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Re: From OASIS digital signatures services TC: request of comments

From: Joseph Reagle <reagle@w3.org>
Date: Fri, 11 Apr 2003 16:59:18 -0400
To: Juan Carlos Cruellas <cruellas@ac.upc.es>, w3c-ietf-xmldsig@w3.org
Cc: dss@lists.oasis-open.org
Message-Id: <200304111659.19025.reagle@w3.org>

On Friday 11 April 2003 11:42, Juan Carlos Cruellas wrote:
> These two issues have been well summarized by
> one of the members in the group as follows:

Juan Carlos,

Ed Simon did a good job of concisely addressing the issues, to which I hope 
to add a few augmentations. This is not an easy topic though, you can find 
much discussion on "document closure" in our archives while we tried to 
come to our present understanding [1].

[1]http://www.w3.org/Search/Mail/Public/search?keywords=document+closure&hdr-1-name=subject&hdr-1-query=&index=Public%2FFULL&index-type=t&type-index=w3c-ietf-xmldsig

> 1. Securing Transform Chain -
> So external transform data needs to be secured through some out-of-band
> method, or needs to be covered by the XML-DSIG itself.  If the data is to
> be covered by the XML-DSIG, should it be:
>    a) referred to by a dsig:Reference in the dsig:Signature (and attached
> as a signed attribute or something, if it's representable in XML)?
>    b) referred to by a dsig:Reference in a dsig:Manifest to the
> dsig:Signature?

I don't think the dsig:Manifest buys you anything. In fact, to me it serves 
as a loose (no reference validation) prototype/example that other 
applications should extend/supercede with more explicit processing rules. A 
serious application should ensure that as much as possible of the material 
that effects the what-is-seen/signed-data *contributes* towards the 
signature value. This depends on the application: is it using external 
entities, inline embeds, images, objects, scripts, XLink, XSLT, etc. One 
approach is to include references in the ds:SignedInfo. Something I've 
proposed in the past is a (proper) secure cache/manifest. It'd be a signed 
instance that defines *correspondences* between *contributing* URIs and 
their dereferenced content (hash values), and maybe even a digest of the 
final (implicit) data object.

 For example:
  <transormManifest id="sign me">
    <references URL="...">
       <dereferenceTime>20020228 15:35  UTC</dereferenceTime>
       <digestValue>cafebabe...</digestValue>
       <optional-object>...</optional-object>
    </reference>
    ...
    <result>
       <createdTime>20020228 15:35  UTC</createdTime>
       <type>text/html</type> 
       <digestValue>cafebabe...</digestValue>
       <optional-object>...</optional-object>
    </reference>
  </transformManifest>
Then the app would make sure that this transformManifest is signed in 
ds:SignedInfo, and that there is a correspondence: a reference for every 
contributing URL within the transform sequence. At the time there didn't 
seem to be much support for this (folks obviously wanted to implement and 
apply the 1st gen of xmlsec) and I've been waiting to see if anything in 
the "XML Processor Pipeline" [2] issue gelled, perhaps an instantiation of 
such a pipeline could be signed.

[2] http://www.w3.org/XML/2001/07/XMLPM.html

Regardless, while the ds:transform mechanism is powerful and flexible, no 
unknown/arbitrary transform should be accepted without evaluation. Each 
transform sequence is an "algorithm" as important to the validity of the 
signature as the strength of the key and the crypto algorithms.

>Even though the signer intends to sign Document 1, the attacker can 
>trick the relying party into relying on Document 2.

Yes, here's an example:

o InputDocument1 (ID1) is a statement from the bank saying, "pay $50"
o Transform1 (T1) is invoked from within ID1 and is an XSLT that turns ID1 
into pretty HTML; it is located at a fixed URI (URI1).

The owner of controlling domain name for URI1 decides to cause a little 
mayhem and decides to serve Transform2 (T2) instead, that adds two zeros to 
the amount before turning it into HTML. Now, when the bank see the 
resulting document, it is signed by themselves, but they pay out more than 
they should. The problem? The bank obligated itself to "unsigned" code 
(i.e., the transform at URI1).

The conclusion is that identifiers/URIs need to be secure/stable, transforms 
need to be evaluated/standardized, and one should sign as much of the 
contributing material as possible, or keep it really simple. (One of the 
reasons I *don't* like URNs is that they are difficult to dereference and 
consequently hash, even though URN proponents argue they are more 
secure...)

> 2. Signing Human-Readable data and XML -
>
> The client may want to sign both an XML document and its human-readable
> transformed version, for use in dispute resolution.  How should this be
> done?

I discuss this question somewhat in [3].

[3]http://www.w3.org/TR/md-policy-design#_Misunderstanding_Metadata_Agreements

> One approach is to include two dsig:Reference elements in the signature,
> the first to the XML document, the second to the same document with
> human-readable transforms applied.  Then a "policy identifier" of some
> sort should be added as a signed attribute to indicate the relation of
> these documents.

I'd suggest a "transformManifest" like I suggested above, but this is 
similar.

> This approach will fail if the human-readable transforms behave
> inconsistently when applied by the signing and relying parties (perhaps
> due to implementation differences in the transform engines), and a
> canonicalization transform for the transform output data is
> unavailable.  This is (apparently) the case with XSLT transforms
> producing HTML.

Right, or imagine simple HTML tricks (font size is really small), CSS tricks 
(display:none), and browser bugs. The ambiguity of XML, the number and 
complexity of specifications, and the presence of bugs or lack of interop 
make this a difficult issue. While more formal assertion languages (e.g., 
RDF) might mitigate some of these ambiguities there will always be a "GUI 
gap" that must be addressed by law and human judgment. A court will use 
various principles [3] to determine reasonable expectations, fault, and 
liability. So the more clear, explicit evidence you have 
(signatures/digests over the various documents, logs, etc.) the better. For 
instance, if a particular XSLT is standardized by the banking industry for 
rendering an XML payment into HTML (and they've done their homework with 
all the rendering bugs and such) and its been used for 2 years, including 
by me, I'm going to have a much harder time on my 100th time using it to 
argue what was shown on my CRT was not what I thought I was signing.

[3]http://www.w3.org/TR/md-policy-design#_Breach_and_Liability

> This could be solved by the signer storing the transform output data at a
> URL, or sending it with the signature, and referring to it explicitly
> instead of requiring the verifying party reproduce the transform
> process.  But that may be inconvenient.

Yes, like my <result> element in the <transformManifest>.

> A different approach is to have the signer add a signed attribute
> containing "transforms that reproduce what I was looking at when I chose
> to sign".  Since processing these transforms isn't required to verify the
> signature, it avoids the problem of having the signature fail if the
> signer and verifier's transform engines produce slightly different
> output.  However, if the engines *are* producing different output,
> there's now wiggle-room for the signer to claim "oh, my transform engine
> produced something different, that's not *really* what I was looking at
> when I chose to sign".  So this may just shift the problem of "transform
> variability" to a different place, not remove it.

Yep. This is why I don't see digital signatures as a useful consumer 
technology. IMHO XML Signatures are useful in B2B transactions where the 
transforms and sequences are standardized in their application contexts and 
there is a log/history that demonstrates a norm for the expected 
transactions...

-- 
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 Friday, 11 April 2003 16:59:23 GMT

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