RE: [dss] Groups - dss-requirements-1.0-draft-02.doc uploaded

["Nick Pope" <pope@secstan.com>]

Trevor,

My concern with the signing of the data after an XSLT transform has been
applied is that the chances of two independent implementations of XSLT to
get exactly the same byte-by-byte value for all possible styles is fairly
low, event though they will look the same.

Nick

> -----Original Message-----
> From: Trevor Perrin [mailto:trevp@trevp.net]
> Sent: 26 March 2003 18:58
> To: karel.wouters@esat.kuleuven.ac.be; dss@lists.oasis-open.org
> Subject: RE: [dss] Groups - dss-requirements-1.0-draft-02.doc uploaded
>
>
> At 12:11 PM 3/26/2003 +0000, karel.wouters@esat.kuleuven.ac.be wrote:
>
> >[...]
> >IMHO, the XML and the transform should be signed, and the rest should be
> >left to be specified by people who adopt this standard.
>
>
> I think it's better to sign 2 references, one to the raw
> document, one that
> applies transform(s) to make the raw document human-readable:
>
> <Reference URI="#SomeDocument">
>    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
>    <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
> </Reference>
> <Reference URI="#SomeDocument">
>    <Transforms>
>      <Transform Algorithm="http://www.someplace.org/SomeTransform"/>
>    </Transforms>
>    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
>    <DigestValue>Q52xy4a9289mvDl1up4sbEVU89x=</DigestValue>
> </Reference>
>
> Looking at the XML-DSIG spec, I don't see any mention of signing
> transforms.  Instead, usually transforms shouldn't need to be
> signed/verified, since the data is signed after going through them, so a
> change/corruption of the transform will invalidate the signature.
>
> I guess you're proposing something like this?, where both the
> document and
> transform are signed? -
>
> <Reference URI="#SomeDocument">
>    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
>    <DigestValue>j6lwx3rvEPO0vKtMup4NbeVu8nk=</DigestValue>
> </Reference>
> <Reference URI="#http://www.w3.org/SomeTransform">
>    <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1"/>
>    <DigestValue>Q52xy4a9289mvDl1up4sbEVU89x=</DigestValue>
> </Reference>
>
> However in this case, suppose multiple documents (#SomeDocument1,
> #SomeDocument2, etc.) were signed.  And suppose there are multiple
> transforms.  How would it be clear which transforms apply to which
> documents?  Moreover, if you sign only the transforms, not the
> transformed
> data, then there's wiggle-room for the signer to try to claim that he
> applied the transform differently than the verifier (suppose the
> transform
> references time-sensitive data somehow).
>
> Also, what about in the case above, where the transform might not be a
> stylesheet, but might be described algorithmically by a text document
> referenced by a URI that may not even be a URL - i.e. there may
> not be data
> describing the transform that is signable.  Even if there is, it may be a
> human-readable document subject to revisions (which would invalidate a
> signature), or it may be quite large, which would make signing/verifying
> unwieldy, and in any case add a network dependency, since the
> verifier will
> only be able to verify the document as long as the site stays up.
>
> So I think signing data after it's transformed is much more in
> accord with
> how XML-DSIG was designed to work, and in the case where we want to sign
> both human-readable and machine-readable views on some data, it's easiest
> to do that with different ds:References, each applying whatever
> transforms
> are appropriate to the same underlying data.
>
> Trevor
>
>
>