MCRYPT(3) Library Functions Manual MCRYPT(3)
NAME
libmcrypt - encryption/decryption library
SYNOPSIS
[see also mcrypt.h for more information]
DESCRIPTION
The libmcrypt is a data encryption library. The library is thread safe
and provides encryption and decryption functions. This version of the
library supports many encryption algorithms and encryption modes. Some
algorithms which are supported: SERPENT, RIJNDAEL, 3DES, GOST, SAFER+,
CAST-256, RC2, XTEA, 3WAY, TWOFISH, BLOWFISH, ARCFOUR, WAKE and more.
OFB, CBC, ECB, nOFB, nCFB and CFB are the modes that all algorithms may
function. ECB, CBC, encrypt in blocks but CTR, nCFB, nOFB, CFB and OFB
in bytes (streams). Note that CFB and OFB in the rest of the document
represent the "8bit CFB or OFB" mode. nOFB and nCFB modes represents a
n-bit OFB/CFB mode, n is used to represent the algorithm's block size.
The library supports an extra STREAM mode to include some stream algo-
rithms like WAKE or ARCFOUR.
In this version of the library all modes and algorithms are modular,
which means that the algorithm and the mode is loaded at run-time.
This way you can add algorithms and modes faster, and much easier.
LibMcrypt includes the following symmetric (block) algorithms:
DES: The traditional DES algorithm designed by IBM and US NSA. Uses 56
bit key and 64 bit block. It is now considered a weak algorithm, due to
its small key size (it was never intended for use with classified
data).
3DES or Triple DES: DES but with multiple (triple) encryption. It
encrypts the plaintext once, then decrypts it with the second key, and
encrypts it again with the third key (outer cbc mode used for cbc).
Much better than traditional DES since the key is now 168 bits (actu-
ally the effective key length is 112 bits due to the meet-in-the-middle
attack).
CAST-128: CAST was designed in Canada by Carlisle Adams and Stafford
Tavares. The original algorithm used a 64bit key and block. The algo-
rithm here is CAST-128 (also called CAST5) which has a 128bit key and
64bit block size.
CAST-256: CAST-256 was designed by Carlisle Adams. It is a symmetric
cipher designed in accordance with the CAST design procedure. It is an
extention of the CAST-128, having a 128 bit block size, and up to 256
bit key size.
xTEA: TEA stands for the Tiny Encryption Algorithm. It is a feistel
cipher designed by David Wheeler & Roger M. Needham. The original TEA
was intended for use in applications where code size is at a premium,
or where it is necessary for someone to remember the algorithm and code
it on an arbitrary machine at a later time. The algorithm used here is
extended TEA and has a 128bit key size and 64bit block size.
3-WAY: The 3way algorithm designed by Joan Daemen. It uses key and
block size of 96 bits.
SKIPJACK: SKIPJACK was designed by the US NSA. It was part of the ill-
fated "Clipper" Escrowed Encryption Standard (EES) (FIPS 185) proposal.
It operates on 64bit blocks and uses a key of 80 bits. SKIPJACK is pro-
vided only as an extra module to libmcrypt.
BLOWFISH: The Blowfish algorithm designed by Bruce Schneier. It is bet-
ter and faster than DES. It can use a key up to 448 bits.
TWOFISH: Twofish was designed by Bruce Schneier, Doug Whiting, John
Kelsey, Chris Hall, David Wagner for Counterpane systems. Intended to
be highly secure and highly flexible. It uses a 128bit block size and
128,192,256 bit key size. (Twofish is the default algorithm)
LOKI97: LOKI97 was designed by Lawrie Brown and Josef Pieprzyk. It has
a 128-bit block length and a 256bit key schedule, which can be initial-
ized using 128, 192 or 256 bit keys. It has evolved from the earlier
LOKI89 and LOKI91 64-bit block ciphers, with a strengthened key sched-
ule and a larger keyspace.
RC2: RC2 (RC stands for Rivest Cipher) was designed by Ron Rivest. It
uses block size of 64 bit and a key size from 8 to 1024 bits. It is
optimized for 16bit microprocessors (reflecting its age). It is
described in the RFC2268.
