[CAP] Then Again... (was Re: CAP Security Using Digital Signatures)
David Aylward (Comcare)
daylward at comcare.org
Thu Mar 12 12:17:27 PDT 2009
It is indeed a very significant issue. Thanks to Art for raising it, and to
Rex for expanding the scope.
I think what has been raised is end to end identity management, which is a
central problem across the emergency/safety eco-system, for lots of use
cases, and is closely related to access control (the right to send or
receive certain messages, and assignment of identities).
CAP is an important use case, but only one of those that need these related
capabilities. And it is an important standard, but only one of those that
need these.
As Art correctly points out, the nature of emergency interoperability is a
wide variety of emergency messages, going across multiple
networks/applications which may or may not be secure. And it is "n"
originators, sending messages to "n" recipients. Because of this we in
COMCARE came to the conclusion reached by a lot of folks that we need
federated, standardized solutions for these needs. Rather than the ad hoc,
use-centric approach that has been used to date.
I hope as you address the particular needs of CAP, you will do so with this
broader set of needs in mind.
-----Original Message-----
From: cap-list-bounces at lists.incident.com
[mailto:cap-list-bounces at lists.incident.com] On Behalf Of Rex Brooks
Sent: Thursday, March 12, 2009 3:01 PM
To: matt hoffman; Art Botterell
Cc: cap-list at lists.incident.com
Subject: Re: [CAP] Then Again... (was Re: CAP Security Using Digital
Signatures)
Just wanted to let you all know that I have forwarded this thread to
the Messages and Notification Subcommittee of the OASIS EM TC because
I think it is significant, and I wanted them to be aware of the
conversation.
Thanks,
Rex
At 2:46 PM -0400 3/12/09, matt hoffman wrote:
>Sorry, didn't see this post before replying to the other.
>Whether we specify it or not (and, it could be that canonicalization
>standards have reached a point that we no longer need to -- I don't want to
>be too hopeful, but it is certainly possible) there are certainly
>performance benefits in forwarding the message as you received it, if there
>were no local changes.
>At least, in my implementation experiences. Saves one serialization step,
>and one set of possible compatibility issues.
>
>Now, whether we want to mandate sign-the-blob... I agree that, from a
>signature point of view, it simplifies things considerably (or seems to --
I
>haven't tried it with the Java toolkits, but it makes sense that it would
>work as you're suggesting). But other implementers might want to speak up
>there about the feasibility of ALWAYS passing exactly what they received if
>there is a digital signature included. That's mandating a particular
>implementation step that might not be trivial for all users.
>
>
>
>On Thu, Mar 12, 2009 at 12:08 PM, Art Botterell <acb at incident.com> wrote:
>
>> Thinking about it a bit more... am I mistaken, or wouldn't a simple way
to
>> implement a "sign the blob" approach be just to use a "null"
>> canonicalization in otherwise-normal XMLSIG?
>>
>> As I understand it, the reason for C14N is to deal with variations in
>> whitespace, attribute order and such that can occur when XML is parsed
and
>> then re-serialized. So we try to factor out every possible change that
>> might occur in those processes... with, it appears, limited success.
>>
>> But to simply forward/publish someone else's CAP alert, seems like
passing
>> it along precisely, byte-for-byte, would be relatively simple... and (as
the
>> Gutmann paper points out) much more auditable. Of course that wouldn't
>> prevent any node from parsing the alert and acting on the basis of the
>> contents. It would only mean that if that node decides to pass the
alert to
>> other nodes, it must give them the precise version it received.
>>
>> In other words, if C14N is murky bathwater, maybe we could dispense with
it
>> without needing to toss the baby too. All we'd need would be a
>> clarification in the OASIS spec (and an implementers' convention till
then)
>> that canonicalization SHALL NOT be applied during signing. (And to fix
the
>> schema, of course.)
>>
>> Or am I missing something here?
>>
>> - Art
>>
>>
>> On Mar 12, 2009, at 3/12/09 8:32 AM, Art Botterell wrote:
>>
>> 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
>>>>
>>>>
>>>> _______________________________________________
>>>> This list is for public discussion of the Common Alerting Protocol.
This
>>>> list is NOT part of the formal record of the OASIS Emergency
>>>>Management TC.
>>>> Comments for the OASIS record should be posted using the form at
>>>>
http://www.oasis-open.org/committees/comments/form.php?wg_abbrev=emergency
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>>>>
>>>> This list is not for announcements, advertising or advocacy of any
> >>> particular program or product other than the CAP itself.
>>>>
>>>>
>>> _______________________________________________
>>> This list is for public discussion of the Common Alerting Protocol.
This
>>> list is NOT part of the formal record of the OASIS Emergency Management
TC.
>>> Comments for the OASIS record should be posted using the form at
>>>
http://www.oasis-open.org/committees/comments/form.php?wg_abbrev=emergency
> >> CAP-list mailing list
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>>>
>>> This list is not for announcements, advertising or advocacy of any
>>> particular program or product other than the CAP itself.
>>>
>>
>> _______________________________________________
>> This list is for public discussion of the Common Alerting Protocol.
This
>> list is NOT part of the formal record of the OASIS Emergency Management
TC.
>> Comments for the OASIS record should be posted using the form at
>>
http://www.oasis-open.org/committees/comments/form.php?wg_abbrev=emergency
>> CAP-list mailing list
>> CAP-list at lists.incident.com
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>>
>> This list is not for announcements, advertising or advocacy of any
>> particular program or product other than the CAP itself.
>>
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--
Rex Brooks
President, CEO
Starbourne Communications Design
GeoAddress: 1361-A Addison
Berkeley, CA 94702
Tel: 510-898-0670
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