with handle @var{h} to update the digest values. This
	function should be used for large blocks of data.
	@end deftypefun

	@deftypefun void gcry_md_putc (gcry_md_hd_t @var{h}, int @var{c})

	Pass the byte in @var{c} to the digest object with handle @var{h} to
	update the digest value. This is an efficient function, implemented as
	a macro to buffer the data before an actual update.
	@end deftypefun

	The semantics of the hash functions don't allow to read out intermediate
	message digests because the calculation must be finalized fist. This
	finalization may for example include the number of bytes hashed in the
	message digest.

	@deftypefun void gcry_md_final (gcry_md_hd_t @var{h})

	Finalize the message digest calculation. This is not really needed
	because @code{gcry_md_read} does this implicitly. After this has been
	done no further updates (by means of @code{gcry_md_write} or
	@code{gcry_md_putc} are allowed. Only the first call to this function
	has an effect. It is implemented as a macro.
	@end deftypefun

	The way to read out the calculated message digest is by using the
	function:

	@deftypefun unsigned char *gcry_md_read (gcry_md_hd_t @var{h}, int @var{algo})

	@code{gcry_md_read} returns the message digest after finalizing the
	calculation. This function may be used as often as required but it will
	always return the same value for one handle. The returned message digest
	is allocated within the message context and therefore valid until the
	handle is released or reseted (using @code{gcry_md_close} or
	@code{gcry_md_reset}. @var{algo} may be given as 0 to return the only
	enabled message digest or it may specify one of the enabled algorithms.
	The function does return @code{NULL} if the requested algorithm has not
	been enabled.
	@end deftypefun

	Because it is often necessary to get the message digest of one block of
	memory, a fast convenience function is available for this task:

	@deftypefun void gcry_md_hash_buffer (int @var{algo}, void @var{digest}, const cvoid @var{buffer}, size_t @var{length});

	@code{gcry_md_hash_buffer} is a shortcut function to calculate a message
	digest of a buffer. This function does not require a context and
	immediately returns the message digest of the @var{length} bytes at
	@var{buffer}. @var{digest} must be allocated by the caller, large
	enough to hold the message digest yielded by the the specified algorithm
	@var{algo}. This required size may be obtained by using the function
	@code{gcry_md_get_algo_dlen}.

	Note, that this function will abort the process if an unavailable
	algorithm is used.
	@end deftypefun

	@c ***********************************
	@c *** MD info functions *********
	@c ***********************************

	Hash algorithms are identified by internal algorithm numbers (see
	@code{gcry_md_open} for a list. However, in most applications they are
	used by names, so 2 functions are available to map between string
	representations and hash algorithm identifiers.

	@deftypefun const char *gcry_md_algo_name (int @var{algo})

	Map the digest algorithm id @var{algo} to a string representation of the
	algorithm name. For unknown algorithms this functions returns an
	empty string. This function should not be used to test for the
	availability of an algorithm.
	@end deftypefun

	@deftypefun int gcry_md_map_name (const char *@var{name})

	Map the algorithm with @var{name} to a digest algorithm identifier.
	Returns 0 if the algorithm name is not known. Names representing
	@acronym{ASN.1} object identifiers are recognized if the @acronym{IETF}
	dotted format is used and the OID is prefixed with either "@code{oid.}"
	or "@code{OID.}". For a list of supported OIDs, see the source code at
	@file{cipher/md.c}. This function should not be used to test for the
	availability of an algorithm.
	@end deftypefun

	@deftypefun gcry_error_t gcry_md_get_asnoid (int @var{algo}, void @var{buffer}, size_t @var{length})

	Return an DER encoded ASN.1 OID for the algorithm @var{algo} in the
	user allocated @var{buffer}. @var{length} must point to variable with
	the available size of @var{buffer} and receives after return the
	actual size of the returned OID. The returned error code may be
	@code{GPG_ERR_TOO_SHORT} if the provided buffer is to short to receive
	the OID; it is possible to call the function with @code{NULL} for
	@var{buffer} to have it only return the required size. The function
	returns 0 on success.

	@end deftypefun


	To test whether an algorithm is actually available for use, the
	following macro should be used:

	@deftypefun gcry_error_t gcry_md_test_algo (int @var{algo})

	The macro returns 0 if the algorithm @var{algo} is available for use.
	@end deftypefun

	If the length of a message digest is not known, it can be retrieved
	using the following function:

	@deftypefun unsigned int gcry_md_get_algo_dlen (int @var{algo})

	Retrieve the length in bytes of the digest yielded by algorithm
	@var{algo}. This is often used prior to @code{gcry_md_read} to allocate
	sufficient memory for the digest.
	@end deftypefun


	In some situations it might be hard to remember the algorithm used for
	the ongoing hashing. The following function might be used to get that
	information:

	@deftypefun int gcry_md_get_algo (gcry_md_hd_t @var{h})

	Retrieve the algorithm used with the handle @var{h}. Note, that this
	does not work reliable if more than one algorithm is enabled in @var{h}.
	@end deftypefun

	The following macro might also be useful:

	@deftypefun int gcry_md_is_secure (gcry_md_hd_t @var{h})

	This function returns true when the digest object @var{h} is allocated
	in "secure memory"; i.e. @var{h} was created with the
	@code{GCRY_MD_FLAG_SECURE}.
	@end deftypefun

	@deftypefun int gcry_md_is_enabled (gcry_md_hd_t @var{h}, int @var{algo})

	This function returns true when the algorithm @var{algo} has been
	enabled for the digest object @var{h}.
	@end deftypefun



	Tracking bugs related to hashing is often a cumbersome task which
	requires to add a lot of printf statements into the code. @acronym{Libgcrypt}
	provides an easy way to avoid this. The actual data hashed can be
	written to files on request. The following 2 macros should be used to
	implement such a debugging facility:

	@deftypefun void gcry_md_start_debug (gcry_md_hd_t @var{h}, const char *@var{suffix})

	Enable debugging for the digest object with handle @var{h}. This
	creates create files named @file{dbgmd-<n>.<string>} while doing the
	actual hashing. @var{suffix} is the string part in the filename. The
	number is a counter incremented for each new hashing. The data in the
	file is the raw data as passed to @code{gcry_md_write} or
	@code{gcry_md_putc}.
	@end deftypefun


	@deftypefun void gcry_md_stop_debug (gcry_md_hd_t @var{h}, int @var{reserved})

	Stop debugging on handle @var{h}. @var{reserved} should be specified as
	0. This function is usually not required because @code{gcry_md_close}
	does implicitly stop debugging.
	@end deftypefun


	@c **********************************************************
	@c ***************** Public Key ***********************
	@c **********************************************************
	@node Public Key cryptography (I)
	@chapter Public Key cryptography (I)

	Public key cryptography, also known as asymmetric cryptography, is an
	easy way for key management and to provide digital signatures.
	@acronym{Libgcrypt} provides two completely different interfaces to
	public key cryptography, this chapter explains the one based on
	S-expressions.

	@menu
	* Available algorithms:: Algorithms supported by the library.
	* Used S-expressions:: Introduction into the used S-expression.
	* Public key modules:: How to work with public key modules.
	* Cryptographic Functions:: Functions for performing the cryptographic actions.
	* General public-key related Functions:: General functions, not implementing any cryptography.
	@end menu

	@node Available algorithms
	@section Available algorithms

	@acronym{Libgcrypt} supports the RSA (Rivest-Shamir-Adleman) algorithms as well
	as DSA (Digital Signature Algorithm) and ElGamal. The versatile
	interface allows to add more algorithms in the future.

	@node Used S-expressions
	@section Used S-expressions

	@acronym{Libgcrypt}'s API for asymmetric cryptography is based on data
	structures called S-expressions (see XXXX) and does not work with
	contexts as most of the other building blocks of @acronym{Libgcrypt}
	do.

	The following information are stored in S-expressions:

	@table @asis
	@item keys

	@item plain text data

	@item encrypted data

	@item signatures

	@end table

	@noindent
	To describe how @acronym{Libgcrypt} expect keys, we use some examples. Note that
	words in
	@ifnottex
	uppercase
	@end ifnottex
	@iftex
	italics
	@end iftex
	indicate parameters whereas lowercase words are literals.

	@example
	(private-key
	(dsa
	(p @var{p-mpi})
	(q @var{q-mpi})
	(g @var{g-mpi})
	(y @var{y-mpi})
	(x @var{x-mpi})))
	@end example

	@noindent
	This specifies a DSA private key with the following parameters:

	@table @var
	@item p-mpi
	DSA prime @math{p}.
	@item q-mpi
	DSA group order @math{q} (which is a prime divisor of @math{p-1}).
	@item g-mpi
	DSA group generator @math{g}.
	@item y-mpi
	DSA public key value @math{y = g^x \bmod p}.
	@item x-mpi
	DSA secret exponent x.
	@end table

	All the MPI values are expected to be in @code{GCRYMPI_FMT_USG} format.
	The public key is similar with "private-key" replaced by "public-key"
	and no @var{x-mpi}.

	An easy way to create such an S-expressions is by using
	@code{gcry_sexp_build} which allows to pass a string with printf-like
	escapes to insert MPI values.

