A method of establishing keys for at least partially securing media plane data exchanged between first and second end users via respective first and second media plane network nodes. The method comprises sending session set-up signalling from said first end point towards said second end point, said session set-up signalling including a session key generated by said first end point. The set-up signalling is intercepted at a first signalling plane network node and a determination made as to whether or not a signalling plane key has already been established for securing the signalling plane between said first end point and said first signalling plane network node. If a signalling plane key has already been established, then a media plane key is derived from that signalling plane key, and the media plane key sent to said first media plane network node for securing the media plane between said first end user and said first media plane network node. If a signalling plane key has not already been established, then an alternative media plane key is derived from said session key and sent to said first media plane network node for securing the media plane between said first end user and said first media plane network node.