Alice and Bob

From Academic Kids

Alice and Bob are conventional placeholder terms referring to 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, as they represent both genders. The concrete motive for using such names was that it helps with writing because it gives the personal pronouns unambiguous meanings. The specific names were chosen to match the first letters of the alphabet.

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.

In typical implementations of these protocols, it is understood that the actions attributed to characters such as Alice or Bob would not normally be carried out by human parties directly, but rather by an automated agent (such as a computer program) on their behalf.


List of characters

  • Alice and Bob. Generally Alice wants to send a message to Bob. These names were invented by Ron Rivest for the 1978 Communications of the ACM article presenting the RSA cryptosystem. (The 1977 technical report on RSA did not yet use these names.) Rivest denies that these names have any relation with the 1969 movie Bob & Carol & Ted & Alice as occasionally suggested by others.
  • 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, as in malicious), is an active attacker; unlike Eve, Mallory can modify messages, substitute her 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).
  • Peggy (or Pat), 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 television 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

See also


  • C.H. Lindsey, Regulation of Investigatory Powers Bill: Some Scenarios, 2000, [1] (

External links

de:Alice und Bob he:אליס ובוב


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