A
stream cipher encrypts one bit or byte at a time, XORing with a pseudorandom keystream. Fast and memory-efficient. Examples: RC4 (broken), ChaCha20 (modern, used in TLS 1.3).
RC4 is broken — never use it. ChaCha20-Poly1305 is the modern stream cipher — used in TLS 1.3 and WireGuard VPN. Reusing the same key+nonce = catastrophic (keystreams cancel out, revealing plaintext XOR plaintext).