is message depends on the requirements of the application. Time can be read from an internal clock or from a time server, which may be next door or at the other side the planet. The concepts of how this time server is trusted and how its answer is kept untampered are related to the information security discipline, which is amply discussed in the references. The idea however, that if a payment instruction to a financial institution is timestamped, hashed, and digitally signed, then upon signature verification, no discussion can occur about whether the order was sent or whether it was actually originated by the person who digitally signed the order. Cryptography there- fore enables nonrepudiation and in some cases this can be an issue of very serious importance. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. SECURITY COPROCESSORS SECURITY COPROCESSORS 377 Rekeying Sophisticated hardware-based encryption systems are capable of changing the underlying symmetric encryption key several times during the same session. Some systems can do it periodically, whereas others can do it at quasirandom points in time. This dramatically enhances the security of the munication, but it causes a significant increase in the cost of implementation. This is because spe- cialized signaling protocols (usually working in-band) are needed to safely and reliably halt the exchange of live traffic with the necessary buffering and without loss of information, to renegotiate or generate a new symmetric key, to initialize the appropriate internal states at both the transmitting and receiving portions of the communicating parties, and to resume traffic from the correct point using the new cryptographic context and key. Security Associations (SAs) In a network-processing environment, where numerous sessions are set up and maintained, it is vitally important to preserve the cr