A mobile device and an authentication server are configured to re-establish a security context that was previously established using an Authentication Key Agreement (AKA) procedure. The re-establishment advantageously uses re-use information saved from the preceding AKA procedure, including using synchronization information for each such re-establishment that occurs between AKA procedures. The synchronization information particularly identifies each instance of re-establishment and depends on a sequence number assigned to the preceding AKA procedure and on any previous instances of re-establishing the security context.