Alice and Bob
Placeholder characters
- 4 min read
- 3 chapters
Related on Lably
Through Physics
Overview
Alice and Bob are the names of fictional characters used for convenience and to aid comprehension. For example, "How can Bob send a private message M to Alice in a public-key cryptosystem?" is believed to be easier to describe and understand than if the hypothetical people were simply named A and B as in "How can B send a private message M to A in a public-key cryptosystem?"
The names are conventional, and where relevant may use an alliterative mnemonic such as "Mallory" for "malicious" to associate the name with the typical role of that person.
Alice and Bob also have different genders, making it easier to differentiate them using gendered pronouns alone.
Sign in to keep reading
2 more chapters are free with a Lably account.
History
Scientific papers about thought experiments with several participants often used letters to identify them: A, B, C, etc.
The first mention of Alice and Bob in the context of cryptography was in Rivest, Shamir, and Adleman's 1978 article "A method for obtaining digital signatures and public-key cryptosystems." They wrote, "For our scenarios we suppose that A and B (also known as Alice and Bob) are two users of a public-key cryptosystem". Previous to this article, cryptographers typically referred to message senders and receivers as A and B, or other simple symbols. In fact, in the two previous articles by Rivest, Shamir, and Adleman, introducing the RSA cryptosystem, there is no mention of Alice and Bob. The choice of the first three names may have come from the film Bob & Carol & Ted & Alice.
Within a few years, however, references to Alice and Bob in cryptological literature became a common trope. Cryptographers would often begin their academic papers with reference to Alice and Bob. For instance, Michael Rabin began his 1981 paper, "Bob and Alice each have a secret, SB and SA, respectively, which they want to exchange." Early on, Alice and Bob were starting to appear in other domains, such as in Manuel Blum's 1981 article, "Coin Flipping by Telephone: A Protocol for Solving Impossible Problems," which begins, "Alice and Bob want to flip a coin by telephone."
Although Alice and Bob were invented with no reference to their personality, authors soon began adding colorful descriptions. In 1983, Blum invented a backstory about a troubled relationship between Alice and Bob, writing, "Alice and Bob, recently divorced, mutually distrustful, still do business together. They live on opposite coasts, communicate mainly by telephone, and use their computers to transact business over the telephone." In 1984, John Gordon delivered his famous "After Dinner Speech" about Alice and Bob, which he imagines to be the first "definitive biography of Alice and Bob."
In addition to adding backstories and personalities to Alice and Bob, authors soon added other characters, with their own personalities. The first to be added was Eve, the "eavesdropper." Eve was invented in 1988 by Charles Bennet, Gilles Brassard, and Jean-Marc Robert, in their paper, "Privacy Amplification by Public Discussion." In Bruce Schneier's book Applied Cryptography, other characters are listed.
Cast of characters
Cryptographic systems
The most common characters are Alice and Bob. Eve, Mallory, and Trent are also common names, and have fairly well-established "personalities" (or functions). The names often use alliterative mnemonics (for example, Eve, "eavesdropper"; Mallory, "malicious") where different players have different motives. Other names are much less common and more flexible in use. Sometimes the genders are alternated: Alice, Bob, Carol, Dave, Eve, etc.
Interactive proof systems
For interactive proof systems there are other characters:
Physics
The names Alice and Bob are often used to name the participants in thought experiments in physics. More alphabetical names, usually of alternating gender, are used as required, e.g. "Alice and Bob (and Carol and Dave and Eve)".
In experiments involving robotic systems, the terms "Alice Robot" and "Bob Robot" refer to mobile platforms responsible for transmitting quantum information and receiving it with quantum detectors, respectively, within the context of the field of quantum robotics.