why the nonces(N1 and N2) and Timestamps(T1 and T2) are used in Public key Authority? which of them prevent replay attack?

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  • $\begingroup$ Hint: remove them one by one and think attacks... $\endgroup$
    – kelalaka
    Commented Apr 29, 2019 at 21:09

1 Answer 1


To understand how nonces and timestamps are used to prevent a replay attack, you must understand generally what a replay attack is. A replay attack is a network attack in which a data transmission (that is valid) is repeated or delayed. An attack like this is usually done by an adversary who happens to intercept this data and re-transmits it.

If Alice (Initiator A) and Bob (Responder B) are conversing, Alice can send over a nonce with a MAC which can be checked by Bob to verify that it is Alice and not an adversary sending it.

Timestamping also is important because if Alice sends a message with one, Bob can choose to only accept messages with reasonable timestamps.

So for 'Certificate-Based Key exchanges', Alice provides nonce, or a timestamp along with her certificate. This will help preventing Replay Attacks!


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