In security engineering, a nonce is a number used once. It is often a random or pseudo-random number issued in an authentication protocol to ensure that old communications cannot be reused in replay attacks. For instance, nonces are used in HTTP digest access authentication to calculate an MD5 digest of the password. The nonces are different each time that the 401 authentication challenge response code is presented, and each client request has a unique sequence number, thus making the replay attack virtually impossible. To meet Wikipedias quality standards, this article or section may require cleanup. ... The word random is used to express lack of purpose, cause, order, or predictability in non-scientific parlance. ... A pseudo-random number is a number belonging to a sequence which appears to be random, but can in fact be generated by a finite computation. ... In computing, the Challenge-Handshake Authentication Protocol (CHAP) is an authentication protocol used to log on a user to an Internet access provider. ... A replay attack is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. ... HTTP (for HyperText Transfer Protocol) is the primary method used to convey information on the World Wide Web. ... Digest access authentication is one of the agreed methods a web page can use to negotiate credentials with a web user (using the HTTP protocol). ... In cryptography, MD5 (Message-Digest algorithm 5) is a widely-used cryptographic hash function with a 128-bit hash value. ... A password is a form of secret authentication data that is used to control access to a resource. ... The following is a list of HTTP response status codes and standard associated phrases, intended to give a short textual description of the status. ...

Some also refer to initialization vectors as nonces for the above reasons. To ensure that a nonce is used only once, it should be time-variant (including a suitably granular timestamp in its value), or generated with enough random bits to ensure a probabilistically insignificant chance of repeating a previously generated value. In cryptography, an initialization vector (IV) is a block of bits that is required to allow a stream cipher or a block cipher executed in any of several streaming modes of operation to produce a unique stream independent from other streams produced by the same encryption key, without having to... Timestamp can refer to a time code or to a digitally signed timestamp whose signer vouches for the existence of the signed document or content at the time given as part of the digital signature. ...

See also

• Initialization vector
• Salt (cryptography)

In cryptography, an initialization vector (IV) is a block of bits that is required to allow a stream cipher or a block cipher executed in any of several streaming modes of operation to produce a unique stream independent from other streams produced by the same encryption key, without having to... In cryptography, a salt consists of random bits used as one of the inputs to a key derivation function. ...

External links

• Sam Ruby Blogging on Nonce with an implementation
• HTTP Authentication: Basic and Digest Access Authentication
• Robust Explicit Congestion Notification (ECN) Signaling with Nonces
• UMAC: Message Authentication Code using Universal Hashing