 FIGURE 17.8 The packet structure after application of the ESP protocol IKE is used to establish shared security parameters and authenticated keys, which amounts to establishing SAs between peer systems that are engaged into an IPsec session. More specifically, IKE is a hybrid of the Oakley (from which it has adopted multiple modes) and SKEME (from which it has adopted sharing and rekeying techniques) protocols. It operates in the Internet Security Association and Key Management Protocol (ISAKMP) framework, which fully defines packet formats, retrans- mission timers, and message construction requirements. As a result, IKE defines its own way of deriv- ing authenticated keying material and negotiating shared policy. A few words are appropriate here about the module names that we just mentioned. First, Oakley was designed by University of Arizona cryptographer Hilarie Orman. It is a free-form protocol that essentially enables parties to advance the state of the protocol at their own pace. It allows an authen- ticated key exchange. IKE actually borrowed the idea of some of its modes from Oakley and codified them into concretely specified handshake exchanges. SKEME is a key exchange protocol that was originally designed by cryptographer Hugo Krawczyk. It defines a mechanism of authenticated key exchange, whereby the communicating par- ties use public-key encryption to authenticate each other and then they share some randomly gener- ated components of the exchange, which the parties exchange among themselves using public encryption. It may be combined with a regular DH handshake. ISAKMP is a framework that was developed originally by the NSA and defines how peer devices engage into a secure communications session. This includes how the exchanged messages are con- structed and formatted, and the exact state transitions that have to be undergone in order to secure the communication session. It provides means that allow a device to properly authenticate a peer, to exchange information needed for a key exchange, and to ne