res. Both are needed for a system to handle authentication; however, the former is computationally slower than the latter. When a vendor says that their equipment handles digital signatures in X milliseconds, find out which one of the two operations they refer to. Embedded Sequence Counters We often take for granted that in packetized networks higher-level protocols generate counters and map their content into header fields. The advantage of this is that the receiver knows upon reception of a packet that this is the kth packet out of a total of m packets. If packets arrive out of order, which 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 376 NETWORK PROCESSORS is extremely probable in a routed world as they are traversing many different routes on their way to the same destination, the appropriate intelligence will reorder the packets at the receiver in the cor- rect sequence, stripping away the headers and trailers. The reordered recovered payload is then pre- sented to the higher layers that await to process it (the presentation and application layers). To ensure that unintended receivers (eavesdroppers) do not acquire any knowledge about how many packets intended for a destination are parts of a specific transmission sequence, protocol-sen- sitive encryptors (and now IPsec) embed internal counters inside the secure payload with a sliding window over the bitstream. The receiver upon decryption will know that this is the kth packet from a transmission of m consecutive packets that should be accepted according to the current window. If the counter is connected to a well-thought-out timestamping process, it protects against playback attacks, even if someone knows the cryptographic context and tries to alter the packets. In simpler words, we cannot be receiving the 141st packet for payment clearance as all the packet-rela