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The XML-DSig Non-standard, or Location/Transforms as 'hints'

From: John Boyer <jboyer@uwi.com>
Date: Thu, 18 Nov 1999 15:26:40 -0800
To: "DSig Group" <w3c-ietf-xmldsig@w3.org>
One of the main points that has caused much of the recent debate over
signing location and transforms is that some of us believe that

1) the ObjectReference's Location and Transforms will tell core code how to
obtain the bucket of bits digested in DigestValue.

while others of us believe that

2) the ObjectReference's Location and Transforms are a hint that 'may' help
the application find the bits that the core code will need to do the

I'm having difficulty buying into this latter point of view because I think
that far too much work is being pushed off to the application, which to me
means that most signatures will not validate outside of their application
domains.  I don't see the point in having a 'standard' if the result is that
applications don't interoperate.

From an API point of view, proponents of the first idea seem to want to call
CreateSignature() or VerifySignature() and give a pointer to a Signature
element.  Proponents of the second idea seem to want the same thing, except
that they must first set up an application-specific callback function that
CreateSignature() and VerifySignature() can use to help dig up the required
bits.  Therein lies the rub.  Callbacks are a wonderful way to solve
problems if you don't care about globally secure resources, application
interoperability, and so forth.  The first idea is in many of our minds
because we associate 'standard' with interoperability.

When the signer creates a signature, we are saying that Location and
Transforms provide 'hints' that indicate how the signer created the bucket
of bits.  Presumably, when the signer signed, the Location and Transforms
describe precisely what happened.  So, we are basically saying that the
verifier can treat these as hints rather than precise steps.  So, the
meaning of these *signed* bits has changed without breaking the signature.
I agree that it will work in any single application context, but it has an
unappealing engineering aesthetic.

Finally, when proponents of the second idea say that Transforms are 'hints',
does this mean that we will be making each application responsible for
resolving the Transforms too?  In other words, going back to the idea of the
callback function, must the callback function resolve the Location or must
it resolve the Location and Transforms, giving to core code the exact set of
bits that should match the DigestValue once the DigestMethod is applied?

John Boyer
Software Development Manager
UWI.Com -- The Internet Forms Company
Received on Thursday, 18 November 1999 18:27:53 UTC

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