In the description of cryptographic protocols, the same roles keep appearing (e.g., sender, receiver, evesdropper). It’s convenient to give these roles memorable names, with pride of place going to Alice and Bob. What follows is a list of the names of these cryptographic role players along with an explanation of their roles.
Alice and Bob. Ron Rivest, Adi Shamir, and Leonard Adleman (of RSA fame) immortalized Alice and Bob in their 1978 paper outlining their scheme for public-key cryptography: “A Method for Obtaining Digital Signatures and Public-key Cryptosystems.” Alice, the sender, wants to transmit a message securely to Bob, the receiver. The challenge is to determine what Alice and Bob need to make their communications secure, especially against various types of attack against their cryptographic communication system.
Eve. Eve is an eavesdropper who tries to monitor the communications channel between Alice and Bob. Eve is a passive attacker in the sense that she is capable only of listening in on messages being sent, not on changing or disrupting them.
Sybil / Sybil attack. Sybil is an attacker who uses multiple personalities (i.e., invented fake personas) to attack a communications system. Sybil comes from the pseudonym for Shirley Ardell Mason, one of the most famous people to suffer multiple personality disorder (a book and film were made of her case). A Sybil attack attempts to disrupt the network on which Alice and Bob are communicating by creating multiple fake personas and using them to subvert any consensus mechanisms, thereby influencing the network to take actions it would not otherwise take.