A method of generating a key by a first correspondent. The key is computable by a second correspondent. The method comprises the steps of: a) making available to the second correspondent a first short term public key; b) obtaining a second short term public key from the second correspondent; c) computing a first exponent derived from the first short term private key, the first short term public key, and the first long term private key; d) computing a second exponent derived from the first short term private key, the first short term public key, the second short term public key and the first long term private key; e) computing a simultaneous exponentiation of the first exponent with the second short term public key and the second exponent with the second long term public key.