Alice and Bob are common archetypal characters used in explanations in fields such as cryptography and physics. The names are used for convenience, since explanations such as "Person A wants to send a message to person B" rapidly become difficult to follow. The names are also said to be politically correct, since they are from both sexes, and were chosen only because of the alphabetical order.

In cryptography and computer security, there are a number of widely-used names for the participants in discussions and presentations about various protocols. The names are conventional, somewhat self-suggestive, sometimes humorous, and are, more or less, metasyntactic variables. They are shorthand only and have no other significance.

One problem with this literary convention is that it oversimplifies reality. Alice almost never makes the computations ascribed to her. Instead she clicks icons on the screen of a computer she purchased from a retailer, who received it from a factory in Asia, where it was assembled from chips designed, fabricated and packaged in half a dozen countries. The software intended to perform the mathematical function (Alice computes...) was downloaded over the Internet accompanied with a cryptographic hash verification value she never checked. It typically runs on a closed source operating system with known security flaws, some of which were corrected in patches she never installed. Her computer is connected to the Internet where it is constantly attacked, and often penetrated, by various forms of malware. The computer sits on a desk where it is easily accessed by coworkers, cleaning people and others. Of what it is really calculating, Alice has no certainty.

List of characters

• Alice and Bob. Generally Alice wants to send a message to Bob. Schneier suggests that these names were drawn from the 1969 movie, Bob & Carol & Ted & Alice [1] (http://arxiv.org/pdf/cs.CR/0109089).
  • Carol, as a third participant in communications. Thereafter, we often have Dave, a fourth participant, and so on alphabetically.
• Eve, an eavesdropper, is a passive attacker. While she can listen in on messages between Alice and Bob, she cannot modify them.
• Isaac, an Internet Service Provider (ISP).
• Ivan, an issuer (as in financial cryptography).
• Justin, from the justice system; specifically a lawyer.
• Mallory, (sometimes Mallet), is an active attacker; unlike Eve, Mallory can modify messages, substitute his own messages, replay old messages, and so on. The problem of securing a system against Mallory is much greater than against Eve.
• Matilda, a merchant (as in ecommerce or financial cryptography).
• Pat, see Peggy.
• Peggy, a prover, and Victor ($), a verifier, often must interact in some way to show that the intended transaction has actually taken place. They are often found in zero knowledge proofs. Another name pair sometimes used is Pat and Vanna (after the host and letter_turner on the Wheel of Fortune televison show).
• Plod, a police officer (or, as the case may be, a customs officer, or a member of the intelligence services).
• Oscar, an opponent, is usually taken as equivalent to Mallory.
• Trudy, an intruder, is more dangerous than Eve because she can modify messages in transit. Bob and Alice should ideally be able to detect any such modification and either ignore the changed message, or retrieve the correct message despite the intrusion. If they can't, Trudy can cause much mischief.
• Trent, a trusted arbitrator, is some kind of neutral third party, whose exact role varies with the protocol under discussion.
• Vanna, see Peggy.
• Victor, see Peggy.
• Walter, a warden, may be needed to guard Alice and Bob in some respect, depending on the protocol being discussed.

Although interactive proof systems is not (quite) a cryptographic protocol, it is sufficiently closely related to mention its literature's 'cast of characters':

• Arthur and Merlin: In IPSs, the prover has unbounded computational ability and is hence associated with Merlin, the powerful wizard. He claims the truth of a statement, and Arthur, the wise king, questions him to verify the claim. These two characters also give the name for two complexity classes, namely MA and AM.

Some articles using Alice and Bob explanations

• In physics:
  • EPR paradox
  • Quantum teleportation
• In cryptography:
  • RSA
  • Quantum cryptography
• In communication:
  • Communication complexity

See also

• Metasyntactic variable

References

• Bruce Schneier, Applied Cryptography (2nd ed., 1996, John Wiley & Sons, ISBN 0-471-11709-9).
• C.H. Lindsey, Regulation of Investigatory Powers Bill: Some Scenarios, 2000, [2] (http://www.cs.man.ac.uk/~chl/scenarios.html).

External links

• The Story of Alice and Bob (http://www.conceptlabs.co.uk/alicebob.html)