Stream cipher
Symmetric encryption that generates a pseudo-random keystream and XORs with plaintext. Fast on short messages, no padding issues. Opposite: block cipher.
Classic: RC4 (Ron Rivest, 1987 — popular in the 90s, now broken, deprecated in TLS 1.3). Modern: ChaCha20 (Daniel J. Bernstein, 2008 — TLS 1.3, WireGuard, Signal). Salsa20 is the predecessor. Authenticated encryption (AEAD) variants: ChaCha20-Poly1305 (TLS), XSalsa20Poly1305 (NaCl/libsodium). Stream ciphers are also used in mobile phones (A5/1, A5/3 in GSM). One-time pad is the theoretically perfect stream cipher if the key is random + as long as the message.