     [file version.texi not included]

The Libgcrypt Library
*********************

This manual is for Libgcrypt (version VERSION, UPDATED), which is GNU's library of cryptographic building blocks.

Copyright © 2000, 2002, 2003, 2004 Free Software Foundation, Inc.

     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''.

Introduction
************

`Libgcrypt' is a library providing cryptographic building blocks.

Getting Started
===============

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

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.

Features
========

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

It's Free Software
     Anybody can use, modify, and redistribute it under the terms of the GNU Lesser General Public License (see 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 (see Copying); please see the README file of the distribution for of list of these parts.

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

Overview
========

The `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.

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

Preparation
***********

To use `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.

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:

     #include <gcrypt.h>

The name space of `Libgcrypt' is gcry_* for function and type names and 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 Libgcrypt uses libgpg-error, which uses gpg_err_* as name space for function and type names and GPG_ERR_* for other symbols, including all the error codes.

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 -I option).

However, the path to the include file is determined at the time the source is configured.  To solve this problem, `Libgcrypt' ships with a small helper program 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 --cflags option to libgcrypt-config.  The following example shows how it can be used at the command line:

     gcc -c foo.c `libgcrypt-config --cflags`

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

     gcc -o foo foo.o `libgcrypt-config --libs`

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

     gcc -o foo foo.c `libgcrypt-config --cflags --libs`

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 libgcrypt-config script at all. Libgcrypt provides an extension to Automake that does all the work for you.

 -- Macro: AM_PATH_LIBGCRYPT ([minimum-version], [action-if-found], [action-if-not-found])
     Check whether Libgcrypt (at least version MINIMUM-VERSION, if given) exists on the host system.  If it is found, execute ACTION-IF-FOUND, otherwise do ACTION-IF-NOT-FOUND, if given.

     Additionally, the function defines LIBGCRYPT_CFLAGS to the flags needed for compilation of the program to find the gcrypt.h header file, and LIBGCRYPT_LIBS to the linker flags needed to link the program to the Libgcrypt library.

You can use the defined Autoconf variables like this in your Makefile.am:

     AM_CPPFLAGS = $(LIBGCRYPT_CFLAGS)
     LDADD = $(LIBGCRYPT_LIBS)

Initializing the library
========================

It is often desirable to check that the version of `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.

 -- Function: const char *gcry_check_version (const char *REQ_VERSION)

     The function 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 Libgcrypt.

Multi Threading
===============

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

  * If your application is multi-threaded, you must set the thread support callbacks with the GCRYCTL_SET_THREAD_CBS command 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.

  * The function gcry_check_version must be called before any other function in the library, except the GCRYCTL_SET_THREAD_CBS command (called via the gcry_control function), because it initializes the thread support subsystem in Libgcrypt.  To achieve this in multi-threaded programs, you must synchronize the memory with respect to other threads that also want to use Libgcrypt.  For this, it is sufficient to call gcry_check_version before creating the other threads using Libgcrypt (At least this is true for POSIX threads, as 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.).

  * As with the function gpg_strerror, gcry_strerror is not thread safe.  You have to use gpg_strerror_r instead.

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

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''.

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''.

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.

Generalities
************

Controlling the library
=======================

 -- Function: gcry_error_t gcry_control (enum gcry_ctl_cmds CMD, ...)

     This function can be used to influence the general behavior of Libgcrypt in several ways.  Depending on CMD, more arguments can or have to be provided.

Modules
=======

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

 -- Data type: gcry_module_t
     This data type represents a `module'.

Functions registering modules provided by the user take a `module specification structure' as input and return a value of 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 Libgcrypt until it is unregistered again.

Error Handling
==============

Many functions in 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.

Libgcrypt uses the 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, Libgcrypt does not use its own identifiers for error codes, but uses those provided by libgpg-error.  They usually start with GPG_ERR_.

However, Libgcrypt does provide aliases for the functions defined in libgpg-error, which might be preferred for name space consistency.

Most functions in 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.

Error Values
------------

 -- Data type: gcry_err_code_t
     The gcry_err_code_t type is an alias for the libgpg-error type 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.

 -- Data type: gcry_err_source_t
     The gcry_err_source_t type is an alias for the libgpg-error type 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.

 -- Data type: gcry_error_t
     The gcry_error_t type is an alias for the libgpg-error type 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 (GPG_ERR_NO_ERROR), and that in this case all other parts of the error value are set to 0, too.

     Note that in 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.

 -- Function: gcry_err_code_t gcry_err_code (gcry_error_t ERR)
     The static inline function gcry_err_code returns the gcry_err_code_t component of the error value ERR.  This function must be used to extract the error code from an error value in order to compare it with the GPG_ERR_* error code macros.

 -- Function: gcry_err_source_t gcry_err_source (gcry_error_t ERR)
     The static inline function gcry_err_source returns the gcry_err_source_t component of the error value ERR.  This function must be used to extract the error source from an error value in order to compare it with the GPG_ERR_SOURCE_* error source macros.

 -- Function: gcry_error_t gcry_err_make (gcry_err_source_t SOURCE, gcry_err_code_t CODE)
     The static inline function gcry_err_make returns the error value consisting of the error source SOURCE and the error code CODE.

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

 -- Function: gcry_error_t gcry_error (gcry_err_code_t CODE)
     The static inline function gcry_error returns the error value consisting of the default error source and the error code CODE.

     For GCRY applications, the default error source is GPG_ERR_SOURCE_USER_1.  You can define GCRY_ERR_SOURCE_DEFAULT before including 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.

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

 -- Function: gcry_error_t gcry_err_make_from_errno (gcry_err_source_t SOURCE, int ERR)
     The function gcry_err_make_from_errno is like gcry_err_make, but it takes a system error like errno instead of a gcry_err_code_t error code.

 -- Function: gcry_error_t gcry_error_from_errno (int ERR)
     The function gcry_error_from_errno is like gcry_error, but it takes a system error like errno instead of a gcry_err_code_t error code.

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.

 -- Function: gcry_err_code_t gcry_err_code_from_errno (int ERR)
     The function gcry_err_code_from_errno returns the error code for the system error ERR.  If ERR is not a known system error, the function returns GPG_ERR_UNKNOWN_ERRNO.

 -- Function: int gcry_err_code_to_errno (gcry_err_code_t ERR)
     The function gcry_err_code_to_errno returns the system error for the error code ERR.  If ERR is not an error code representing a system error, or if this system error is not defined on this system, the function returns 0.

Error Sources
-------------

The library 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 0, the whole error value will be 0.  In this case the error source part is of course GPG_ERR_SOURCE_UNKNOWN.

The list of error sources that might occur in applications using Libgctypt is:

GPG_ERR_SOURCE_UNKNOWN
     The error source is not known.  The value of this error source is 0.

GPG_ERR_SOURCE_GPGME
     The error source is GPGME itself.

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

GPG_ERR_SOURCE_GPGSM
     The error source is GPGSM, which is the crypto engine used for the OpenPGP protocol.

GPG_ERR_SOURCE_GCRYPT
     The error source is libgcrypt, which is used by crypto engines to perform cryptographic operations.

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

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

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

GPG_ERR_SOURCE_KEYBOX
     The error source is libkbx, a library used by the crypto engines to manage local keyrings.

GPG_ERR_SOURCE_USER_1

GPG_ERR_SOURCE_USER_2

GPG_ERR_SOURCE_USER_3

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 Libgcrypt can use them to mark error values coming from callback handlers.  Thus GPG_ERR_SOURCE_USER_1 is the default for errors created with gcry_error and gcry_error_from_errno, unless you define GCRY_ERR_SOURCE_DEFAULT before including gcrypt.h.

