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Fwd: Heads-up on proposed breaking change to XML Signature

From: Thomas Roessler <tlr@w3.org>
Date: Tue, 28 Oct 2008 19:36:51 +0100
Cc: John Boyer <boyerj@ca.ibm.com>
Message-Id: <856EE3C3-3356-44A7-8B81-13238E55B74A@w3.org>
To: public-xmlsec@w3.org

Forwarding with permission.
--
Thomas Roessler, W3C  <tlr@w3.org>







Begin forwarded message:

> From: John Boyer <boyerj@ca.ibm.com>
> Date: 28 October 2008 18:23:26 CEST
> To: Thomas Roessler <tlr@w3.org>
> Cc: chairs@w3.org, w3c-ac-forum@w3.org
> Subject: Re: Heads-up on proposed breaking change to XML Signature
>
>
> Hi Thomas,
>
> Thanks for taking the time to make this notification.  I generally  
> agree that some simplifications would be helpful for performance and  
> for making it easier to express signatures of higher security.  As  
> I'll explain below, the focus on selection would be too restrictive,  
> i.e. too much simplification, in that it tends to substantially  
> *reduce* the security value of the signatures.
>
> However, it is easy to maintain the highest possible security while  
> focusing for performance and simplicity on "selection".  The trick  
> is to understand what must be selected and why.  Put simply, a  
> document subset transform is more secure when it selects what must  
> be subtracted from the document before the canonicalization and  
> digest operations.  The subtraction filter, then, is a precise and  
> explicit expression of what part of the document may still be  
> mutated by the rest of a business process that occurs after the  
> signature is affixed.  Any change to a document other than to the  
> subtracted part will invalidate the signature as desired.
>
> For example, suppose you have a document with the familiar "office  
> use only" section. When a user signs the document, the document  
> subset should be the entire document less the "office use only"  
> section.  This way, any change made to the document in any place  
> except the "office use only" section would invalidate the  
> signature.  The purpose of a digital signature is to become invalid  
> when any change is made, except those anticipated by the system.   
> Thus, subtraction filtering is the best fit for a document subset  
> signature.
>
> By comparison, if a document subset signature merely selects the  
> portion of the document to be signed, then additions can be made not  
> only to the "office use only" section but also to any other location  
> in the document that is outside of the selected portions of the  
> document.  It is entirely too easy to exploit the document semantics  
> and inject unintended side effects.
>
> Regarding the issue of performance, I would point out that the  
> original XPath filter should be replace by XPath Filter 2, or  
> something like it that is capable of subtraction.  Whereas the  
> original working group paid insufficient attention to performance as  
> an interoperability criterion in the original recommendation of the  
> first XPath filter, the *fix* to that was the XPath Filter 2  
> recommendation, which was *designed* for performance in the sense  
> that high performance was considered to be a requirement.   
> Specifically, interoperable implementations were expected to be  
> capable of performing a subtraction filter on signature generation  
> and validation operations over a 100K XML document in 1/4 second per  
> operation on an 800MhZ computer.  This time includes not just the  
> subtraction filtering, but the entire transform-canonicalize-digest- 
> digest-encrypt sequence.
>
> The XPath Filter 2 transform should therefore be seen as highly  
> performant, and I would assert it would be difficult indeed to  
> provide actual empirical evidence to the contrary.  That being said,  
> the XPath Filter 2 transform internally allows any number of set  
> operations of subtraction, union and intersection.  Given the fact  
> that XML Signatures itself allows multiple References, it would seem  
> that a single operation of subtraction or selection would be  
> sufficient for all practical purposes.
>
> To be clear, IBM currently ships successful products that are based  
> on XML Signatures and that are particularly dependent for security  
> reasons on the existence of subtraction filtering. However, in the  
> design tools we offer for constructing document signatures, we  
> simplify the interface down to a simpler set of operations, which  
> you now appear to be wanting to achieve at a markup level.   This  
> would make it easier for design tools to offer a visual/gestural  
> experience that is close in concept to the actual markup that is  
> generated, which in turn makes it easier for design tools to figure  
> out what has happened in prior design experiences based on the  
> markup.  There's just a lot fewer unnecessary knobs and dials, so to  
> speak, so this seems like a good direction for the working group to  
> take.  As long as it is clear, in case I haven't said it enough,  
> that we need document subtraction.
>
> As a final point, I realize that this message indicates you are  
> focusing on the transformation-digest-canonicalization sequence.  My  
> greatest disappointment at the conclusion of the original XML  
> signature group pertains to an aspect of canonicalization that the  
> working group was unwilling to take on at the time due to the  
> newness of the technologies needed to address it.  But those  
> technologies are now entrenched, so I would strongly urge the new  
> working group to reconsider.  The issue is that the current  
> canonicalization algorithms make no use of DTD or Schema information  
> that might be available on when whitespace is important and when it  
> isn't.  It is easy with today's technology to determine when an  
> element's content model permits whitespace merely as a convenience  
> versus when the element actually has a PCData or mixed content  
> model.  It would be a great service to the XML community to see a  
> new canonicalizer that could detect and eliminate unnecessary  
> whitespace from the canonical form.
>
> Best regards,
> John M. Boyer, Ph.D.
> STSM, Interactive Documents and Web 2.0 Applications
> Chair, W3C Forms Working Group
> Workplace, Portal and Collaboration Software
> IBM Victoria Software Lab
> E-Mail: boyerj@ca.ibm.com
>
> Blog: http://www.ibm.com/developerworks/blogs/page/JohnBoyer
> Blog RSS feed: http://www.ibm.com/developerworks/blogs/rss/JohnBoyer?flavor=rssdw
>
>
>
>
> From:
> Thomas Roessler <tlr@w3.org>
> To:
> w3c-ac-forum@w3.org, chairs@w3.org
> Date:
> 10/21/2008 08:21 AM
> Subject:
> Heads-up on proposed breaking change to XML Signature
>
>
>
>
>
> Dear Chairs and AC representatives,
>
> The XML Security Working Group had a productive meeting here in Cannes
> this Monday and Tuesday.  In outlining its roadmap for a next version
> of XML Signature, the Working Group - as chartered - reviewed
> possibilities for simplifying the specification.
>
> One area of that is promising for simplification is the Reference
> Processing Model's ability to specify an (almost arbitrary) list of
> transforms between node-sets and octet-streams.
>
> One option under consideration by the Working Group is to modify the
> Reference Processing Model in XML Signature 2.0 to consider only
> "selection", "canonicalization" and "digesting" transformations.  The
> Working Group believes that there are significant security and
> performance advantages to making this change to the XML Signature
> structure, but it does constitute a restriction on behavior that is
> currently permitted by XML Signature 1.0.  Specifically, this change
> would still permit signing of document subsets but would prohibit
> transformations of arbitrary complexity (e.g. unconstrained XSLT) from
> being used within the XMLDSIG sturcture itself.
>
> We plan to outline this design idea in more detail in a Working Group
> Note that we anticipate to put out for broad comment.
>
> At this point, we would be grateful for early feed-back about actual
> use of custom Transforms with XML Signature that might be affected by
> the change that is being considered.
>
> We're available in the hallways at TPAC to talk more about this, and
> appreciate early feed-back.
>
> Regards,
>
> Frederick Hirsch (Nokia), Chair, XML Security WG;
> Thomas Roessler (W3C), Security Activity Lead.
>
>
>
>
Received on Tuesday, 28 October 2008 18:37:04 GMT

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