one-way hash function
message digest function

    (Or "message digest function") A one-way
   function which takes a variable-length message and produces a
   fixed-length hash.  Given the hash it is computationally
   infeasible 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, SHA 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>.

   (2001-05-10)