Error Codes
-----------

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

GPG_ERR_EOF
     This value indicates the end of a list, buffer or file.

GPG_ERR_NO_ERROR
     This value indicates success.  The value of this error code is 0.  Also, it is guaranteed that an error value made from the error code 0 will be 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.

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.

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

GPG_ERR_E...
     System errors are mapped to GPG_ERR_EFOO where FOO is the symbol for the system error.

GPG_ERR_INV_VALUE
     This value means that some user provided data was out of range.

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

GPG_ERR_UNUSABLE_SECKEY
     This value means that some signers were invalid.

GPG_ERR_NO_DATA
     This value means that data was expected where no data was found.

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

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.

GPG_ERR_DECRYPT_FAILED
     This value indicates that a decryption operation was unsuccessful.

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

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

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

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

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

GPG_ERR_USER_1

GPG_ERR_USER_2

...

GPG_ERR_USER_16
     These error codes are not used by any GnuPG component and can be freely used by other software.  Applications using 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.

Error Strings
-------------

 -- Function: const char * gcry_strerror (gcry_error_t ERR)
     The function gcry_strerror returns a pointer to a statically allocated string containing a description of the error code contained in the error value ERR.  This string can be used to output a diagnostic message to the user.

 -- Function: const char * gcry_strsource (gcry_error_t ERR)
     The function gcry_strerror returns a pointer to a statically allocated string containing a description of the error source contained in the error value ERR.  This string can be used to output a diagnostic message to the user.

The following example illustrates the use of the functions described above:

     {
       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));
         }
     }

Handler Functions
*****************

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

Progress handler
================

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

 -- Data type: gcry_handler_progress_t
     Progress handler functions have to be of the type gcry_handler_progress_t, which is defined as:

     void (*gcry_handler_progress_t) (void *, const char *, int, int, int)

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

 -- Function: void gcry_set_progress_handler (gcry_handler_progress_t CB, void *CB_DATA)

     This function installs CB as the `Progress handler' function. CB must be defined as follows:

          void
          my_progress_handler (void *CB_DATA, const char *WHAT,
                               int PRINTCHAR, int CURRENT, int TOTAL)
          {
            /* Do something.  */
          }

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

     cb_data
          The argument provided in the call to gcry_set_progress_handler.

     what
          A string identifying the type of the progress output.  The following values for WHAT are defined:

          need_entropy
               Not enough entropy is available.  TOTAL holds the number of required bytes.

          primegen
               Values for PRINTCHAR:

               \n
                    Prime generated.

               !
                    Need to refresh the pool of prime numbers.

               <, >
                    Number of bits adjusted.

               ^
                    Searching for a generator.

               .
                    Fermat test on 10 candidates failed.

               :
                    Restart with a new random value.

               +
                    Rabin Miller test passed.

Allocation handler
==================

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

Memory allocation functions are of the following types:
 -- Data type: gcry_handler_alloc_t
     This type is defined as: void *(*gcry_handler_alloc_t) (size_t n).

 -- Data type: gcry_handler_secure_check_t
     This type is defined as: int *(*gcry_handler_secure_check_t) (const void *).

 -- Data type: gcry_handler_realloc_t
     This type is defined as: void *(*gcry_handler_realloc_t) (void *p, size_t n).

 -- Data type: gcry_handler_free_t
     This type is defined as: void *(*gcry_handler_free_t) (void *).

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

 -- Function: void gcry_set_allocation_handler (gcry_handler_alloc_t FUNC_ALLOC, gcry_handler_alloc_t FUNC_ALLOC_SECURE, gcry_handler_secure_check_t FUNC_SECURE_CHECK, gcry_handler_realloc_t FUNC_REALLOC, gcry_handler_free_t FUNC_FREE)
     Install the provided functions and use them instead of the built-in functions for doing memory allocation.

Error handler
=============

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

 -- Data type: gcry_handler_no_mem_t
     This type is defined as: void (*gcry_handler_no_mem_t) (void *, size_t, unsigned int)

 -- Function: void gcry_set_outofcore_handler (gcry_handler_no_mem_t FUNC_NO_MEM, void *CB_DATA)
     This function registers FUNC_NO_MEM as `out-of-core handler', which means that it will be called in the case of not having enough memory available.

 -- Data type: gcry_handler_error_t
     This type is defined as: void (*gcry_handler_error_t) (void *, int, const char *)

 -- Function: void gcry_set_fatalerror_handler (gcry_handler_error_t FUNC_ERROR, void *CB_DATA)
     This function registers FUNC_ERROR as `error handler', which means that it will be called in error conditions.

Logging handler
===============

 -- Data type: gcry_handler_log_t
     This type is defined as: void (*gcry_handler_log_t) (void *, int, const char *, va_list)

 -- Function: void gcry_set_log_handler (gcry_handler_log_t FUNC_LOG, void *CB_DATA)
     This function registers FUNC_LOG as `logging handler', which means that it will be called in case Libgcrypt wants to log a message.

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 Libgcrypt.

Available ciphers
=================

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

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.

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.

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

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

GCRY_CIPHER_SAFER_SK128
     Reserved and not currently implemented.

GCRY_CIPHER_DES_SK
     Reserved and not currently implemented.

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

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

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

GCRY_CIPHER_TWOFISH
     The Twofish algorithm with a 256 bit key.

GCRY_CIPHER_TWOFISH128
     The Twofish algorithm with a 128 bit key.

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.

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.

Cipher modules
==============

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 Modules.

 -- 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:

     const char *name
          The primary name of the algorithm.

     const char **aliases
          A list of strings that are `aliases' for the algorithm.  The list must be terminated with a NULL element.

     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.

     size_t blocksize
          The block size of the algorithm, in bytes.

     size_t keylen
          The length of the key, in bits.

     size_t contextsize
          The size of the algorithm-specific `context', that should be allocated for each handle.

     gcry_cipher_setkey_t setkey
          The function responsible for initializing a handle with a provided key.  See below for a description of this type.

     gcry_cipher_encrypt_t encrypt
          The function responsible for encrypting a single block.  See below for a description of this type.

     gcry_cipher_decrypt_t decrypt
          The function responsible for decrypting a single block.  See below for a description of this type.

     gcry_cipher_stencrypt_t stencrypt
          Like `encrypt', for stream ciphers.  See below for a description of this type.

     gcry_cipher_stdecrypt_t stdecrypt
          Like `decrypt', for stream ciphers.  See below for a description of this type.

 -- 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:

     const char *oid
          Textual representation of the OID.

     int mode
          Cipher mode for which this OID is valid.

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- Function: gcry_error_t gcry_cipher_register (gcry_cipher_spec_t *CIPHER, unsigned int *algorithm_id, gcry_module_t *MODULE)

     Register a new cipher module whose specification can be found in CIPHER.  On success, a new algorithm ID is stored in ALGORITHM_ID and a pointer representing this module is stored in MODULE.

 -- Function: void gcry_cipher_unregister (gcry_module_t MODULE)
     Unregister the cipher identified by MODULE, which must have been registered with gcry_cipher_register.

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

Available cipher modes
======================

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

GCRY_CIPHER_MODE_ECB
     Electronic Codebook mode.

GCRY_CIPHER_MODE_CFB
     Cipher Feedback mode.

GCRY_CIPHER_MODE_CBC
     Cipher Block Chaining mode.

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

GCRY_CIPHER_MODE_OFB
     Outer Feedback mode.

GCRY_CIPHER_MODE_CTR
     Counter mode.

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:

 -- Function: gcry_error_t gcry_cipher_open (gcry_cipher_hd_t *HD, int ALGO, int MODE, unsigned int 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 ALGO.  See Available ciphers, for a list of supported ciphers and the according constants.

