MD5(3) BSD Library Functions Manual MD5(3)
NAME
MD5Init, MD5Update, MD5Pad, MD5Final, MD5Transform, MD5End, MD5File,
MD5FileChunk, MD5Data — calculate the RSA Data Security, Inc., “MD5” mes-
sage digest
LIBRARY
Message Digest (MD4, MD5, etc.) Support Library (libmd, -lmd)
SYNOPSIS
#include <sys/types.h>
#include <md5.h>
void
MD5Init(MD5_CTX *context);
void
MD5Update(MD5_CTX *context, const uint8_t *data, size_t len);
void
MD5Pad(MD5_CTX *context);
void
MD5Final(uint8_t digest[MD5_DIGEST_LENGTH], MD5_CTX *context);
void
MD5Transform(uint32_t state[4], uint8_t block[MD5_BLOCK_LENGTH]);
char *
MD5End(MD5_CTX *context, char *buf);
char *
MD5File(const char *filename, char *buf);
char *
MD5FileChunk(const char *filename, char *buf, off_t offset,
off_t length);
char *
MD5Data(const uint8_t *data, size_t len, char *buf);
DESCRIPTION
The MD5 functions calculate a 128-bit cryptographic checksum (digest) for
any number of input bytes. A cryptographic checksum is a one-way hash-
function, that is, you cannot find (except by exhaustive search) the in-
put corresponding to a particular output. This net result is a
“fingerprint” of the input-data, which doesn't disclose the actual input.
MD2 is the slowest, MD4 is the fastest and MD5 is somewhere in the mid-
dle. MD2 can only be used for Privacy-Enhanced Mail. MD4 has been crit-
icized for being too weak, so MD5 was developed in response as ``MD4 with
safety-belts''. MD4 and MD5 have been broken; they should only be used
where necessary for backward compatibility. The attacks on both MD4 and
MD5 are both in the nature of finding “collisions” - that is, multiple
inputs which hash to the same value; it is still unlikely for an attacker
to be able to determine the exact original input given a hash value.
The MD5Init(), MD5Update(), and MD5Final() functions are the core func-
tions. Allocate an MD5_CTX, initialize it with MD5Init(), run over the
data with MD5Update(), and finally extract the result using MD5Final().
The MD5Pad() function can be used to apply padding to the message digest
as in MD5Final(), but the current context can still be used with
MD5Update().
The MD5Transform() function is used by MD5Update() to hash 512-bit blocks
and forms the core of the algorithm. Most programs should use the inter-
face provided by MD5Init(), MD5Update() and MD5Final() instead of calling
MD5Transform() directly.
MD5End() is a wrapper for MD5Final() which converts the return value to
an MD5_DIGEST_STRING_LENGTH-character (including the terminating '\0')
ASCII string which represents the 128 bits in hexadecimal.
MD5File() calculates the digest of a file, and uses MD5End() to return
the result. If the file cannot be opened, a null pointer is returned.
MD5FileChunk() behaves like MD5File() but calculates the digest only for
that portion of the file starting at offset and continuing for length
bytes or until end of file is reached, whichever comes first. A zero
length can be specified to read until end of file. A negative length or
offset will be ignored. MD5Data() calculates the digest of a chunk of
data in memory, and uses MD5End() to return the result.
When using MD5End(), MD5File(), MD5FileChunk(), or MD5Data(), the buf ar-
gument can be a null pointer, in which case the returned string is allo-
cated with malloc(3) and subsequently must be explicitly deallocated us-
ing free(3) after use. If the buf argument is non-null it must point to
at least MD5_DIGEST_STRING_LENGTH characters of buffer space.
SEE ALSO
md2(3), md4(3), md5(3), rmd160(3), sha1(3), sha2(3)
B. Kaliski, The MD2 Message-Digest Algorithm, RFC 1319.
R. Rivest, The MD4 Message-Digest Algorithm, RFC 1186.
R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321.
RSA Laboratories, Frequently Asked Questions About today's Cryptography,
<http://www.rsa.com/rsalabs/faq/>.
H. Dobbertin, “Alf Swindles Ann”, CryptoBytes, 1(3):5, 1995.
MJ. B. Robshaw, “On Recent Results for MD4 and MD5”, RSA Laboratories
Bulletin, 4, November 12, 1996.
Hans Dobbertin, Cryptanalysis of MD5 Compress.
HISTORY
These functions appeared in OpenBSD 2.0 and NetBSD 1.3.
AUTHORS
The original MD5 routines were developed by RSA Data Security, Inc., and
published in the above references. This code is derived from a public
domain implementation written by Colin Plumb.
The MD5End(), MD5File(), MD5FileChunk(), and MD5Data() helper functions
are derived from code written by Poul-Henning Kamp.
BUGS
Collisions have been found for the full versions of both MD4 and MD5.
The use of sha2(3) is recommended instead.
BSD July 13, 2010 BSD
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