ARCFOUR: RC4 was designed by Ron Rivest. For several years this algo-
rithm was considered a trade secret and details were not available. In
September 1994 someone posted the source code in the cypherpunks mail-
ing list. Although the source code is now available RC4 is trademarked
by RSADSI so a compatible cipher named ARCFOUR is included in the
mcrypt distribution. It is a stream cipher and has a maximum key of
2048 bits.
RC6: RC6 was designed by Ron Rivest for RSA labs. In mcrypt it uses
block size of 128 bit and a key size of 128/192/256 bits. Refer to RSA
Labs and Ron Rivest for any copyright, patent or license issues for the
RC6 algorithm. RC6 is provided only as an extra module to libmcrypt.
RIJNDAEL: Rijndael is a block cipher, designed by Joan Daemen and Vin-
cent Rijmen, and was approved for the USA's NIST Advanced Encryption
Standard, FIPS-197. The cipher has a variable block length and key
length. Rijndael can be implemented very efficiently on a wide range of
processors and in hardware. The design of Rijndael was strongly influ-
enced by the design of the block cipher Square. There exist three ver-
sions of this algorithm, namely: RIJNDAEL-128 (the AES winner) , RIJN-
DAEL-192 , RIJNDAEL-256 The numerals 128, 192 and 256 stand for the
length of the block size.
MARS: MARS is a 128-bit block cipher designed by IBM as a candidate for
the Advanced Encryption Standard. Refer to IBM for any copyright,
patent or license issues for the MARS algorithm. MARS is provided only
as an extra module to libmcrypt.
PANAMA: PANAMA is a cryptographic module that can be used both as a
cryptographic hash function and as a stream cipher. It designed by Joan
Daemen and Craig Clapp. PANAMA (the stream cipher) is included in libm-
crypt.
WAKE: WAKE stands for Word Auto Key Encryption, and is an encryption
system for medium speed encryption of blocks and of high security.
WAKE was designed by David J. Wheeler. It is intended to be fast on
most computers and relies on repeated table use and having a large
state space.
SERPENT: Serpent is a 128-bit block cipher designed by Ross Anderson,
Eli Biham and Lars Knudsen as a candidate for the Advanced Encryption
Standard. Serpent's design was limited to well understood mechanisms,
so that could rely on the wide experience of block cipher cryptanaly-
sis, and achieve the highest practical level of assurance that no
shortcut attack will be found. Serpent has twice as many rounds as are
necessary, to block all currently known shortcut attacks. Despite these
exacting design constraints, Serpent is faster than DES.
IDEA: IDEA stands for International Data Encryption Algorithm and was
designed by Xuejia Lai and James Massey. It operates on 64bit blocks
and uses a key of 128 bits. Refer to Ascom-Tech AG for any copyright,
patent or license issues for the IDEA algorithm. IDEA is provided only
as an extra module to libmcrypt.
ENIGMA (UNIX crypt): A one-rotor machine designed along the lines of
Enigma but considerable trivialized. Very easy to break for a skilled
cryptanalyst. I suggest against using it. Added just for completeness.
GOST: A former soviet union's algorithm. An acronym for "Gosudarstven-
nyi Standard" or Government Standard. It uses a 256 bit key and a 64
bit block.
The S-boxes used here are described in the Applied Cryptography book
by Bruce Schneier. They were used in an application for the Central
Bank of the Russian Federation.
Some quotes from gost.c: The standard is written by A. Zabotin
(project leader), G.P. Glazkov, and V.B. Isaeva. It was accepted and
introduced into use by the action of the State Standards Committee of
the USSR on 2 June 1989 as No. 1409. It was to be reviewed in 1993,
but whether anyone wishes to take on this obligation from the USSR is
questionable.
