blockcipher

A block cipher is a symmetric-key cipher that encrypts and decrypts fixed-size blocks of plaintext, typically 64 or 128 bits, using a secret key. The core algorithm provides a reversible transformation that, given the same key, can convert plaintext blocks into ciphertext blocks and back again. Block ciphers by themselves are not designed to handle messages of arbitrary length; they require a mode of operation to process longer data and to provide security properties beyond the block level.

Modes of operation define how to apply a block cipher to multiple blocks. Common modes include electronic codebook (ECB), which encrypts each block independently and is vulnerable to pattern leakage; cipher block chaining (CBC), which links blocks with chaining and requires a unique initialization vector; and counter (CTR), which turns the block cipher into a stream of keystream blocks and requires a unique nonce for each encryption under the same key. Authenticated encryption with associated data (AEAD) modes such as GCM combine confidentiality with integrity.

Block ciphers also need padding schemes when the message length is not a multiple of the block size, with PKCS#7 being a common choice. Security depends on a large enough block size to minimize block collisions, a strong key schedule, and careful implementation to avoid side-channel leaks. Widely used modern block ciphers include AES (Rijndael) with a 128-bit block size and 128-, 192-, or 256-bit keys, DES and its successor 3DES, Blowfish, and Camellia. Block ciphers are foundational in symmetric cryptography and are often paired with specific modes to meet confidentiality, integrity, and performance requirements.