	@noindent
	Here is an example for an RSA key:

	@example
	(private-key
	(rsa
	(n @var{n-mpi})
	(e @var{e-mpi})
	(d @var{d-mpi})
	(p @var{p-mpi})
	(q @var{q-mpi})
	(u @var{u-mpi})
	@end example

	@noindent
	with

	@table @var
	@item n-mpi
	RSA public modulus @math{n}.
	@item e-mpi
	RSA public exponent @math{e}.
	@item d-mpi
	RSA secret exponent @math{d = e^{-1} \bmod (p-1)(q-1)}.
	@item p-mpi
	RSA secret prime @math{p}.
	@item q-mpi
	RSA secret prime @math{q} with @math{q > p}.
	@item u-mpi
	multiplicative inverse @math{u = p^{-1} \bmod q}.
	@end table

	@node Public key modules
	@section Public key modules

	@acronym{Libgcrypt} makes it possible to load additional `public key
	modules'; these public key algorithms can be used just like the
	algorithms that are built into the library directly. For an
	introduction into extension modules, see @xref{Modules}.

	@deftp {Data type} gcry_pk_spec_t
	This is the `module specification structure' needed for registering
	public key modules, which has to be filled in by the user before it
	can be used to register a module. It contains the following members:

	@table @code
	@item const char *name
	The primary name of this algorithm.
	@item char **aliases
	A list of strings that are `aliases' for the algorithm. The list
	must be terminated with a NULL element.
	@item const char *elements_pkey
	String containing the one-letter names of the MPI values contained in
	a public key.
	@item const char *element_skey
	String containing the one-letter names of the MPI values contained in
	a secret key.
	@item const char *elements_enc
	String containing the one-letter names of the MPI values that are the
	result of an encryption operation using this algorithm.
	@item const char *elements_sig
	String containing the one-letter names of the MPI values that are the
	result of a sign operation using this algorithm.
	@item const char *elements_grip
	String containing the one-letter names of the MPI values that are to
	be included in the `key grip'.
	@item int use
	The bitwise-OR of the following flags, depending on the abilities of
	the algorithm:
	@table @code
	@item GCRY_PK_USAGE_SIGN
	The algorithm supports signing and verifying of data.
	@item GCRY_PK_USAGE_ENCR
	The algorithm supports the encryption and decryption of data.
	@end table
	@item gcry_pk_generate_t generate
	The function responsible for generating a new key pair. See below for
	a description of this type.
	@item gcry_pk_check_secret_key_t check_secret_key
	The function responsible for checking the sanity of a provided secret
	key. See below for a description of this type.
	@item gcry_pk_encrypt_t encrypt
	The function responsible for encrypting data. See below for a
	description of this type.
	@item gcry_pk_decrypt_t decrypt
	The function responsible for decrypting data. See below for a
	description of this type.
	@item gcry_pk_sign_t sign
	The function responsible for signing data. See below for a description
	of this type.
	@item gcry_pk_verify_t verify
	The function responsible for verifying that the provided signature
	matches the provided data. See below for a description of this type.
	@item gcry_pk_get_nbits_t get_nbits
	The function responsible for returning the number of bits of a provided
	key. See below for a description of this type.
	@end table
	@end deftp

	@deftp {Data type} gcry_pk_generate_t
	Type for the `generate' function, defined as: gcry_err_code_t
	(*gcry_pk_generate_t) (int algo, unsigned int nbits, unsigned long
	use_e, gcry_mpi_t skey, gcry_mpi_t *retfactors)
	@end deftp

	@deftp {Data type} gcry_pk_check_secret_key_t
	Type for the `check_secret_key' function, defined as: gcry_err_code_t
	(gcry_pk_check_secret_key_t) (int algo, gcry_mpi_t skey)
	@end deftp

	@deftp {Data type} gcry_pk_encrypt_t
	Type for the `encrypt' function, defined as: gcry_err_code_t
	(gcry_pk_encrypt_t) (int algo, gcry_mpi_t resarr, gcry_mpi_t data,
	gcry_mpi_t *pkey, int flags)
	@end deftp

	@deftp {Data type} gcry_pk_decrypt_t
	Type for the `decrypt' function, defined as: gcry_err_code_t
	(gcry_pk_decrypt_t) (int algo, gcry_mpi_t result, gcry_mpi_t *data,
	gcry_mpi_t *skey, int flags)
	@end deftp

	@deftp {Data type} gcry_pk_sign_t
	Type for the `sign' function, defined as: gcry_err_code_t
	(gcry_pk_sign_t) (int algo, gcry_mpi_t resarr, gcry_mpi_t data,
	gcry_mpi_t *skey)
	@end deftp

	@deftp {Data type} gcry_pk_verify_t
	Type for the `verify' function, defined as: gcry_err_code_t
	(gcry_pk_verify_t) (int algo, gcry_mpi_t hash, gcry_mpi_t data,
	gcry_mpi_t pkey, int (cmp) (void , gcry_mpi_t), void opaquev)
	@end deftp

	@deftp {Data type} gcry_pk_get_nbits_t
	Type for the `get_nbits' function, defined as: unsigned
	(gcry_pk_get_nbits_t) (int algo, gcry_mpi_t pkey)
	@end deftp

	@deftypefun gcry_error_t gcry_pk_register (gcry_pk_spec_t @var{pubkey}, unsigned int algorithm_id, gcry_module_t *@var{module})

	Register a new public key module whose specification can be found in
	@var{pubkey}. On success, a new algorithm ID is stored in
	@var{algorithm_id} and a pointer representing this module is stored
	in @var{module}.
	@end deftypefun

	@deftypefun void gcry_pk_unregister (gcry_module_t @var{module})
	Unregister the public key module identified by @var{module}, which
	must have been registered with gcry_pk_register.
	@end deftypefun

	@deftypefun gcry_error_t gcry_pk_list (int @var{list}, int @var{list_length})
	Get a list consisting of the IDs of the loaded pubkey modules. If
	@var{list} is zero, write the number of loaded pubkey modules to
	@var{list_length} and return. If @var{list} is non-zero, the first
	*@var{list_length} algorithm IDs are stored in @var{list}, which must
	be of according size. In case there are less pubkey modules than
	@var{list_length}, @var{list_length} is updated to the correct
	number.
	@end deftypefun

	@node Cryptographic Functions
	@section Cryptographic Functions

	@noindent
	Note, that we will in future allow to use keys without p,q and u
	specified and may also support other parameters for performance
	reasons.

	@noindent

	Some functions operating on S-expressions support `flags', that
	influence the operation. These flags have to be listed in a
	sub-S-expression named `flags'; the following flags are known:

	@table @var
	@item pkcs1
	Use PKCS#1 block type 2 padding.
	@item no-blinding
	Do not use a technique called `blinding', which is used by default in
	order to prevent leaking of secret information. Blinding is only
	implemented by RSA, but it might be implemented by other algorithms in
	the future as well, when necessary.
	@end table

	@noindent
	Now that we know the key basics, we can carry on and explain how to
	encrypt and decrypt data. In almost all cases the data is a random
	session key which is in turn used for the actual encryption of the real
	data. There are 2 functions to do this:

	@deftypefun gcry_error_t gcry_pk_encrypt (@w{gcry_sexp_t *@var{r_ciph},} @w{gcry_sexp_t @var{data},} @w{gcry_sexp_t @var{pkey}})

	Obviously a public key must be provided for encryption. It is
	expected as an appropriate S-expression (see above) in @var{pkey}.
	The data to be encrypted can either be in the simple old format, which
	is a very simple S-expression consisting only of one MPI, or it may be
	a more complex S-expression which also allows to specify flags for
	operation, like e.g. padding rules.

	@noindent
	If you don't want to let @acronym{Libgcrypt} handle the padding, you must pass an
	appropriate MPI using this expression for @var{data}:

	@example
	(data
	(flags raw)
	(value @var{mpi}))
	@end example

	@noindent
	This has the same semantics as the old style MPI only way. @var{MPI} is
	the actual data, already padded appropriate for your protocol. Most
	systems however use PKCS#1 padding and so you can use this S-expression
	for @var{data}:

	@example
	(data
	(flags pkcs1)
	(value @var{block}))
	@end example

	@noindent
	Here, the "flags" list has the "pkcs1" flag which let the function know
	that it should provide PKCS#1 block type 2 padding. The actual data to
	be encrypted is passed as a string of octets in @var{block}. The
	function checks that this data actually can be used with the given key,
	does the padding and encrypts it.

	If the function could successfully perform the encryption, the return
	value will be 0 and a a new S-expression with the encrypted result is
	allocated and assign to the variable at the address of @var{r_ciph}.
	The caller is responsible to release this value using
	@code{gcry_sexp_release}. In case of an error, an error code is
	returned and @var{r_ciph} will be set to @code{NULL}.

