Abstract or Summary:
The use of timestamps in key distribution protocols was suggested by Denning and Sacco. Timestamps are now used in most production authentication services including Kerberos. Concerns have been raised about the security implications of this practice. Timestamps are necessary in authentication protocols that support multiple authentication without multiple requests to an authentication server. Kehne, Schonwalder, and Langendorfer have proposed a nonce-based protocol for multiple authentications that they claim improved upon the Kerberos protocol because it does not depend on the presence of synchronized clocks.

This note discusses the use of timestamps as nonces and demonstrates a nonce-based mutual-authentication protocol requiring only four messages, one less than described in [KSL92], and the same number of messages required for mutual-authentiction in Kerberos. The note concludes by suggesting extensions to our protocol that allow the use of verifier issued timestamps as nonces while recovering some (though not all) of the benefits of traditional timestamps.