The use of suitable measures in a method for agreeing on a security key between at least one first and one second communication station to secure a communication link is improved so that the security level for the communication is increased and the improved method can be combined with already available methods. A first parameter is determined from an authentication and key derivation protocol. In addition, an additional parameter is sent securely from the second to the first communications station. A security key is then determined from the first parameter and the additional parameter.