     Besides using the constants directly, the function 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 MODE.  See 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 FLAGS can either be passed as 0 or as the bit-wise OR of the following constants.

     GCRY_CIPHER_SECURE
          Make sure that all operations are allocated in secure memory.  This is useful, when the key material is highly confidential.

     GCRY_CIPHER_ENABLE_SYNC
          This flag enables the CFB sync mode, which is a special feature of Libgcrypt's CFB mode implementation to allow for OpenPGP's CFB variant. See gcry_cipher_sync.

     GCRY_CIPHER_CBC_CTS
          Enable cipher text stealing (CTS) for the CBC mode.  Cannot be used simultaneous as GCRY_CIPHER_CBC_MAC

     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.

Use the following function to release an existing handle:

 -- Function: void gcry_cipher_close (gcry_cipher_hd_t H)

     This function releases the context created by gcry_cipher_open.

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

 -- Function: gcry_error_t gcry_cipher_setkey (gcry_cipher_hd_t H, void *K, size_t L)

     Set the key K used for encryption or decryption in the context denoted by the handle H.  The length L of the key 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.

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:

 -- Function: gcry_error_t gcry_cipher_setiv (gcry_cipher_hd_t H, void *K, size_t L)

     Set the initialization vector used for encryption or decryption. The vector is passed as the buffer K of length 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.

 -- Function: gcry_error_t gcry_cipher_setctr (gcry_cipher_hd_t H, void *C, size_t L)

     Set the counter vector used for encryption or decryption. The counter is passed as the buffer C of length 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.

 -- Function: gcry_error_t gcry_cipher_reset (gcry_cipher_hd_t 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.

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.

 -- Function: gcry_error_t gcry_cipher_encrypt (gcry_cipher_hd_t H, void *out, size_t OUTSIZE, const void *IN, size_t INLEN)

     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 H.  There are 2 ways to use the function: If IN is passed as NULL and INLEN is 0, in-place encryption of the data in OUT or length OUTSIZE takes place.  With IN being not NULL, INLEN bytes are encrypted to the buffer OUT which must have at least a size of INLEN.  OUTLEN must be set to the allocated size of 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 0 on success or an error code.

 -- Function: gcry_error_t gcry_cipher_decrypt (gcry_cipher_hd_t H, void *out, size_t OUTSIZE, const void *IN, size_t INLEN)

     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 H.  There are 2 ways to use the function: If IN is passed as NULL and INLEN is 0, in-place decryption of the data in OUT or length OUTSIZE takes place.  With IN being not NULL, INLEN bytes are decrypted to the buffer OUT which must have at least a size of INLEN.  OUTLEN must be set to the allocated size of 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 0 on success or an error code.

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

 -- Function: gcry_error_t gcry_cipher_sync (gcry_cipher_hd_t H)

     Perform the OpenPGP sync operation on context H. Note, that this is a no-op unless the context was created with the flag GCRY_CIPHER_ENABLE_SYNC

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:

 -- Function: gcry_error_t gcry_cipher_ctl (gcry_cipher_hd_t H, int CMD, void *BUFFER, size_t BUFLEN)

     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 CMD and the passed context handle H.  Please see the comments in the source code (src/global.c) for details.

 -- Function: gcry_error_t gcry_cipher_info (gcry_cipher_hd_t H, int WHAT, void *BUFFER, size_t *NBYTES)

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

     Currently no information is available.

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.

 -- Function: gcry_error_t gcry_cipher_algo_info (int ALGO, int WHAT, void *BUFFER, size_t *NBYTES)

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

     Here is a list of supported codes for WHAT:

     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 NBYTES; BUFFER must be zero.

     GCRYCTL_GET_BLKLEN:
          Return the block length of the algorithm.  The length is returned as a number of octets in NBYTES; BUFFER must be zero.

     GCRYCTL_TEST_ALGO:
          Returns 0 when the specified algorithm is available for use. BUFFER and NBYTES must be zero.

 -- Function: const char *gcry_cipher_algo_name (int ALGO)

     gcry_cipher_algo_name returns a string with the name of the cipher algorithm ALGO.  If the algorithm is not known or another error occurred, an empty string is returned.  This function will never return NULL.

 -- Function: int gcry_cipher_map_name (const char *NAME)

     gcry_cipher_map_name returns the algorithm identifier for the cipher algorithm described by the string NAME.  If this algorithm is not available 0 is returned.

 -- Function: int gcry_cipher_mode_from_oid (const char *STRING)

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

Hashing
*******

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 Libgcrypt.

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

Available hash algorithms
=========================

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 0.

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

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.

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

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

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

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

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.

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

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.

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.

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

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

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

Hash algorithm modules
======================

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 Modules.

 -- 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:

     const char *name
          The primary name of this algorithm.

     unsigned char *asnoid
          Array of bytes that form the ASN OID.

     int asnlen
          Length of bytes in `asnoid'.

     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.

     int mdlen
          Length of the message digest algorithm.  See below for an explanation of this type.

     gcry_md_init_t init
          The function responsible for initializing a handle.  See below for an explanation of this type.

     gcry_md_write_t write
          The function responsible for writing data into a message digest context.  See below for an explanation of this type.

     gcry_md_final_t final
          The function responsible for `finalizing' a message digest context. See below for an explanation of this type.

     gcry_md_read_t read
          The function responsible for reading out a message digest result.  See below for an explanation of this type.

     size_t contextsize
          The size of the algorithm-specific `context', that should be allocated for each handle.

 -- 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:

     const char *oidstring
          Textual representation of the OID.

 -- Data type: gcry_md_init_t
     Type for the `init' function, defined as: void (*gcry_md_init_t) (void *c)

 -- 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)

 -- Data type: gcry_md_final_t
     Type for the `final' function, defined as: void (*gcry_md_final_t) (void *c)

 -- Data type: gcry_md_read_t
     Type for the `read' function, defined as: unsigned char *(*gcry_md_read_t) (void *c)

 -- Function: gcry_error_t gcry_md_register (gcry_md_spec_t *DIGEST, unsigned int *algorithm_id, gcry_module_t *MODULE)

     Register a new digest module whose specification can be found in DIGEST.  On success, a new algorithm ID is stored in ALGORITHM_ID and a pointer representing this module is stored in MODULE.

 -- Function: void gcry_md_unregister (gcry_module_t MODULE)
     Unregister the digest identified by MODULE, which must have been registered with gcry_md_register.

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

Working with hash algorithms
============================

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

 -- Function: gcry_error_t gcry_md_open (gcry_md_hd_t *HD, int ALGO, unsigned int FLAGS)

     Create a message digest object for algorithm ALGO.  FLAGS may be given as an bitwise OR of constants described below.  ALGO may be given as 0 if the algorithms to use are later set using gcry_md_enable. HD is guaranteed to either receive a valid handle or NULL.

     For a list of supported algorithms, see Available hash algorithms.

     The flags allowed for MODE are:

     GCRY_MD_FLAG_SECURE
          Allocate all buffers and the resulting digest in "secure memory".  Use this is the hashed data is highly confidential.

     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 gcry_md_setkey must be used set the MAC key.  If you want CBC message authentication codes based on a cipher, see Working with cipher handles.

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

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

 -- Function: gcry_error_t gcry_md_enable (gcry_md_hd_t H, int ALGO)

     Add the message digest algorithm ALGO to the digest object described by handle H.  Duplicated enabling of algorithms is detected and ignored.

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

 -- Function: gcry_error_t gcry_md_setkey (gcry_md_hd_t H, const void *KEY, size_t KEYLEN)

     For use with the HMAC feature, set the MAC key to the value of KEY of length KEYLEN.