	@noindent
	The returned S-expression has this format when used with RSA:

	@example
	(enc-val
	(rsa
	(a @var{a-mpi})))
	@end example

	@noindent
	Where @var{a-mpi} is an MPI with the result of the RSA operation. When
	using the ElGamal algorithm, the return value will have this format:

	@example
	(enc-val
	(elg
	(a @var{a-mpi})
	(b @var{b-mpi})))
	@end example

	@noindent
	Where @var{a-mpi} and @var{b-mpi} are MPIs with the result of the
	ElGamal encryption operation.
	@end deftypefun
	@c end gcry_pk_encrypt

	@deftypefun gcry_error_t gcry_pk_decrypt (@w{gcry_sexp_t *@var{r_plain},} @w{gcry_sexp_t @var{data},} @w{gcry_sexp_t @var{skey}})

	Obviously a private key must be provided for decryption. It is expected
	as an appropriate S-expression (see above) in @var{skey}. The data to
	be decrypted must match the format of the result as returned by
	@code{gcry_pk_encrypt}, but should be enlarged with a @code{flags}
	element:

	@example
	(enc-val
	(flags)
	(elg
	(a @var{a-mpi})
	(b @var{b-mpi})))
	@end example

	@noindent
	Note, that this function currently does not know of any padding
	methods and the caller must do any un-padding on his own.

	@noindent
	The function returns 0 on success or an error code. The variable at the
	address of @var{r_plain} will be set to NULL on error or receive the
	decrypted value on success. The format of @var{r_plain} is a
	simple S-expression part (i.e. not a valid one) with just one MPI if
	there was no @code{flags} element in @var{data}; if at least an empty
	@code{flags} is passed in @var{data}, the format is:

	@example
	(value @var{plaintext})
	@end example
	@end deftypefun
	@c end gcry_pk_decrypt


	Another operation commonly performed using public key cryptography is
	signing data. In some sense this is even more important than
	encryption because digital signatures are an important instrument for
	key management. @acronym{Libgcrypt} supports digital signatures using
	2 functions, similar to the encryption functions:

	@deftypefun gcry_error_t gcry_pk_sign (@w{gcry_sexp_t *@var{r_sig},} @w{gcry_sexp_t @var{data},} @w{gcry_sexp_t @var{skey}})

	This function creates a digital signature for @var{data} using the
	private key @var{skey} and place it into the variable at the address of
	@var{r_sig}. @var{data} may either be the simple old style S-expression
	with just one MPI or a modern and more versatile S-expression which
	allows to let @acronym{Libgcrypt} handle padding:

	@example
	(data
	(flags pkcs1)
	(hash @var{hash-algo} @var{block}))
	@end example

	@noindent
	This example requests to sign the data in @var{block} after applying
	PKCS#1 block type 1 style padding. @var{hash-algo} is a string with the
	hash algorithm to be encoded into the signature, this may be any hash
	algorithm name as supported by @acronym{Libgcrypt}. Most likely, this will be
	"sha1", "rmd160" or "md5". It is obvious that the length of @var{block}
	must match the size of that message digests; the function checks that
	this and other constraints are valid.

	@noindent
	If PKCS#1 padding is not required (because the caller does already
	provide a padded value), either the old format or better the following
	format should be used:

	@example
	(data
	(flags raw)
	(value @var{mpi}))
	@end example

	@noindent
	Here, the data to be signed is directly given as an @var{MPI}.

	@noindent
	The signature is returned as a newly allocated S-expression in
	@var{r_sig} using this format for RSA:

	@example
	(sig-val
	(rsa
	(s @var{s-mpi})))
	@end example

	Where @var{s-mpi} is the result of the RSA sign operation. For DSA the
	S-expression returned is:

	@example
	(sig-val
	(dsa
	(r @var{r-mpi})
	(s @var{s-mpi})))
	@end example

	Where @var{r-mpi} and @var{s-mpi} are the result of the DSA sign
	operation. For ElGamal signing (which is slow, yields large numbers
	and probably is not as secure as the other algorithms), the same format is
	used with "elg" replacing "dsa".
	@end deftypefun
	@c end gcry_pk_sign

	@noindent
	The operation most commonly used is definitely the verification of a
	signature. @acronym{Libgcrypt} provides this function:

	@deftypefun gcry_error_t gcry_pk_verify (@w{gcry_sexp_t @var{sig}}, @w{gcry_sexp_t @var{data}}, @w{gcry_sexp_t @var{pkey}})

	This is used to check whether the signature @var{sig} matches the
	@var{data}. The public key @var{pkey} must be provided to perform this
	verification. This function is similar in its parameters to
	@code{gcry_pk_sign} with the exceptions that the public key is used
	instead of the private key and that no signature is created but a
	signature, in a format as created by @code{gcry_pk_sign}, is passed to
	the function in @var{sig}.

	@noindent
	The result is 0 for success (i.e. the data matches the signature), or an
	error code where the most relevant code is @code{GCRYERR_BAD_SIGNATURE}
	to indicate that the signature does not match the provided data.

	@end deftypefun
	@c end gcry_pk_verify

	@node General public-key related Functions
	@section General public-key related Functions

	@noindent
	A couple of utility functions are available to retrieve the length of
	the key, map algorithm identifiers and perform sanity checks:

	@deftypefun {const char *} gcry_pk_algo_name (int @var{algo})

	Map the public key algorithm id @var{algo} to a string representation of
	the algorithm name. For unknown algorithms this functions returns an
	empty string.
	@end deftypefun

	@deftypefun int gcry_pk_map_name (const char *@var{name})

	Map the algorithm @var{name} to a public key algorithm Id. Returns 0 if
	the algorithm name is not known.
	@end deftypefun

	@deftypefun int gcry_pk_test_algo (int @var{algo})

	Return 0 if the public key algorithm @var{algo} is available for use.
	Note, that this is implemented as a macro.
	@end deftypefun


	@deftypefun {unsigned int} gcry_pk_get_nbits (gcry_sexp_t @var{key})

	Return what is commonly referred as the key length for the given
	public or private in @var{key}.
	@end deftypefun

	@deftypefun {unsigned char } gcry_pk_get_keygrip (@w{gcry_sexp_t @var{key}}, @w{unsigned char @var{array}})

	Return the so called "keygrip" which is the SHA-1 hash of the public key
	parameters expressed in a way depended on the algorithm. @var{array}
	must either provide space for 20 bytes or @code{NULL;}. In the latter
	case a newly allocated array of that size is returned. On success a
	pointer to the newly allocated space or to @var{array} is returned.
	@code{NULL} is returned to indicate an error which is most likely an unknown
	algorithm or one where a "keygrip" has not yet been defined.
	The function accepts public or secret keys in @var{key}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_pk_testkey (gcry_sexp_t @var{key})

	Return zero if the private key @var{key} is `sane', an error code otherwise.
	Note, that it is not possible to chek the `saneness' of a public key.

	@end deftypefun


	@deftypefun int gcry_pk_algo_info (@w{int @var{algo}}, @w{int @var{what}}, @w{void @var{buffer}}, @w{size_t @var{nbytes}})

	Depending on the value of @var{what} return various information about
	the public key algorithm with the id @var{algo}. Note, that the
	function returns @code{-1} on error and the actual error code must be
	retrieved using the function @code{gcry_errno}. The currently defined
	values for @var{what} are:

	@table @code
	@item GCRYCTL_TEST_ALGO:
	Return 0 when the specified algorithm is available for use.
	@var{buffer} must be @code{NULL}, @var{nbytes} may be passed as
	@code{NULL} or point to a variable with the required usage of the
	algorithm. This may be 0 for "don't care" or the bit-wise OR of these
	flags:

	@table @code
	@item GCRY_PK_USAGE_SIGN
	Algorithm is usable for signing.
	@item GCRY_PK_USAGE_ENCR
	Algorithm is usable for encryption.
	@end table

	@item GCRYCTL_GET_ALGO_USAGE:
	Return the usage flags for the given algorithm. An invalid algorithm
	return 0. Disabled algorithms are ignored here because we
	want to know whether the algorithm is at all capable of a certain usage.

	@item GCRYCTL_GET_ALGO_NPKEY
	Return the number of elements the public key for algorithm @var{algo}
	consist of. Return 0 for an unknown algorithm.

	@item GCRYCTL_GET_ALGO_NSKEY
	Return the number of elements the private key for algorithm @var{algo}
	consist of. Note that this value is always larger than that of the
	public key. Return 0 for an unknown algorithm.

	@item GCRYCTL_GET_ALGO_NSIGN
	Return the number of elements a signature created with the algorithm
	@var{algo} consists of. Return 0 for an unknown algorithm or for an
	algorithm not capable of creating signatures.

	@item GCRYCTL_GET_ALGO_NENC
	Return the number of elements a encrypted message created with the algorithm
	@var{algo} consists of. Return 0 for an unknown algorithm or for an
	algorithm not capable of encryption.
	@end table

	@noindent
	Please note that parameters not required should be passed as @code{NULL}.
	@end deftypefun
	@c end gcry_pk_algo_info


	@deftypefun gcry_error_t gcry_pk_ctl (@w{int @var{cmd}}, @w{void *@var{buffer}}, @w{size_t @var{buflen}})

	This is a general purpose function to perform certain control
	operations. @var{cmd} controls what is to be done. The return value is
	0 for success or an error code. Currently supported values for
	@var{cmd} are:

	@table @code
	@item GCRYCTL_DISABLE_ALGO
	Disable the algorithm given as an algorithm id in @var{buffer}.
	@var{buffer} must point to an @code{int} variable with the algorithm id
	and @var{buflen} must have the value @code{sizeof (int)}.

