DotNetOpenAuth - What do "Nonce" and "OpenIDAssociation" tables do?

This is a specific question to DotNetOpenAuth. I opened up the .NET template, and found that there are 2 tables "Nonce" and "OpenIDAssociation" and there are 2 SPs that do the cleanup upon on expiration date.

What exactly do these 2 tables do? Why do we need to k开发者_如何转开发eep track of the association?

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Nonces provides replay protection and OpenID associations allow for faster logins and (in some cases) enhanced security. While nonces and associations can be stored in memory without a database table behind it, a database is more secure because nonces won't be lost from memory when a web app restart causes memory to be cleared. Also, in web farms a database is the primary way to share state between web servers in the farm, so these tables facilitate that.