Alice and Bob

Alice and Bob, also just A and B, are the standard example users in writing on cryptography, coding theory, communication complexity theory etc. Carol and Dave often join them for protocols that require more than two players.

Bruce Schneier extends these with two kinds of attacker: and several other types of player required in various protocols:
 * Eve the Eavesdropper, using passive attacks, just trying to read messages
 * Mallory the Malicious, using active attacks, attempting to deceive A and B by altering messages or sending bogus ones
 * Victor the Verifier
 * Peggy the Prover
 * Trent the Trusted third party
 * Walter the Warden, who watches over A and B in some protocols

Schneier's extensions seem to be in the process of becoming standard as well.

It is also moderately common to add additional characters as needed for a particular protocol. For example, in discussing a e-commerce system, one might need Matlida the Merchant and Ivan the Issuer of credentials.

History
Alice and Bob were introduced in the original paper on the RSA algorithm for public key cryptography.

The similar name of the film Bob & Carol & Ted & Alice and subsequent TV show appears to be just a coincidence. Rivest denies that there is a connection.

Alice and Bob have an amusing biography on the web.

Other pairs
While Alice and Bob are standard in cryptography and coding theory, other pairs of players are used in other domains.

In interactive proof systems, 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.

In combinatorial game theory, the characters are often Paul and Carole. The characters were introduced in the solution of the Twenty Questions problem, where "Paul", who asked questions, stood for Paul Erdős and "Carole", who answered them, was an anagram of "oracle". They were further used in certain games in the roles of Pusher and Chooser respectively, and have since been used in various roles.

In English grammar discussions, notably in Noam Chomsky's writings, John and Mary are often used.