This is gcrypt.info, produced by makeinfo version 4.2 from
../../doc/gcrypt.texi.

INFO-DIR-SECTION GNU Libraries
START-INFO-DIR-ENTRY
* libgcrypt: (gcrypt) Cryptographic function library.
END-INFO-DIR-ENTRY

   This file documents the `Libgcrypt' library.

   This is Edition 1.1.12, last updated 19 January 2003, of `The
`Libgcrypt' Reference Manual', for Version 1.1.12.

   Copyright (C) 2000, 2002, 2003 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.1 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no the Front-Cover texts, and with no
Back-Cover Texts.  A copy of the license is included in the section
entitled "GNU Free Documentation License".


File: gcrypt.info,  Node: Top,  Next: Introduction,  Up: (dir)

Main Menu
*********

   This is Edition 1.1.12, last updated 19 January 2003, of `The
`Libgcrypt' Reference Manual', for Version 1.1.12 of the GPGME library.

* Menu:

* Introduction::                How to use this manual.
* Preparation::                 What you should do before using the library.
* Cipher Functions::            All about ciphers.
* Hash Functions::              How to use cryptographic hash functions.
* Public Key Functions::        How to use asymmetric encryption.
* Random Numbers::              How to create random.
* S-expressions::               How to manage S-expressions.
* MPI Functions::               How to work with big integers.
* Utilities::                   Helper functions.
* Error Handling::              Error codes and such.

Appendices

* Library Copying::             The GNU Lesser General Public License
                                says how you can copy and share `Libgcrypt'.
* Copying::                     The GNU General Public License says how you
                                can copy and share some parts of `Libgcrypt'.
* Free Documentation License::  This manual is under the GNU Free
                                Documentation License.

Indices

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


File: gcrypt.info,  Node: Introduction,  Next: Preparation,  Prev: Top,  Up: Top

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

   `Libgcrypt' is a library to provide cryptographic building blocks.

* Menu:

* Getting Started::
* Features::
* Overview::


File: gcrypt.info,  Node: Getting Started,  Next: Features,  Up: Introduction

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

   This manual documents the `Libgcrypt' library programming interface.
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.


File: gcrypt.info,  Node: Features,  Next: Overview,  Prev: Getting Started,  Up: Introduction

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 (*note 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
     (*note Copying::); please see the README file of the distribution
     for of list of these parts.

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


File: gcrypt.info,  Node: Overview,  Prev: Features,  Up: Introduction

Overview
========

The `Libgcrypt' library is thread-safe.  Well, we hope so ;-).  Frankly,
`gcry_errno' is not yet thread-safe.  Most others are believed to be.
Libgcrypt automagically detects whether an applications uses no
threading, pthreads or GNU Pth.


File: gcrypt.info,  Node: Preparation,  Next: Cipher Functions,  Prev: Introduction,  Up: Top

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.

* Menu:

* Header::
* Version Check::
* Building the source::


File: gcrypt.info,  Node: Header,  Next: Version Check,  Up: Preparation

Header
======

   All interfaces (data types and functions) of the library are defined
in the header file `gcrypt.h'.  You must include this in all programs
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 names,
`Gcry*' for data types 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.


File: gcrypt.info,  Node: Version Check,  Next: Building the source,  Prev: Header,  Up: Preparation

Version Check
=============

   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)
     Check that the the version of the library is at minimum the one
     given as a string in REQ_VERSION and return the actual version
     string of the library; return NULL if the condition is not met.
     If `NULL' is passed to this function no check is done and only the
     version string is returned.  It is a pretty good idea to run this
     function as soon as possible, because it may also initializes some
     subsystems.  In a multi-threaded environment if should be called
     before any more threads are created.


File: gcrypt.info,  Node: Building the source,  Prev: Version Check,  Up: Preparation

Building the source
===================

   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`


File: gcrypt.info,  Node: Cipher Functions,  Next: Hash Functions,  Prev: Preparation,  Up: Top

Cipher Functions
****************

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

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

 - Function: GcryCipherHd gcry_cipher_open (int ALGO, int MODE,
          unsigned int FLAGS)
     This function creates the context required for most of the other
     cipher functions and returns a handle to it.  In case of an error
     `NULL' is returned.  You must tell this function which algorithm
     and what mode you want to use.  The function
     `gcry_cipher_map_name' may be used to get the a value for the ALGO
     from a textual name or one of the predefined constants can be used:

    `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_RIJNDAEL128'
          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_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 is can be broken in reasonable time
          using a brute force approach.

     The second argument is MODE which describes the mode the algorithm
     is to be used in.  Note, that some modes don't work together with
     all algorithms. The following modes are available:

    `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.

     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 for the CBS mode.


   To release the context allocated with this function,
`gcry_cipher_close' should be used:

 - Function: void gcry_cipher_close (GcryCipherHd H)
     This function releases the context created by `gcry_cipher_open'.