This code is based on the 25 November 1993 draft translation by Alek-
sandr Malchik, with Whitfield Diffie, of the Government Standard of the
U.S.S.R. GOST 28149-89, "Cryptographic Transformation Algorithm",
effective 1 July 1990. (Whitfield.Diffie AT eng.com) Some details
have been cleared up by the paper "Soviet Encryption Algorithm" by
Josef Pieprzyk and Leonid Tombak of the University of Wollongong, New
South Wales. (josef/leo AT cs.au)
SAFER: SAFER (Secure And Fast Encryption Routine) is a block cipher
developed by Prof. J.L. Massey at the Swiss Federal Institute of Tech-
nology. There exist four versions of this algorithm, namely: SAFER
K-64 , SAFER K-128 , SAFER SK-64 and SAFER SK-128. The numerals 64 and
128 stand for the length of the user-selected key, 'K' stands for the
original key schedule and 'SK' stands for the strengthened key schedule
(in which some of the "weaknesses" of the original key schedule have
been removed). In mcrypt only SAFER SK-64 and SAFER SK-128 are used.
SAFER+: SAFER+ was designed by Prof. J.L. Massey, Prof. Gurgen H.
Khachatrian and Dr. Melsik K. Kuregian for Cylink. SAFER+ is based on
the existing SAFER family of ciphers and provides for a block size of
128bits and 128, 192 and 256 bits key length.
A short description of the modes supported by libmcrypt:
STREAM: The mode used with stream ciphers. In this mode the keystream
from the cipher is XORed with the plaintext. Thus you should NOT ever
use the same key.
ECB: The Electronic CodeBook mode. It is the simplest mode to use with
a block cipher. Encrypts each block independently. It is a block mode
so plaintext length should be a multiple of blocksize (n*blocksize).
CBC: The Cipher Block Chaining mode. It is better than ECB since the
plaintext is XOR'ed with the previous ciphertext. A random block should
be placed as the first block (IV) so the same block or messages always
encrypt to something different. It is a block mode so plaintext length
should be a multiple of blocksize (n*blocksize).
CFB: The Cipher-Feedback Mode (in 8bit). This is a self-synchronizing
stream cipher implemented from a block cipher. This is the best mode to
use for encrypting strings or streams. This mode requires an IV.
OFB: The Output-Feedback Mode (in 8bit). This is a synchronous stream
cipher implemented from a block cipher. It is intended for use in noisy
lines, because corrupted ciphertext blocks do not corrupt the plaintext
blocks that follow. Insecure (because used in 8bit mode) so it is rec-
ommended not to use it. Added just for completeness.
nOFB: The Output-Feedback Mode (in nbit). n Is the size of the block of
the algorithm. This is a synchronous stream cipher implemented from a
block cipher. It is intended for use in noisy lines, because corrupted
ciphertext blocks do not corrupt the plaintext blocks that follow. This
mode operates in streams.
nCFB: The Cipher-Feedback Mode (in nbit). n Is the size of the block of
the algorithm. This is a self synchronizing stream cipher implemented
from a block cipher. This mode operates in streams.
CTR: The Counter Mode. This is a stream cipher implemented from a block
cipher. This mode uses the cipher to encrypt a set of input blocks,
called counters, to produce blocks that will be XORed with the plain-
text. In libmcrypt the counter is the given IV which is incremented at
each step. This mode operates in streams.
Error Recovery in these modes: If bytes are removed or lost from the
file or stream in ECB, CTR, CBC and OFB modes, are impossible to
recover, although CFB and nCFB modes will recover. If some bytes are
altered then a full block of plaintext is affected in ECB, nOFB and CTR
modes, two blocks in CBC, nCFB and CFB modes, but only the correspond-
ing byte in OFB mode.
Encryption can be done as follows:
A call to function: MCRYPT mcrypt_module_open( char *algorithm, char*
algorithm_directory, char* mode, char* mode_directory);
This function associates the algorithm and the mode specified. The
name of the algorithm is specified in algorithm, eg "twofish", and the
algorithm_directory is the directory where the algorithm is (it may be
null if it is the default). The same applies for the mode. The library
is closed by calling mcrypt_module_close(), but you should not call
that function if mcrypt_generic_end() is called before. Normally it
returns an encryption descriptor, or MCRYPT_FAILED on error.
