es. These sessions use different encryption keys, and entire cryptographic contexts must be maintained. This is the first indication of a potential need for a dedicated coprocessor where the main baseband processor of the base transceiver station will offload cryptographic computations in order to pre- serve its own capability of managing a cell or a family of cells in the first place. 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 363 Mobile Client WAP Gateway Internet-based Server Encoded Request Request Script Filtering Agent Security Encoder/ security Decoder Encoded Request Response Web pages Images File Content Air-link Land-based FIGURE 17.1 The security approach in a WAP environment based on a gateway that decrypts and reencrypts in both directions while operating between incompatible mobile and land-based network infrastructures. No end-to-end security can be guaranteed in this scheme. CRYPTOGRAPHY: SOME BASIC NOTIONS This book examines network processing. Therefore, we will not to spend too much time on the ject of cryptography. For more information on this topic, refer to the several outstanding texts we list at the end of the chapter. For the sake of convenience and because a significant portion of the readers most likely do not possess any special expertise on the subject, we will introduce some fundamental concepts and state some facts without any further elaboration. The notions we explain will also put the capabilities of security coprocessor chips into context, and the uninitiated reader will be able to appreciate the impact of what vendors integrate inside their chips and why. At the transmitting station, encryption or enciphering is a deterministic mathematical process amounting to the controlled alteration of an input bit sequence called plaintext, which a user intends to send 