The encryption key ck esp is the same for the two

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The encryption key CK ESP is the same for the two pairs of simultaneously established SAs. The encryption key CK ESP is obtained from the key CK IM established as a result of the AKA procedure, specified in clause M.6.1, using a suitable key expansion function. The encryption key expansion on the user side is done in the UE. The encryption key expansion on the network side is done in the P-CSCF. M.6.3 Integrity mechanisms IPsec ESP as specified in reference RFC 4303 [54] shall provide integrity protection of SIP signalling between the UE and the P-CSCF, protecting all SIP signalling messages at the IP level. IPsec ESP general concepts on Security Policy management, Security Associations and IP traffic processing as described in reference RFC 4301 [53] shall also be considered. ESP integrity shall be applied in transport mode between UE and P-CSCF either in transport mode if no NAT is present or – if NAT traversal shall be supported – in UDP encapsulated tunnel mode. The method to set up ESP security associations (SAs) during the SIP registration procedure is specified in clause M.7. As a result of an authenticated registration procedure, two pairs of unidirectional SAs between the UE and the P-CSCF, all shared by TCP and UDP, shall be established in the P-CSCF and later in the UE. One SA pair is for traffic between a client port at the UE and a server port at the P-CSCF and the other SA is for traffic between a client port at the P-CSCF and a server port at the UE. For a detailed description of the establishment of these security associations see clause M.7. The integrity key IK ESP is the same for the two pairs of simultaneously established SAs. The integrity key IK ESP is obtained from the key IK IM established as a result of the AKA procedure, specified in clause M.6.1, using a suitable key expansion function. This key expansion function depends on the ESP integrity algorithm and is specified in Annex I of this specification. The integrity key expansion on the user side is done in the UE. The integrity key expansion on the network side is done in the P-CSCF. 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 61 Release 12
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The anti-replay service shall be enabled in the UE and the P-CSCF on all established SAs. M.6.4 Hiding mechanisms The text in section 6.4 applies without changes. M.6.5 CSCF interoperating with proxy located in a non-IMS network The text in section 6.5 applies without changes. M.7 Security association set-up procedure M.7.0 General The security association set-up procedure is necessary in order to decide what security services to apply and when the security services start. In the IMS authentication of users is performed during registration as specified in clause M.6.1. Subsequent signalling communications in this session will be integrity protected based on the keys derived during the authentication process.
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