The initiator is initialized with a pre-shared long-term static key, which is assumed to be pre-authenticated out of band by the responder.
Message A, sent by the initiator, benefits from receiver authentication but is vulnerable to Key Compromise Impersonation. If the responder's long-term private key has been compromised, this authentication can be forged. However, if the initiator carries out a separate session with a separate, compromised responder, this other session can be used to forge the authentication of this message with this session's responder. 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. 1,2
Message B, 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 weak forward secrecy under a passive attacker: if the responder's long-term static keys were previously compromised, the later compromise of the initiator's long-term static keys can lead to message contents being decrypted by an attacker. 4,3
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
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