Jon Callas <jon(_at_)callas(_dot_)org> writes:
On 4/25/2002 5:54 PM, "Len Sassaman" <rabbi(_at_)quickie(_dot_)net> wrote:
I'd like to be able to run a service wherein a user submits a signed
document, and the service signs the signature. This is done to allow for
verification that the signature was made prior to the timestamp provided
by my service (the trusted notary).
Not the document, only the signature packet? I'm trying to envision what one
would do with this mechanically, as well as syntactically and semantically.
Yes. The notary verifies the signature, and then signs the
_signature_, not the document. This is why it's a signature on a
signature. The notary is trusted to have verified the contents before
it actually creates the new signature.
Note that you still cannot change the document, because to change the
document you would need to change the signature (unless you break the
Hash function). If you change the signature, then the notary
signature fails. Therefore, transitively, the notary is verifying
the document.
At least, this was the theory we had when wrtiing 1991.
Jon
-derek
--
Derek Atkins
Computer and Internet Security Consultant
derek(_at_)ihtfp(_dot_)com www.ihtfp.com