A method for key generation at a terminal device supporting at least two communication modes of operation is described. The method comprises, at a terminal device comprising a key derivation function: receiving at least one input parameter from a first wireless communication system; generating a first key in the key derivation function using the received at least one input parameter and at least one first locally stored input parameter in order to transmit in a first mode of operation on the first wireless communication system; and generating a second key in the key derivation function solely using at least one second locally stored input parameter in order to transmit in a second mode of operation on a second wireless communication system; wherein the at least one second locally stored input parameter is arranged to enable the second key to be used as the first key.