Cyptographic key generation

A method of generating a cryptographic key (120) to protect a mobile communication between two entities (202, 204), wherein the method is carried out by the first entity (202, 302) as part of an Authentication and Agreement procedure Key <AKA> based on a UMTS AKA protocol initiated by the second entity (204, 304), the method comprising the steps of: - providing (306) an input to a key derivation function, the input comprising at least two parameters (106, 108), wherein the first parameter (106) comprises or is derived from a set of cryptographic keys (110, 112 ) that have been calculated by the first entity (202) executing the UMTS AKA procedure, and the second parameter comprises or is derived from a token (116) calculated by the second entity (204, 304) executing the UMTS AKA procedure for the first entity (202, 302); wherein the first parameter comprises a concatenation of an encryption key <CK> (110) and an integrity key <IK> (112); and - applying (308) the key derivation function to generate the cryptographic key (120) based on the input provided, wherein the cryptographic key (120) is an Access Security Management Entity (ASME) key, KASME ; wherein the token (116) comprises or is derived from a sequence number <SQN> indicating the number of times the UMTS AKA procedure has been initiated by the second entity (204, 304) for the first entity (202 , 302). wherein the entry is unique each time the UMTS AKA procedure is initiated by the second entity (204, 304) for the first entity (202, 302); and wherein the token (116) is an SQN or comprises an exclusive OR of the SQN and an anonymity key <AK>, wherein the AK is a cryptographic key produced by a key generation function f5 using a random challenge conforming to the AKA protocol of UMTS.