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

 -- Function: void gcry_md_close (gcry_md_hd_t H)

     Release all resources of hash context H.  H should not be used after a call to this function.  A NULL passed as H is ignored.

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:

 -- Function: void gcry_md_reset (gcry_md_hd_t 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.

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:

 -- Function: gcry_error_t gcry_md_copy (gcry_md_hd_t *HANDLE_DST, gcry_md_hd_t HANDLE_SRC)

     Create a new digest object as an exact copy of the object described by handle HANDLE_SRC and store it in HANDLE_DST.  The context is not reset and you can continue to hash data using this context and independently using the original context.

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.

 -- Function: void gcry_md_write (gcry_md_hd_t H, const void *BUFFER, size_t LENGTH)

     Pass LENGTH bytes of the data in BUFFER to the digest object with handle H to update the digest values. This function should be used for large blocks of data.

 -- Function: void gcry_md_putc (gcry_md_hd_t H, int C)

     Pass the byte in C to the digest object with handle H to update the digest value.  This is an efficient function, implemented as a macro to buffer the data before an actual update.

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.

 -- Function: void gcry_md_final (gcry_md_hd_t H)

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

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

 -- Function: unsigned char *gcry_md_read (gcry_md_hd_t H, int ALGO)

     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 gcry_md_close or gcry_md_reset.  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 NULL if the requested algorithm has not been enabled.

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

 -- Function: void gcry_md_hash_buffer (int ALGO, void *DIGEST, const cvoid *BUFFER, size_t LENGTH);

     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 LENGTH bytes at BUFFER.  DIGEST must be allocated by the caller, large enough to hold the message digest yielded by the the specified algorithm ALGO.  This required size may be obtained by using the function gcry_md_get_algo_dlen.

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

Hash algorithms are identified by internal algorithm numbers (see 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.

 -- Function: const char *gcry_md_algo_name (int ALGO)

     Map the digest algorithm id 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.

 -- Function: int gcry_md_map_name (const char *NAME)

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

 -- Function: gcry_error_t gcry_md_get_asnoid (int ALGO, void *BUFFER, size_t *LENGTH)

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

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

 -- Function: gcry_error_t gcry_md_test_algo (int ALGO)

     The macro returns 0 if the algorithm ALGO is available for use.

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

 -- Function: unsigned int gcry_md_get_algo_dlen (int ALGO)

     Retrieve the length in bytes of the digest yielded by algorithm ALGO.  This is often used prior to gcry_md_read to allocate sufficient memory for the digest.

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:

 -- Function: int gcry_md_get_algo (gcry_md_hd_t H)

     Retrieve the algorithm used with the handle H. Note, that this does not work reliable if more than one algorithm is enabled in H.

The following macro might also be useful:

 -- Function: int gcry_md_is_secure (gcry_md_hd_t H)

     This function returns true when the digest object H is allocated in "secure memory"; i.e. H was created with the GCRY_MD_FLAG_SECURE.

 -- Function: int gcry_md_is_enabled (gcry_md_hd_t H, int ALGO)

     This function returns true when the algorithm ALGO has been enabled for the digest object H.

Tracking bugs related to hashing is often a cumbersome task which requires to add a lot of printf statements into the code.  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:

 -- Function: void gcry_md_start_debug (gcry_md_hd_t H, const char *SUFFIX)

     Enable debugging for the digest object with handle H.  This creates create files named dbgmd-<n>.<string> while doing the actual hashing.  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 gcry_md_write or gcry_md_putc.

 -- Function: void gcry_md_stop_debug (gcry_md_hd_t H, int RESERVED)

     Stop debugging on handle H.  RESERVED should be specified as 0.  This function is usually not required because gcry_md_close does implicitly stop debugging.

Public Key cryptography (I)
***************************

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

Available algorithms
====================

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.

Used S-expressions
==================

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 Libgcrypt do.

The following information are stored in S-expressions:

keys

plain text data

encrypted data

signatures

To describe how Libgcrypt expect keys, we use some examples. Note that words in uppercase indicate parameters whereas lowercase words are literals.

     (private-key
       (dsa
         (p P-MPI)
         (q Q-MPI)
         (g G-MPI)
         (y Y-MPI)
         (x X-MPI)))

This specifies a DSA private key with the following parameters:

p-mpi
     DSA prime p.

q-mpi
     DSA group order q (which is a prime divisor of p-1).

g-mpi
     DSA group generator g.

y-mpi
     DSA public key value y = g^x \bmod p.

x-mpi
     DSA secret exponent x.

All the MPI values are  expected to be in GCRYMPI_FMT_USG format. The public key is similar with "private-key" replaced by "public-key" and no X-MPI.

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

Here is an example for an RSA key:

     (private-key
       (rsa
         (n N-MPI)
         (e E-MPI)
         (d D-MPI)
         (p P-MPI)
         (q Q-MPI)
         (u U-MPI)

with

n-mpi
     RSA public modulus n.

e-mpi
     RSA public exponent e.

d-mpi
     RSA secret exponent d = e^-1 \bmod (p-1)(q-1).

p-mpi
     RSA secret prime p.

q-mpi
     RSA secret prime q with q > p.

u-mpi
     multiplicative inverse u = p^-1 \bmod q.

Public key modules
==================

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 Modules.

 -- 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:

     const char *name
          The primary name of this algorithm.

     char **aliases
          A list of strings that are `aliases' for the algorithm.  The list must be terminated with a NULL element.

     const char *elements_pkey
          String containing the one-letter names of the MPI values contained in a public key.

     const char *element_skey
          String containing the one-letter names of the MPI values contained in a secret key.

     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.

     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.

     const char *elements_grip
          String containing the one-letter names of the MPI values that are to be included in the `key grip'.

     int use
          The bitwise-OR of the following flags, depending on the abilities of the algorithm:

          GCRY_PK_USAGE_SIGN
               The algorithm supports signing and verifying of data.

          GCRY_PK_USAGE_ENCR
               The algorithm supports the encryption and decryption of data.

     gcry_pk_generate_t generate
          The function responsible for generating a new key pair.  See below for a description of this type.

     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.

     gcry_pk_encrypt_t encrypt
          The function responsible for encrypting data.  See below for a description of this type.

     gcry_pk_decrypt_t decrypt
          The function responsible for decrypting data.  See below for a description of this type.

     gcry_pk_sign_t sign
          The function responsible for signing data.  See below for a description of this type.

     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.

     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.

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- 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)

 -- Function: gcry_error_t gcry_pk_register (gcry_pk_spec_t *PUBKEY, unsigned int *algorithm_id, gcry_module_t *MODULE)

     Register a new public key module whose specification can be found in PUBKEY.  On success, a new algorithm ID is stored in ALGORITHM_ID and a pointer representing this module is stored in MODULE.

 -- Function: void gcry_pk_unregister (gcry_module_t MODULE)
     Unregister the public key module identified by MODULE, which must have been registered with gcry_pk_register.

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

Cryptographic Functions
=======================

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.

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:

pkcs1
     Use PKCS#1 block type 2 padding.

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.

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:

 -- Function: gcry_error_t gcry_pk_encrypt (gcry_sexp_t *R_CIPH, gcry_sexp_t DATA, gcry_sexp_t PKEY)

     Obviously a public key must be provided for encryption.  It is expected as an appropriate S-expression (see above) in 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.

     If you don't want to let Libgcrypt handle the padding, you must pass an appropriate MPI using this expression for DATA:

          (data
            (flags raw)
            (value MPI))

     This has the same semantics as the old style MPI only way.  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 DATA:

          (data
            (flags pkcs1)
            (value BLOCK))

     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 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 R_CIPH. The caller is responsible to release this value using gcry_sexp_release.  In case of an error, an error code is returned and R_CIPH will be set to NULL.

