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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 17:11:16 +0100
To: <reagle@w3.org>, "'John Boyer'" <JBoyer@PureEdge.com>, "'merlin'" <merlin@baltimore.ie>
Cc: <w3c-ietf-xmldsig@w3.org>
Message-ID: <004201c1a816$6a720a10$64981b81@iaik.at>
hi all,

because it was my mail and problem, i want to share my further
experience on this topic. 
as mentioned we use Xalan to evalutate XPath expressions. even if we
cached all possible Xalan objects and reused them, we could not achieve
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.
thus, i decided to implement the necessary subset of XPointer using
Xalan. this subset implementation is quite small, ~200 lines of code and
accepts simple XPointers of the form 
"xmlns(aida=http://www.iaik.at/aida)
xpointer(./ancestor::aida:eDocument[1]/... ". with this implementation
the performance raises dramatically, because it evaluates the XPath
expression in the XPointer only once resulting in the final node-set to
be serialized. with this XPointer, the signing/verification takes only a
fraction of a second.
i agree that if you have a tuned version of an XPath engine, you might
get a much better performance, but it will never be the same, because
the filter always needs to check each node of the document; i.e. its
won't ever perform better than O(n), where n is the numebr of nodes in
the document. for XPointer you should get O(log n), because you just
step down the document tree until you reach the subtree's root.

as a consequence of my experience with XPath filters of the form

      <dsig:Reference URI="">
        <dsig:Transforms>
          <dsig:Transform
Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
            <dsig:XPath xmlns:aida="http://www.iaik.at/aida"
 
xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">count(here()/ancestor::a
ida:eDocument[1]/child::aida:signedContent[1]/descendant-or-self::node()
...

and XPointers of the form

	<dsig:Reference
URI="#xmlns(aida=http://www.iaik.at/aida)%20xpointer(./ancestor...

, i would never use the Xpath filter version.

i recommend to use XPointers instead of XPath filters wherever possible.

regards

  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
http://jcewww.iaik.at
Phone: (+43) (316) 873-5540