A call to function: int mcrypt_generic_init( MCRYPT td, void *key, int
lenofkey, void *IV);
This function initializes all buffers for the specified thread The max-
imum value of lenofkey should be the one obtained by calling
mcrypt_get_key_size() and every value smaller than this is legal. Note
that Lenofkey should be specified in bytes not bits. The IV should
normally have the size of the algorithms block size, but you must
obtain the size by calling mcrypt_get_iv_size(). IV is ignored in ECB.
IV MUST exist in CFB, CBC, STREAM, nOFB and OFB modes. It needs to be
random and unique (but not secret). The same IV must be used for
encryption/decryption. After calling this function you can use the
descriptor for encryption or decryption (not both). Returns a negative
value on error.
To encrypt now call:
int mcrypt_generic( MCRYPT td, void *plaintext, int len);
This is the main encryption function. td is the encryption descriptor
returned by mcrypt_generic_init(). Plaintext is the plaintext you wish
to encrypt and len should be the length (in bytes) of the plaintext and
it should be k*algorithms_block_size if used in a mode which operated
in blocks (cbc, ecb, nofb), or whatever when used in cfb or ofb which
operate in streams. The plaintext is replaced by the ciphertext.
Returns 0 on success.
To decrypt you can call:
int mdecrypt_generic( MCRYPT td, void *ciphertext, int len);
The decryption function. It is almost the same with mcrypt_generic.
Returns 0 on success.
When you're finished you should call:
int mcrypt_generic_end( MCRYPT td);
This function terminates encryption specified by the encryption
descriptor (td). Actually it clears all buffers, and closes all the
modules used. Returns a negative value on error. This function is
deprecated. Use mcrypt_generic_deinit() and mcrypt_module_close()
instead.
int mcrypt_generic_deinit( MCRYPT td);
This function terminates encryption specified by the encryption
descriptor (td). Actually it clears all buffers. The difference with
mcrypt_generic_end() is that this function does not close the modules
used. Thus you should use mcrypt_module_close(). Using this function
you gain in speed if you use the same modules for several encryptions.
Returns a negative value on error.
int mcrypt_module_close( MCRYPT td);
This function closes the modules used by the descriptor td.
These are some extra functions that operate on modules that have been
opened: These functions have the prefix mcrypt_enc_*.
int mcrypt_enc_set_state(MCRYPT td, void *state, int size); This func-
tion sets the state of the algorithm. Can be used only with block algo-
rithms and certain modes like CBC, CFB etc. It is usefully if you want
to restart or start a different encryption quickly. Returns zero on
success. The state is the output of mcrypt_enc_get_state().
int mcrypt_enc_get_state(MCRYPT td, void *state, int *size); This func-
tion returns the state of the algorithm. Can be used only certain modes
and algorithms. The size will hold the size of the state and the state
must have enough bytes to hold it. Returns zero on success.
int mcrypt_enc_self_test( MCRYPT td);
This function runs the self test on the algorithm specified by the
descriptor td. If the self test succeeds it returns zero.
int mcrypt_enc_is_block_algorithm_mode( MCRYPT td);
Returns 1 if the mode is for use with block algorithms, otherwise it
returns 0. (eg. 0 for stream, and 1 for cbc, cfb, ofb)
int mcrypt_enc_is_block_algorithm( MCRYPT td);
Returns 1 if the algorithm is a block algorithm or 0 if it is a stream
algorithm.
int mcrypt_enc_is_block_mode( MCRYPT td);
Returns 1 if the mode outputs blocks of bytes or 0 if it outputs bytes.
(eg. 1 for cbc and ecb, and 0 for cfb and stream)
int mcrypt_enc_get_block_size( MCRYPT td);
Returns the block size of the algorithm specified by the encryption
descriptor in bytes. The algorithm MUST be opened using mcrypt_mod-
ule_open().
int mcrypt_enc_get_key_size( MCRYPT td);
Returns the maximum supported key size of the algorithm specified by
the encryption descriptor in bytes. The algorithm MUST be opened using
mcrypt_module_open().
int* mcrypt_enc_get_supported_key_sizes( MCRYPT td, int* sizes)
Returns the key sizes supported by the algorithm specified by the
encryption descriptor. If sizes is zero and returns NULL then all key
sizes between 1 and mcrypt_get_key_size() are supported by the algo-
rithm. If it is 1 then only the mcrypt_get_key_size() size is supported
and sizes[0] is equal to it. If it is greater than 1 then that number
specifies the number of elements in sizes which are the key sizes that
the algorithm supports. The returned value is allocated with malloc, so
you should not forget to free it.