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

          (enc-val
            (rsa
              (a A-MPI)))

     Where A-MPI is an MPI with the result of the RSA operation.  When using the ElGamal algorithm, the return value will have this format:

          (enc-val
            (elg
              (a A-MPI)
              (b B-MPI)))

     Where A-MPI and B-MPI are MPIs with the result of the ElGamal encryption operation.

 -- Function: gcry_error_t gcry_pk_decrypt (gcry_sexp_t *R_PLAIN, gcry_sexp_t DATA, gcry_sexp_t SKEY)

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

          (enc-val
            (flags)
            (elg
              (a A-MPI)
              (b B-MPI)))

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

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

          (value PLAINTEXT)

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.  Libgcrypt supports digital signatures using 2 functions, similar to the encryption functions:

 -- Function: gcry_error_t gcry_pk_sign (gcry_sexp_t *R_SIG, gcry_sexp_t DATA, gcry_sexp_t SKEY)

     This function creates a digital signature for DATA using the private key SKEY and place it into the variable at the address of R_SIG.  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 Libgcrypt handle padding:

           (data
            (flags pkcs1)
            (hash HASH-ALGO BLOCK))

     This example requests to sign the data in BLOCK after applying PKCS#1 block type 1 style padding.  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 Libgcrypt.  Most likely, this will be "sha1", "rmd160" or "md5".  It is obvious that the length of BLOCK must match the size of that message digests; the function checks that this and other constraints are valid.

     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:

          (data
            (flags raw)
            (value MPI))

     Here, the data to be signed is directly given as an MPI.

     The signature is returned as a newly allocated S-expression in R_SIG using this format for RSA:

          (sig-val
            (rsa
              (s S-MPI)))

     Where S-MPI is the result of the RSA sign operation.  For DSA the S-expression returned is:

          (sig-val
            (dsa
              (r R-MPI)
              (s S-MPI)))

     Where R-MPI and 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".

The operation most commonly used is definitely the verification of a signature.  Libgcrypt provides this function:

 -- Function: gcry_error_t gcry_pk_verify (gcry_sexp_t SIG, gcry_sexp_t DATA, gcry_sexp_t PKEY)

     This is used to check whether the signature SIG matches the DATA.  The public key PKEY must be provided to perform this verification.  This function is similar in its parameters to 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 gcry_pk_sign, is passed to the function in SIG.

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

General public-key related Functions
====================================

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

 -- Function: const char * gcry_pk_algo_name (int ALGO)

     Map the public key algorithm id ALGO to a string representation of the algorithm name.  For unknown algorithms this functions returns an empty string.

 -- Function: int gcry_pk_map_name (const char *NAME)

     Map the algorithm NAME to a public key algorithm Id.  Returns 0 if the algorithm name is not known.

 -- Function: int gcry_pk_test_algo (int ALGO)

     Return 0 if the public key algorithm ALGO is available for use. Note, that this is implemented as a macro.

 -- Function: unsigned int gcry_pk_get_nbits (gcry_sexp_t KEY)

     Return what is commonly referred as the key length for the given public or private in KEY.

 -- Function: unsigned char * gcry_pk_get_keygrip (gcry_sexp_t KEY, unsigned char *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.  ARRAY must either provide space for 20 bytes or 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 ARRAY is returned. 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 KEY.

 -- Function: gcry_error_t gcry_pk_testkey (gcry_sexp_t KEY)

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

 -- Function: int gcry_pk_algo_info (int ALGO, int WHAT, void *BUFFER, size_t *NBYTES)

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

     GCRYCTL_TEST_ALGO:
          Return 0 when the specified algorithm is available for use. BUFFER must be NULL, NBYTES may be passed as 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:

          GCRY_PK_USAGE_SIGN
               Algorithm is usable for signing.

          GCRY_PK_USAGE_ENCR
               Algorithm is usable for encryption.

     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.

     GCRYCTL_GET_ALGO_NPKEY
          Return the number of elements the public key for algorithm ALGO consist of.  Return 0 for an unknown algorithm.

     GCRYCTL_GET_ALGO_NSKEY
          Return the number of elements the private key for algorithm ALGO consist of.  Note that this value is always larger than that of the public key.  Return 0 for an unknown algorithm.

     GCRYCTL_GET_ALGO_NSIGN
          Return the number of elements a signature created with the algorithm ALGO consists of.  Return 0 for an unknown algorithm or for an algorithm not capable of creating signatures.

     GCRYCTL_GET_ALGO_NENC
          Return the number of elements a encrypted message created with the algorithm ALGO consists of.  Return 0 for an unknown algorithm or for an algorithm not capable of encryption.

     Please note that parameters not required should be passed as NULL.

 -- Function: gcry_error_t gcry_pk_ctl (int CMD, void *BUFFER, size_t BUFLEN)

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

     GCRYCTL_DISABLE_ALGO
          Disable the algorithm given as an algorithm id in BUFFER. BUFFER must point to an int variable with the algorithm id and BUFLEN must have the value sizeof (int).

Libgcrypt also provides a function for generating public key pairs:

 -- Function: gcry_error_t gcry_pk_genkey (gcry_sexp_t *R_KEY, gcry_sexp_t PARMS)

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

     Here is an example for PARMS for creating a 1024 bit RSA key:

          (genkey
            (rsa
              (nbits 4:1024)))

     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:

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

     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:

          0
               Use a secure and fast value.  This is currently the number 41.

          1
               Use a secure value as required by some specification.  This is currently the number 65537.

          2
               Reserved

          If this parameter is not used, Libgcrypt uses for historic reasons 65537.

     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.

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

          (key-data
            (public-key
              (elg
                (p P-MPI)
                (g G-MPI)
                (y Y-MPI)))
            (private-key
              (elg
                (p P-MPI)
                (g G-MPI)
                (y Y-MPI)
                (x X-MPI)))
            (misc-key-info
              (pm1-factors N1 N2 ... NN)))

     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. N1 N2 ... NN is a list of prime numbers used to composite P-MPI; this is in general not a very useful information.

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.

Available asymmetric algorithms
===============================

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.

 -- Data type: gcry_ac_id_t

     The following constants are defined for this type:

     GCRY_AC_RSA
          Riven-Shamir-Adleman

     GCRY_AC_DSA
          Digital Signature Algorithm

     GCRY_AC_ELG
          ElGamal

     GCRY_AC_ELG_E
          ElGamal, encryption only.

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.

 -- Data type: gcry_ac_data_t
     A data set, that is simply a list of named MPI values.

The following flags are supported:

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

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.

 -- Function: gcry_error_t gcry_ac_data_new (gcry_ac_data_t *DATA)
     Creates a new, empty data set and stores it in DATA.

 -- Function: void gcry_ac_data_destroy (gcry_ac_data_t DATA)
     Destroys the data set DATA.

 -- Function: gcry_error_t gcry_ac_data_set (gcry_ac_data_t DATA, unsigned int FLAGS, char *NAME, gcry_mpi_t MPI)
     Add the value MPI to DATA with the label NAME.  If FLAGS contains GCRY_AC_FLAG_DATA_COPY, the data set will contain copies of NAME and MPI.  If 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.

 -- Function: gcry_error_t gcry_ac_data_copy (gcry_ac_data_t *DATA_CP, gcry_ac_data_t DATA)
     Create a copy of the data set DATA and store it in DATA_CP.

 -- Function: unsigned int gcry_ac_data_length (gcry_ac_data_t DATA)
     Returns the number of named MPI values inside of the data set DATA.

