My question maybe will be stupid, but my problem is that I do not understand why stream ciphers need a key and also a nonce. As far as I understand, the keystream is generated with the nonce. The same key can be reused with a different nonce.

Wouldn't be simpler just to use a new key each time? If I understand good, to decrypt the message the recipient needs the nonce. Is communicating the nonce simpler than communicating the key?


1 Answer 1


By the modern definition of a cipher, it must be possible to encipher several messages with the same secret key. That's also a practical necessity, due to the difficulty of securely establishing a shared secret key. That issue is solved with the nonce, which is not secret, and can be transferred as part of the ciphertext (typically: at the beginning).

Without nonce, or if the nonce repeats, the keystream would repeat, and that would allow breaking the cipher, e.g. with a single known plaintext.

Because the nonce is not secret, communicating the nonce is simpler than securely communicating the key. Altering the nonce would alter the deciphered message, but a generic stream cipher is not supposed to allow detection of alterations anyway.

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    $\begingroup$ Can you elaborate a little further on why the keystream repeating would allow you to break it? Do you need both the keystream repeating and a guess about the plaintext? How much plaintext would you need to break the cipher? $\endgroup$ Commented Jul 24, 2017 at 2:40
  • $\begingroup$ @Kevin Burke: the answer does NOT state that "the keystream repeating would allow you to break it", with it referring to keystream; that would be incorrect. The answer states that a repeating keystream would allow breaking the cipher. That stands per nearly all formal definition of cipher security, including IND-CPA (a common academic definition), and Known Plaintext. Fixed keystream also allows recovering most plaintext with minimal statistical knowledge about plaintext (being English text in ASCII) and a few ciphertexts. $\endgroup$
    – fgrieu
    Commented Jul 24, 2017 at 8:53

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