MDX(3)

NAME
MDXInit, MDXUpdate, MDXPad, MDXFinal, MDXEnd, MDXFile, MDXFileChunk, MDXData - calculate the RSA Data Security, Inc., ``MDX'' message digest

LIBRARY
libmd

SYNOPSIS
#include <sys/types.h>
#include <mdX.h>
void
MDXInit(MDX_CTX *context);
void
MDXUpdate(MDX_CTX *context, const void *data, unsigned int len);
void
MDXPad(MDX_CTX *context);
void
MDXFinal(unsigned char digest[16], MDX_CTX *context);
char *
MDXEnd(MDX_CTX *context, char *buf);
char *
MDXFile(const char *filename, char *buf);
char *
MDXFileChunk(const char *filename, char *buf, off_t offset, off_t length);
char *
MDXData(const void *data, unsigned int len, char *buf);

DESCRIPTION
The MDX 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.

MD2 is the slowest, MD4 is the fastest and MD5 is somewhere in the middle. MD2 can only be used for Privacy-Enhanced Mail. 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 MDXInit(), MDXUpdate(), and MDXFinal() functions are the core functions. Allocate an MDX_CTX, initialize it with MDXInit(), run over the data with MDXUpdate(), and finally extract the result using MDXFinal().

The MDXPad() function can be used to pad message data in same way as done by MDXFinal() without terminating calculation.

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

The MDXFile() function calculates the digest of a file, and uses MDXEnd() to return the result. If the file cannot be opened, a null pointer is returned. The MDXFileChunk() function is similar to MDXFile(), 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, MDXFileChunk() calculates the digest from offset to the end of file. The MDXData() function calculates the digest of a chunk of data in memory, and uses MDXEnd() to return the result.

When using MDXEnd(), MDXFile(), or MDXData(), 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
md2(3), md4(3), md5(3), sha(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. 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 MDX 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.

MD2 has only been licensed for use in Privacy Enhanced Mail. Use MD4 or MD5 if that is not what you are doing.