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Lets say there are 3 people.

  • 1st signs a message and sends it to 2nd using public key encryption
  • 1st deletes the message
  • 1st tells 2nd to forward his message
  • 2nd forwards the message to 3rd

How can 1st verify the message was forwarded untampered without trusting 2nd or 3rd and without finding out the message contents again?

A terrible idea is to have 3rd give his private key to a 4th who will confirm to 1st that his message was untampered.

I am new to this so if what I want to do is impossible a heads up would be deeply appreciated.

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  • $\begingroup$ Would simply storing a - possibly salted - hash over the message at A work? Then to verify C can recalculate the hash over the message and return it to A - possibly including the salt send to him by A into the hash calculation, of course. This would work even without a private key, although C could also sign the hash or return the signature & hash after verification to make it more secure. $\endgroup$ – Maarten Bodewes Nov 16 '17 at 15:24
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I see this as a mixture of easy and impossible. A standard digital signature will work to verify the contents, as long as the document+signature is sent to person 1 (or whoever wants to check the document)

The impossible part comes if you only want person 1 to only have the signature and not the document. Even if person 1 keeps a copy of every signature, Person 2 can send a tampered document with the original signature. Person 3 would know the document has been tampered, but if 3 wants to be part of the deception they can just send the original signature to person 1.

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