Useful Notes: Alice and Bob
If we put together snippets from lots of papers, we get a fascinating picture of their lives.This may be the first time a definitive biography of Alice and Bob has been given. In papers written by American authors Bob is frequently selling stock to speculators. From the number of stock market deals Bob is involved in we infer that he is probably a stockbroker. However from his concern about eavesdropping he's probably active in some subversive enterprise as well. And from the number of times Alice tries to buy stock from him we infer she is probably a speculator. Alice is also concerned that her financial dealings with Bob are not brought to the attention of her husband. So Bob is a subversive stockbroker and Alice is a two-timing speculator.
But Alice has a number of serious problems.
She and Bob only get to talk by telephone or by electronic mail. In the country where they live the telephone service is very expensive. And Alice and Bob are cheapskates. So the first thing Alice must do is MINIMIZE THE COST OF THE PHONE CALL. The telephone is also very noisy. Often the interference is so bad that Alice and Bob can hardly hear each other. So another of Alice's problems is DEALING WITH NOISE. On top of that Alice and Bob have very powerful enemies. One of their enemies is the Tax Authority. Another is the Secret Police. This is a pity since their favourite topics of discussion are tax frauds and overthrowing the government. These enemies have almost unlimited resources. They always listen in to telephone conversations between Alice and Bob. So Alice also has the problem of ENSURING CONFIDENTIALITY. And these enemies are very sneaky. One of their favourite tricks is to telephone Alice and pretend to be Bob. Well, you think, so all Alice has to do is listen very carefully to be sure she recognises Bob's voice. But no. You see Alice has never met Bob. She has no idea what his voice sounds like. So IDENTIFICATION is another of Alice's problems.
So you see Alice has a whole bunch of problems to face. Oh yes, and there is one more thing I forgot so say - Alice doesn't trust Bob. We don't know why she doesn't trust him, but at some time in the past there has been an incident. So she's got an AUTHENTICATION problem too. Now most people in Alice's position would give up. Not Alice. She has courage which can only be described as awesome. Against all odds, over a noisy telephone line, tapped by the tax authorities and the secret police, Alice will happily attempt, with someone she doesn't trust, whom she cannot hear clearly, and who is probably someone else, to fiddle her tax returns and to organise a coup d'etat, while at the same time minimising the cost of the phone call.
A coding theorist is someone who doesn't think Alice is crazy.
—John Gordon, explaining the many and varied proclivities and problems of everyone's favorite fictional duo.
If you've wandered through TV Tropes
, you may have noticed that when people reach for two generic names to give to characters in an example, the names will be, almost unerringly, Alice
. And as you might suspect, this isn't a coincidence - Alice and Bob have traditionally been used as the names of the two parties involved in examples in a number of fields, most famously in cryptography. In fact, it was the seminal paper outlining the RSA asynchronous encryption algorithm that introduced Alice and Bob to the world.
The reason that this duo is used continually is because they're a simple yet elegant shorthand for saying "Party A" and "Party B". When you start talking about "Alice wants to send a message to Bob", it immediately sets up the situation without requiring a lot of exposition. And as their usage became more popular in cryptography and then geek circles, they've popped up in more and more mainstream venues.
Unsurprisingly, there's a whole cast of characters involved in Alice and Bob's adventures, and they too have been standardized:
- Carol/Carlos/Charlie/Chuck: The third party ("Party C") in three way communications. Chuck is usually reserved for situations where the third party has malicious intent.
- Dave: The fourth party ("Party D", natch) in four way communications.
- Eve: A passive eavesdropper. Tries to listen in, but doesn't attempt to actively alter the communication like Mallory (see below).
- Mallory: A malicious intruder. Unlike her passive counterpart Eve, Mallory doesn't just read messages, but can actively interfere in the communications (e.g. edit, delay, or prevent delivery of messages).
- Peggy: A prover. Her job is to act as third party proof of transactions taking place.
- Trent: A trusted arbitrator. A third party known as trustworthy and neutral to the other parties involved. His exact role may vary based on the system being used.
- Walter: A warden. His job is to protect the participants in the communication.
- Victor: A verifier. He provides third party verification, similar to Peggy.