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XSL WG comments on XML Signatures

From: by way of <jmarsh@microsoft.com>
Date: Tue, 14 Mar 2000 14:42:38 -0500
Message-Id: <>
To: "IETF/W3C XML-DSig WG" <w3c-ietf-xmldsig@w3.org>
Cc: w3c-xsl-wg@w3.org
The XSL WG took a look at the http://www.w3.org/TR/xmldsig-core/ draft,
especially at the XPath and XSLT transformation sections, and had some
serious concerns.

The section on XPath Filtering provides a capability that has often been
requested - the ability to take shape a nodeset returned by the XPath query
into an XML result.  We feel the model you provide will be a valuable
contribution.  However, as consituted, the current formulation does not use
XPath in a way consistent with the XPath recommendation.  We recommend a
substantial redesign of this section.

Notes on particular issues follow.

[6.3.3 XPath Filtering] "The XPath transform output is the result of
applying an XPath expression to an input string."

Conceptually this is odd.  XPaths is designed to be applied to XML
documents, not to strings.  Serious problems arise from this later on.

"The XPath expression appears in a parameter element named XPath."

No examples of this are shown; I assume the syntax is:

  <Transform Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">

But the XPath element does not appear to be in the DTD.  An example would be

"The primary purpose of this transform is to omit information from the input
document that must be allowed to vary after the signature is affixed to the
input document."

Despite this claim, the mechanism uses an XPath to describe the information
that is to be retained, instead of the information that is to be omitted.
Either the mechanism or this description of what is going on should be
adjusted for consistency.

[ Evaluation Context Initialization] "A context node, initialized to

Without a context node, the XPath cannot be applied against an XML tree.  We
suggest that an XPath transform parses the document in all cases (not just
when the parse() function is called), and the context node be set to the
root of the parsed XML document.  The context size and position can then be
initialized to 1, consistent with XPointer and XSLT.

"(Typically, $input is passed directly to parse(), but if $input does not
contain a well-formed XML document, XPath functions such as concat() can be
used before passing the result to parse())."

The need for this functionality is unclear, but seems to be a motivating
factor in XPath abuse throughout this section.  If indeed this need must be
fulfilled, it should be accomplished by a separate mechanism prior to
application of XPath to the parsed (thus guaranteed well-formed) document.

"An empty set of namespace declarations. (Note: It is possible to address a
node by its qualified name, even though the evaluation context has not been
initialized with a declaration of the namespace. The XPath language provides
the functions namespace-uri() and local-name() for this purpose)."

It appears quite easy in this syntax (being XML) to allow the user to
declare namespaces for use within XPaths.  XSLT and XLink both provide this
capability.  Using namespace-uri() and local-name() hinders readability and
impacts performance significantly.  This workaround should only be used as a
last resort, and even then many feel that this mechanism is too unwieldy.
We strongly recommend that a syntax for passing author-declared namespace
bindnigs to the XPath evaluation context be developed.

[ Function Library Additions] "Function: node-set parse (stringInput,
boolean LexOrder)"

This function will not work as intended.  The XPath BNF prevents functions
from being used as a location step - they can only appear within predicates.
Thus parse()/x (which appears to be fundamental to your design) is an
illegal use of a function.  We recommend that the parsed XML be provided to
an XPath processor through the context node instead, with any necessary
parsing controls specified on the XPath element (for example) and applied
prior to XPath execution.

"Function:string serialize(node-set); This function converts a node-set into
a string by generating the representative text for each node in the

Under what circumstances would the serialize function NOT be called on a
node-set return?  Since it appears that the vast majority (if not the
entirety) of XPath Filtering operations will need to call this function,
this capability should probably be built in instead of requiring the author
to call it explicitly.

Are the serialization constraints consistent with canonicalization?  Is it
inappropriate simply to say that the output is canonicalized instead of
defining the exact representation here?

[ XPath Transform Output] "serialize(parse($input,
     not(self::SignatureValue and parent::Signature[@id="S1"]) and
     not(self::KeyInfo and parent::Signature[@id="S1"]) and
     not(self::DigestValue and ancestor::*[3 and @id="S1"])]"

If this is the intended usage scenario (omitting descendants), perhaps a
mechanism based on XSLT match patterns (a subset of XPath) should be
pursued.  Combined with an omission semantic instead of a retention
semantic, the above might be simplified to:

  Signature[@id="S1"]/SignatureValue | Signature[@id="S1"]/KeyInfo |

[6.6.4 XSLT Transform] "Identifier:

Why is this identifier used instead of the XSLT namespace?  All XSLT
stylesheets contain version info already.

"The Transform element contains a single parameter child element called
XSLT, whose content MUST conform to the XSL Transforms [XSLT] language
syntax. The processing rules for the XSLT transform are stated in the XSLT
specification [XSLT]."

This seems quite underspecified after the XPath Filtering section.  For
instance, are similar parsing controls needed?  If not, why are they
necessary in the XPath case?  Are similar serialization constraints needed?
If not, why are they necessary in the XPath case?  Are certain output
methods required (they are optional in XSLT).

[ The Transforms Element] "<!ELEMENT Transform (#PCDATA)>"

The definition of the "Transform" element is #PCDATA.  This will not allow
an XSLT stylesheet to be included.  The XML Schema defines it as "element
only" but does not define the content.  This definition would not allow a
naked XPath if that is your intent.

In addition, the section mentions that the sequence of transformations can
be XPath, XSLT, or some custom Java algorithm.  It seems rather
underspecified how this sequence of transformations interact (e.g. XSLT
and XPath operate on nodes and the Java operates on ???).

We look forward to working with you to resolve these issues in a way that
meets your needs and is consistent, implementable, and interoperable.

- Jonathan Marsh

(With contributions from Alex Milowski.)
Received on Tuesday, 14 March 2000 14:42:39 UTC

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