block. This drawback is called error extension, and it can be detrimental in some applications. After these two errors (first a block and then a bit are deciphered incorrectly), the system recovers auto- matically, and CBC is said to be self-recovering. However, this is only true as long as the block-level synchronization is maintained. If a bit is lost or an extra bit is inserted into the ciphertext due to some interference, the receiver will continue decrypting traffic to garbage and the algorithm will be unable to recover. This is a good example of a classic case where higher-layer communication protocols cou- pled with error correction codes (ECCs) must be combined at the right place to preserve the integrity of CBC-based cryptographic operations. In the IP network world, this can mean that the security requirements of a streaming audio or video application that runs with the Real-Time Protocol (RTP) over User Datagram Protocol (UDP) (which is less reliable than Transmission Control Protocol [TCP]) may actually negate the use of underlying encryption if the latter is configured to run in an undesirable cryptographic mode, whereas file trans- fer or e-mail sent simultaneously with the former may have absolutely no problem with it, as they using TCP's reliability of connection. Some people are surprised to suddenly find out that the choice of encryption technique is often not dictated by security concerns and requirements only, but by the apparently irrelevant network proto- col stack and software infrastructure, which seem completely unrelated factors at first glance. IMPORTANT CRYPTOGRAPHIC CONSIDERATIONS IN COMMUNICATIONS Several techniques are being systematically used in the implementation of cryptographically equipped communication systems. Some of these techniques have become mainstream ideas, whereas some remain less well known to users. We will mention several of these techniques to give an idea of what Downloaded from Digital Engineering Library McGraw-Hill (www.digitalengineeringlibrary.com) Copyrigh