English Computing Dictionary
one-way hash function
(Or "message digest function") A function which
takes a variable-length message and produces a fixed-length
hash. Given the hash it is computationally impossible to find
a message with that hash; in fact one can't determine any
usable information about a message with that hash, not even a
single bit. For some one-way hash functions it's also
computationally impossible to determine two messages which
produce the same hash.
A one-way hash function can be private or public, just like an
{encryption} function. {MD5} and {Snefru} are examples of
public one-way hash functions.
A public one-way hash function can be used to speed up a
public-key {digital signature} system. Rather than sign a
long message which can take a long time, compute the one-way
hash of the message, and sign the hash.
{sci.crypt FAQ