   Now that a context has been allocated, the key to be used for
decryption or encryption must be set.  This is done with the following
function:

 - Function: int gcry_cipher_setkey (GcryCipherHd 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.
To set this IV, use the function:

 - Function: int gcry_cipher_setiv (GCRY_CIPHER_HD 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.

   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: int gcry_cipher_encrypt (GCRY_CIPHER_HD H, unsigned char
          *out, size_t OUTSIZE, const unsigned char *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: int gcry_cipher_decrypt (GCRY_CIPHER_HD H, unsigned char
          *out, size_t OUTSIZE, const unsigned char *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: int gcry_cipher_sync (GCRY_CIPHER_HD 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: int gcry_cipher_ctl (GCRY_CIPHER_HD 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.

   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: int gcry_cipher_info (GCRY_CIPHER_HD 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.

 - Function: int 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.

     The function returns `-1' on error;  `gcry_errno' may be used to
     get the actual error code.

     Here is a list of supported codes for WHAT:

    `GCRYCTL_GET_KEYLEN:'
          Return the length of the key. If the algorithm supports
          multiple key length, the maximum supported value is returned.
          The length is returned as number of octets (bytes) and not
          as number of bits.  BUFFER and NBYTES must be zero.

    `GCRYCTL_GET_BLKLEN:'
          Return the block length of the algorithm counted in octets.
          BUFFER and NBYTES 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.


File: gcrypt.info,  Node: Hash Functions,  Next: Public Key Functions,  Prev: Cipher Functions,  Up: Top

Hash Functions
**************

   How to use cryptographic hash functions.  Libgcrypt provides an easy
and consistent to use interface to hash functions.  Hashing is buffered
and several hash algorithms can be updated at once.  It is possible to
calculate a MAC using the same routines.

   For convenience reasons, we plan to integrate simple CRC algorithms
too

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

 - Function: GcryMDHd gcry_md_open (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'.

     The following algorithms are supported:

    `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 reserved for SHA-2 with 256 bits. It yields a message
          digest of 32 bytes.  Note that there is no implementation yet
          available.

    `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.

     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.  Note that the function `gcry_md_setkey' must be
          used set the MAC key.


   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: int gcry_md_enable (GcryMDHd 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: int gcry_md_setkey (GcryMDHd 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 (GcryMDHd 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 (GcryMDHd 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: GcryMDHd gcry_md_copy (GcryMDHd H)
     Create a new digest object as an exact copy of the object
     described by handle H.  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 (GcryMDHd 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 (GcryMDHd 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 (GcryMDHd 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 (GcryMDHd 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: int 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 return value may be `GCRYERR_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: int 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 (GcryMDHd 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 (GcryMDHd H)
     This macro return true when the digest object H is allocated in
     "secure memory"; i.e. H was created with the `GCRY_MD_FLAG_SECURE'.

   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 (GcryMDHd 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 (GcryMDHd 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.


File: gcrypt.info,  Node: Public Key Functions,  Next: Random Numbers,  Prev: Hash Functions,  Up: Top

Public Key Functions
********************

   Public key encryption, also known as asymmetric encryption, is am
easy way for key management and to provide digital signatures.
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.

   The API is based on data structures called S-expressions (see XXXX)
and does not work with contexts as most of the other building blocks
Libgcrypt provides.

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 an 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.

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.

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: int gcry_pk_encrypt (GcrySexp *R_CIPH, GcrySexp DATA,
          GcrySexp 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
     padding rules.

     If you don't want to let Libgcrypt handle the padding, you must
     pass an appropriate MPI using the 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: int gcry_pk_decrypt (GcrySexp *R_PLAIN, GcrySexp DATA,
          GcrySexp 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'.  Note that this function does not
     know of any padding 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
     currently a simple S-expression with just one MPI.

   Another operation commonly performed using public keys are digital
signature.  In some sense they are even more important than the
encryption because digital signature are an important instrument for key
management.  Libgcrypt support digital signatures using 2 functions,
similar to the encryption functions:

 - Function: int gcry_pk_sign (GcrySexp *R_SIG, GcrySexp DATA,
          GcrySexp 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: int gcry_pk_verify (GcrySexp SIG, GcrySexp DATA,
          GcrySexp 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.


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.

 - Function: unsigned int gcry_pk_get_nbits (GcrySexp 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 (GcrySexp 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: int gcry_pk_testkey (GcrySexp KEY)
     Return 0 if KEY (either private or public) is sane.

 - 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: int 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 to generate public key pairs:

 - Function: int gcry_pk_genkey (GcrySexp *R_KEY, GcrySexp 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 1024)))

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

     The key pair is returned in a format depending on the algorithm.
     Both, private and secret, keys are returned and my be accompanied
     by some miscellaneous information.  The format resembles closely
     the one of the public respectively the private key.  Frankly, they
     are put into one container, so that they can easily be extracted.

     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.


File: gcrypt.info,  Node: Random Numbers,  Next: S-expressions,  Prev: Public Key Functions,  Up: Top

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.