[CAP] Then Again... (was Re: CAP Security Using DigitalSignatures)
Rex Buddenberg
budden at nps.navy.mil
Thu Mar 12 13:23:50 PDT 2009
Matt,
Note that XML sign can be applied to the whole message (making it
equivalent to a whole-message e-mail signature, MIME this time instead
of XML). What Art is calling the blob.
But that does not prevent signing subsets of the message too.
Individual tagged fields or groups of them.
Order of business. I'd suggest that the greater granularity should be
pursued only after we get a workable blob authentication working.
matt hoffman wrote:
> To be clear, the alteration we're talking about here isn't any kind of
> content modification -- adding or altering fields, for example. It's simply
> formatting, primarily whitespace. For example, the two XML fragments:
> <alert>
> <identifier>MyMessageIdentifier</identifier>
> <sender>me at mycompany.com</sender>
>
> and:
>
> <alert><identifier>MyMessageIdentifier</identifier><sender>me at mycompany.com
> </sender>
>
> are functionally identical, but would result in different XML signatures as
> they are. Thus the question of either altering the whitespace of every XML
> message coming in to some standard, canonical form before verifying
> signatures, or agreeing by convention to never alter a single byte of the
> message whether it affects the content or not. Both methods are fragile in
> their own way.
>
> I assume that the modification that would concern you, as an originator of
> alerts, would be modifications of content, not necessarily modifications to
> XML formatting.
>
>
>
>
> On Thu, Mar 12, 2009 at 3:05 PM, Hughes, William <
> William.Hughes at co.ramsey.mn.us> wrote:
>
>
>> Hi time top weigh in on this on the non-technical side.
>>
>> Since I may be an originator of CAP alerts, I and I am sure the majority of
>> my colleagues would prefer that alerts / warnings be relayed with out
>> alteration in anyway.
>>
>> Subsequent enhancement / modification of the alert by other authorities,
>> should be a new separate message.
>>
>> That would help keep the audit trail clean.
>>
>> Thanks
>>
>> Bill Hughes, Mobile 651-325-5867
>>
>> ----- Original Message -----
>> From: cap-list-bounces at lists.incident.com <
>> cap-list-bounces at lists.incident.com>
>> To: Art Botterell <acb at incident.com>
>> Cc: cap-list at lists.incident.com <cap-list at lists.incident.com>
>> Sent: Thu Mar 12 13:46:34 2009
>> Subject: Re: [CAP] Then Again... (was Re: CAP Security Using
>> DigitalSignatures)
>>
>> 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
>>
>>>>> CAP-list mailing list
>>>>> CAP-list at lists.incident.com
>>>>> http://lists.incident.com/mailman/listinfo/cap-list
>>>>>
>>>>> 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
>>>> http://lists.incident.com/mailman/listinfo/cap-list
>>>>
>>>> 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
>>> http://lists.incident.com/mailman/listinfo/cap-list
>>>
>>> This list is not for announcements, advertising or advocacy of any
>>> particular program or product other than the CAP itself.
>>>
>>>
>> -------------- next part --------------
>> An HTML attachment was scrubbed...
>> URL: <
>> http://lists.incident.com/pipermail/cap-list/attachments/20090312/29909d94/attachment.htm
>>
>> _______________________________________________
>> 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
>> http://lists.incident.com/mailman/listinfo/cap-list
>>
>> This list is not for announcements, advertising or advocacy of any
>> particular program or product other than the CAP itself.
>> -------------- next part --------------
>> An HTML attachment was scrubbed...
>> URL: <
>> http://lists.incident.com/pipermail/cap-list/attachments/20090312/b5d00ccf/attachment.htm
>>
>> _______________________________________________
>> 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
>> http://lists.incident.com/mailman/listinfo/cap-list
>>
>> This list is not for announcements, advertising or advocacy of any
>> particular program or product other than the CAP itself.
>>
>>
> -------------- next part --------------
> An HTML attachment was scrubbed...
> URL: <http://lists.incident.com/pipermail/cap-list/attachments/20090312/9b33f5a2/attachment.htm>
> _______________________________________________
> 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
> http://lists.incident.com/mailman/listinfo/cap-list
>
> This list is not for announcements, advertising or advocacy of any particular program or product other than the CAP itself.
>
>
--
Rex Buddenberg
Senior Lecturer
Naval Postgraduate School
Monterey, Ca 93943
831/656-3576
More information about the CAP-list
mailing list