Alice And Bob

See also: Cryptography

A more colorful description of “point A to point B,” using people. This example has been used so often, its become a standard.

In most cryptography and security literature, stories are told where a character named “Alice” wishes to exchange secret messages with someone named “Bob.” This is very convienient, as in addition to beginning with the first two letters of the English alphabet, Alice and Bob are of different genders and so references to “he” and “she” are unambiguous. Keeping with the A-B idea, Alice usually initiates the communication: she is the one (at least the first one) with a message to send to Bob.

In addition to these two, a cast of other characters is often seen throughout the literature:

  • Carol, Dave, and so on may be involved when more than two parties are communicating.
  • Eve, an evesdropper, can spy on messages between Alice and Bob but generally cannot modify them. It is important that she not be able to determine the content of the message being sent.
  • Mallory, a manipulator, is more dangerous than Eve because she can modify messages in transit. It is desired that Alice and Bob be able to detect Mallory’s influence, and either ignore the changed message or somehow retreive the correct message anyway.
  • Trent, a trusted arbitrator, is some kind of neutral third party, whose exact role varies with the protocol under discussion.
  • Peggy, a prover, and Victor, a verifier, often must interact in some way to show that the intended transaction has actually taken place.
  • Walter, a warden, may be needed to guard Alice and Bob in some respect.

After reading enough papers, one begins to wonder what, exactly, Alice and Bob have that is such an interesting secret…

In Bruce Schneier‘s definitive introductory text Applied Cryptography (2nd ed., 1996, John Wiley & Sons, ISBN 0-471-11709-9) he documented a table of dramatis personae headed by Alice and Bob. A whole mythology has grown up around the metasyntactic names; see

External Links

  • Alice (
  • Protocols, etc. (

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