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Re: Encryption Subset Scenario

From: Jiandong Guo <jguo@phaos.com>
Date: Thu, 16 May 2002 15:17:15 -0400
Message-ID: <3CE405BA.18F65B00@phaos.com>
To: "Dournaee, Blake" <bdournaee@rsasecurity.com>
CC: xml-encryption@w3.org, "Hammond, Ben" <bhammond@rsasecurity.com>


"Dournaee, Blake" wrote:

> Jiandong - I don't think I understand your objection. I think the original
> problem contained an (unstated) assumption that XML document subset is
> continuous. I think I see what you meant - you were concerned about empty
> text nodes inbetween perhaps? If we encrypt a continuous document subset -
> is there a plaintext replacement problem upon encryption or decryption that
> I'm not seeing?
>

I was thinking about encrypting a set of elements not necessarily continuos. To
encrypt
a set of continuous elements is too special a case to be treated  separately in
the
specification as you suggested below.  How much more chances you need to encrypt

two adjacent elements than two non-adjecent elements in application?

Jiandong Guo
Phaos Technology

> I think I might be looking for a Type value that has "document subset"
> semantics, or "Type='http://www.w3.org/20001/04/xmlenc#DocumentSubset". I am
> of course just throwing this out there for the sake of argument at this
> point. There may be good reasons *not* to have this.
>
> Consider Document D below:
>
> <doc>
>         Mary
>         <elem1> Had </elem1>
>         <elem2> A little </elem2>
>         <![CDATA[ lamb for dinner ]]>
>         <!-- This is a SECRET comment -->
>         <cleartext>
>                 This information should not be encrypted
>         </cleartext>
> </doc>
>
> Now, suppose I have a single encryption key E_k, that I want to use to
> encrypt a document subset. Further, I want the same algorithm and parameters
> for everything.
>
> The document subset that I want to encrypt is everything but the element
> <cleartext> and its contents. The normative (soon to be!) solution, using a
> single encryption key, might look like this:
>
> <doc>
>       <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#Content'...>...</EncryptedData>
>       <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#Element'...>...</EncryptedData>
>         <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#Element'...>...</EncryptedData>
>         <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#Content'...>...</EncryptedData>
>         <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#Content'...>...</EncryptedData>
>         <cleartext>
>                 This information should not be encrypted
>         </cleartext>
> </doc>
>
> I'm fairly sure the comment and CDATA sections get treated as type
> "Content". In any case, this seems to be a less than optimal solution in
> terms of wasted space because a good deal of redundant information is
> included. I also believe that from an implementation point of view it will
> cause uneeded overhead (yes, this is out of scope!), but I want to mention
> it anyway :)
>
> Consider what happens when the document subset to be encrypted grows to
> hundreds or thousands of lines? The redundancy surely adds up as the scale
> increases.
>
> Why not have something like this instead?
>
> <doc>
>       <EncryptedData
> Type='http://www.w3.org/2001/04/xmlenc#DocumentSubset'...>...</EncryptedData
> >
>       <cleartext>
>                 This information should not be encrypted
>         </cleartext>
> </doc>
>
> Not only is this more space efficient, but it preserves the semantics of
> what is encrypted as an XML document subset instead of just plain text. This
> argument really only holds water if it is *useful* to preserve the document
> subset as an "XML document subset" and not just octets.
>
> Ed's solution (an out-of-scope XPath transform or equivalent) works, but it
> is not written into the standard. Also, as we're learning in XML Dsig, XPath
> can be messy and slow... :)
>
> The other solution is to force the plaintext creator to do something like
> this:
>
> <doc>
>         <secureSection>
>          Mary
>          <elem1> Had </elem1>
>          <elem2> A little </elem2>
