\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_RIJNDAEL128

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