streamciphers

Stream ciphers are a class of symmetric key ciphers that encrypt plaintext by combining it with a pseudorandom keystream, typically using an XOR operation. They operate on data as a stream rather than in fixed-size blocks, allowing low-latency encryption of inputs of varying length. The security of a stream cipher rests on the keystream being unpredictable and non-repeating; if the same keystream is ever reused with different plaintext, information can be leaked through simple algebraic relations.

Most stream ciphers generate a keystream from a secret key and an initialization vector (IV) or nonce. Architectures typically combine small, fast state elements such as linear feedback shift registers with nonlinear filters or generators to produce the keystream. There are two main families: synchronous stream ciphers, where the keystream is advanced independently of the ciphertext, and self-synchronizing stream ciphers, where parts of the ciphertext feed back into the keystream state to resynchronize after errors or losses.

Common examples include RC4 (now deprecated due to known biases), the Grain and Trivium families, and modern designs such as Salsa20 and ChaCha, which are widely used in practice. Some designs also exist in the ChaCha20-Poly1305 construction for authenticated encryption. Compared to block ciphers used in CTR mode, stream ciphers generally offer very low latency and reduced memory footprint but require careful IV management to avoid keystream reuse.

Security notes emphasize that reusing a keystream is catastrophic, and implementations must avoid key/IV reuse, side-channel leaks, or poor randomness. Stream ciphers find use in TLS, SSH, and wireless and embedded communications, particularly where speed and simplicity are advantageous.