Re: Choosing key to verify someone else's sig?

From: David Wagner (daw_at_mozart.cs.berkeley.edu)
Date: 09/12/03 

Date: Fri, 12 Sep 2003 00:20:58 +0000 (UTC)

Trevor Perrin wrote:
> - Given a signed document from Bob, you shouldn't assume that Bob was
>the original author.
>
> - Given a signature that verifies with Bob's key, you shouldn't
>assume that Bob's key was the original signer.

I guess what I really mean is the second one. The following is probably
also good advice:
  - Given a signature that verifies with Bob's key, you shouldn't assume
  that Bob was the original author.

There's no way for the receiver to tell whether it was *really* Bob who
signed the document. For example, Bob could have given his private
key to Charlie, and the document could have been signed by Charlie.
Or, the document might originally have been signed by Zacchary, and Bob
might have stripped Zacchary's signature off and added his own.

All that a receiver can do is check whether the signature verifies using
Bob's public key, so the most we can verify is that some holder of Bob's
private key endorsed or approved of the document. We can't ensure that
Bob was the original author.

You're right that it is important to be precise here. I apologize for
my imprecision.

In any case, these number-theoretic properties of RSA that let you match
an existing signature are only really relevant when Bob's signature is
sent on a different, "more authentic" channel than Bob's public key.
That's pretty rare in practice -- or if it isn't, it ought to be!

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