DES Authentication on the Server Side

The method for determining the validity of a client's time stamp depends on which transaction is under consideration. For the first transaction, the server checks only that the time stamp has not expired.

For subsequent transactions, the server verifies that the time stamp is greater than the previous time stamp from the same client, and that the time stamp has not expired. A time stamp has expired if the server's time is later than the sum of the client's time stamp plus the client's window. The sum of the time stamp plus the client's window can be thought of as the lifetime of the credential.