> -----Original Message-----
> From: w3c-ietf-xmldsig-request@w3.org 
> [mailto:w3c-ietf-xmldsig-request@w3.org] On Behalf Of Joseph Reagle
> Sent: Wednesday, January 23, 2002 9:28 PM
> To: John Boyer; merlin
> Cc: w3c-ietf-xmldsig@w3.org
> Subject: History: Question on C14N list of nodes instead of subtrees
> 
> 
> A bit of archeology ... I've been corresponding with someone about 
> performance and dependence on XPtr recently and brought up 
> message [1]. I 
> characterized (perhaps mistakenly) the issue as follows, but 
> this sort of 
> begs the question of why did we "burst/flatten" our nodelist 
> in the first 
> place? Was it so we could easily create subsets as a list of 
> nodes (making 
> the "node in subset test" easy?), so that we could easily 
> remove comments?
> 
> ---- Summary Characterization ----
> 
> 1. When the signature is in the document being signed 
> (enveloped) it has to 
> except itself from the document: a signature can't contain 
> itself (paradox). 2. One approach is to use XPath to throw 
> out all the dsig element from the 
> document and then sign it. BUT what if I'm an enveloped 
> signature in a 
> document that has *other* signatures that I want to sign. I 
> need a way to 
> say "only throw out the Signatutre element containing *me*" 
> There's no easy 
> way to do that so we invented the here() XPath function -- 
> which is also in 
> XPtr now.
>  http://www.w3.org/TR/xmldsig-core/#function-here
> 3. The example is someone noting that when someone takes the 
> enveloped-signature-in-a-document and embeds *that* document 
> into another 
> document, the here() expression and processing gets even 
> heavier! (4. Also note that in our use of XPath we have lists 
> of XPath nodes 
> (flattened/burst), not trees. So if we have an XPath element 
> node *that's* 
> all it is. "because XPointer typically indicates a subtree of an XML 
> document's parse tree using just the element node at the root of the 
> subtree, whereas Canonical XML treats a node-set as a set of 
> nodes in which 
> absence of descendant nodes results in absence of their 
> representative text 
> from the canonical form." 
>   http://www.w3.org/TR/xmldsig-core/#sec-Same-Document
> )
> 5. The result is our expression gets really ugly (cause we 
> need to always 
> ask for (//. | //@* | //namespace::*) and the processing from 
> the XPath is 
> *very* slow: parsing the tree and doing counts. Merlin was 
> indicating that 
> in XPointer, the expression would look  prettier (since it 
> points to a 
> subtree, not just a flat node list) and I'm wondering about 
> the performance 
> of the XPointer here. Now that I think about it here() probably isn't 
> something you'd want in a simple profile, but having it be 
> there and have 
> it be implemented by fast implementations could've been of 
> use to us -- or 
> could in a future version of xmldsig....
> 
> ----- Old Email ----
> 
> [1] 
> http://lists.w3.org/Archives/Public/w3c-ietf-xmldsig/2001AprJu
> n/0081.html
> 
> Subject: Re: Problem: referring to a complete sub-tree using XPath
> Date: Wed, 25 Apr 2001 09:48:53 +0100
> From: merlin <merlin@baltimore.ie>
> To: "Karl Scheibelhofer" <Karl.Scheibelhofer@iaik.at>
> Cc: "XMLSigWG" <w3c-ietf-xmldsig@w3.org>
> 
> Hi Karl,
> 
> XPointer is the tool for this job, not XPath. It can do this 
> reference in a single path evaluation. The xmldsig standard 
> supports full same-document XPointer references; it is simply 
> an issue of implementing it ;}
> 
> Merlin
> 
> r/Karl.Scheibelhofer@iaik.at/2001.04.19/12:57:45
> 
> >Hi,
> >
> >i use XPath in a reference to select a element of the same 
> document and
> > all its descendants, attributes,... - simply the subtree with the
> > specific element as its root.
> >i already have a XPath that works. however, its awfully 
> slow, because 
> >its quite long for this simple task it perfoms.
> >
> >here a short example
> >
> ><?xml version="1.0" encoding="UTF-8"?>
> ><aida:eDocument xmlns:aida="http://www.iaik.at/aida"
> >xmlns:xsi="http://www.w3.org/2000/10/XMLSchema-instance"
> >xsi:schemaLocation="http://www.iaik.at/aida eDocument.xsd">
> >  <aida:signedContent>
> >    <personnel 
> xmlns:xsi="http://www.w3.org/2000/10/XMLSchema-instance"
> >xsi:noNamespaceSchemaLocation="personal.xsd">
> >      <person contr="false" id="Big.Boss">
> >        <name>
> >          <family>Boss</family>
> >          <given>Big</given>
> >        </name>
> >        <email>chief@foo.com</email>
> >        <link subordinates="one.worker two.worker three.worker 
> >four.worker five.worker"/>
> >      </person>
> >       ... (omitted some data)
> >    </personnel>
> >  </aida:signedContent>
> >  <dsig:Signature Id="eDocumentSignature-1" 
> >xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">
> >    <dsig:SignedInfo>
> >      <dsig:CanonicalizationMethod 
> >Algorithm="http://www.w3.org/TR/2000/WD-xml-c14n-20000907"/>
> >      <dsig:SignatureMethod 
> >Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1"/>
> >      <dsig:Reference URI="">
> >        <dsig:Transforms>
> >          <dsig:Transform 
> >Algorithm="http://www.w3.org/TR/1999/REC-xpath-19991116">
> >            <dsig:XPath xmlns:aida="http://www.iaik.at/aida"
> >xmlns:dsig="http://www.w3.org/2000/09/xmldsig#">count((here()
> /ancestor:
> >:ai
> >da
> >
> >:eDocument[1]/child::aida:signedContent[1]//. |
> >
> >here()/ancestor::aida:eDocument[1]/child::aida:signedContent[
> 1]//@* | 
> >here()/ancestor::aida:eDocument[1]/child::aida:signedContent[
> 1]//namesp
> >ace
> >:: *) | self::node()) =
> >count((here()/ancestor::aida:eDocument[1]/child::aida:signedC
> ontent[1]//.
> > | 
> here()/ancestor::aida:eDocument[1]/child::aida:signedContent[1]//@* |
> > 
> here()/ancestor::aida:eDocument[1]/child::aida:signedContent[1
> ]//namespac
> >e:: *))</dsig:XPath>
> >          </dsig:Transform>
> >        </dsig:Transforms>
> >        <dsig:DigestMethod
> >Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
> >        
> <dsig:DigestValue>ssbkbDM6VCUTYyzXMK06RKcbFHQ=</dsig:DigestValue>
> >      </dsig:Reference>
> >    </dsig:SignedInfo>
> >
> ><dsig:SignatureValue>PFkUqjNCq9Ujyl/K/5c62vyEeExIVNFwrrH2ZuxL
> bmXjH9dQOF
> >rVw
> >Po dMb1xUY1Y 
> > 8iHpAcl8Z6xP3mMCK60ROtVCcDRS2v0ydULhJ+IZFjotIgwtGECy9lxZy4LDkeUJ
> > RIvtzlDHBnp5jMb1+iLO1aTvkBzNLWbrAGo+rbqely4=</dsig:SignatureValue>
> > <dsig:KeyInfo>
> >      <dsig:X509Data>
> >        <dsig:X509Certificate>MIIC .... (omitted some data)
> ></dsig:X509Certificate>
> >      </dsig:X509Data>
> >    </dsig:KeyInfo>
> >  </dsig:Signature>
> ></aida:eDocument>
> >
> >i need the here() functionality to ensure that the signature even
> > verifies, if i embed the whole document into another xml 
> document. the 
> >long XPath the you see in the example just selects the 
> ><aida:signedContent> element with everything contained within this
> > element. does anyone know a simpler XPath that does the 
> same job? the
> > performance of this is unacceptable: up to some minutes if i have a
> > medium XML document in the signed content running without 
> JIT. (i use
> > Xerces 1.3.0 [with some patches])
> >i did not want to use IDs, to be able to arbitrary include signed
> > documents into other documents.
> >
> >regards,
> >
> >  Karl Scheibelhofer
> 
> 
> 
Received on Monday, 28 January 2002 11:07:38 GMT

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