library.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 373 Protocol-Sensitive Encryption The first use of communications encryptors on digital lines was applied to point-to-point links, where all the transmitted bits including the useful information, the headers, and the trailers of frames (in the case of old BSC/SDLC protocols) are encrypted. These encryptors were called link encryptors and did not care what protocol was used on the link they purported to secure. Assuming that the receiver was always listening to the channel and that it was in possession of the symmetric used for the encryption, decryption was not a problem. For obvious reasons, people applied this tech- nique predominantly on private leased lines or on wireless radio links (in land-mobile or space communications). However, with the arrival of packetized networks such as X.25 and frame relay, as well as with the subsequent asynchronous transfer mode IP/ATM explosion, the act of encrypting indiscriminately all traffic on a link bit stopped. Instead, only the payload of packets or frames would be encrypted by leaving the headers (which contain source and destination addresses) untouchable SO that the encrypted packets or frames could still be routed and switched by the network infrastructure without problems. That provides the benefit of packet-switched networks to customers who needed to move away from the old regime of leased lines for financial reasons while providing them with the ability to operate and exchange information with other sites in a more secure environment than before. From a security standpoint, the idea was that decryption would only be possible at the intended destination receiver, and any other party accidentally or intentionally intercepting the packets or frames could not decipher its contents. Of course, the CRC codes of the new packets/frames would have to be regenerated after the payload encryp