Block size (cryptography)
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In modern
symmetric key ciphers are generally divided into stream ciphers and block ciphers. Block ciphers operate on a fixed length string of bits. The length of this bit string is the block size.[1] Both the input (plaintext) and output (ciphertext) are the same length; the output cannot be shorter than the input – this follows logically from the pigeonhole principle
and the fact that the cipher must be reversible – and it is undesirable for the output to be longer than the input.
Until the announcement of
cipher mode
does not properly randomise the input, the limit is even lower.
Consequently, AES candidates were required to support a block length of 128 bits (16 bytes). This should be acceptable for up to 264 × 16 B = 256
Rijndael, supports block and key sizes of 128, 192, and 256 bits, but in AES the block size is always 128 bits. The extra block sizes were not adopted by the AES
standard.
Many block ciphers, such as RC5, support a variable block size. The Luby-Rackoff construction and the Outerbridge construction can both increase the effective block size of a cipher.
Joan Daemen's 3-Way and BaseKing have unusual block sizes of 96 and 192 bits, respectively.
