Signing a message to make sure that it will not be tampered with when forwarded, without trusting the receivers? 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.
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.