AKA stands for Authentication and Key Agreement. It is based on RFC 3310 and is adopted by 3GPP for GSM /UMTS/LTE Authentication Algorithm and Authentication Process in IMS Registration. This is basically a shared Key mechanism. The shared key is 'K' value that is distributed to each UE via UICC / ISIM and managed in AuC in Server (Network) side. For the detailed meachanism of AKA besing used in Cellular Network and IMS Registration, refer to following pages.