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From RFC 4880 - Open PGP Message Format (emphasis added) OpenPGP implementations SHOULD compress the message after applying the signature but before encryption. As the signature provides authentication, and you specifically ask about authentication, I think that quote from the RFC should answer your question. Compression does not affect ...


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Authentication can either mean entity authentication or data authentication. Data authentication is a means to demonstrate that some specific data originates from a specific source and has not been modified in transit/on storage. It can be achieved by the use of digital signatures in a public key, i.e., asymmetric, setting or message authentication codes ...


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Seems like the answer is that your goal is obviously impossible, without Bob's cooperation. How would Alice distinguish between (1) a write to the disk by Bob's PC, initiated at Bob's request, vs (2) a write to the disk by Bob's PC, initiated by the malware? She can't. They look identical from Alice's perspective. For instance, having Bob's PC sign ...


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Yes. $\:$ Since "Eva sees everything written on Bob's PC", the process needs to involve something that's not written on Bob's PC. $\:$ They could use either some sort-of MAC that can be computed with pencil-and paper, or a signature scheme whose private key is held on a hardware token. I don't have any particular such MACs in mind, although my immediate ...



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