With the popularity of mobile devices, the mobile service requirements are now changing rapidly. This implies that the micro network operator dedicated to a specific sector of users has the potential to improve the 5G architecture in terms of scalability and autonomy. However, the traditional AKA protocol does not allow the micro operator to authenticate mobile users independently. To solve this problem, we propose the Fast AKA protocol, which disseminates a subscriber's profile among base stations via a Blockchain and mutually authenticates the subscriber and serving base station locally for roaming. The proposed architecture speeds up the authentication process, provides forward/backward secrecy, and resists replay attack as well as man-in-The-middle attack. We believe that Fast AKA can serve as a cornerstone for B5G.