 -- Function: gcry_error_t gcry_ac_data_get_name (gcry_ac_data_t DATA, unsigned int FLAGS, char *NAME, gcry_mpi_t *MPI)
     Store the value labelled with NAME found in DATA in MPI.  If FLAGS contains GCRY_AC_FLAG_COPY, store a copy of the MPI value contained in the data set.  MPI may be NULL.

 -- Function: gcry_error_t gcry_ac_data_get_index (gcry_ac_data_t DATA, unsigned int flags, unsigned int INDEX, const char **NAME, gcry_mpi_t *MPI)
     Stores in NAME and MPI the named MPI value contained in the data set DATA with the index IDX.  If FLAGS contains GCRY_AC_FLAG_COPY, store copies of the values contained in the data set. NAME or MPI may be NULL.

 -- Function: void gcry_ac_data_clear (gcry_ac_data_t DATA)
     Destroys any values contained in the data set DATA.

Working with handles
====================

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

 -- Function: gcry_error_t gcry_ac_open (gcry_ac_handle_t *HANDLE, int ALGORITHM, int FLAGS)

     Creates a new handle for the algorithm ALGORITHM and stores it in HANDLE.  FLAGS is not used yet.

     ALGORITHM must be a valid algorithm ID, see Available algorithms, for a list of supported algorithms and the according constants.  Besides using the listed constants directly, the functions gcry_ac_name_to_id may be used to convert the textual name of an algorithm into the according numeric ID.

 -- Function: void gcry_ac_close (gcry_ac_handle_t HANDLE)
     Destroys the handle HANDLE.

Working with keys
=================

 -- Data type: gcry_ac_key_type_t
     Defined constants:

     GCRY_AC_KEY_TYPE_SECRET
          Specifies a secret key.

     GCRY_AC_KEY_TYPE_PUBLIC
          Specifies a public key.

 -- Data type: gcry_ac_key_t
     This type represents a single `key', either a secret one or a public one.

 -- Data type: gcry_ac_key_pair_t
     This type represents a `key pair' containing a secret and a public key.

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.

 -- Function: gcry_error_t gcry_ac_key_init (gcry_ac_key_t *KEY, gcry_ac_handle_t HANDLE, gcry_ac_key_type_t TYPE, gcry_ac_data_t DATA)
     Creates a new key of type TYPE, consisting of the MPI values contained in the data set DATA and stores it in KEY.

 -- Function: gcry_error_t gcry_ac_key_pair_generate (gcry_ac_handle_t HANDLE, unsigned int NBITS, void *KEY_SPEC, gcry_ac_key_pair_t *KEY_PAIR, gcry_mpi_t **MISC_DATA)

     Generates a new key pair via the handle HANDLE of NBITS bits and stores it in KEY_PAIR.

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

     gcry_ac_key_spec_rsa_t

          gcry_mpi_t e
               Generate the key pair using a special e.  The value of e has the following meanings:

               = 0
                    Let Libgcrypt device what exponent should be used.

               = 1
                    Request the use of a ``secure'' exponent; this is required by some specification to be 65537.

               > 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.

     Example code:

          {
            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);
          }

 -- Function: gcry_ac_key_t gcry_ac_key_pair_extract (gcry_ac_key_pair_t KEY_PAIR, gcry_ac_key_type_t WHICH)
     Returns the key of type WHICH out of the key pair KEY_PAIR.

 -- Function: void gcry_ac_key_destroy (gcry_ac_key_t KEY)
     Destroys the key KEY.

 -- Function: void gcry_ac_key_pair_destroy (gcry_ac_key_pair_t KEY_PAIR)
     Destroys the key pair KEY_PAIR.

 -- Function: gcry_ac_data_t gcry_ac_key_data_get (gcry_ac_key_t KEY)
     Returns the data set contained in the key KEY.

 -- Function: gcry_error_t gcry_ac_key_test (gcry_ac_handle_t HANDLE, gcry_ac_key_t KEY)
     Verifies that the private key KEY is sane via HANDLE.

 -- Function: gcry_error_t gcry_ac_key_get_nbits (gcry_ac_handle_t HANDLE, gcry_ac_key_t KEY, unsigned int *NBITS)
     Stores the number of bits of the key KEY in NBITS via HANDLE.

 -- Function: gcry_error_t gcry_ac_key_get_grip (gcry_ac_handle_t HANDLE, gcry_ac_key_t KEY, unsigned char *KEY_GRIP)
     Writes the 20 byte long key grip of the key KEY to KEY_GRIP via HANDLE.

Using cryptographic functions
=============================

The following flags might be relevant:

GCRY_AC_FLAG_NO_BLINDING
     Disable any blinding, which might be supported by the chosen algorithm; blinding is the default.

 -- Function: gcry_error_t gcry_ac_data_encrypt (gcry_ac_handle_t HANDLE, unsigned int FLAGS, gcry_ac_key_t KEY, gcry_mpi_t DATA_PLAIN, gcry_ac_data_t **DATA_ENCRYPTED)
     Encrypts the plain text MPI value DATA_PLAIN with the key public KEY under the control of the flags FLAGS and stores the resulting data set into DATA_ENCRYPTED.

 -- Function: gcry_error_t gcry_ac_data_decrypt (gcry_ac_handle_t HANDLE, unsigned int FLAGS, gcry_ac_key_t KEY, gcry_mpi_t *DATA_PLAIN, gcry_ac_data_t DATA_ENCRYPTED)
     Decrypts the encrypted data contained in the data set DATA_ENCRYPTED with the secret key KEY under the control of the flags FLAGS and stores the resulting plain text MPI value in DATA_PLAIN.

 -- Function: gcry_error_t gcry_ac_data_sign (gcry_ac_handle_t HANDLE, gcry_ac_key_t KEY, gcry_mpi_t DATA, gcry_ac_data_t *DATA_SIGNATURE)
     Signs the data contained in DATA with the secret key KEY and stores the resulting signature in the data set DATA_SIGNATURE.

 -- Function: gcry_error_t gcry_ac_data_verify (gcry_ac_handle_t HANDLE, gcry_ac_key_t KEY, gcry_mpi_t DATA, gcry_ac_data_t DATA_SIGNATURE)
     Verifies that the signature contained in the data set DATA_SIGNATURE is indeed the result of signing the data contained in DATA with the secret key belonging to the public key KEY.

Handle-independent functions
============================

 -- Function: gcry_error_t gcry_ac_id_to_name (gcry_ac_id_t ALGORITHM, const char **NAME)
     Stores the textual representation of the algorithm whose id is given in ALGORITHM in NAME.

 -- Function: gcry_error_t gcry_ac_name_to_id (const char *NAME, gcry_ac_id_t *ALGORITHM)
     Stores the numeric ID of the algorithm whose textual representation is contained in NAME in ALGORITHM.

Random Numbers
**************

Quality of random numbers
=========================

Libgcypt offers random numbers of different quality levels:

 -- Data type: enum gcry_random_level
     The constants for the random quality levels are of this type.

GCRY_WEAK_RANDOM
     This should not anymore be used.  It has recently been changed to an alias of GCRY_STRONG_RANDOM.  Use gcry_create_nonce instead.

GCRY_STRONG_RANDOM
     Use this level for e.g. session keys and similar purposes.

GCRY_VERY_STRONG_RANDOM
     Use this level for e.g. key material.

Retrieving random numbers
=========================

 -- Function: void gcry_randomize (unsigned char *BUFFER, size_t LENGTH, enum gcry_random_level LEVEL)

     Fill BUFFER with LENGTH random bytes using a random quality as defined by LEVEL.

