tion SK { } indicates that the blocks are completely encrypted and protected (Figure 7.8). The responder communicates its identity in the IDr block. It may send a certificate. It authenticates its identity and protects the integrity of the second message of the IKE_SA_INIT exchange. It completes the negotiation of the ESP/AH security association with the SAr2 block (Figure 7.8). 7.1.3.3. CREATE_CHILD_SA exchange The CREATE_CHILD_SA exchange is used to create the ESP/AH security association and to renew the keys of the IKE and ESP/AH security association. For the exchange involving the creation of the ESP/AH security association, the initiator finalizes it in the SA block and the traffic selectors proposed for the SA in the TSi and TSr blocks. It transmits a random number in the Ni block and optionally a Diffie-Hellman value in the KEi block (Figure 7.9). HDR, SK {SA, Ni, [KEi], TSi, TSr} Initiator HDR, SK {SA, Nr, [KEr], TSi, TSr} Responder Figure 7.9. CREATE_CHILD_SA exchange creation of ESP/AH SA The responder confirms the offer in the SA block. It transmits a Diffie- Hellman value in the KEr block, if the KEi block has been included in the request (Figure 7.9). SWu Tunnel Establishment The KEYMAT key, used for the ESP/AH security association, is derived from the SK d key and the random numbers Ni and Nr. If the exchange contains Diffie-Hellman values, then the secret D-H key obtained also participates in the creation of the KEYMAT key: KEYMAT = PRF (SK_d, Ni Nr) KEYMAT = PRF (SK_d, D-H key | Ni | Nr) To renew the IKE SA key, the initiator sends the parameters in the SA block, a random number in the Ni block and a Diffie-Hellman value in the KEi block. The initiator's new SPI is provided in the SA block (Figure 7.10). The responder confirms the offer in the SA block. It transmits a Diffie- Hellman value in the KEr block. The responder's new SPI is provided in the SA block (Figure 7.10). HDR, SK {SA, Ni, KEi} Initiator HDR, SK {SA, Nr, KEr} Responder Figure 7.10. CREATE CHILD SA exchange renewal of IKE SA key Th