Alice and Bob

alice and bob by John Richardson

The names Alice and Bob are commonly used placeholder names for archetypal characters in fields such as cryptography and physics. The names are used for convenience; for example, ‘Alice sends a message to Bob encrypted with his public key’ is easier to follow than ‘Party A sends a message to Party B encrypted by Party B’s public key.’ Following the alphabet, the specific names have evolved into common parlance within these fields—helping technical topics to be explained in a more understandable fashion.

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 effectively act as metasyntactic variables. In typical implementations of these protocols, it is understood that the actions attributed to characters such as Alice or Bob need not always be carried out by human parties directly, but also by a trusted automated agent (such as a computer program) on their behalf. Despite the advantage of Alice and Bob’s distinct genders in reducing ambiguity, there has been little tendency to introduce inanimate parties so they could be referred by neuter pronouns.

Other’s include: Chuck, as a third participant usually of malicious intent. Eve, an eavesdropper, is usually a passive attacker. While she can listen in on messages between Alice and Bob, she cannot modify them. In quantum cryptography, Eve may also represent the environment. Mallory, a malicious attacker (less commonly called Trudy, an intruder.); unlike Eve, Mallory can modify messages, substitute her own messages, replay old messages, and so on. The difficulty of securing a system against Mallory is much greater than against Eve. 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. Trent, a trusted arbitrator, is some kind of neutral third party, whose exact role varies with the protocol under discussion. Walter, a warden, may be needed to guard Alice and Bob in some respect, depending on the protocol being discussed.

Interactive proof systems are models of computation as the exchange of messages between two parties. The prover has unbounded computational ability and is called Merlin. 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. A similar pair of characters is Paul and Carole. The characters were introduced in the solution of a Twenty Questions problem, where ‘Paul,’ who asked questions, stood for Hungarian mathematician Paul Erdős and ‘Carole,’ who answered them, was an anagram of ‘oracle.’ They were further used in certain combinatorial games in the roles of Pusher and Chooser respectively, and have since been used in various roles.

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