Stream cipher encryption and message authentication. Stream cipher encryption is performed by generating a keystream at the transmitting end from a state value, applying the keystream to plaintext to generate an encrypted message block having at least a portion of the plaintext converted to ciphertext, and updating the state value as a function of said at least a portion of the plaintext. Stream cipher decryption is performed by generating a keystream at the receiving end from the same state value, applying the keystream to the encrypted message block to convert the ciphertext to plaintext, and updating the state value as a function of the plaintext. Message authentication techniques are also described.