The responder is initialized with a pre-shared long-term static key, which is assumed to be pre-authenticated out of band by the initiator.
Message A, sent by the initiator, does not benefit from sender authentication and does not provide message integrity. It could have been sent by any party, including an active attacker. Message contents benefit from message secrecy and some forward secrecy: the compromise of the responder's long-term private keys, even at a later date, will lead to message contents being decrypted by the attacker. 0,2
Message B, sent by the responder, benefits from sender authentication and is resistant to Key Compromise Impersonation. Assuming the corresponding private keys are secure, this authentication cannot be forged. However, if the responder carries out a separate session with a separate, compromised initiator, this other session can be used to forge the authentication of this message with this session's initiator. Message contents benefit from some message secrecy and some forward secrecy, but not sufficiently to resist any active attacker. 2,1
Message C, sent by the initiator, benefits from sender and receiver authentication and is resistant to Key Compromise Impersonation. Assuming the corresponding private keys are secure, this authentication cannot be forged. Message contents benefit from message secrecy and weak forward secrecy under a passive attacker: if the initiator's long-term static keys were previously compromised, the later compromise of the responder's long-term static keys can lead to message contents being decrypted by an attacker. 4,3
Message D, sent by the responder, benefits from sender and receiver authentication and is resistant to Key Compromise Impersonation. Assuming the corresponding private keys are secure, this authentication cannot be forged. Message contents benefit from message secrecy and strong forward secrecy: if the ephemeral private keys are secure and the initiator is not being actively impersonated by an active attacker, message contents cannot be decrypted. 4,5
Message E, sent by the initiator, benefits from sender and receiver authentication and is resistant to Key Compromise Impersonation. Assuming the corresponding private keys are secure, this authentication cannot be forged. Message contents benefit from message secrecy and strong forward secrecy: if the ephemeral private keys are secure and the responder is not being actively impersonated by an active attacker, message contents cannot be decrypted. 4,5
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