EAP-AKA' includes optional support for identity privacy mechanism that protects the privacy against passive eavesdropping. The mechanism is described in Section 188.8.131.52 of RFC 4187, and it uses pseudonyms that are delivered from the EAP server to the peer as part of an EAP-AKA exchange. The privacy mechanism described in RFC 4187 corresponds to the privacy provided by 5G-GUTI, however, assignment of 5G-GUTI is done outside the EAP framework in 5GS.
TS 33.501 assumes that the SUCI is sent outside the EAP messages, however, the peer may still receive EAP-Request/Identity or EAP-Request/AKA-Identity messages. Table F.2-1 specifies how the 5G UE shall behave when receiving such requests.
EAP-Response/AKA-Client-Error with the error code "unable to process packet"
AT_IDENTITY=fast re-auth identity OR
RFC 3748 allows the peer to respond with abbreviated Identity Response where the peer-name portion of the NAI has been omitted. The 5G UE responds with SUCI in the same format as sent in the Registration Request, where the peer name has been encrypted.
RFC 4187 allows the peer to respond with a pseudonym (cf. 5G-GUTI) or the permanent identity (i.e. SUPI). The 5G UE follows the "conservative" policy that has been described in Section 4.1.6 of RFC 4187 (Attacks against Identity Privacy) for the pseudonym based privacy, i.e. the peer shall not reveal its permanent identity. Instead, the peer shall send the EAP-Response/AKA-Client-Error packet with the error code "unable to process packet", and the authentication exchange terminates. The peer assumes that the EAP-Request/AKA-Identity originates from an attacker that impersonates the network, and for this reason refuses to send the cleartext SUPI.
RFC 4187 allows the peer to respond with a pseudonym (cf. 5G-GUTI) or the permanent identity (i.e. SUPI). The 5G UE responds with SUCI.
RFC 4187 allows the peer to respond with a fast re-authentication identity, pseudonym (cf. 5G-GUTI) or the permanent identity (i.e. SUPI). If the 5G UE supports fast re-authentication, it responds with the fast re-authentication identity, and if the 5G UE does not support fast re-authentication, it responds with SUCI.
EAP-AKA' uses the subscriber identity (Identity) as an input to the key derivation when the key derivation function has value 1 (i.e. MK = PRF'(IK'|CK',"EAP-AKA'"|Identity)). Section 7 of RFC 4187 describes that the Identity is taken from the EAP-Response/Identity or EAP-Response/AKA-Identity AT_IDENTITY attribute sent by the peer. This principle is not applied to the 5GS.
If the AT_KDF_INPUT parameter contains the prefix "5G:", the AT_KDF parameter has the value 1 and the authentication is not related to fast re-authentication, then the UE shall set as the Identity for key derivation. When the SUPI Type is IMSI, the Identity shall be set to IMSI as defined in clause 2.2 of TS 23.003. When the SUPI type is network specific identifier, the Identity shall be set to Network Access Identifier (NAI) as defined in clause 28.7.2 of TS 23.003. When the SUPI type is GLI, the Identity shall be set to GLI taking format of NAI as defined in clause 28.15.2 of TS 23.003. When the SUPI type is GCI, the Identity shall be set to GLI taking format of NAI as defined in clause 28.16.2 of TS 23.003. This principle applies to all full EAP-AKA' authentications, even if the UE sent SUCI in NAS protocol or if the UE sent SUCI in the respose to the EAP identity requests as described in Table F.2-1 or if no identity was sent because the network performed re-authentication. The only exception is fast re-authentication when the UE follows the key derivation as described in RFC 5448 for fast re-authentication.