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RE: Quick Comments on Types/Encoding of XML

From: Mark Bartel <mbartel@thistle.ca>
Date: Fri, 15 Oct 1999 17:28:33 -0400
Message-ID: <91F20911A6C0D2118DF80040056D77A2032A56@arren.roke.thistle.ca>
To: "'John Boyer '" <jboyer@uwi.com>, "'IETF/W3C XML-DSig WG '" <w3c-ietf-xmldsig@w3.org>
I agree with John here.  I must admit that the whole argument about types &
transformations confused me until I realized that the idea was to redefine
what a transformation was.

So, in the interest of clarifying the problem, I'll explain my thinking.

As I understand it, the original idea was that the transformations were a
pipeline taking in an octet stream at one end and producing an output octet
stream.  Very simple and straightforward; one input and one output.

The new idea is to augment the octet stream, by adding a parallel type.  In
other words, each algorithm would have two inputs and two outputs, one of
which is an octet stream and the other a "type".  The idea comes from the
http world where one retrieves a document (analogous to our octet stream)
and also gets a whole whack of other information along with it in the http
headers, including the mime type of the document.  Browsers also sometimes
utilize the file extension to determine type.

The motivation for the new concept is that some transformations require
knowledge of the input type.  The main example is the fragment id, which is
defined per type.  Another example is character set encodings.

I strongly feel that transformations should have one input and one output.
This makes the specification and implementation for each transformation much
simpler and therefore simplifies interoperability and testing.  The document
being transformed is not going to switch from being text/html to image/jpeg
betwixt the signing and the verification, and if it does the signature
wouldn't verify anyway.  The html or jpeg parser needs to throw errors for
invalid input regardless of what we choose.  For transformations that need
the input type, the input type or character set can be specified as a
parameter on that transformation when the signature is created.

For character set encodings, I don't think there is much we can do for the
general case.  Frequently it won't be an issue, the character set will stay
constant between signing and verification.  If they have an entire XML
document, the character set can be determined from the prolog.  Otherwise,
there is no way of knowing.  The http protocol may give us some information
that could be passed through in the dual-input/output model, but any other
protocol probably won't give us useful information.

In the XML case, we could recommend dealing with this issue by
canonicalizing the entire document into UTF-8, and then picking out the
appropriate fragment.  Doing it the other way around would lose the
character set encoding information.

-Mark Bartel

-----Original Message-----
From: John Boyer
Sent: 10/15/99 1:09 PM
Subject: RE: Quick Comments on Types/Encoding of XML

Hi Joseph, Don and others on the teleconference,

I was not completely with it yesterday because my wife's been in
the day before (she had a miscarriage (which really hurts) with some

Anyway, I wanted to further clarify the 'input/output type' problem we
discussing on the telecon.

It seems that there was some confusion because there were two issues at
hand. First was the charset encoding and second was the MIME type.

For charset encoding in the XML transformations, I solved the problem
(interestingly in the way that Peter Norman suggested).  Some on the
indicated that this was not a solution they preferred.  OK, so we can
it a bit, but the charset issue is solved.

By the way, the charset transformations require knowledge of how the
document type deals with changes in charset (unlike base64 which
care less).  I classified them as c14n algorithms to be defined in
7.5 because XML c14n seems to have the precedent on charset changes, and
should stick to the standards as much as possible.

For other types of documents, such as MIME type, it seemed exceedingly
pedantic to me to define an input type for the algorithm when the input
is evident from the algorithm itself.  I do not believe that the same
transform can process multiple types of documents unless the transform
doesn't actually parse the document (like base64) in which case it does
need to know the input type.  A JPEG decompressor just isn't going to
on an XFDL form.

It is the responsibility of the entity that creates the transformation
sequence to match up the transformations based on the expected input and
output of the types of algorithms that *the entity* strings together.
signature system simply will not have need of this information.  If the
transformation code receives invalid data, it will generate errors (e.g.
you push non-xml data into an xpath transform, the xml processor will

Basically, the transformations I defined did not need this information
all, and the application specific transformations are 1) quite unlikely
need the type information, and 2) quite capable of placing the type
information in the Transformation element content even if they do need

It's really a question of who does the error checking.  When I say the
information is not needed, I'm not saying that data of the wrong type
shouldn't generate errors, I'm just saying that the core signature
and processing rules should not be burdened with trying to figure out
input document type of a Transformation (which would be necessary in
to check whether they match the recorded input type).

It has been said repeatedly that we need to be quite specific about the
processing rules of these transformations.  I was quite specific, but I
think some mistook my not restating the rules of XML or other
for ambiguity.  I would encourage a reread of the section before further
discussion, and please address concerns about the actual proposed
to me.  It doesn't make sense to have a telecon call where we spend 40
minutes discussing things that are solved either by the spec or
by the specs to which the spec refers.

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

Note, if the XML is anything other than UTF-8 or UTF-16, the encoding
be present:


        Because each XML entity not in UTF-8 or UTF-16 format must begin
with an
        XML encoding declaration, in which the first characters must be
'<?xml', any
        conforming processor can detect, after two to four octets of
which of the
        following cases apply.

Joseph Reagle Jr.
Policy Analyst           mailto:reagle@w3.org
XML-Signature Co-Chair   http://w3.org/People/Reagle/
Received on Friday, 15 October 1999 17:28:35 UTC

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