Defines the methods for protocol message signing using a session signing key. This requires signatures to include information about which session key is being used, as well as an authorization by a long-term key through an additional signature. This extra signature covers the session key, its validity period, and the synchronizer for which it is valid. This allows us to use the session key, within a specific time frame and synchronizer, to sign protocol messages instead of using the long-term key. Session keys are intended to be used with a KMS/HSM-based provider to reduce the number of signing calls and, consequently, lower the latency costs associated with such external key management services.