	@end table
	@end deftypefun
	@c end gcry_pk_ctl

	@noindent
	@acronym{Libgcrypt} also provides a function for generating public key
	pairs:

	@deftypefun gcry_error_t gcry_pk_genkey (@w{gcry_sexp_t *@var{r_key}}, @w{gcry_sexp_t @var{parms}})

	This function create a new public key pair using information given in
	the S-expression @var{parms} and stores the private and the public key
	in one new S-expression at the address given by @var{r_key}. In case of
	an error, @var{r_key} is set to @code{NULL}. The return code is 0 for
	success or an error code otherwise.

	@noindent
	Here is an example for @var{parms} for creating a 1024 bit RSA key:

	@example
	(genkey
	(rsa
	(nbits 4:1024)))
	@end example

	@noindent
	To create an ElGamal key, substitute "elg" for "rsa" and to create a DSA
	key use "dsa". Valid ranges for the key length depend on the
	algorithms; all commonly used key lengths are supported. Currently
	supported parameters are:

	@table @code
	@item nbits
	This is always required to specify the length of the key. The argument
	is a string with a number in C-notation.

	@item rsa-use-e
	This is only used with RSA to give a hint for the public exponent. The
	value will be used as a base to test for a usable exponent. Some values
	are special:

	@table @samp
	@item 0
	Use a secure and fast value. This is currently the number 41.
	@item 1
	Use a secure value as required by some specification. This is currently
	the number 65537.
	@item 2
	Reserved
	@end table

	@noindent
	If this parameter is not used, @acronym{Libgcrypt} uses for historic reasons
	65537.

	@end table
	@c end table of parameters

	@noindent
	The key pair is returned in a format depending on the algorithm. Both
	private and public keys are returned in one container and may be
	accompanied by some miscellaneous information.

	@noindent
	As an example, here is what the ElGamal key generation returns:

	@example
	(key-data
	(public-key
	(elg
	(p @var{p-mpi})
	(g @var{g-mpi})
	(y @var{y-mpi})))
	(private-key
	(elg
	(p @var{p-mpi})
	(g @var{g-mpi})
	(y @var{y-mpi})
	(x @var{x-mpi})))
	(misc-key-info
	(pm1-factors @var{n1 n2 ... nn})))
	@end example

	@noindent
	As you can see, some of the information is duplicated, but this provides
	an easy way to extract either the public or the private key. Note that
	the order of the elements is not defined, e.g. the private key may be
	stored before the public key. @var{n1 n2 ... nn} is a list of prime
	numbers used to composite @var{p-mpi}; this is in general not a very
	useful information.
	@end deftypefun
	@c end gcry_pk_genkey

	@node Public Key cryptography (II)
	@chapter Public Key cryptography (II)

	This chapter documents the alternative interface to asymmetric
	cryptography (ac) that is not based on S-expressions, but on native C
	data structures. As opposed to the pk interface described in the
	former chapter, this one follows an open/use/close paradigm like other
	building blocks of the library.

	@menu
	* Available asymmetric algorithms:: List of algorithms supported by the library.
	* Working with sets of data:: How to work with sets of data.
	* Working with handles:: How to use handles.
	* Working with keys:: How to work with keys.
	* Using cryptographic functions:: How to perform cryptographic operations.
	* Handle-independent functions:: General functions independent of handles.
	@end menu

	@node Available asymmetric algorithms
	@section Available asymmetric algorithms

	@acronym{Libgcrypt} supports the RSA (Rivest-Shamir-Adleman)
	algorithms as well as DSA (Digital Signature Algorithm) and ElGamal.
	The versatile interface allows to add more algorithms in the future.

	@deftp {Data type} gcry_ac_id_t

	The following constants are defined for this type:

	@table @code
	@item GCRY_AC_RSA
	Riven-Shamir-Adleman
	@item GCRY_AC_DSA
	Digital Signature Algorithm
	@item GCRY_AC_ELG
	ElGamal
	@item GCRY_AC_ELG_E
	ElGamal, encryption only.
	@end table
	@end deftp

	@node Working with sets of data
	@section Working with sets of data

	In the context of this interface the term `data set' refers to a list
	of `named MPI values' that is used by functions performing
	cryptographic operations.

	Such data sets are used for representing keys, since keys simply
	consist of a variable amount of numbers. Furthermore some functions
	return data sets to the caller that are to be provided to other
	functions.

	This section documents the data types, symbols and functions that are
	relevant for working with such data sets.

	@deftp {Data type} gcry_ac_data_t
	A data set, that is simply a list of named MPI values.
	@end deftp

	The following flags are supported:

	@table @code
	@item GCRY_AC_FLAG_DEALLOC
	Used for storing data in a data set. If given, the data will be
	released by the library.

	@item GCRY_AC_FLAG_COPY
	Used for storing/retrieving data in/from a data set. If given, the
	library will create copies of the provided/contained data, which will
	then be given to the user/associated with the data set.
	@end table

	@deftypefun gcry_error_t gcry_ac_data_new (gcry_ac_data_t *@var{data})
	Creates a new, empty data set and stores it in @var{data}.
	@end deftypefun

	@deftypefun void gcry_ac_data_destroy (gcry_ac_data_t @var{data})
	Destroys the data set @var{data}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_set (gcry_ac_data_t @var{data}, unsigned int @var{flags}, char *@var{name}, gcry_mpi_t @var{mpi})
	Add the value @var{mpi} to @var{data} with the label @var{name}. If
	@var{flags} contains GCRY_AC_FLAG_DATA_COPY, the data set will contain
	copies of @var{name} and @var{mpi}. If @var{flags} contains
	GCRY_AC_FLAG_DATA_DEALLOC or GCRY_AC_FLAG_DATA_COPY, the values
	contained in the data set will be deallocated when they are to be
	removed from the data set.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_copy (gcry_ac_data_t *@var{data_cp}, gcry_ac_data_t @var{data})
	Create a copy of the data set @var{data} and store it in @var{data_cp}.
	@end deftypefun

	@deftypefun unsigned int gcry_ac_data_length (gcry_ac_data_t @var{data})
	Returns the number of named MPI values inside of the data set
	@var{data}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_get_name (gcry_ac_data_t @var{data}, unsigned int @var{flags}, char @var{name}, gcry_mpi_t @var{mpi})
	Store the value labelled with @var{name} found in @var{data} in
	@var{mpi}. If @var{flags} contains GCRY_AC_FLAG_COPY, store a copy of
	the @var{mpi} value contained in the data set. @var{mpi} may be NULL.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_get_index (gcry_ac_data_t @var{data}, unsigned int flags, unsigned int @var{index}, const char *@var{name}, gcry_mpi_t @var{mpi})
	Stores in @var{name} and @var{mpi} the named @var{mpi} value contained
	in the data set @var{data} with the index @var{idx}. If @var{flags}
	contains GCRY_AC_FLAG_COPY, store copies of the values contained in
	the data set. @var{name} or @var{mpi} may be NULL.
	@end deftypefun

	@deftypefun void gcry_ac_data_clear (gcry_ac_data_t @var{data})
	Destroys any values contained in the data set @var{data}.
	@end deftypefun

	@node Working with handles
	@section Working with handles

	In order to use an algorithm, an according handle must be created.
	This is done using the following function:

	@deftypefun gcry_error_t gcry_ac_open (gcry_ac_handle_t *@var{handle}, int @var{algorithm}, int @var{flags})

	Creates a new handle for the algorithm @var{algorithm} and stores it
	in @var{handle}. @var{flags} is not used yet.

	@var{algorithm} must be a valid algorithm ID, see @xref{Available
	algorithms}, for a list of supported algorithms and the according
	constants. Besides using the listed constants directly, the functions
	@code{gcry_ac_name_to_id} may be used to convert the textual name of
	an algorithm into the according numeric ID.
	@end deftypefun

	@deftypefun void gcry_ac_close (gcry_ac_handle_t @var{handle})
	Destroys the handle @var{handle}.
	@end deftypefun

	@node Working with keys
	@section Working with keys

	@deftp {Data type} gcry_ac_key_type_t
	Defined constants:

	@table @code
	@item GCRY_AC_KEY_TYPE_SECRET
	Specifies a secret key.
	@item GCRY_AC_KEY_TYPE_PUBLIC
	Specifies a public key.
	@end table
	@end deftp

	@deftp {Data type} gcry_ac_key_t
	This type represents a single `key', either a secret one or a public
	one.
	@end deftp

	@deftp {Data type} gcry_ac_key_pair_t
	This type represents a `key pair' containing a secret and a public key.
	@end deftp

	Key data structures can be created in two different ways; a new key
	pair can be generated, resulting in ready-to-use key. Alternatively a
	key can be initialized from a given data set.

	@deftypefun gcry_error_t gcry_ac_key_init (gcry_ac_key_t *@var{key}, gcry_ac_handle_t @var{handle}, gcry_ac_key_type_t @var{type}, gcry_ac_data_t @var{data})
	Creates a new key of type @var{type}, consisting of the MPI values
	contained in the data set @var{data} and stores it in @var{key}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_key_pair_generate (gcry_ac_handle_t @var{handle}, unsigned int @var{nbits}, void @var{key_spec}, gcry_ac_key_pair_t @var{key_pair}, gcry_mpi_t **@var{misc_data})

	Generates a new key pair via the handle @var{handle} of @var{NBITS}
	bits and stores it in @var{key_pair}.

