1. A method of generating a shared key at one of a pair of correspondents participating in a public key cryptosystem, said shared key to be used by said one correspondent in communicating with another correspondent over a data communication channel, each of said correspondents respectively having a long-term private key and a corresponding long-term public key, and said shared key having the form of a combination of a long-term private key of one of said correspondents with a long-term public key and an ephemeral public key of another of said correspondents, said method comprising: a) said one correspondent obtaining said long-term public key of said other correspondent; b) said one correspondent determining whether said ephemeral public key of said other correspondent is the same as said long-term public key of said other correspondent; c) upon determining that said long-term public key and said ephemeral public key of said other correspondent are the same, said one correspondent utilising said long-term public key of said other correspondent as both said long-term public key and said ephemeral public key of said other correspondent in generating said shared key, and d) utilising said shared key to exchange information between said correspondents.