Actor Key Compromise: Consequences and Countermeasures

Despite Alice's best efforts, her long-term secret keys may be revealed to an adversary. Possible reasons include weakly generated keys, compromised key storage, subpoena, and coercion. However, Alice may still be able to communicate securely with other parties, depending on the protocol used. We call the associated property resilience against Actor Key Compromise (AKC). We formalise this property in a symbolic model and identify conditions under which it can and cannot be achieved. In case studies that include TLS and SSH, we find that many protocols are not resilient against AKC. We implement a concrete AKC attack on the mutually authenticated TLS protocol.