int mcrypt_enc_get_iv_size( MCRYPT td);
Returns size of the IV of the algorithm specified by the encryption
descriptor in bytes. The algorithm MUST be opened using mcrypt_mod-
ule_open(). If it is '0' then the IV is ignored in that algorithm. IV
is used in CBC, CFB, OFB modes, and in some algorithms in STREAM mode.
int mcrypt_enc_mode_has_iv( MCRYPT td);
Returns 1 if the mode needs an IV, 0 otherwise. Some 'stream' algo-
rithms may need an IV even if the mode itself does not need an IV.
char* mcrypt_enc_get_algorithms_name( MCRYPT td);
Returns a character array containing the name of the algorithm. The
returned value is allocated with malloc, so you should not forget to
free it.
char* mcrypt_enc_get_modes_name( MCRYPT td);
Returns a character array containing the name of the mode. The
returned value is allocated with malloc, so you should not forget to
free it.
These are some extra functions that operate on modules: These functions
have the prefix mcrypt_module_*.
int mcrypt_module_self_test (char* algorithm, char* directory);
This function runs the self test on the specified algorithm. If the
self test succeeds it returns zero.
int mcrypt_module_is_block_algorithm_mode( char* algorithm, char*
directory);
Returns 1 if the mode is for use with block algorithms, otherwise it
returns 0. (eg. 0 for stream, and 1 for cbc, cfb, ofb)
int mcrypt_module_is_block_algorithm( char* mode, char* directory);
Returns 1 if the algorithm is a block algorithm or 0 if it is a stream
algorithm.
int mcrypt_module_is_block_mode( char* mode, char* directory);
Returns 1 if the mode outputs blocks of bytes or 0 if it outputs bytes.
(eg. 1 for cbc and ecb, and 0 for cfb and stream)
int mcrypt_module_get_algo_block_size( char* algorithm, char* direc-
tory);
Returns the block size of the algorithm.
int mcrypt_module_get_algo_key_size( char* algorithm, char* directory);
Returns the maximum supported key size of the algorithm.
int* mcrypt_module_get_algo_supported_key_sizes( char* algorithm, char*
directory, int* sizes);
Returns the key sizes supported by the algorithm. If sizes is zero and
returns NULL then all key sizes between 1 and mcrypt_get_key_size() are
supported by the algorithm. If it is 1 then only the
mcrypt_get_key_size() size is supported and sizes[0] is equal to it. If
it is greater than 1 then that number specifies the number of elements
in sizes which are the key sizes that the algorithm supports. This
function differs to mcrypt_enc_get_supported_key_sizes(), because the
return value here is allocated (not static), thus it should be freed.
char** mcrypt_list_algorithms ( char* libdir, int* size);
Returns a pointer to a character array containing all the mcrypt algo-
rithms located in the libdir, or if it is NULL, in the default direc-
tory. The size is the number of the character arrays. The arrays are
allocated internally and should be freed by using mcrypt_free_p().
char** mcrypt_list_modes ( char* libdir, int *size);
Returns a pointer to a character array containing all the mcrypt modes
located in the libdir, or if it is NULL, in the default directory. The
size is the number of the character arrays. The arrays should be freed
by using mcrypt_free_p().
void mcrypt_free_p (char **p, int size);
Frees the pointer to array returned by previous functions.
void mcrypt_free (void *ptr);
Frees the memory used by the pointer.
void mcrypt_perror(int err);
This function prints a human readable description of the error 'err' in
the stderr. The err should be a value returned by
mcrypt_generic_init().
const char* mcrypt_strerror(int err);
This function returns a human readable description of the error 'err'.
The err should be a value returned by mcrypt_generic_init().
int mcrypt_mutex_register ( void (*mutex_lock)(void) , void
(*mutex_unlock)(void) );
This function is only used in multithreaded application and only if
compiled with dynamic module loading support. This is actually used
internally in libltdl. Except for the dynamic module loading libmcrypt
is thread safe.
