[CAP] CAP Security Using Digital Signatures

[Art Botterell]

Interesting link, Matt.  Sounds like an object lesson on why we should  
resist the temptation to standardize what we don't yet understand.

I was thinking we might wind up proposing an erratum to OASIS to fix  
the schema issue, but hadn't appreciated that cannonicalization was  
still proving so intractable.  Although that article is from 2004, I  
take it the C14N situation hasn't improved.  So maybe it would make  
more sense to identify (and demonstrate!) alternate approaches that  
could be fed back into the standard.

My concern is that if we don't address the end-to-end signature  
problem as a community there might not be a business incentive for any  
particular implementer to design for that level of interoperability.   
And while the OASIS process usually does a good job of refining and  
ratifying contributed designs, it seems not to be as effective as a  
framework for developing those designs in the first place.

- Art



On Mar 12, 2009, at 3/12/09 5:59 AM, matt hoffman wrote:

> I did a proof-of-concept implementation of this on a previous DHS  
> system. Although, as you say, there was no way of verifying it with  
> other providers or consumers at the time.
> However, a couple of points that I recall:
>
> a.) The document mentions that the signature element can be a child  
> of "alert", but unless I'm mistaken it is not specified in the  
> schema.  So messages containing signatures fail schema  
> validation ... I had to explicitly add it to the schema.
>
> b.) The signatures are delicate, and tricky:  http://www.cs.auckland.ac.nz/~pgut001/pubs/xmlsec.txt
>
> Unfortunately I no longer have access to that code, but I'd be happy  
> to help others if they're looking into it.
>
>
> - matt
>
> On Wed, Mar 11, 2009 at 11:20 PM, Art Botterell <acb at incident.com>  
> wrote:
> Friends -
>
> We're moving rapidly toward an important threshold in CAP  
> implementations.  So far, most CAP-based systems have been self- 
> contained, single vendor/implementer arrangements.  But soon we're  
> going to need to "federate" CAP traffic among multiple interoperable  
> systems.  And that has important implications for security.
>
> Most current systems use a trusted-link/trusted-host mode based on  
> encrypted network links and password access control at a central  
> server.  That's a familiar Web 1.0 approach and it works fine for  
> "single-hop" implementations.  But it has a major drawback: As soon  
> as messages are forwarded from one server to another, a security  
> compromise anywhere can compromise the authentication and integrity  
> of all CAP messages downstream.
>
> The alternative, of course, is to apply digital signatures to CAP  
> messages at their origin, and to verify them at receiving points.   
> That way, it doesn't matter if the links or intervening nodes are  
> secure or not; any recipient can verify independently that the  
> message is a) from who it says it's from, and b) hasn't been  
> modified in transit.
>
> There's a standard way of doing this for XML, as cited in the CAP  
> Specification:
>
> >3.3.2.1 Digital Signatures
> >The alert element of a CAP Alert Message MAY have an Enveloped  
> Signature, as described by XML Signature and
> >Syntax Processing [XMLSIG]. Other XML signature mechanisms MUST NOT  
> be used in CAP Alert Messages.  Processors
> >MUST NOT reject a CAP Alert Message containing such a signature  
> simply because they are not capable of verifying
> >it; they MUST continue processing and MAY inform the user of their  
> failure to validate the signature.
>
> But I'm not aware of anyone that's implemented it yet... partly  
> because it hasn't been necessary in stand-alone systems, and partly  
> because it involves a type of programming a lot of folks haven't had  
> occasion to explore yet.
>
> But ultimately, it's going to be essential for interoperability.  So  
> I'd be interested in hearing, has anyone implemented XMLSIG on CAP  
> yet?  And would anyone be interested in doing some interoperability  
> experiments?  The standard is there, the technology is there (in  
> Java and a number of other languages) and I see the requirement  
> bearing down on us quickly.
>
> What say?
>
> - Art
>
>
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