What is a nonce and why is it valid for a short time period?

A nonce is a random number that uniquely identifies each attestation request. The server creates and returns a nonce, which you use to identify an attestation request. Each nonce is valid for a short time period, after which the Attestation Server fails any request made using that nonce. This is to avoid a replay attack that could allow an attacker to reuse a past attestation result.