# Tag Info

4

No. Cryptography alone cannot solve this problem. Solving this problem requires a combination of technical (e.g., cryptography, systems security) and non-technical (e.g., legal, regulatory, contractual) solutions. Even the technical part is not solely a cryptography question; it as much about systems security.

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A theoretical concept for that is covered by so called contract signing protocols. There are quite some research papers into this direction, such as the seminal paper and follow up works in the field of (optimistic) contract signing. For instance, this one or this one. Such protocols always involve a trusted third party, although this party might not be ...

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I think destroying the private key and using a notary could be some kind of solution to that problem. Both Parties create a private and public key. The public keys are signed by a CA. Both parties sign the document with their private key. After signing the document both parties destroy their private key. After step 4 nobody can claim that he lost the ...

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You can use the well established TLS (Transport Layer Security) protocol to achieve the first three properties and modify it to include a digital signature for non-repudiation*. However, strictly speaking, non-repudiation requires the use of certificates from a CA so that the signature can be verified by any third party.

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Here's another idea, which seems to be quite similar to poncho's solution, but uses RSA keys: Bob writes his message $P$, creates a random symmetric key $K$ and uses a MAC to calculate a MAC tag $T$ for the message. He signs only this key $K$ with his private RSA key, and also encrypts it with Alice's public key. The transmitted message consists of the ...

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