Cryptographic nonce

Cryptographic nonce

In cryptography, a nonce is an arbitrarynumber that may only be used once. It is similar in spirit to a nonce word,hence the name. It is often a random or pseudo-random number issued in anauthentication protocol to ensure that old communications cannot be reused inreplay attack. They can also be useful as initialization vectors and incryptographic hash function.

Usage

Authentication

Authentication protocol may use nonces toensure that old communications cannot be reused in replay attacks. Forinstance, nonces are used in HTTP digest access authentication to calculate anMD5 digest of the password. The nonces are different each time the 401authentication challenge response code is presented, thus making replay attacksvirtually impossible. The scenario of ordering products over the Internet canprovide an example of the usefulness of nonces in replay attacks. An attackercould take the encrypted information and –without needing to decrypt – couldcontinue to send a particular order to the supplier, thereby ordering productsover and over again under the same name and purchase information. The nonce isused to give ‘originality’ to a given message so that if the company receivesany other orders from the same person with the same nonce, it will discardthose as invalid orders.

A nonce may be used to ensure security fora stream cipher. Where the same key is used for more than one message and thena different nonce is used to ensure that the keystream is different fordifferent messages encrypted with that key; often the message number is used.