 -- Function: void * gcry_random_bytes (size_t NBYTES, enum gcry_random_level LEVEL)

     Allocate a memory block consisting of NBYTES fresh random bytes using a random quality as defined by LEVEL.

 -- Function: void * gcry_random_bytes_secure (size_t NBYTES, enum gcry_random_level LEVEL)

     Allocate a memory block consisting of NBYTES fresh random bytes using a random quality as defined by LEVEL.  This function differs from gcry_random_bytes in that the returned buffer is allocated in a ``secure'' area of the memory.

 -- Function: void gcry_create_nonce (void *BUFFER, size_t LENGTH)

     Fill BUFFER with 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.

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 Libgcrypt provides functions to parse and construct them.  For detailed information, see Ron Rivest, code and description of S-expressions, http://theory.lcs.mit.edu/~rivest/sexp.html.

Data types for S-expressions
============================

 -- Data type: gcry_sexp_t
     The gcry_sexp_t type describes an object with the Libgcrypt internal representation of an S-expression.

Working with S-expressions
==========================

There are several functions to create an 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:

 -- Function: gcry_error_t gcry_sexp_new (gcry_sexp_t *R_SEXP, const void *BUFFER, size_t LENGTH, int AUTODETECT)

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

 -- Function: gcry_error_t gcry_sexp_create (gcry_sexp_t *R_SEXP, void *BUFFER, size_t LENGTH, int AUTODETECT, void (*FREEFNC)(void*))

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

 -- Function: gcry_error_t gcry_sexp_sscan (gcry_sexp_t *R_SEXP, size_t *ERROFF, const char *BUFFER, size_t LENGTH)

     This is another variant of the above functions.  It behaves nearly identical but provides an ERROFF argument which will receive the offset into the buffer where the parsing stopped on error.

 -- Function: gcry_error_t gcry_sexp_build (gcry_sexp_t *R_SEXP, size_t *ERROFF, const char *FORMAT, ...)

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

     %m
          The next argument is expected to be of type gcry_mpi_t and a copy of its value is inserted into the resulting S-expression.

     %s
          The next argument is expected to be of type char * and that string is inserted into the resulting S-expression.

     %d
          The next argument is expected to be of type int and its value ist inserted into the resulting S-expression.

     %b
          The next argument is expected to be of type int directly followed by an argument of type char *.  This represents a buffer of given length to be inserted into the resulting regular expression.

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

 -- Function: void gcry_sexp_release (gcry_sexp_t SEXP)

     Release the S-expression object SEXP.

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.

 -- Function: size_t gcry_sexp_sprint (gcry_sexp_t SEXP, int MODE, void *BUFFER, size_t MAXLENGTH)

     Copies the S-expression object SEXP into BUFFER using the format specified in MODE.  MAXLENGTH must be set to the allocated length of BUFFER.  The function returns the actual length of valid bytes put into BUFFER or 0 if the provided buffer is too short.  Passing NULL for BUFFER returns the required length for BUFFER.  For convenience reasons an extra byte with value 0 is appended to the buffer.

     The following formats are supported:

     GCRYSEXP_FMT_DEFAULT
          Returns a convenient external S-expression representation.

     GCRYSEXP_FMT_CANON
          Return the S-expression in canonical format.

     GCRYSEXP_FMT_BASE64
          Not currently supported.

     GCRYSEXP_FMT_ADVANCED
          Returns the S-expression in advanced format.

 -- Function: void gcry_sexp_dump (gcry_sexp_t SEXP)

     Dumps SEXP in a format suitable for debugging to Libgcrypt's logging stream.

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"

 -- Function: size_t gcry_sexp_canon_len (const unsigned char *BUFFER, size_t LENGTH, size_t *ERROFF, int *ERRCODE)

     Scan the canonical encoded 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 LENGTH is not 0, the maximum length to scan is given; this can be used for syntax checks of data passed from outside.  ERRCODE and ERROFF may both be passed as NULL.

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

 -- Function: gcry_sexp_t gcry_sexp_find_token (const gcry_sexp_t LIST, const char *TOKEN, size_t TOKLEN)

     Scan the S-expression for a sublist with a type (the car of the list) matching the string TOKEN.  If 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 NULL when not found.

 -- Function: int gcry_sexp_length (const gcry_sexp_t LIST)

     Return the length of the LIST.  For a valid S-expression this should be at least 1.

 -- Function: gcry_sexp_t gcry_sexp_nth (const gcry_sexp_t LIST, int NUMBER)

     Create and return a new S-expression from the element with index NUMBER in LIST.  Note that the first element has the index 0.  If there is no such element, NULL is returned.

 -- Function: gcry_sexp_t gcry_sexp_car (const gcry_sexp_t LIST)

     Create and return a new S-expression from the first element in LIST; this called the "type" and should always exist and be a string. NULL is returned in case of a problem.

 -- Function: gcry_sexp_t gcry_sexp_cdr (const gcry_sexp_t 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 NULL on error.

 -- Function: const char * gcry_sexp_nth_data (const gcry_sexp_t LIST, int NUMBER, size_t *DATALEN)

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

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

          size_t len;
          const char *name;

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

 -- Function: gcry_mpi_t gcry_sexp_nth_mpi (gcry_sexp_t LIST, int NUMBER, int MPIFMT)

     This function is used to get and convert data from a LIST. This data is assumed to be an MPI stored in the format described by MPIFMT and returned as a standard Libgcrypt MPI.  The caller must release this returned value using 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, NULL is returned.

MPI library
***********

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 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.

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

Data types
==========

 -- Data type: gcry_mpi_t
     The gcry_mpi_t type represents an object to hold an MPI.

Basic functions
===============

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

 -- Function: gcry_mpi_t gcry_mpi_new (unsigned int NBITS)

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

 -- Function: gcry_mpi_t gcry_mpi_snew (unsigned int NBITS)

     This is identical to 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.

 -- Function: gcry_mpi_t gcry_mpi_copy (const gcry_mpi_t A)

     Create a new MPI as the exact copy of A.

 -- Function: void gcry_mpi_release (gcry_mpi_t A)

     Release the MPI A and free all associated resources.  Passing NULL is allowed and ignored.  When a MPI stored in the "secure memory" is released, that memory gets wiped out immediately.

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

 -- Function: gcry_mpi_t gcry_mpi_set (gcry_mpi_t W, const gcry_mpi_t U)

     Assign the value of U to W and return W.  If NULL is passed for W, a new MPI is allocated, set to the value of U and returned.

 -- Function: gcry_mpi_t gcry_mpi_set_ui (gcry_mpi_t W, unsigned long U)

     Assign the value of U to W and return W.  If NULL is passed for W, a new MPI is allocated, set to the value of U and returned.  This function takes an unsigned int as type for U and thus it is only possible to set W to small values (usually up to the word size of the CPU).

 -- Function: void gcry_mpi_swap (gcry_mpi_t A, gcry_mpi_t B)

     Swap the values of A and B.

MPI formats
===========

The following functions are used to convert between an external representation of an MPI and the internal one of Libgcrypt.

 -- Function: int gcry_mpi_scan (gcry_mpi_t *R_MPI, enum gcry_mpi_format FORMAT, const void *BUFFER, size_t BUFLEN, size_t *NSCANNED)

     Convert the external representation of an integer stored in BUFFER with a length of BUFLEN into a newly created MPI returned which will be stored at the address of R_MPI.  For certain formats the length argument is not required and may be passed as 0.  After a successful operation the variable NSCANNED receives the number of bytes actually scanned unless NSCANNED was given as NULL. FORMAT describes the format of the MPI as stored in BUFFER:

     GCRYMPI_FMT_STD
          2-complement stored without a length header.

