7 Freshness

7.4 Nonces

This third method of freshness indication uses an unpredictable value in a challenge–response sequence. The sequence of events is illustrated in Figure 11. Bob wants to communicate with Alice but she needs reassurance that his message is not an old one that is simply being replayed. She generates some random number, which she encrypts and sends to Bob. He then binds the decrypted version of the random number to his message to Alice. On receipt she checks that the returned number is indeed the one she recently issued and sent to Bob. This number, which is used only once by Alice, is called a nonce (derived from ‘number used once’). The term ‘nonce’ is also often used in a wider sense to indicate any freshness indicator.

Figure 11: Using a nonce as a freshness indicator