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.
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" [en] 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
.Vt 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.
md2(3), md4(3), md5(3), sha(3)
.Rs The MD2 Message-Digest Algorithm
.Rs The MD4 Message-Digest Algorithm
.Rs The MD5 Message-Digest Algorithm
.Rs Frequently Asked Questions About todays Cryptography
.Rs Alf Swindles Ann
.Rs On Recent Results for MD2, MD4 and MD5