Initialization vector

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 go through a (usually lengthy) re-keying process. The size of IV depends on the encryption algorithm and on the cryptographic protocol in use and is normally as large as the block or as large as the encryption key. It is often chosen at random or incrementally, and some times includes other information such as packet/sector/cluster/file ID. It must be known to the recepient of encrypted information.

Related Topics:
Cryptography - Stream cipher - Block cipher - Streaming modes of operation - Encryption - Random

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IVs are implemented differently in block ciphers and in stream ciphers. In straight-forward operation of block ciphers or so-called Electronic Code Book (ECB) mode, encryption of the same plain text with the same key results in the same ciphertext, which is a considerable threat to security. Use of an initialization vector linearly added to (XORed with) the first block of plaintext or included in front of the plaintext prior to encryption in one of the streaming modes of operation resolves this problem.

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In stream ciphers, IVs are loaded into the keyed internal secret state of the cipher, after which a number of cipher rounds is executed prior to releasing the first bit of output. For performance reasons, designers of stream ciphers try to keep that number of rounds as small as possible, but because determining the minimal secure number of rounds for stream ciphers is not a trivial task, and considering other issues such as entropy loss, unique to each cipher construction, related-IVs and other IV-related attacks are a known security issue for stream ciphers, which makes IV loading in stream ciphers a subject of ongoing research.

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