	In case non-standard settings are wanted, a pointer to a structure of
	type @code{gcry_ac_key_spec_<algorithm>_t}, matching the selected
	algorithm, can be given as @var{key_spec}. @var{misc_data} is not
	used yet. Such a structure does only exist for RSA. A descriptions
	of the members of the supported structures follows.

	@table @code
	@item gcry_ac_key_spec_rsa_t
	@table @code
	@item gcry_mpi_t e
	Generate the key pair using a special @code{e}. The value of @code{e}
	has the following meanings:
	@table @code
	@item = 0
	Let @acronym{Libgcrypt} device what exponent should be used.
	@item = 1
	Request the use of a ``secure'' exponent; this is required by some
	specification to be 65537.
	@item > 2
	Try starting at this value until a working exponent is found. Note,
	that the current implementation leaks some information about the
	private key because the incrementation used is not randomized. Thus,
	this function will be changed in the future to return a random
	exponent of the given size.
	@end table
	@end table
	@end table

	Example code:
	@example
	@{
	gcry_ac_key_pair_t key_pair;
	gcry_ac_key_spec_rsa rsa_spec;

	rsa_spec.e = gcry_mpi_new (0);
	gcry_mpi_set_ui (rsa_spec.e, 1)

	err = gcry_ac_open (&handle, GCRY_AC_RSA, 0);
	assert (! err);

	err = gcry_ac_key_pair_generate (handle, &key_pair, 1024, (void *) &rsa_spec);
	assert (! err);
	@}
	@end example
	@end deftypefun


	@deftypefun gcry_ac_key_t gcry_ac_key_pair_extract (gcry_ac_key_pair_t @var{key_pair}, gcry_ac_key_type_t @var{which})
	Returns the key of type @var{which} out of the key pair
	@var{key_pair}.
	@end deftypefun

	@deftypefun void gcry_ac_key_destroy (gcry_ac_key_t @var{key})
	Destroys the key @var{key}.
	@end deftypefun

	@deftypefun void gcry_ac_key_pair_destroy (gcry_ac_key_pair_t @var{key_pair})
	Destroys the key pair @var{key_pair}.
	@end deftypefun

	@deftypefun gcry_ac_data_t gcry_ac_key_data_get (gcry_ac_key_t @var{key})
	Returns the data set contained in the key @var{key}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_key_test (gcry_ac_handle_t @var{handle}, gcry_ac_key_t @var{key})
	Verifies that the private key @var{key} is sane via @var{handle}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_key_get_nbits (gcry_ac_handle_t @var{handle}, gcry_ac_key_t @var{key}, unsigned int *@var{nbits})
	Stores the number of bits of the key @var{key} in @var{nbits} via @var{handle}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_key_get_grip (gcry_ac_handle_t @var{handle}, gcry_ac_key_t @var{key}, unsigned char *@var{key_grip})
	Writes the 20 byte long key grip of the key @var{key} to
	@var{key_grip} via @var{handle}.
	@end deftypefun

	@node Using cryptographic functions
	@section Using cryptographic functions

	The following flags might be relevant:

	@table @code
	@item GCRY_AC_FLAG_NO_BLINDING
	Disable any blinding, which might be supported by the chosen
	algorithm; blinding is the default.
	@end table

	@deftypefun gcry_error_t gcry_ac_data_encrypt (gcry_ac_handle_t @var{handle}, unsigned int @var{flags}, gcry_ac_key_t @var{key}, gcry_mpi_t @var{data_plain}, gcry_ac_data_t **@var{data_encrypted})
	Encrypts the plain text MPI value @var{data_plain} with the key public
	@var{key} under the control of the flags @var{flags} and stores the
	resulting data set into @var{data_encrypted}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_decrypt (gcry_ac_handle_t @var{handle}, unsigned int @var{flags}, gcry_ac_key_t @var{key}, gcry_mpi_t *@var{data_plain}, gcry_ac_data_t @var{data_encrypted})
	Decrypts the encrypted data contained in the data set
	@var{data_encrypted} with the secret key KEY under the control of the
	flags @var{flags} and stores the resulting plain text MPI value in
	@var{DATA_PLAIN}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_sign (gcry_ac_handle_t @var{handle}, gcry_ac_key_t @var{key}, gcry_mpi_t @var{data}, gcry_ac_data_t *@var{data_signature})
	Signs the data contained in @var{data} with the secret key @var{key}
	and stores the resulting signature in the data set
	@var{data_signature}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_data_verify (gcry_ac_handle_t @var{handle}, gcry_ac_key_t @var{key}, gcry_mpi_t @var{data}, gcry_ac_data_t @var{data_signature})
	Verifies that the signature contained in the data set
	@var{data_signature} is indeed the result of signing the data
	contained in @var{data} with the secret key belonging to the public
	key @var{key}.
	@end deftypefun

	@node Handle-independent functions
	@section Handle-independent functions

	@deftypefun gcry_error_t gcry_ac_id_to_name (gcry_ac_id_t @var{algorithm}, const char **@var{name})
	Stores the textual representation of the algorithm whose id is given
	in @var{algorithm} in @var{name}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_ac_name_to_id (const char @var{name}, gcry_ac_id_t @var{algorithm})
	Stores the numeric ID of the algorithm whose textual representation is
	contained in @var{name} in @var{algorithm}.
	@end deftypefun

	@c **********************************************************
	@c ***************** Random ***************************
	@c **********************************************************
	@node Random Numbers
	@chapter Random Numbers

	@menu
	* Quality of random numbers:: @acronym{Libgcrypt} uses different quality levels.
	* Retrieving random numbers:: How to retrieve random numbers.
	@end menu

	@node Quality of random numbers
	@section Quality of random numbers

	@acronym{Libgcypt} offers random numbers of different quality levels:

	@deftp {Data type} enum gcry_random_level
	The constants for the random quality levels are of this type.
	@end deftp

	@table @code
	@item GCRY_WEAK_RANDOM
	This should not anymore be used. It has recently been changed to an
	alias of GCRY_STRONG_RANDOM. Use @code{gcry_create_nonce} instead.
	@item GCRY_STRONG_RANDOM
	Use this level for e.g. session keys and similar purposes.
	@item GCRY_VERY_STRONG_RANDOM
	Use this level for e.g. key material.
	@end table

	@node Retrieving random numbers
	@section Retrieving random numbers

	@deftypefun void gcry_randomize (unsigned char *@var{buffer}, size_t @var{length}, enum gcry_random_level @var{level})

	Fill @var{buffer} with @var{length} random bytes using a random quality
	as defined by @var{level}.
	@end deftypefun

	@deftypefun void * gcry_random_bytes (size_t @var{nbytes}, enum gcry_random_level @var{level})

	Allocate a memory block consisting of @var{nbytes} fresh random bytes
	using a random quality as defined by @var{level}.
	@end deftypefun

	@deftypefun void * gcry_random_bytes_secure (size_t @var{nbytes}, enum gcry_random_level @var{level})

	Allocate a memory block consisting of @var{nbytes} fresh random bytes
	using a random quality as defined by @var{level}. This function
	differs from @code{gcry_random_bytes} in that the returned buffer is
	allocated in a ``secure'' area of the memory.
	@end deftypefun

	@deftypefun void gcry_create_nonce (void *@var{buffer}, size_t @var{length})

	Fill @var{buffer} with @var{length} unpredictable bytes. This is
	commonly called a nonce and may also be used for initialization
	vectors and padding. This is an extra function nearly independent of
	the other random function for 3 reasons: It better protects the
	regular random generator's internal state, provides better performance
	and does not drain the precious entropy pool.

	@end deftypefun



	@c **********************************************************
	@c ***************** S-Expressions *********************
	@c **********************************************************
	@node S-expressions
	@chapter S-expressions

	S-expressions are used by the public key functions to pass complex data
	structures around. These LISP like objects are used by some
	cryptographic protocols (cf. RFC-2692) and @acronym{Libgcrypt} provides functions
	to parse and construct them. For detailed information, see
	@cite{Ron Rivest, code and description of S-expressions,
	@uref{http://theory.lcs.mit.edu/~rivest/sexp.html}}.

