9.1. Session and Interchange Keys

Definition 91. An interchange key is a cryptographic key associated with a principal to a communication. A session key is a cryptographic key associated with the communication itself.

This distinction reflects the difference between a communication and a user involved in that communication. Alice has a cryptographic key used specifically to exchange information with Bob. This key does not change over interactions with Bob. However, if Alice communicates twice with Bob (and "communication" can be with, for example, an e-mail or a Web browser), she does not want to use the same key to encipher the messages. This limits the amount of data enciphered by a single key and reduces the likelihood of an eavesdropper being able to break the cipher. It also hinders the effectiveness of replay attacks. Instead, she will generate a key for that single session. That key enciphers the data only; it does not authenticate either principal, and it is discarded when the session ends. Hence, the name "session key."

Session keys also prevent forward searches [830]. A forward search attack occurs when the set of plaintext messages is small. The adversary enciphers all plaintexts using the target's public key. When ciphertext is intercepted, it is compared with the precomputed texts. This quickly gives the corresponding plaintext. A randomly generated session key, used once, would prevent this attack. (See Exercise 1 for another approach.)

EXAMPLE: Suppose Alice is a client of Bob's stockbrokering firm. She needs to send Bob one of two messages: BUY or SELL. The attacker, Cathy, enciphers both messages with Bob's public key. When Alice sends her message, Cathy compares it with her messages and sees which one it matches.

An interchange key is associated with a principal. Alice can use the key she shares with Bob to convince Bob that the sender is Alice. She uses this key for all sessions. It changes independently of session initiation and termination.