Stream Cipher

A stream cipher is a symmetric-key cryptographic algorithm that combines plaintext digits with a pseudorandom keystream one bit or byte at a time, typically by XOR, rather than operating on fixed-size blocks. The keystream is generated from a secret key and, often, a nonce, so the same key never reuses the same keystream. RC4 and Salsa20 are commonly cited stream ciphers, and ChaCha20 is a widely deployed modern successor to Salsa20 used in TLS. Because encryption and decryption are the same simple combining operation, stream ciphers are prized for speed and low hardware cost in constrained or high-throughput settings, at the cost of strict nonce-reuse discipline.