ey can range from efficiency 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 NETWORK PROCESSORS Encryption Encrypt State Encrypt State Encrypt State Decrypt State Decrypt State Decrypt State Decryption FIGURE 17.4 CBC mode applied on a block cipher like DES cipher and the associated need for an IV and fault tolerance to capabilities to guard against the loss of synchronization all the way to resistance to specific types of attacks (such as chosen plaintext and error injection in the ciphertext). As the requirements are diverse based on each system implementation, the cryptographic community developed several types of cryptographic modes of operation. These have been exhaustively analyzed and documented, and their behavior is well understood. A cryptographic mode is usually the combi- nation of the underlying cipher algorithm along with some feedback mechanisms and other associ- ated operations that produce a chain-like behavior. The most widely known and used cryptographic modes are electronic codebook (ECB), cipher block chaining (CBC), cipher feedback (CFB) (also known as cipher text auto key mode [CTAK]), and output feedback (OFB) (also known as key auto key [KAK] mode. Many other less widely known modes such as propagating CBC mode, OFB with a nonlinear function, and plaintext feedback are available, but we will not elaborate on those here. Bruce Schneier's book Applied Cryptography: Protocols, Algorithms, and Source Code in C (New York: John Wiley, 1996) examines the pros and cons of many cryptographic modes. The important point for our discussion is that different crypto- graphic modes are available and the choice of which one to use in a system or session may very well decide whether two devices can communicate. As mentioned earlier, the interoperability of crypto- graphic systems has traditionally been a problem