ect messages in the other direction are SK ar and SK_er. Other keys are also derived from the SKEYSEED key. The SK d key is used to derive the keys used in the ESP/AH security association. The SKEYSEED key is calculated using the Diffie-Hellman secret (D-H key) and the random numbers Ni and Nr: SKEYSEED = PRF (D-H key, Ni Nr) SWu Tunnel Establishment The keys SK_d, SK_ai, SK_ar, SK_ei, SK_er, SK_pi and SK_pr are generated as follows: SK_d = PRF (SKEYSEED, Ni | Nr | SPIi SPIr 0x01); SK_ai = PRF (SKEYSEED, SK_d|Ni| Nr SPIi SPIr | 0x02); SK_ar = PRF (SKEYSEED, SK_ai Ni Nr | SPIi SPIr 0x03); SK_ei = PRF (SKEYSEED, SK_ar Ni | Nr I SPIi | SPIr 0x04); SK_er = PRF (SKEYSEED, SK_ei|Ni Nr SPIi SPIr 0x05). 7.1.3.2. KE_AUTH exchange The second exchange, IKE_AUTH, is used to authenticate previous IKE messages and communicate identities and possibly exchange certificates, as well as to establish the first AH/ESP security association. These messages are completely encrypted and protected by the keys established during the IKE_SA_INIT exchange. The initiator indicates its identity with the IDi block. It authenticates its identity and protects the integrity of the first message in the IKE_SA_INIT exchange using the AUTH block. It can possibly send its certificate in the CERT block and the certification authority's identity in the CERTREQ block. In this case, the AUTH block contains the initiator's signature (Figure 7.8). HDR, SK {IDi, [CERT,] [CERTREQ,] [IDr,] AUTH, SAi2, TSi, TSr} Initiator HDR, SK {IDr, [CERT,] AUTH, SAr2, TSi, TSr} Responder Figure 7.8. IKE_AUTH exchange The optional IDr block enables the initiator to specify which of the responder's identities it wishes to communicate with (Figure 7.8). 140 Wi-Fi Integration to the 4G Mobile Network The initiator starts the negotiation of the AH/ESP security association with the SAi2 block. The TSi block specifies the characteristics of the packets transferred by the initiator. The TSr block specifies the address for packets transferred to the responder (Figure 7.8). The nota