Some example programs follow here. Compile as "cc prog.c -lmcrypt", or
"cc prog.c -lmcrypt -lltdl" depending on your installation. Libltdl is
used for opening dynamic libraries (modules).
/* First example: Encrypts stdin to stdout using TWOFISH with 128 bit key and CFB */
#include <mcrypt.h>
#include <stdio.h>
#include <stdlib.h>
/* #include <mhash.h> */
main() {
MCRYPT td;
int i;
char *key;
char password[20];
char block_buffer;
char *IV;
int keysize=16; /* 128 bits */
key=calloc(1, keysize);
strcpy(password, "A_large_key");
/* Generate the key using the password */
/* mhash_keygen( KEYGEN_MCRYPT, MHASH_MD5, key, keysize, NULL, 0, password, strlen(password));
*/
memmove( key, password, strlen(password));
td = mcrypt_module_open("twofish", NULL, "cfb", NULL);
if (td==MCRYPT_FAILED) {
return 1;
}
IV = malloc(mcrypt_enc_get_iv_size(td));
/* Put random data in IV. Note these are not real random data,
* consider using /dev/random or /dev/urandom.
*/
/* srand(time(0)); */
for (i=0; i< mcrypt_enc_get_iv_size( td); i++) {
IV[i]=rand();
}
i=mcrypt_generic_init( td, key, keysize, IV);
if (i<0) {
mcrypt_perror(i);
return 1;
}
/* Encryption in CFB is performed in bytes */
while ( fread (&block_buffer, 1, 1, stdin) == 1 ) {
mcrypt_generic (td, &block_buffer, 1);
/* Comment above and uncomment this to decrypt */
/* mdecrypt_generic (td, &block_buffer, 1); */
fwrite ( &block_buffer, 1, 1, stdout);
}
/* Deinit the encryption thread, and unload the module */
mcrypt_generic_end(td);
return 0;
}
/* Second Example: encrypts using CBC and SAFER+ with 192 bits key */
#include <mcrypt.h>
#include <stdio.h>
#include <stdlib.h>
main() {
MCRYPT td;
int i;
char *key; /* created using mcrypt_gen_key */
char *block_buffer;
char *IV;
int blocksize;
int keysize = 24; /* 192 bits == 24 bytes */
key = calloc(1, keysize);
strcpy(key, "A_large_and_random_key");
td = mcrypt_module_open("saferplus", NULL, "cbc", NULL);
blocksize = mcrypt_enc_get_block_size(td);
block_buffer = malloc(blocksize);
/* but unfortunately this does not fill all the key so the rest bytes are
* padded with zeros. Try to use large keys or convert them with mcrypt_gen_key().
*/
IV=malloc(mcrypt_enc_get_iv_size(td));
/* Put random data in IV. Note these are not real random data,
* consider using /dev/random or /dev/urandom.
*/
/* srand(time(0)); */
for (i=0; i < mcrypt_enc_get_iv_size(td); i++) {
IV[i]=rand();
}
mcrypt_generic_init( td, key, keysize, IV);
/* Encryption in CBC is performed in blocks */
while ( fread (block_buffer, 1, blocksize, stdin) == blocksize ) {
mcrypt_generic (td, block_buffer, blocksize);
/* mdecrypt_generic (td, block_buffer, blocksize); */
fwrite ( block_buffer, 1, blocksize, stdout);
}
/* deinitialize the encryption thread */
mcrypt_generic_deinit (td);
/* Unload the loaded module */
mcrypt_module_close(td);
return 0;
}
The library does not install any signal handler.
Questions about libmcrypt should be sent to:
mcrypt-dev AT lists.gr or, if this fails, to the author
addresses given below. The mcrypt home page is:
http://mcrypt.hellug.gr
AUTHORS
Version 2.4 Copyright (C) 1998-1999 Nikos Mavroyanopoulos (nmav@hel-
lug.gr).
Thanks to all the people who reported problems and suggested various
improvements for mcrypt; who are too numerous to cite here.
10 March 2002 MCRYPT(3)
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