A block cipher is a symmetric-key cipher that breaks a plaintext message into segments of fixed length. Each segment, called a plaintext block, consists of a fixed number of plaintext symbols. Using a secret key k known to both encryptor and decryptor, each plaintext block is encrypted independently into a ciphertext block of blocklength n. In many common block ciphers, but not all, the length n of the ciphertext block is equal to the length of the plaintext block. The ciphertext blocks are concatenated to form the ciphertext message, which is then sent to the decryptor. The decryptor receives the ciphertext message, then breaks it into the sequence of ciphertext blocks of length n and, using a corresponding key k, decrypts each ciphertext block of length n independently into the corresponding plaintext block. The plaintext blocks leaving the decryptor are concatenated to reform the plaintext message, which is then sent to the user.

The encryption function and the key k can remain the same from block to block, but the encryption of the lth block does not depend on the data within other blocks. Each block is encrypted independently. A symmetric-key block cipher requires that, for the same block, the encryptor and the decryptor use the same key. This means that a secure method of distributing this key to both the encryptor and decryptor, or of exchanging this key between them, is required. Any method of key distribution, including public-key cryptography, can be used.