keystream

Keystream is a sequence of bits or bytes used in certain encryption schemes to transform plaintext into ciphertext by combining the keystream with the plaintext, typically by the XOR operation. It is produced by a keystream generator and depends on a secret key and usually a nonce or initialization vector (IV) to ensure uniqueness across messages. The security of keystream-based schemes rests on the unpredictability and non-repetition of the keystream for a given key/IV pair.

In synchronous stream ciphers, the keystream is generated independently of the plaintext and then XORed with the plaintext to yield ciphertext. In self-synchronizing schemes, the keystream may be influenced by recent ciphertext. Modern implementations derive the keystream from a pseudorandom number generator or a block cipher in a mode that produces a keystream, such as ChaCha20 or AES in counter mode (CTR). The keystream must be kept secure in the sense that its generation is deterministic from the secret key; the keystream itself is not a secret value separate from the key, but revealing it compromises security.

A fundamental risk is keystream reuse: using the same keystream with two different messages enables attackers to obtain the XOR of the two plaintexts, often allowing recovery of both messages. One-time pads use a truly random, non-repeating keystream as long as the message, yielding information-theoretic security, but require secure generation and distribution of the keystream. In practice, secure keystream generation is essential to prevent keystream leakage and related attacks.

See also: stream cipher, ChaCha20, CTR mode, one-time pad.