1. A method at a mobile device configured for operation in a communication network comprising: establishing Authentication and Key Agreement (AKA) based security contexts with the communication network from time to time, based on performing corresponding AKA procedures with the communication network, each AKA procedure involving the derivation of corresponding session keys at the mobile device and at the communication network according to a shared secret known a priori at the mobile device and at the communication network, said corresponding session keys used for securing data exchanged during the established security context; and at one or more instances after the performance of a preceding AKA procedure and before the performance of a next, succeeding AKA procedure, re-establishing the security context from the preceding AKA procedure using re-use information saved from the preceding AKA procedure, wherein the re-use information depends on a sequence number assigned by the communication network to identify the preceding AKA procedure, and wherein re-establishing the security context from the preceding AKA procedure comprises, for each instance of re-establishment: forming a context regeneration request that includes synchronization information particularly identifying each instance of re-establishment and dependent on the sequence number assigned to the preceding AKA procedure and on any previous instances of re-establishing the security context; sending the context regeneration request to the communication network; receiving a nonce or other authentication value from the communication network in response to the context regeneration request; and generating a new session key to use at the mobile device for securing data exchanged during the re-established security context, based on the received authentication value and the shared secret.