This is the seventh of ten parts of the sci.crypt FAQ. The parts are
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Notes such as ``[KAH67]'' refer to the reference list in the last part.

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Contents:

7.1. What is a one-way hash function?
7.2. What is the difference between public, private, secret, shared, etc.?
7.3. What are MD4 and MD5?
7.4. What is Snefru?


7.1. What is a one-way hash function?

  A typical one-way hash function 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. Here's one application of a public one-way hash
  function, like MD5 or Snefru. Most public-key signature systems are
  relatively slow. To sign a long message may take longer than the user
  is willing to wait. Solution: Compute the one-way hash of the message,
  and sign the hash, which is short. Now anyone who wants to verify the
  signature can do the same thing.

  Another name for one-way hash function is message digest function.

7.2. What is the difference between public, private, secret, shared, etc.?

  There is a horrendous mishmash of terminology in the literature for a
  very small set of concepts. Here are the concepts: (1) When an
  algorithm depends on a key which isn't published, we call it a private
  algorithm; otherwise we call it a public algorithm. (2) We have
  encryption functions E and decryption functions D, so that D(E(M)) = M
  for any message M. (3) We also have hashing functions H and
  verification functions V, such that V(M,X) = 1 if and only if X = H(M).

  A public-key cryptosystem has public encryption and private
  decryption. Checksums, such as the application mentioned in the
  previous question, have public hashing and public verification.
  Digital signature functions have private hashing and public
  verification: only one person can produce the hash for a message,
  but everyone can verify that the hash is correct.

  Obviously, when an algorithm depends on a private key, it's meant to
  be unusable by anyone who doesn't have the key. There's no real
  difference between a ``shared'' key and a private key: a shared key
  isn't published, so it's private. If you encrypt data for a friend
  rather than ``for your eyes only'', are you suddenly doing
  ``shared-key encryption'' rather than private-key encryption? No.

7.3. What are MD4 and MD5?

  MD4 and MD5 are message digest functions developed by Ron Rivest.
  Definitions appear in RFC 1320 and RFC 1321 (see part 10). Code is
  available from [FTPMD].

  Note that a transcription error was found in the original MD5 draft
  RFC. The corrected algorithm should be called MD5a, though some
  people refer to it as MD5.

7.4. What is Snefru?

  Snefru is a family of message digest functions developed by Ralph
  Merkle. Snefru-8 is an 8-round function, the newest in the family.
  Definitions appear in Merkle's paper [ME91a]. Code is available from
  [FTPSF].