	@menu
	* Data types for S-expressions:: Data types related with S-expressions.
	* Working with S-expressions:: How to work with S-expressions.
	@end menu

	@node Data types for S-expressions
	@section Data types for S-expressions

	@deftp {Data type} gcry_sexp_t
	The @code{gcry_sexp_t} type describes an object with the @acronym{Libgcrypt} internal
	representation of an S-expression.
	@end deftp

	@node Working with S-expressions
	@section Working with S-expressions

	@noindent
	There are several functions to create an @acronym{Libgcrypt} S-expression object
	from its external representation or from a string template. There is
	also a function to convert the internal representation back into one of
	the external formats:


	@deftypefun gcry_error_t gcry_sexp_new (@w{gcry_sexp_t @var{r_sexp}}, @w{const void @var{buffer}}, @w{size_t @var{length}}, @w{int @var{autodetect}})

	This is the generic function to create an new S-expression object from
	its external representation in @var{buffer} of @var{length} bytes. On
	success the result is stored at the address given by @var{r_sexp}.
	With @var{autodetect} set to 0, the data in @var{buffer} is expected to
	be in canonized format, with @var{autodetect} set to 1 the parses any of
	the defined external formats. If @var{buffer} does not hold a valid
	S-expression an error code is returned and @var{r_sexp} set to
	@code{NULL}.
	Note, that the caller is responsible for releasing the newly allocated
	S-expression using @code{gcry_sexp_release}.
	@end deftypefun

	@deftypefun gcry_error_t gcry_sexp_create (@w{gcry_sexp_t @var{r_sexp}}, @w{void @var{buffer}}, @w{size_t @var{length}}, @w{int @var{autodetect}}, @w{void (@var{freefnc})(void)})

	This function is identical to @code{gcry_sexp_new} but has an extra
	argument @var{freefnc}, which, when not set to @code{NULL}, is expected
	to be a function to release the @var{buffer}; most likely the standard
	@code{free} function is used for this argument. This has the effect of
	transferring the ownership of @var{buffer} to the created object in
	@var{r_sexp}. The advantage of using this function is that @acronym{Libgcrypt}
	might decide to directly use the provided buffer and thus avoid extra
	copying.
	@end deftypefun

	@deftypefun gcry_error_t gcry_sexp_sscan (@w{gcry_sexp_t @var{r_sexp}}, @w{size_t @var{erroff}}, @w{const char *@var{buffer}}, @w{size_t @var{length}})

	This is another variant of the above functions. It behaves nearly
	identical but provides an @var{erroff} argument which will receive the
	offset into the buffer where the parsing stopped on error.
	@end deftypefun

	@deftypefun gcry_error_t gcry_sexp_build (@w{gcry_sexp_t @var{r_sexp}}, @w{size_t @var{erroff}}, @w{const char *@var{format}, ...})

	This function creates an internal S-expression from the string template
	@var{format} and stores it at the address of @var{r_sexp}. If there is a
	parsing error, the function returns an appropriate error code and stores
	the offset into @var{format} where the parsing stopped in @var{erroff}.
	The function supports a couple of printf-like formatting characters and
	expects arguments for some of these escape sequences right after
	@var{format}. The following format characters are defined:

	@table @samp
	@item %m
	The next argument is expected to be of type @code{gcry_mpi_t} and a copy of
	its value is inserted into the resulting S-expression.
	@item %s
	The next argument is expected to be of type @code{char *} and that
	string is inserted into the resulting S-expression.
	@item %d
	The next argument is expected to be of type @code{int} and its
	value ist inserted into the resulting S-expression.
	@item %b
	The next argument is expected to be of type @code{int} directly
	followed by an argument of type @code{char *}. This represents a
	buffer of given length to be inserted into the resulting regular
	expression.
	@end table

	@noindent
	No other format characters are defined and would return an error. Note,
	that the format character @samp{%%} does not exists, because a percent
	sign is not a valid character in an S-expression.
	@end deftypefun

	@deftypefun void gcry_sexp_release (@w{gcry_sexp_t @var{sexp}})

	Release the S-expression object @var{sexp}.
	@end deftypefun


	@noindent
	The next 2 functions are used to convert the internal representation
	back into a regular external S-expression format and to show the
	structure for debugging.

	@deftypefun size_t gcry_sexp_sprint (@w{gcry_sexp_t @var{sexp}}, @w{int @var{mode}}, @w{void *@var{buffer}}, @w{size_t @var{maxlength}})

	Copies the S-expression object @var{sexp} into @var{buffer} using the
	format specified in @var{mode}. @var{maxlength} must be set to the
	allocated length of @var{buffer}. The function returns the actual
	length of valid bytes put into @var{buffer} or 0 if the provided buffer
	is too short. Passing @code{NULL} for @var{buffer} returns the required
	length for @var{buffer}. For convenience reasons an extra byte with
	value 0 is appended to the buffer.

	@noindent
	The following formats are supported:

	@table @code
	@item GCRYSEXP_FMT_DEFAULT
	Returns a convenient external S-expression representation.

	@item GCRYSEXP_FMT_CANON
	Return the S-expression in canonical format.

	@item GCRYSEXP_FMT_BASE64
	Not currently supported.

	@item GCRYSEXP_FMT_ADVANCED
	Returns the S-expression in advanced format.
	@end table
	@end deftypefun

	@deftypefun void gcry_sexp_dump (@w{gcry_sexp_t @var{sexp}})

	Dumps @var{sexp} in a format suitable for debugging to @acronym{Libgcrypt}'s
	logging stream.
	@end deftypefun

	@noindent
	Often canonical encoding is used in the external representation. The
	following function can be used to check for valid encoding and to learn
	the length of the S-expression"

	@deftypefun size_t gcry_sexp_canon_len (@w{const unsigned char @var{buffer}}, @w{size_t @var{length}}, @w{size_t @var{erroff}}, @w{int *@var{errcode}})

	Scan the canonical encoded @var{buffer} with implicit length values and
	return the actual length this S-expression uses. For a valid S-expression
	it should never return 0. If @var{length} is not 0, the maximum
	length to scan is given; this can be used for syntax checks of
	data passed from outside. @var{errcode} and @var{erroff} may both be
	passed as @code{NULL}.

	@end deftypefun


	@noindent
	There are a couple of functions to parse S-expressions and retrieve
	elements:

	@deftypefun gcry_sexp_t gcry_sexp_find_token (@w{const gcry_sexp_t @var{list}}, @w{const char *@var{token}}, @w{size_t @var{toklen}})

	Scan the S-expression for a sublist with a type (the car of the list)
	matching the string @var{token}. If @var{toklen} is not 0, the token is
	assumed to be raw memory of this length. The function returns a newly
	allocated S-expression consisting of the found sublist or @code{NULL}
	when not found.
	@end deftypefun


	@deftypefun int gcry_sexp_length (@w{const gcry_sexp_t @var{list}})

	Return the length of the @var{list}. For a valid S-expression this
	should be at least 1.
	@end deftypefun


	@deftypefun gcry_sexp_t gcry_sexp_nth (@w{const gcry_sexp_t @var{list}}, @w{int @var{number}})

	Create and return a new S-expression from the element with index @var{number} in
	@var{list}. Note that the first element has the index 0. If there is
	no such element, @code{NULL} is returned.
	@end deftypefun

	@deftypefun gcry_sexp_t gcry_sexp_car (@w{const gcry_sexp_t @var{list}})

	Create and return a new S-expression from the first element in
	@var{list}; this called the "type" and should always exist and be a
	string. @code{NULL} is returned in case of a problem.
	@end deftypefun

	@deftypefun gcry_sexp_t gcry_sexp_cdr (@w{const gcry_sexp_t @var{list}})

	Create and return a new list form all elements except for the first one.
	Note, that this function may return an invalid S-expression because it
	is not guaranteed, that the type exists and is a string. However, for
	parsing a complex S-expression it might be useful for intermediate
	lists. Returns @code{NULL} on error.
	@end deftypefun


	@deftypefun {const char } gcry_sexp_nth_data (@w{const gcry_sexp_t @var{list}}, @w{int @var{number}}, @w{size_t @var{datalen}})

	This function is used to get data from a @var{list}. A pointer to the
	actual data with index @var{number} is returned and the length of this
	data will be stored to @var{datalen}. If there is no data at the given
	index or the index represents another list, @code{NULL} is returned.
	@strong{Take care:} The returned pointer is valid as long as @var{list} is
	not modified or released.

	@noindent
	Here is an example on how to extract and print the surname (Meier) from
	the S-expression @samp{(Name Otto Meier (address Burgplatz 3))}:

	@example
	size_t len;
	const char *name;

	name = gcry_sexp_nth_data (list, 2, &len);
	printf ("my name is %.*s\n", (int)len, name);
	@end example
	@end deftypefun

	@deftypefun gcry_mpi_t gcry_sexp_nth_mpi (@w{gcry_sexp_t @var{list}}, @w{int @var{number}}, @w{int @var{mpifmt}})

	This function is used to get and convert data from a @var{list}. This
	data is assumed to be an MPI stored in the format described by
	@var{mpifmt} and returned as a standard @acronym{Libgcrypt} MPI. The caller must
	release this returned value using @code{gcry_mpi_release}. If there is
	no data at the given index, the index represents a list or the value
	can't be converted to an MPI, @code{NULL} is returned.
	@end deftypefun


	@c **********************************************************
	@c ***************** MPIs **** *********************
	@c **********************************************************
	@node MPI library
	@chapter MPI library

	@menu
	* Data types:: MPI related data types.
	* Basic functions:: First steps with MPI numbers.
	* MPI formats:: External representation of MPIs.
	* Calculations:: Performing MPI calculations.
	* Comparisons:: How to compare MPI values.
	* Bit manipulations:: How to access single bits of MPI values.
	* Miscellaneous:: Miscellaneous MPI functions.
	@end menu

	Public key cryptography is based on mathematics with large numbers. To
	implement the public key functions, a library for handling these large
	numbers is required. Because of the general usefulness of such a
	library, its interface is exposed by @acronym{Libgcrypt}. The implementation is
	based on an old release of GNU Multi-Precision Library (GMP) but in the
	meantime heavily modified and stripped down to what is required for
	cryptography. For a lot of CPUs, high performance assembler
	implementations of some very low level functions are used to gain much
	better performance than with the standard C implementation.

