In a HTTP scheme where the HMAC-SHA256 signature is sent as part of the Authoriz

Question

In a HTTP scheme where the HMAC-SHA256 signature is sent as part of the Authorization header, and the message input contains:

- The request method
- URL
- Post data
- Nonce
- Date HTTP header

How unique does the nonce need to be, or, for how long should the nonce be disallowed to be re-used?

For example, if I enforce uniqueness of (Date, nonce) rather than enforcing absolute uniqueness of the nonce on its own, have I introduced any weakness to replay or other kinds of attack?

When another second of time elapses, the signature changes (due to the Date header), so if I re-use a nonce from 1 second ago, it should not be a problem, right?

Explanation / Answer

You need to take in consideration the allowable time window (to compensate for Clock drift). If you allow Date's with a time window of plus/minus 10 minutes (total: 20 minutes) you need to use nonces that are unique on the whole time window, and you also need to store spent nonces from that time period.

To avoid a race situation where a nonce is being deleted from the "spent" list Before the Date: header of the used nonce has became invalid, it would be recommended to store the spent nonces from the last 40 minutes, and also use nonces that are unique for 40 minutes.

Then you have a practically a watertight solution.

In other Words, if you would allow a nonce to be reused after 1 second, a attacker can just resend the old request with the old date header, since if you allow a time window - which you must because you cannot ensure the client Clock is synced with the server, then you would accept a request with a spent nonce.

If you reallywant a good nonce, use H(Time, Nonce) where H is a decent hash function. Then you can use "non-unique" nonces as long as you dont generate 2 equal nonces the same second. Then you get unique hashes to use as nonces.

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