>          <![CDATA[ lamb for dinner ]]>
>          <!-- This is a SECRET comment -->
>         </secureSection>
>         <cleartext>
>                 This information should not be encrypted
>         </cleartext>
> </doc>
>
> This makes the whole problem go away, as we can now just use
> Type="...#Content".   This, however, may not be practical in the real world
> if someone has a poorly designed XML markup language (XML "Application" or
> whatever you want to call it)
>
> Can anyone articulate some good arguments against a DocumentSubset Type? Are
> there processing rules or plaintext replacement issues that I am not seeing
> here? Does this bias the XML Encryption spec towards a DOM tree based view ?
> (don't think so, could be wrong though, an XML document subset is just a
> subset, regardless of how the document is modeled).
>
> Regards,
>
> Blake Dournaee
> Toolkit Applications Engineer
> RSA Security
>
> "The only thing I know is that I know nothing" - Socrates
>
>
>
> -----Original Message-----
> From: Ed Simon [mailto:edsimon@xmlsec.com]
> Sent: Thursday, May 16, 2002 8:21 AM
> To: Jiandong Guo; Dournaee, Blake
> Cc: xml-encryption@w3.org; Hammond, Ben; edsimom@xmlsec.com
> Subject: Re: Encryption Subset Scenario
>
> If one needs to combine adjacent elements before encrypting so that one ends
> up with a single <EncryptedData> element, then one can always use a
> transform (eg. XSLT) just before encryption and just after decryption.  Note
> that these transforms fall outside the core encryption and decryption
> processes and are NOT related to the <Transforms> element in XML Encryption.
>
> Note that the above technique doesn't require elements to be adjacent.  For
> example, if one wanted to encrypt all <Age> and <Address> elements in a
> document, regardless of their location, into one <EncryptedData> element,
> one could do so.
>
> Of course, the critical question is not "can I do this?" but "should I do
> this?".   In other words, always ask if using a workaround to get a desired
> result is really justified.  I would recommend initially assuming the answer
> is NO until proven otherwise.
>
> Ed
>
> ----- Original Message -----
> From: "Jiandong Guo" <jguo@phaos.com>
> To: "Dournaee, Blake" <bdournaee@rsasecurity.com>
> Cc: <xml-encryption@w3.org>; "Hammond, Ben" <bhammond@rsasecurity.com>;
> <edsimom@xmlsec.com>
> Sent: Thursday, May 16, 2002 10:17 AM
> Subject: Re: Encryption Subset Scenario
>
> >
> >
> > "Dournaee, Blake" wrote:
> >
> > > All -
> > >
> > > Given an input Document D:
> > >
> > > <doc>
> > >   <elem1> foo1 </elem1>
> > >   <elem2> foo2 </elem2>
> > >   <elem3> foo3 </elem3>
> > > </doc>
> > >
> > > I want to encrypt just the first two child elements (<elem1> and
> <elem2>).
> > > This doesn't appear to fit the definition of
> > > Type='http://www.w3.org/2001/04/xmlenc#Element', which suggests a single
> > > element, or Type='http://www.w3.org/20001/04/xmlenc#Content'
> > > which suggests that all three elements must be encrypted (elem1, elem2
> and
> > > elem3).
> > >
> > > Choosing to treat the first two elements as arbitrary plaintext also
> seems
> > > overkill, and if so, this ruins the XML semantics. I cannot
> > > treat it as text/xml, because this document subset is not well-formed.
> > > Treating it as text/plain looses all of the XML semantics.
> > >
> > > The obvious solution is to create two <EncryptedData> elements, but this
> is
> > > redundant. Another solution is an XPath transform, but this
> > > doesn't exist for XML Encryption.
> > >
> > > Am I missing something here? Is there an obvious solution to this? It
> seems
> > > like a simple case that might have been overlooked.
> >
> > If you want to encrypt two elements in one EncryptedData, the question is
> > that how do you handle the "replace" process in encryption and later in
> > decryption,
> > considering there could have other nodes (text nodes or other elements)
> between
> >
> > these two elements?
> >
> > -----------------------------------------
> > Jiandong Guo
> > Phaos Technology
> > www.phaos.com
> >
> >
> >
> >
> >
Received on Thursday, 16 May 2002 15:13:14 GMT

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