	@noindent
	In the context of @acronym{Libgcrypt} and in most other applications, these large
	numbers are called MPIs (multi-precision-integers).

	@node Data types
	@section Data types

	@deftp {Data type} gcry_mpi_t
	The @code{gcry_mpi_t} type represents an object to hold an MPI.
	@end deftp

	@node Basic functions
	@section Basic functions

	@noindent
	To work with MPIs, storage must be allocated and released for the
	numbers. This can be done with one of these functions:

	@deftypefun gcry_mpi_t gcry_mpi_new (@w{unsigned int @var{nbits}})

	Allocate a new MPI object, initialize it to 0 and initially allocate
	enough memory for a number of at least @var{nbits}. This pre-allocation is
	only a small performance issue and not actually necessary because
	@acronym{Libgcrypt} automatically re-allocates the required memory.
	@end deftypefun

	@deftypefun gcry_mpi_t gcry_mpi_snew (@w{unsigned int @var{nbits}})

	This is identical to @code{gcry_mpi_new} but allocates the MPI in the so
	called "secure memory" which in turn will take care that all derived
	values will also be stored in this "secure memory". Use this for highly
	confidential data like private key parameters.
	@end deftypefun

	@deftypefun gcry_mpi_t gcry_mpi_copy (@w{const gcry_mpi_t @var{a}})

	Create a new MPI as the exact copy of @var{a}.
	@end deftypefun


	@deftypefun void gcry_mpi_release (@w{gcry_mpi_t @var{a}})

	Release the MPI @var{a} and free all associated resources. Passing
	@code{NULL} is allowed and ignored. When a MPI stored in the "secure
	memory" is released, that memory gets wiped out immediately.
	@end deftypefun

	@noindent
	The simplest operations are used to assign a new value to an MPI:

	@deftypefun gcry_mpi_t gcry_mpi_set (@w{gcry_mpi_t @var{w}}, @w{const gcry_mpi_t @var{u}})

	Assign the value of @var{u} to @var{w} and return @var{w}. If
	@code{NULL} is passed for @var{w}, a new MPI is allocated, set to the
	value of @var{u} and returned.
	@end deftypefun

	@deftypefun gcry_mpi_t gcry_mpi_set_ui (@w{gcry_mpi_t @var{w}}, @w{unsigned long @var{u}})

	Assign the value of @var{u} to @var{w} and return @var{w}. If
	@code{NULL} is passed for @var{w}, a new MPI is allocated, set to the
	value of @var{u} and returned. This function takes an @code{unsigned
	int} as type for @var{u} and thus it is only possible to set @var{w} to
	small values (usually up to the word size of the CPU).
	@end deftypefun

	@deftypefun void gcry_mpi_swap (@w{gcry_mpi_t @var{a}}, @w{gcry_mpi_t @var{b}})

	Swap the values of @var{a} and @var{b}.
	@end deftypefun

	@node MPI formats
	@section MPI formats

	@noindent
	The following functions are used to convert between an external
	representation of an MPI and the internal one of @acronym{Libgcrypt}.

	@deftypefun int gcry_mpi_scan (@w{gcry_mpi_t @var{r_mpi}}, @w{enum gcry_mpi_format @var{format}}, @w{const void @var{buffer}}, @w{size_t @var{buflen}}, @w{size_t *@var{nscanned}})

	Convert the external representation of an integer stored in @var{buffer}
	with a length of @var{buflen} into a newly created MPI returned which
	will be stored at the address of @var{r_mpi}. For certain formats the
	length argument is not required and may be passed as @code{0}. After a
	successful operation the variable @var{nscanned} receives the number of
	bytes actually scanned unless @var{nscanned} was given as
	@code{NULL}. @var{format} describes the format of the MPI as stored in
	@var{buffer}:

	@table @code
	@item GCRYMPI_FMT_STD
	2-complement stored without a length header.

	@item GCRYMPI_FMT_PGP
	As used by OpenPGP (only defined as unsigned). This is basically
	@code{GCRYMPI_FMT_STD} with a 2 byte big endian length header.

	@item GCRYMPI_FMT_SSH
	As used in the Secure Shell protocol. This is @code{GCRYMPI_FMT_STD}
	with a 4 byte big endian header.

	@item GCRYMPI_FMT_HEX
	Stored as a C style string with each byte of the MPI encoded as 2 hex
	digits.

	@item GCRYMPI_FMT_USG
	Simple unsigned integer.
	@end table

	@noindent
	Note, that all of the above formats store the integer in big-endian
	format (MSB first).
	@end deftypefun


	@deftypefun int gcry_mpi_print (@w{enum gcry_mpi_format @var{format}}, @w{unsigned char @var{buffer}}, @w{size_t @var{buflen}}, @w{size_t @var{nwritten}}, @w{const gcry_mpi_t @var{a}})

	Convert the MPI @var{a} into an external representation described by
	@var{format} (see above) and store it in the provided @var{buffer}
	which has a usable length of at least the @var{buflen} bytes. If
	@var{nwritten} is not NULL, it will receive the number of bytes
	actually stored in @var{buffer} after a successful operation.
	@end deftypefun

	@deftypefun int gcry_mpi_aprint (@w{enum gcry_mpi_format @var{format}}, @w{unsigned char *@var{buffer}}, @w{size_t @var{nbytes}}, @w{const gcry_mpi_t @var{a}})

	Convert the MPI @var{a} into an external representation described by
	@var{format} (see above) and store it in a newly allocated buffer which
	address will be stored in the variable @var{buffer} points to. The
	number of bytes stored in this buffer will be stored in the variable
	@var{nbytes} points to, unless @var{nbytes} is @code{NULL}.
	@end deftypefun

	@deftypefun void gcry_mpi_dump (@w{const gcry_mpi_t @var{a}})

	Dump the value of @var{a} in a format suitable for debugging to
	Libgcrypt's logging stream. Note that one leading space but no trailing
	space or linefeed will be printed. It is okay to pass @code{NULL} for
	@var{a}.
	@end deftypefun


	@node Calculations
	@section Calculations

	@noindent
	Basic arithmetic operations:

	@deftypefun void gcry_mpi_add (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}})

	@math{@var{w} = @var{u} + @var{v}}.
	@end deftypefun


	@deftypefun void gcry_mpi_add_ui (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{unsigned long @var{v}})

	@math{@var{w} = @var{u} + @var{v}}. Note, that @var{v} is an unsigned integer.
	@end deftypefun


	@deftypefun void gcry_mpi_addm (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}}, @w{gcry_mpi_t @var{m}})

	@math{@var{w} = @var{u} + @var{v} \bmod @var{m}}.
	@end deftypefun

	@deftypefun void gcry_mpi_sub (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}})

	@math{@var{w} = @var{u} - @var{v}}.
	@end deftypefun

	@deftypefun void gcry_mpi_sub_ui (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{unsigned long @var{v}})

	@math{@var{w} = @var{u} - @var{v}}. @var{v} is an unsigned integer.
	@end deftypefun

	@deftypefun void gcry_mpi_subm (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}}, @w{gcry_mpi_t @var{m}})

	@math{@var{w} = @var{u} - @var{v} \bmod @var{m}}.
	@end deftypefun

	@deftypefun void gcry_mpi_mul (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}})

	@math{@var{w} = @var{u} * @var{v}}.
	@end deftypefun

	@deftypefun void gcry_mpi_mul_ui (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{unsigned long @var{v}})

	@math{@var{w} = @var{u} * @var{v}}. @var{v} is an unsigned integer.
	@end deftypefun

	@deftypefun void gcry_mpi_mulm (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{gcry_mpi_t @var{v}}, @w{gcry_mpi_t @var{m}})

	@math{@var{w} = @var{u} * @var{v} \bmod @var{m}}.
	@end deftypefun

	@deftypefun void gcry_mpi_mul_2exp (@w{gcry_mpi_t @var{w}}, @w{gcry_mpi_t @var{u}}, @w{unsigned long @var{e}})

	@c FIXME: I am in need for a real TeX{info} guru:
	@c I don't know why TeX can grok @var{e} here.
	@math{@var{w} = @var{u} * 2^e}.
	@end deftypefun

	@deftypefun void gcry_mpi_div (@w{gcry_mpi_t @var{q}}, @w{gcry_mpi_t @var{r}}, @w{gcry_mpi_t @var{dividend}}, @w{gcry_mpi_t @var{divisor}}, @w{int @var{round}})

	@math{@var{q} = @var{dividend} / @var{divisor}}, @math{@var{r} =
	@var{dividend} \bmod @var{divisor}}. @var{q} and @var{r} may be passed
	as @code{NULL}. @var{round} should be negative or 0.
	@end deftypefun

	@deftypefun void gcry_mpi_mod (@w{gcry_mpi_t @var{r}}, @w{gcry_mpi_t @var{dividend}}, @w{gcry_mpi_t @var{divisor}})