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

     GCRYMPI_FMT_SSH
          As used in the Secure Shell protocol.  This is GCRYMPI_FMT_STD with a 4 byte big endian header.

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

     GCRYMPI_FMT_USG
          Simple unsigned integer.

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

 -- Function: int gcry_mpi_print (enum gcry_mpi_format FORMAT, unsigned char *BUFFER, size_t BUFLEN, size_t *NWRITTEN, const gcry_mpi_t A)

     Convert the MPI A into an external representation described by FORMAT (see above) and store it in the provided BUFFER which has a usable length of at least the BUFLEN bytes. If NWRITTEN is not NULL, it will receive the number of bytes actually stored in BUFFER after a successful operation.

 -- Function: int gcry_mpi_aprint (enum gcry_mpi_format FORMAT, unsigned char **BUFFER, size_t *NBYTES, const gcry_mpi_t A)

     Convert the MPI A into an external representation described by FORMAT (see above) and store it in a newly allocated buffer which address will be stored in the variable BUFFER points to.  The number of bytes stored in this buffer will be stored in the variable NBYTES points to, unless NBYTES is NULL.

 -- Function: void gcry_mpi_dump (const gcry_mpi_t A)

     Dump the value of 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 NULL for A.

Calculations
============

Basic arithmetic operations:

 -- Function: void gcry_mpi_add (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V)

     W = U + V.

 -- Function: void gcry_mpi_add_ui (gcry_mpi_t W, gcry_mpi_t U, unsigned long V)

     W = U + V.  Note, that V is an unsigned integer.

 -- Function: void gcry_mpi_addm (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V, gcry_mpi_t M)

     W = U + V \bmod M.

 -- Function: void gcry_mpi_sub (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V)

     W = U - V.

 -- Function: void gcry_mpi_sub_ui (gcry_mpi_t W, gcry_mpi_t U, unsigned long V)

     W = U - V.  V is an unsigned integer.

 -- Function: void gcry_mpi_subm (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V, gcry_mpi_t M)

     W = U - V \bmod M.

 -- Function: void gcry_mpi_mul (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V)

     W = U * V.

 -- Function: void gcry_mpi_mul_ui (gcry_mpi_t W, gcry_mpi_t U, unsigned long V)

     W = U * V.  V is an unsigned integer.

 -- Function: void gcry_mpi_mulm (gcry_mpi_t W, gcry_mpi_t U, gcry_mpi_t V, gcry_mpi_t M)

     W = U * V \bmod M.

 -- Function: void gcry_mpi_mul_2exp (gcry_mpi_t W, gcry_mpi_t U, unsigned long E)

     W = U * 2^e.

 -- Function: void gcry_mpi_div (gcry_mpi_t Q, gcry_mpi_t R, gcry_mpi_t DIVIDEND, gcry_mpi_t DIVISOR, int ROUND)

     Q = DIVIDEND / DIVISOR, R = DIVIDEND \bmod DIVISOR.  Q and R may be passed as NULL.  ROUND should be negative or 0.

 -- Function: void gcry_mpi_mod (gcry_mpi_t R, gcry_mpi_t DIVIDEND, gcry_mpi_t DIVISOR)

     R = DIVIDEND \bmod DIVISOR.

 -- Function: void gcry_mpi_powm (gcry_mpi_t W, const gcry_mpi_t B, const gcry_mpi_t E, const gcry_mpi_t M)

     W = B^e \bmod M.

 -- Function: int gcry_mpi_gcd (gcry_mpi_t G, gcry_mpi_t A, gcry_mpi_t B)

     Set G to the greatest common divisor of A and B. Return true if the G is 1.

 -- Function: int gcry_mpi_invm (gcry_mpi_t X, gcry_mpi_t A, gcry_mpi_t M)

     Set X to the multiplicative inverse of A \bmod M. Return true if the inverse exists.

Comparisons
===========

The next 2 functions are used to compare MPIs:

 -- Function: int gcry_mpi_cmp (const gcry_mpi_t U, const gcry_mpi_t V)

     Compare the big integer number U and V returning 0 for equality, a positive value for U > V and a negative for U < V.

 -- Function: int gcry_mpi_cmp_ui (const gcry_mpi_t U, unsigned long V)

     Compare the big integer number U with the unsigned integer V returning 0 for equality, a positive value for U > V and a negative for U < V.

Bit manipulations
=================

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

 -- Function: unsigned int gcry_mpi_get_nbits (gcry_mpi_t A)

     Return the number of bits required to represent A.

 -- Function: int gcry_mpi_test_bit (gcry_mpi_t A, unsigned int N)

     Return true if bit number N (counting from 0) is set in A.

 -- Function: void gcry_mpi_set_bit (gcry_mpi_t A, unsigned int N)

     Set bit number N in A.

 -- Function: void gcry_mpi_clear_bit (gcry_mpi_t A, unsigned int N)

     Clear bit number N in A.

 -- Function: void gcry_mpi_set_highbit (gcry_mpi_t A, unsigned int N)

     Set bit number N in A and clear all bits greater than N.

 -- Function: void gcry_mpi_clear_highbit (gcry_mpi_t A, unsigned int N)

     Clear bit number N in A and all bits greater than N.

 -- Function: void gcry_mpi_rshift (gcry_mpi_t X, gcry_mpi_t A, unsigned int N)

     Shift the value of A by N bits to the right and store the result in X.

Miscellanous
============

 -- Function: gcry_mpi_t gcry_mpi_set_opaque (gcry_mpi_t A, void *P, unsigned int NBITS)

     Store NBITS of the value P points to in A and mark 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 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.

 -- Function: void * gcry_mpi_get_opaque (gcry_mpi_t A, unsigned int *NBITS)

     Return a pointer to an opaque value stored in A and return its size in NBITS.  Note, that the returned pointer is still owned by A and that the function should never be used for an non-opaque MPI.

 -- Function: void gcry_mpi_set_flag (gcry_mpi_t A, enum gcry_mpi_flag FLAG)

     Set the FLAG for the MPI A.  Currently only the flag GCRYMPI_FLAG_SECURE is allowed to convert A into an MPI stored in "secure memory".

 -- Function: void gcry_mpi_clear_flag (gcry_mpi_t A, enum gcry_mpi_flag FLAG)

     Clear FLAG for the big integer A.  Note, that this function is currently useless as no flags are allowed.

 -- Function: int gcry_mpi_get_flag (gcry_mpi_t A, enum gcry_mpi_flag FLAG)

     Return true when the FLAG is set for A.

 -- Function: void gcry_mpi_randomize (gcry_mpi_t W, unsigned int NBITS, enum gcry_random_level LEVEL)

     Set the big integer W to a random value of NBITS, using random data quality of level LEVEL.  In case NBITS is not a multiple of a byte, NBITS is rounded up to the next byte boundary.

Utilities
*********

Memory allocation
=================

 -- Function: void *gcry_malloc (size_t N)

     This function tries to allocate N bytes of memory.  On success it returns a pointer to the memory area, in an out-of-core condition, it returns NULL.

 -- Function: void *gcry_malloc_secure (size_t N)
     Like gcry_malloc, but uses secure memory.

 -- Function: void *gcry_calloc (size_t N)

     This function tries to allocate 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.

 -- Function: void *gcry_calloc_secure (size_t N)
     Like gcry_calloc, but uses secure memory.

 -- Function: void *gcry_realloc (void *P, size_t N)

     This function tries to resize the memory area pointed to by P to 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 P is secure memory or not, gcry_realloc tries to use secure memory as well.

 -- Function: void gcry_free (void *P)
     Release the memory area pointed to by P.

     [file lgpl.texi not included]

     [file gpl.texi not included]