MD5(3)

NAME
MD5Init, MD5Update, MD5Pad, MD5Final, MD5End, MD5File, MD5FileChunk, MD5Data - calculate the RSA Data Security, Inc., ``MD5'' message digest

LIBRARY
libmd

SYNOPSIS
#include <sys/types.h>
#include <md5.h>
void
MD5Init(MD5_CTX *context);
void
MD5Update(MD5_CTX *context, const void *data, unsigned int len);
void
MD5Pad(MD5_CTX *context);
void
MD5Final(unsigned char digest[16], MD5_CTX *context);
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 void *data, unsigned int 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 input corresponding to a particular output. This net result is a “fingerprint” of the input-data, which does not disclose the actual input.

MD4 is the fastest and MD5 is somewhat slower. MD4 has now been broken; it should only be used where necessary for backward compatibility. MD5 has not yet (1999-02-11) been broken, but sufficient attacks have been made that its security is in some doubt. 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 functions. 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 pad message data in same way as done by MD5Final() without terminating calculation.

The MD5End() function is a wrapper for MD5Final() which converts the return value to a 33-character (including the terminating '\0') ASCII string which represents the 128 bits in hexadecimal.

The MD5File() function calculates the digest of a file, and uses MD5End() to return the result. If the file cannot be opened, a null pointer is returned. The MD5FileChunk() function is similar to MD5File(), but it only calculates the digest over a byte-range of the file specified, starting at offset and spanning length bytes. If the length parameter is specified as 0, or more than the length of the remaining part of the file, MD5FileChunk() calculates the digest from offset to the end of file. The MD5Data() function calculates the digest of a chunk of data in memory, and uses MD5End() to return the result.

When using MD5End(), MD5File(), or MD5Data(), the buf argument can be a null pointer, in which case the returned string is allocated with malloc(3) and subsequently must be explicitly deallocated using free(3) after use. If the buf argument is non-null it must point to at least 33 characters of buffer space.

SEE ALSO
md4(3), md5(3), sha(3) R. Rivest, The MD4 Message-Digest Algorithm, RFC 1186. R. Rivest, The MD5 Message-Digest Algorithm, RFC 1321. H. Dobbertin, Alf Swindles Ann, CryptoBytes, 1(3):5, 1995. MJ. B. Robshaw, On Recent Results for MD2, MD4 and MD5, RSA Laboratories Bulletin, 4, November 12, 1996.

HISTORY
These functions appeared in FreeBSD 2.0.

AUTHORS
The original MD5 routines were developed by RSA Data Security, Inc., and published in the above references. This code is derived directly from these implementations by Poul-Henning Kamp <phk@FreeBSD.org>

Phk ristede runen.

BUGS
No method is known to exist which finds two files having the same hash value, nor to find a file with a specific hash value. There is on the other hand no guarantee that such a method does not exist. Copyright (C) 1991-2, RSA Data Security, Inc. Created 1991. All rights reserved.

License to copy and use this software is granted provided that it is identified as the "RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing this software or this function.

License is also granted to make and use derivative works provided that such works are identified as "derived from the RSA Data Security, Inc. MD5 Message-Digest Algorithm" in all material mentioning or referencing the derived work.

RSA Data Security, Inc. makes no representations concerning either the merchantability of this software or the suitability of this software for any particular purpose. It is provided "as is" without express or implied warranty of any kind.

These notices must be retained in any copies of any part of this documentation and/or software.