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Re: Initial draft note for Transform processing

From: Sean Mullan <Sean.Mullan@Sun.COM>
Date: Mon, 08 Dec 2008 12:01:21 -0500
To: Pratik Datta <pratik.datta@oracle.com>
Cc: Frederick Hirsch <frederick.hirsch@nokia.com>, XMLSec WG Public List <public-xmlsec@w3.org>
Message-id: <493D52E1.8060407@sun.com>

 > The "type" atttribute will be the cornerstone of extensibility. Instead
 > of adding new transforms, higher level specs would add new types, and
 > then define new attributes/subelements that are valid for that type.

I agree it would help if we try to adhere to the existing syntax as much 
as possible. Think of all the deployed implementations and APIs that are 
already in use.

Why not reuse the existing Reference type attribute to encapsulate the 
Reference/Transform chain that identifies what is signed? Then it would 
be up to a standard XML Signature 2.0 implementation to reject unknown 
types or references whose transform chains did not conform to the 
registered type. This doesn't solve all of the issues such as the XPath 
case of identifying what is signed before you execute the transforms, 
but maybe what we really need then is another XPath Transform (yes, yet 
another):

<Reference URI=".." Type=".../xpath-xml">
   <Transforms>
     <Transform Algorithm=".../xpath3"
       includedXPath=".."
       excludedXPath="..."
       reincludedXPath="..."
       envelopedSignature="true/false"/>
   </Transforms>
</Reference>

