Manpages - md5.3

message digest

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

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 middle. MD2 can only be used for Privacy-Enhanced Mail. MD4 has been criticized 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

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

and

functions are the core functions. Allocate an MD5_CTX, initialize it with

run over the data with

and finally extract the result using

The

function can be used to apply padding to the message digest as in

but the current context can still be used with

The

function is used by

to hash 512-bit blocks and forms the core of the algorithm. Most programs should use the interface provided by

and

instead of calling

directly.

is a wrapper for

which converts the return value to an MD5_DIGEST_STRING_LENGTH-character (including the terminating ’\0’)

string which represents the 128 bits in hexadecimal.

calculates the digest of a file, and uses

to return the result. If the file cannot be opened, a null pointer is returned.

behaves like

but calculates the digest only for that portion of the file starting at

and continuing for

bytes or until end of file is reached, whichever comes first. A zero

can be specified to read until end of file. A negative

will be ignored.

calculates the digest of a chunk of data in memory, and uses

to return the result.

When using

the

argument can be a null pointer, in which case the returned string is allocated with

and subsequently must be explicitly deallocated using

after use. If the

argument is non-null it must point to at least MD5_DIGEST_STRING_LENGTH characters of buffer space.

These functions appeared in

and

The original MD5 routines were developed by

Data Security, Inc., and published in the above references. This code is derived from a public domain implementation written by Colin Plumb.

The

and

helper functions are derived from code written by Poul-Henning Kamp.

Collisions have been found for the full versions of both MD4 and MD5. The use of

is recommended instead.