Subject: Re: [smime] [pkix] Research question: Witnessing by digital signature

From: Matt McCutchen <[email-masked]>
Date: Tue, 15 Jun 2010 00:15:38 -0400

On Fri, 2010-06-11 at 06:01 +1000, Stephen Wilson wrote:
> Has any work been done in PKIX or elsewhere on formal witnessing of 
> digital signatures?  And/or ... does anyone in the group know of real 
> life instances where a digital signature is witnesses and attested to 
> using another dig sig?

How would one "witness" a digital signature?  To witness a physical
signature means that one has personal knowledge that the signer
intentionally signed the document.  Simply adding a digital signature to
a document bearing someone else's digital signature is not witnessing in
this sense.

-- 
Matt

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From: Peter Gutmann <[email-masked]>
Date: Thu, 17 Jun 2010 16:53:46 +1200

Matt McCutchen <[email-masked]> writes:
>How would one "witness" a digital signature?  To witness a physical signature
>means that one has personal knowledge that the signer intentionally signed the
>document.  Simply adding a digital signature to a document bearing someone
>else's digital signature is not witnessing in this sense.

  "When I use a word it means just what I choose it to mean -- neither more
   nor less"
   -- Humpty Dumpty (and PKI).

Peter.

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From: Matt McCutchen <[email-masked]>
Date: Thu, 17 Jun 2010 01:14:15 -0400

>    -- Humpty Dumpty (and PKI).

And what is it chosen to mean?  Maybe I can propose a better term for
the concept.

-- 
Matt

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From: Peter Gutmann <[email-masked]>
Date: Thu, 17 Jun 2010 18:16:14 +1200

Matt McCutchen <[email-masked]> writes:
>And what is it chosen to mean?  Maybe I can propose a better term for the
>concept.

I was commenting on the way PKI borrows standard, well-defined terms and uses
them in strange ways, "nonrepudiation" being another example (and,
incidentally, agreeing with your comment).  So perhaps "PKI-witness" can be
defined as:

  "A service designed to ensure PKI-nonrepudiation"

and then all you need to do is come up with a definition of PKI-nonrepudiation.

(My suggestion for the latter would be "A legal term misused for PKI in order
to give potential buyers of PKI products and services warm fuzzies").

Peter.

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From: Michael Ströder <[email-masked]>
Date: Tue, 22 Jun 2010 09:46:51 +0200

> a document bearing someone else's digital signature is not witnessing in

Matt, you're right.

That's another example of trying to solve something with PKI technology
instead of examining how processes work in the real world and whether there's
a technical solution at all. Just like the famous "warranty" draft.

Ciao, Michael.

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From: Peter Gutmann <[email-masked]>
Date: Tue, 22 Jun 2010 21:09:26 +1200

>a technical solution at all. Just like the famous "warranty" draft.

Hey, don't criticise that, it provided a valuable service by pointing out 
that the default warranty for certs is:

  Warranty  ::=  CHOICE  {
     none                 NULL,            -- No warranty provided
     ...  }

or more specifically (in not-quite-ASN.1):

  Warranty ::= DEFAULT NULL

So if you don't find a warranty extensions, you know that the above is the
warranty attached to your cert.

Peter.