A method of subscriber identity authentication in an Internet Protocol Multimedia Subsystem, IMS, core network, comprising: receiving, at an IMS Centralized Services Gateway, ICS GW, an access request from a User Equipment, UE, having a Subscriber Identity Module, SIM; requesting, by a node in the IMS core network, IMS Authentication and Key Agreement, AKA, authentication vector information from a Home Subscriber Server HSS; retrieving, by the HSS, A3 authentication algorithm/A8 confidentiality key creation authentication vector information, A3/A8 authentication vector information, including an A3/A8 authentication response from a Home Location Register HLR; encoding, by the HSS, the A3/A8 authentication vector information into parameters defined for IMS AKA by adding sufficient number of filler bits of a known value or adding sufficient number of 0 bits to achieve required bit lengths of parameters of IMS AKA and transmitting the encoded A3/A8 authentication vector information having parameters defined for IMS AKA along with a random number, RAND, to the node in the IMS core network; receiving, by the ICS GW, an authentication challenge from the node in the IMS core network with the encoded A3/A8 authentication vector information and the RAND; detecting, by the ICS GW, the A3/A8 authentication vector information and issuing an A3/A8 authentication challenge containing the RAND to the UE; receiving, by the ICS GW, an A3/A8 authentication response from the UE; encoding, by the ICS GW, the A3/A8 authentication response from the UE into parameters defined for an IMS AKA authentication response; and transmitting, by the ICS GW, the encoded A3/A8 authentication response from the UE having parameters defined for an IMS AKA authentication response to the node in the IMS core network for comparing, by the node, the A3/A8 authentication response from the HLR with the A3/A8 authentication response from the UE. receiving, at an IMS Centralized Services Gateway, ICS GW, an access request from a User Equipment, UE, having a Subscriber Identity Module, SIM; requesting, by a node in the IMS core network, IMS Authentication and Key Agreement, AKA, authentication vector information from a Home Subscriber Server HSS; retrieving, by the HSS, A3 authentication algorithm/A8 confidentiality key creation authentication vector information, A3/A8 authentication vector information, including an A3/A8 authentication response from a Home Location Register HLR; encoding, by the HSS, the A3/A8 authentication vector information into parameters defined for IMS AKA by adding sufficient number of filler bits of a known value or adding sufficient number of 0 bits to achieve required bit lengths of parameters of IMS AKA and transmitting the encoded A3/A8 authentication vector information having parameters defined for IMS AKA along with a random number, RAND, to the node in the IMS core network; receiving, by the ICS GW, an authentication challenge from the node in the IMS core network with the encoded A3/A8 authentication vector information and the RAND; detecting, by the ICS GW, the A3/A8 authentication vector information and issuing an A3/A8 authentication challenge containing the RAND to the UE; receiving, by the ICS GW, an A3/A8 authentication response from the UE; encoding, by the ICS GW, the A3/A8 authentication response from the UE into parameters defined for an IMS AKA authentication response; and transmitting, by the ICS GW, the encoded A3/A8 authentication response from the UE having parameters defined for an IMS AKA authentication response to the node in the IMS core network for comparing, by the node, the A3/A8 authentication response from the HLR with the A3/A8 authentication response from the UE.