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Consider app using say, HMAC-SHA2, with securely pre-shared symmetric key. Are the nonces of exchanged messages considered secret as well?
Or in other words, does its easy predictability or even outright knowledge(plaintext) of nonce to attacker compromise/reduce HMAC security? Some sources suggest using cryptographically secure PRNG for nonce generation as if the PRNG algorithm/seed was part of preshared secret which confuses me.

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    nonces are not secret, but hmac doesn't use nonces, so it's a trick question. – dandavis Feb 2 '18 at 10:09
  • @dandavis hmm, true. Wondering how should I rephrase the question - when using nonce for messages it is in fact more of a protocol(?) secured by HMAC rather than HMAC itself, isnt it? – wondra Feb 2 '18 at 10:17
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NIST (National Institute of Standards in Technology) defines an nonce as a time varying, non-repeating value to help prevent 'replay attacks.

The nonce doesn't necessarily have to be secret but should be a random sourced non-repeating value used in block ciphers for symmetric and asymmetric encryption.

Please reference NIST SP 800 series of documentation for additions details of best practices, implementation & reasons for use.

  • There seem to be conflicting definitions, e.g. NIST SP 800-63-2: ...a nonce is not necessarily unpredictable. while NIST SP 800-44 Version 2: ...Because the sender randomly generated the nonce, this defeats playback attacks because the replayer cannot know in advance the nonce the sender will generate. Which would imply that predictable nonce, like timestamp, does not prevent replay attacks(why use such?). Could you clarify this a bit? – wondra Feb 2 '18 at 13:38
  • While not necessarily unpredictable an nonce should never be used more than once. The nonce can be best described as the equivalent of a salt when hashing passwords. While not always a value from a PRNG, it should never be used more than once for each key. Some uses of an nonce don't necessarily require the security a truly random sequence of bytes offers for keys – jas- Feb 2 '18 at 19:02

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