ltimately enables the parties to map the secu- rity services they require on specific links based on established security policies (as contained in a policy database) as well as on multiple cryptographic contexts (keys, sequence numbers, destination and source addresses, and the choice of protocol used). The AH protocol provides data integrity, data source authentication, and protection against play- back (replay) attacks. As it does not provide confidentiality, it involves a new packet header but not a new trailer. The ESP protocol provides confidentiality, data integrity, and data source authentication of IP packets as well as protection against replay attacks. We saw earlier that a new ESP header is created. Part of the ESP packet payload is encrypted and a portion is in the clear to allow the recipient to prop- erly process the packet. All the operations needed to create this new material obviously require a new trailer. The result is a new combination of plaintext-ciphertext-and-authenticated material, as shown in Figure 17.8. It goes without saying that as both confidentiality and authentication are ensured by the ESP protocol, the SPI, which identifies an SA, refers to a combination of cipher and authenticator algo- rithms. In addition to using block ciphers in CBC mode, communication of the necessary IV is also foreseen. The AH and ESP protocols can be used independently or jointly in some sessions. Security and computational issues are involved depending on which one is performed first and on which mode IPsec is configured to perform (transport or tunnel) in that specific case. The pros and cons go beyond our scope and the reader is referred to the literature. 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 387 Encrypted IP Header ESP Header Protected Original Payload Trailer Authenticated