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RE: History: Question on C14N list of nodes instead of subtrees

From: Karl Scheibelhofer <Karl.Scheibelhofer@iaik.at>
Date: Mon, 28 Jan 2002 19:17:12 +0100
To: <reagle@w3.org>, "'John Boyer'" <JBoyer@PureEdge.com>, "'merlin'" <merlin@baltimore.ie>
Cc: <w3c-ietf-xmldsig@w3.org>
Message-ID: <006601c1a828$02086f70$64981b81@iaik.at>

> -----Original Message-----
> From: Joseph Reagle [mailto:reagle@w3.org] 
> Sent: Monday, January 28, 2002 5:57 PM
> To: Karl Scheibelhofer; 'John Boyer'; 'merlin'
> Cc: w3c-ietf-xmldsig@w3.org
> Subject: Re: History: Question on C14N list of nodes instead 
> of subtrees
> On Monday 28 January 2002 11:11, Karl Scheibelhofer wrote:
> > an acceptable performance. the signature creation/verification took 
> > several seconds for an XML signature with three references using a 
> > XPath filter; one for the signed content, one for the key 
> info and one 
> > for signed properties.
> Karl, thanks for this info. I have a few questions.
> 1. With your present implementation when you sign these three 
> things, do 
> you then create 3 references each with an XPointer to the 
> subtree? 2. It's clear we can get very good performance for 

yes, i use three references in each signature. those look like this:

  <dsig:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"
  <dsig:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"
  <dsig:DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"

each of them selects a complete subtree with all attribute and namespace

the structure of such a singed document looks like this

<aida:eDocument xmlns:aida="http://www.iaik.at/aida" ...>

  <aida:signedContent> ... arbitrary well-formed XML ...

  <dsig:Signature xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
    <dsig:SignedInfo> ... (the three XPointer references go here) ...
      <aida:properties xmlns:aida="http://www.iaik.at/aida">

  <dsig:Signature xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
    <dsig:SignedInfo> ... (the three XPointer references go here) ...
      <aida:properties xmlns:aida="http://www.iaik.at/aida">



this structure/XPointer combination has the advantage that it allows an
arbitrary number of parallel signatures. each of these parallel
signatures uses the same XPointer references, because the XPointers are
relative. beeing relative additionally allows embedding the complete
aida:eDocument into the aida:signedContent of antoher aida:eDocument,
and that without breaking any existing signatures. thus, the structure
supprots parallel and embedded signatures always using the same XPointer

> subtree selection. Have 
> any of your scenario' required omission filters? If so, how 
> would you think 
> they be best addressed in the future (e.g., 1.1) 

most use cases only need a simple subtree-selection mechanism, which
selects a complete subtree with all attributes and namespece nodes. if
you carefully design you system, you do not even need anything else.
of course, one can create super-funky-complex signing schemes, but who
really needs these things when you can realize it with a simple

> A. Presently we say be careful with XPointers in URIs because of 
> implementation issues: XPointer is not widely implemented, 
> not yet REC, and 
> we don't quite trust consistent application interpretations 
> of fragment 
> identifiers. Should we relax the discouragement of XPointer? (I'd be 
> concerned about its lack of wide-spread implementation and 
> status, but is 
> the fragment interpretation not a problem in your 

an earlier version of the Xerces parser had problems handling XPointers
in URIs, but it was simple to fix. anyway, if they would have
implemented their URI class according to the URI specification, it
wouldn't have been a problem at all.
i did not see a complete XPointer implementation yet. i searched for one
last year, when i needed XPointer first. i did not find one easily, so i
cxonsidered to implement this part of XPointer which 'makes sense'; i.e.
just the XPath related stuff, but not any of these features like
selecting parts of text, ranges, .... these features might be of use for
text-processing systems but not really for data-processing systems which
use XML to encode structured data.

referencing elements by IDs, i would discourage. if you use IDs, you
cannot extract signed content from one document and combine it with
other signed parts into a new document. you might get ID clashes, which
you cannot fix, because the IDs are covered by the signatures. one would
break the signatures in renameing the IDs.

i would not discourage XPointer in general. it is similar as for other
standards. a fraction of this standard is useful for the majority of its
users, but a good portion is "i am wondering who really needs this".

> experience?) B. Specify a new XPath transform that is at 
> least very fast for subtree 
> selection and hopefully faster for omission filters -- I'm 
> not sure how to 
> do the latter yet.

if you design a new transform, i would recommend to specify one which
allows to specify a complete sub-tree with all attributes and namespace
nodes. i guess this is the most important type of reference required.

omission filters, i do not use any longer. i jsut used them to exclude
the own signature during signing, but with the type of XPointer
references i use now, i do not need this omission filter any longer.
i think i could live without this omission filters, because i cannot
imagine a reasonable other use-case for them. who needs a filter like
"just sign all attributes which's name is ..."?

> 3. Since the enveloped signature transform is specified via an XPath 
> expression (though it is not required to be the  means of 
> implementing it) 
> how do you implement that and how is its performance?

the signature is never part of the signed document. consequently, i
structure my documents that this is really the case. this means, the
signature is never the descendant of any of its signed elements. in my
use-case the signature is a sibling of the signed content, if it is
inside the same document. and if the signature is detached, there is no
problem anyway.
putting all singed data into the Object element of a signature and then
signing the complete document excluding the signature itself, is "not a
nice design" putting it mildly.

to sum up: using XPointers instead of XPath filters was a good decision
with hindsight. using an uncritical subset of XPointer, this which is
just a different notation for XPath expressions, is really woth the
effort. the effort to implement this subset of XPointer that you really
need is little, if you already have a XPath preocessor. this was one
Java class with ~200 lines of code in my case.

i hope this helps


  Karl Scheibelhofer


Karl Scheibelhofer, <mailto:Karl.Scheibelhofer@iaik.at>
Institute for Applied Information Processing and Communications (IAIK)
at Graz University of Technology, Austria, http://www.iaik.at and
Phone: (+43) (316) 873-5540

> -- 
> Joseph Reagle Jr.                 http://www.w3.org/People/Reagle/
> W3C Policy Analyst                mailto:reagle@w3.org
> IETF/W3C XML-Signature Co-Chair   http://www.w3.org/Signature/
> W3C XML Encryption Chair          http://www.w3.org/Encryption/2001/
Received on Monday, 28 January 2002 13:13:12 UTC

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