--Sean

Pratik Datta wrote:
> 
> I want to make some small changes to requirements section, but it is in 
> the design section where I have a completely different thing in mind.
> 
> Why should we have transforms at all? The word "Transform" indicates a 
> processing step, and I would like XML signatures do be more 
> "declarative" and leave the processing to the implementation. This is 
> what I imagine each reference to be.
> 
> <Reference>
>  <WhatIsSigned>
>  </WhatIsSigned>
> 
>  <HowItIsCanonicalized>
>  </HowItIsCanonicalized>
> </Reference>
> 
> (Note: I am not really proposing this syntax - I know that if v2.0 is 
> completely different from v1.1 people won't adopt it. But for now lets 
> ignore that problem, and assume that we are starting from scratch)
> 
> As the name implies, WhatIsSigned just indicates what is being signed.
> <WhatIsSigned   type="xml"
>  URI=" ..."
>  includedXPath=".."
>  excludedXPath="..."
>  reincludedXPath="..."
> 
>  envelopedSignature="true/false"/>
> 
> 
> Note
> * This syntax is equivalent to an XPath transform and an 
> EnvelopedSignature transform. The key difference is that there is no 
> ordering of the transforms. The Signature is not giving a sequence of 
> steps to the SignatureProcessor and asking it to perform them, instead 
> it is specifying the intent of the signature.  This makes it easier for 
> a Policy processor to determine what is signed. With the transform 
> approach - suppose there are three transforms t1, t2, and t3; one has to 
> actually execute the transforms to determine what was signed, but in 
> this approach it is readily apparent
> 
> * reincludedXPath comes from the discussion with John Boyer of IBM. In 
> that email chain with him, we concluded that all we want is first an 
> includedXPath to select nodes, then an excludedXPath to take away some 
> nodes from the original selection, and finally a reincludeXPath to put 
> back some nodes that were taken away.
> 
> * This mechanism takes away a lot of variability which makes the 
> signatures more secure and robust. For example envelopedSignature is now 
> just a true/false attribute, so you cannot have two enveloped signature 
> transforms (which to me is completely meaningless). Also you cannot do 
> tricky things like have Xpath -> EnvelopedSig -> Xpath, where the second 
> XPath brings back the enveloped signature, which was removed by the 
> EnvelopedSig transform. Or you can't go from xml -> binary->xml
> 
> * The "type" attribute: In a transform chain you do not know if you are 
> signing xml or if you are signing binary or something else, unless you 
> run through the transforms. For example if you have an external URI 
> reference and no transforms, then the data is interpreted as binary. But 
> if you have an external URI references followed by an XPath transform, 
> then the data is interpreted as XML. This can be very confusing to a 
> policy processor. Instead it is better if a type attribute clearly 
> specifies what is being done - we can have two different types:  
> type="binaryFromURI" to mean directly fetch binary from an external URI 
> and type="binaryFromBase64Nodes" to mean  use URI and XPath to identify 
> text nodes, and then base64decode them and get the binary from them.
> 
> <WhatIsSigned
>  type="binaryFromURI"
>  URI="..."
>  byteRange="0-20,220-270,320-"
> />
> 
> <WhatIsSigned
>  type="binaryFromBase64Nodes"
>  URI="..."
>  includedXPath=".."
>  excludedXPath="..."
>  reincludedXPath="..."
> 
>  byteRange="0-20,220-270,320-"
> />
> 
> Note I have incorporated Chris Solc's byte range transform as one of the 
> attributes.
> 
> 
> The "type" atttribute will be the cornerstone of extensibility. Instead 
> of adding new transforms, higher level specs would add new types, and 
> then define new attributes/subelements that are valid for that type.
> For Example the WS-Security SWA profile defines these transforms - 
> AttachmentContentOnly and AttachmentComplete. In the new syntax this 
> would be represented as as a type
> 
> <WhatIsSigned
>  type="binarySoapAttachmentContentOnly"
>  URI="cid:.."
> />
> 
> Similary Widget framework could define a type called "widget" and define 
> new attributes for it.
> 
> "dbRecords" could be yet another type :  Konrad's example.
> 
> 
> XSLT transform has potential security problems. But I see Konrad's point 
> - if there is a set of well known XSLT files which the signer and the 
> verifier both know about, then it perfectly ok to use them. But in that 
> case maybe it could be represented as a custom attribute. Let us say 
> there is a well know XSLT that transforms the XML to a displayable 
> format - one could define an attribute convertToDisplayable=true , this 
> will indicate that the verifier has to run this particular XSLT 
> transform. However the policy processor does not need to run that.
> 
> One can argue that if you change the order of operations and verify the 
> SignedInfo, then what is harm is running the XSLT . But I think it is 
> still very risky. Consider this analogy - someone knocks on your door, 
> you open it, there is somebody selling vaccum cleaner, you ask him to 
> show his ID, and if that matches you listen to benefits of that vacuum 
> cleaner, and then decide on whether you want to buy it or not.  The act 
> of checking his ID is similar to checking the SignedInfo, it just means 
> that the signer is who he says he is and so you are willing to listen to 
> him; it doesn't mean that you trust him completely.  In a Web service 
> scenario - the signature may be just based on the user's password, and 
> during the message processing you will look up the type of the user - a 
> sys admin, an employee, a contractor, an registerd external user - and 
> decide how much to trust him.
> 
> Only in the case where the signer and the verifier are the same, e.g. in 
> case of long term signature, then you can have absolute trust on all the 
> transforms.
> 
> -----------------------------------
> 
> For the <HowItIsCanonicalized> section too, I am thinking of similar 
> declarative approach
> 
> <HowItIsCanoncialized
>   inclusive="yes"
>   ignoreComments="yes"
>   noMixedContent="true"/>
> 
> -------------------------------------------------
> 
> Pratik
> 
> Frederick Hirsch wrote:
>>
>> I created an initial draft for a note outlining requirements and 
>> design for XML Signature transform processing simplification [1].
>> I also incorporated some material from the mail list, including 
>> material from Pratik and John Boyer, to help get us started.
>>
>> Please review and help fill in the gaps by proposing material to the 
>> mail list.
>>
>> We may wish to consider an additional document focused on 
>> Canonicalization v.next.
>>
>> This should complete ACTION-93.
>>
>> Thanks
>>
>> regards, Frederick
>>
>> Frederick Hirsch
>> Nokia
>>
>> [1] http://www.w3.org/2008/xmlsec/Drafts/transform-note/Overview.html
>>
>>
> 
> 
Received on Monday, 8 December 2008 17:03:15 GMT

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