Block cipher

In cryptography, a block cipher is a symmetric key cipher which operates on fixed-length groups of bits, termed blocks, with an unvarying transformation. When encrypting, a block cipher might take a (for example) 128-bit block of plaintext as input, and output a corresponding 128-bit block of ciphertext. The exact transformation is controlled using a second input — the secret key. Decryption is similar: the decryption algorithm takes a 128-bit block of ciphertext together with the secret key, and yields the original 128-bit block of plaintext.

Related Topics:
Cryptography - Symmetric key - Cipher - Plaintext - Key

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To encrypt messages longer than the block size (128 bits in the above example), a mode of operation is used.

Related Topics:
Block size - Mode of operation

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Block ciphers can be contrasted with stream ciphers; a stream cipher operates on individual digits one at a time, and the transformation varies during the encryption. The distinction between the two types is not always clear-cut: a block cipher, when used in certain modes of operation, acts effectively as a stream cipher.

Related Topics:
Stream cipher - Modes of operation

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An early and highly influential block cipher design was the Data Encryption Standard (DES), developed at IBM and published as a standard in 1977. A successor to DES, the Advanced Encryption Standard (AES), was adopted in 2001.

Related Topics:
Data Encryption Standard - IBM - 1977 - Advanced Encryption Standard - 2001

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