One of the most challenging aspects in Next Generation Networks (NGNs) is the mechanism through which the different technologies will complement each other to achieve a constant access to networks. This research work deals with the problem of defining fast movements without interruptions (seamless handoffs) in NGNs. This objective is achieved through a secure key distribution process, which enables a fast re-authentication process providing both user anonymity and untraceability. The developed access control system offers a set of features not covered so far by a single solution: (1) applicable for EAP-based NGNs; (2) reduction of the authentication latency in mobile environments irrespective of the type of handoff performed by the user; (3) provision of strong security properties; (4) easy deployment in current networks; (5) compatibility with current standardized technologies; and (6) user privacy support.