Stream Cipher

A stream cipher generates a pseudorandom keystream from a secret key and a nonce (initial value), then XORs that stream byte-by-byte with the plaintext. Decryption regenerates the same keystream and XORs it with the ciphertext. Stream ciphers are well suited to streaming and resource-constrained environments because they have no block boundary and minimal state. Modern designs such as ChaCha20 and Salsa20 are software-friendly, fast, and constant-time, while older designs like RC4 are now broken and forbidden by TLS, WPA, and most standards. The cardinal rule of stream ciphers is never to reuse a (key, nonce) pair: two messages encrypted with the same keystream leak their XOR, which often suffices to recover both plaintexts. They are therefore typically used in authenticated modes such as ChaCha20-Poly1305.