	@math{@var{r} = @var{dividend} \bmod @var{divisor}}.
	@end deftypefun

	@deftypefun void gcry_mpi_powm (@w{gcry_mpi_t @var{w}}, @w{const gcry_mpi_t @var{b}}, @w{const gcry_mpi_t @var{e}}, @w{const gcry_mpi_t @var{m}})

	@c I don't know why TeX can grok @var{e} here.
	@math{@var{w} = @var{b}^e \bmod @var{m}}.
	@end deftypefun

	@deftypefun int gcry_mpi_gcd (@w{gcry_mpi_t @var{g}}, @w{gcry_mpi_t @var{a}}, @w{gcry_mpi_t @var{b}})

	Set @var{g} to the greatest common divisor of @var{a} and @var{b}.
	Return true if the @var{g} is 1.
	@end deftypefun

	@deftypefun int gcry_mpi_invm (@w{gcry_mpi_t @var{x}}, @w{gcry_mpi_t @var{a}}, @w{gcry_mpi_t @var{m}})

	Set @var{x} to the multiplicative inverse of @math{@var{a} \bmod @var{m}}.
	Return true if the inverse exists.
	@end deftypefun


	@node Comparisons
	@section Comparisons

	@noindent
	The next 2 functions are used to compare MPIs:


	@deftypefun int gcry_mpi_cmp (@w{const gcry_mpi_t @var{u}}, @w{const gcry_mpi_t @var{v}})

	Compare the big integer number @var{u} and @var{v} returning 0 for
	equality, a positive value for @var{u} > @var{v} and a negative for
	@var{u} < @var{v}.
	@end deftypefun

	@deftypefun int gcry_mpi_cmp_ui (@w{const gcry_mpi_t @var{u}}, @w{unsigned long @var{v}})

	Compare the big integer number @var{u} with the unsigned integer @var{v}
	returning 0 for equality, a positive value for @var{u} > @var{v} and a
	negative for @var{u} < @var{v}.
	@end deftypefun


	@node Bit manipulations
	@section Bit manipulations

	@noindent
	There are a couple of functions to get information on arbitrary bits
	in an MPI and to set or clear them:

	@deftypefun {unsigned int} gcry_mpi_get_nbits (@w{gcry_mpi_t @var{a}})

	Return the number of bits required to represent @var{a}.
	@end deftypefun

	@deftypefun int gcry_mpi_test_bit (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Return true if bit number @var{n} (counting from 0) is set in @var{a}.
	@end deftypefun

	@deftypefun void gcry_mpi_set_bit (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Set bit number @var{n} in @var{a}.
	@end deftypefun

	@deftypefun void gcry_mpi_clear_bit (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Clear bit number @var{n} in @var{a}.
	@end deftypefun

	@deftypefun void gcry_mpi_set_highbit (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Set bit number @var{n} in @var{a} and clear all bits greater than @var{n}.
	@end deftypefun

	@deftypefun void gcry_mpi_clear_highbit (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Clear bit number @var{n} in @var{a} and all bits greater than @var{n}.
	@end deftypefun

	@deftypefun void gcry_mpi_rshift (@w{gcry_mpi_t @var{x}}, @w{gcry_mpi_t @var{a}}, @w{unsigned int @var{n}})

	Shift the value of @var{a} by @var{n} bits to the right and store the
	result in @var{x}.
	@end deftypefun

	@node Miscellaneous
	@section Miscellanous

	@deftypefun gcry_mpi_t gcry_mpi_set_opaque (@w{gcry_mpi_t @var{a}}, @w{void *@var{p}}, @w{unsigned int @var{nbits}})

	Store @var{nbits} of the value @var{p} points to in @var{a} and mark
	@var{a} as an opaque value (i.e. an value that can't be used for any
	math calculation and is only used to store an arbitrary bit pattern in
	@var{a}).

	WARNING: Never use an opaque MPI for actual math operations. The only
	valid functions are gcry_mpi_get_opaque and gcry_mpi_release. Use
	gcry_mpi_scan to convert a string of arbitrary bytes into an MPI.

	@end deftypefun

	@deftypefun {void } gcry_mpi_get_opaque (@w{gcry_mpi_t @var{a}}, @w{unsigned int @var{nbits}})

	Return a pointer to an opaque value stored in @var{a} and return its
	size in @var{nbits}. Note, that the returned pointer is still owned by
	@var{a} and that the function should never be used for an non-opaque
	MPI.
	@end deftypefun

	@deftypefun void gcry_mpi_set_flag (@w{gcry_mpi_t @var{a}}, @w{enum gcry_mpi_flag @var{flag}})

	Set the @var{flag} for the MPI @var{a}. Currently only the flag
	@code{GCRYMPI_FLAG_SECURE} is allowed to convert @var{a} into an MPI
	stored in "secure memory".
	@end deftypefun

	@deftypefun void gcry_mpi_clear_flag (@w{gcry_mpi_t @var{a}}, @w{enum gcry_mpi_flag @var{flag}})

	Clear @var{flag} for the big integer @var{a}. Note, that this function is
	currently useless as no flags are allowed.
	@end deftypefun

	@deftypefun int gcry_mpi_get_flag (@w{gcry_mpi_t @var{a}}, @w{enum gcry_mpi_flag @var{flag}})

	Return true when the @var{flag} is set for @var{a}.
	@end deftypefun

	@deftypefun void gcry_mpi_randomize (@w{gcry_mpi_t @var{w}}, @w{unsigned int @var{nbits}}, @w{enum gcry_random_level @var{level}})

	Set the big integer @var{w} to a random value of @var{nbits}, using
	random data quality of level @var{level}. In case @var{nbits} is not
	a multiple of a byte, @var{nbits} is rounded up to the next byte
	boundary.
	@end deftypefun

	@node Utilities
	@chapter Utilities

	@menu
	* Memory allocation:: Functions related with memory allocation.
	@end menu

	@node Memory allocation
	@section Memory allocation

	@deftypefun void *gcry_malloc (size_t @var{n})

	This function tries to allocate @var{n} bytes of memory. On success
	it returns a pointer to the memory area, in an out-of-core condition,
	it returns NULL.
	@end deftypefun

	@deftypefun void *gcry_malloc_secure (size_t @var{n})
	Like @code{gcry_malloc}, but uses secure memory.
	@end deftypefun

	@deftypefun void *gcry_calloc (size_t @var{n})

	This function tries to allocate @var{n} bytes of cleared memory
	(i.e. memory that is initialized with zero bytes). On success it
	returns a pointer to the memory area, in an out-of-core condition, it
	returns NULL.
	@end deftypefun

	@deftypefun void *gcry_calloc_secure (size_t @var{n})
	Like @code{gcry_calloc}, but uses secure memory.
	@end deftypefun

	@deftypefun void gcry_realloc (void @var{p}, size_t @var{n})

	This function tries to resize the memory area pointed to by @var{p} to
	@var{n} bytes. On success it returns a pointer to the new memory
	area, in an out-of-core condition, it returns NULL. Depending on
	whether the memory pointed to by @var{p} is secure memory or not,
	gcry_realloc tries to use secure memory as well.
	@end deftypefun

	@deftypefun void gcry_free (void *@var{p})
	Release the memory area pointed to by @var{p}.
	@end deftypefun

	@c **********************************************************
	@c ***************** Appendices ***********************
	@c **********************************************************

	@include lgpl.texi

	@include gpl.texi

	@node Concept Index
	@unnumbered Concept Index

	@printindex cp

	@node Function and Data Index
	@unnumbered Function and Data Index

	@printindex fn

	@bye

	/* Version check should be the very first gcry call because it
	makes sure that constructor functions are run. */
	if (!gcry_check_version (GCRYPT_VERSION))
	die ("version mismatch\n");
	/* Many applications don't require secure memory, so they should
	disable it right away. There won't be a problem unless one makes
	use of a feature which requires secure memory - in that case the
	process would abort because the secmem is not initialized. */
	gcry_control (GCRYCTL_DISABLE_SECMEM, 0);

	/* .. add whatever initialization you want, but better don't make calls
	to libgcrypt from more than one thread ... */

	/* Tell Libgcrypt that initialization has completed. */
	gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);


	If you require secure memory, this code should be used:

	if (!gcry_check_version (GCRYPT_VERSION))
	die ("version mismatch\n");
	/* We don't want to see any warnings, e.g. because we have not yet
	parsed options which might be used to suppress such warnings */
	gcry_control (GCRYCTL_SUSPEND_SECMEM_WARN);

	/* ... */

	/* Allocate a pool of 16k secure memory. This also drops priviliges
	on some systems. */
	gcry_control (GCRYCTL_INIT_SECMEM, 16384, 0);

	/* It is now okay to let Libgcrypt complain when there was/is a problem
	with the secure memory. */
	gcry_control (GCRYCTL_RESUME_SECMEM_WARN);

	/* Tell Libgcrypt that initialization has completed. */
	gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);


	This sounds a bit complicated but has the advantage that the caller
	must decide whether he wants secure memory or not - there is no
	default.

	It is important that this initialization is not done by a library but
	in the application. The library might want to check for finished
	initialization using:

	if (!gcry_control (GCRYCTL_INITIALIZATION_FINISHED_P))
	return MYLIB_ERROR_LIBGCRYPT_NOT_INITIALIZED;


	@c LocalWords: int HD

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