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RE: Locations but not Transforms as hints (was RE: The XML-DSig Non-standard, or Location/Transforms as 'hints')

From: John Boyer <jboyer@uwi.com>
Date: Fri, 19 Nov 1999 13:40:57 -0800
To: "Mark Bartel" <mbartel@thistle.ca>, "'DSig Group '" <w3c-ietf-xmldsig@w3.org>
Hi Mark,

-----Original Message-----
From: Mark Bartel [mailto:mbartel@thistle.ca]
Sent: Friday, November 19, 1999 8:50 AM
To: 'John Boyer '; 'DSig Group '
Subject: Locations but not Transforms as hints (was RE: The XML-DSig
Non-standard, or Location/Transforms as 'hints')

Hi John,

First, preliminary comment:  you say that there are two positions.  You
characterize one of them being that Location and Transforms are only a hint.
Well, I don't think I'm the only one who is perfectly happy with Location as
a hint (with an Encoding attribute which is also a hint) but strongly
opposed to Transforms being a hint.

So, I agree with you that Transforms should always be done but disagree
about Location.

I believe the only problem you have with Location as hint is that a callback
is "required" that otherwise wouldn't be.  All of your other issues seem, to
me, to be only relevant to Transforms as hints.

Aside from our agreement over Transforms not being a hint, my comments were
actually mostly about my expectation that Location should indicate where the
object is rather than be a hint about where the object is.  As you can tell
from the bottom of my letter, I cannot tell at this point whether Transforms
will be handled by core code or also serve as hints.  Naturally, I'd prefer
that they were not hints.

The issues I raised were
1) the Location as hint implies need for application-specific callback
2) meaning of Location can change from signer to verifier without breaking
signature.  To me, this means that the Location shouldn't be signed.

With respect to the callback issue, I don't see it as a problem.  I expect
that most libraries will provide a hook, but "do the normal thing" (grab
whatever is at Location) if you don't provide a callback.  Providing a
callback would only be necessary when you expect documents to move.  Said
callback may be as simple as "here's the URI and Encoding in Location, give
me back the real URI and Encoding".  On the other hand, it may be "here's
the URI and Encoding in Location, give me a byte stream".  Either way, it
doesn't seem an heavy burden to lay on applications.

To me it's not the end of the world if we lay this on applications.  It's
not like security is broken or anything.  It's just that we are in fact
saying that there are classes of applications (given by those scenarios I
presented) for which core validation logic will not work.  These
applications will not be able to have their signatures validated by other

Basically, we sign the Location so that it cannot be changed. Our solution
is that Location can be completely wrong but serves only as a hint that
applications are free to interpret in some completely different way. This is
a very clever way of saying that we don't see how to make core behavior do
what these people want to do.  We broke their apps by signing Location such
that the 'do the normal thing/grab the bytes' behavior doesn't work, and we
are deciding not to fix this break when their clearly is a solution.

People who have these problems I've described would like their signatures to
validate by core behavior just like those of us who don't have the problems
they face.  I would think that anyone involved with a company that has to do
workflow on other companies' documents would be concerned about the
long-term maintenance fiasco that results from having to write a special
module for each document type to deal with the special Location resolution
methods of that application.  Over time, as each company tweaks and changes
its application logic, one might need a small army to keep up with the
potential changes.

Of course, the only time that an application needs to not "do the normal
thing" is when the document may move.  If the document stays put, there is
no issue and no interoperability question.  I believe that the necessity of
being able to move the documents has been quite well expressed.  I also
believe that finding the document once it has moved is very
application-specific.  Viewing the Location as a hint (with an Encoding
attribute that is also a hint) seems to me a wonderfully simple way of
dealing with the travelling document problem.

Nice play on words!  I agree that we have no problem if the documents don't
move.  This whole discussion is about that subset of application that need
to have the travelling document problem solved.  To reiterate, this would

1) someone putting a copy of some document at a different location because
the creator of that document decides to update it yet leave it at the same
URL (e.g. the W3C website, or some page of it).

2) Someone wanting to put multiple signed search results in the same file.

3) Someone wanting to move a signed resource between being inside versus
outside of a document.

Viewing Location (and encoding) as hints don't solve this problem for two
reasons.  First, there are times when the encoding has to be applied after a
transform (e.g. base64 decode after the child::text() Xpath in example 3
above).  Second, you're really saying that a wonderfully simple solution is
to not address these people's problems with core behavior but instead to
tell them that their problem is too weird and must only be addressed by
their application logic, and that their signatures will not be verifiable
outside of their own application domain (unless the host application adds
code to address their specific application).

This seems like an awful mess compared to providing a simple method that
allows these people to solve their problem in a way that will be understood
by all applications that use this digital signature specification.

John Boyer
Software Development Manager
UWI.Com -- The Internet Forms Company

-Mark Bartel
JetForm Corporation

-----Original Message-----
From: John Boyer
To: DSig Group
Sent: 11/18/99 6:26 PM
Subject: The XML-DSig Non-standard, or Location/Transforms as 'hints'

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
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'
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
that far too much work is being pushed off to the application, which to
means that most signatures will not validate outside of their
domains.  I don't see the point in having a 'standard' if the result is
applications don't interoperate.

>From an API point of view, proponents of the first idea seem to want to
CreateSignature() or VerifySignature() and give a pointer to a Signature
element.  Proponents of the second idea seem to want the same thing,
that they must first set up an application-specific callback function
CreateSignature() and VerifySignature() can use to help dig up the
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
of bits.  Presumably, when the signer signed, the Location and
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
I agree that it will work in any single application context, but it has
unappealing engineering aesthetic.

Finally, when proponents of the second idea say that Transforms are
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
callback function, must the callback function resolve the Location or
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 Friday, 19 November 1999 16:42:08 UTC

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