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Experiements with ds:KeyInfo Reuse

From: Joseph M. Reagle Jr. <reagle@w3.org>
Date: Mon, 23 Apr 2001 12:10:53 -0400
Message-Id: <4.3.2.7.2.20010423112918.00b88b10@localhost>
To: "XML Encryption WG " <xml-encryption@w3.org>

I've spent some more time playing with the schema and asking questions of 
Henry. I'm now confident that if we want to use or reuse ds:KeyInfo, my hope 
to co-exist with its ANY and also show how to do cooler extensibility via 
subsitutionGroup, or other stronger typing/extensibility isn't easy.

The problem is a "non-deterministic content model." For instance, if we say 
KeyRetrievalMethod is in the subsitution group of RetrievalMethod this means 
that (1)  KeyRetrievalMethod is a restriction/extension of the 
RetrievalMethod, when it appears in ds:KeyInfo, it should replace 
RetrievalMethod: they both shouldn't occur. This is very nice. 
Unfortunately, since there is an ANY in ds:KeyInfo, the validation doesn't 
know whether to match the KeyRetrievalMethod as a member of the 
subsitutionGroup or as the ANY, resulting in the "non-deterministic" problem.

(I'm using KeyRetrievalMethod as an example of a general extensibility 
issue, I'm not convinced that we actually need KeyRetrievalMethod, don't see 
many reasons we couldn't just use ds:RetrievalMethod to do the trick.)

Regardless, the only way to get around this would be to redefine the 
ds:KeyInfo and remove the ANY -- yes, schema permits someone to redefine the 
schema associated with a namespace in their own context. However, I'm not 
keen on this. And I don't think it's a good idea to try to remove the ANYs 
(and use more sophisticated extensions) from dsig at this point.

So we're stuck with "orphaned" elements that have to rely upon a comment to 
get some sense of where they belong:


http://www.w3.org/Encryption/2001/04/schema/enc-example.xml
<?xml version="1.0" encoding="UTF-8"?>
<EncryptedData xmlns="http://www.w3.org/2001/04/xmlenc#"
                xmlns:ds="http://www.w3.org/2000/09/xmldsig#"
                xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:schemaLocation="http://www.w3.org/2001/04/xmlenc#
                                    20-xenc-schema.xsd">
	<EncryptionMethod Algorithm="foo"/>
	<ds:KeyInfo>
	  <EncryptedKey xmlns="">
               <CipherData>41424344</CipherData></EncryptedKey>
                 <KeyRetrievalMethod URI="http://someKey"
	       Type="http://www.w3.org/2001/04/xmlenc#EncryptedKey"/>
	  <ds:KeyName>Joseph</ds:KeyName>
	</ds:KeyInfo>
	<CipherData>1234</CipherData>
</EncryptedData>

http://www.w3.org/Encryption/2001/04/schema/20-xenc-schema.xsd
...
   <!-- Children of ds:KeyInfo -->

   <element name="EncryptedKey" type="enc:EncryptedKeyType"/>
   <complexType name="EncryptedKeyType">
     <complexContent>
       <extension base="enc:EncryptedType">
         <sequence>
           <element ref="enc:ReferenceList" minOccurs="0"/>
         </sequence>
         <attribute name="NameKey" type="string" use="optional"/>
         <attribute name="Recipient" type="string" use="optional"/>
       </extension>
     </complexContent>
   </complexType>

    <element name="KeyRetrievalMethod" type="enc:KeyRetrievalMethodType" />
    <complexType name="KeyRetrievalMethodType">
      <complexContent>
        <restriction base="ds:RetrievalMethodType">
          <sequence>
            <element name="Transforms" type="ds:TransformsType" 
minOccurs="0"/>
          </sequence>
          <attribute name="URI" type="anyURI"/>
          <attribute name="Type" type="anyURI" use="optional" 
fixed="http://www.w3.org/2001/04/xmlenc#EncryptedKey" />
        </restriction>
      </complexContent>
    </complexType>

   <!-- End Children of ds:KeyInfo -->


__
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, 23 April 2001 12:11:06 GMT

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