Name Constraints, was Re: [caops-wg] Re: ca signing policy file

[David Chadwick]

Frank Siebenlist wrote:
> I don't care about the collisions - that's a non-issue as far as I'm 
> concerned.
> 
> The only sticky point I see, is the ability for any of your trusted CAs 
> to issue certificates outside of their administrative domain and only to 
> rely on essentially paper agreements to prevent this.
> 
> Note that with Kerberos cross-realm authentication, one realm is unable 
> to issue credentials for the director of the other institute...
> 
> With your proposed scheme, any "trusted" CA in Italy, Germany, even 
> Holland..., would have the theoretical opportunity to issue a 
> certificate that would impersonate the director of Berkeley, NCSA, 
> Livermore, Los Alamos... and we would have no way to enforce any policy 
> in real-time that could prevent it.

Using names based on hashes of public keys, the only way I could 
impersonate the director would be to get hold of his private key. And 
then it does not matter which CA issued his cert with whatever name it 
put there. Once I have the private key, I AM the director.

So, if the name is allocated properly, this is not a real risk. It all 
comes back to the same issue I mentioned before, that the CA has to 
prove that the person has the right to assert the name that it is doing.

regards

David


> 
> If this acceptable to all our end user organizations, we should happily 
> adopt the web-browser trust model with paper CA policy statements... and 
> I'm serious here.
> 
> So what are the real "trust-requirements" we are working from?
> 
> Regards, Frank.
> 
> 
> Mike Helm wrote:
> 
>> Frank Siebenlist writes:
>>  
>>
>>> In other words, the Subject's DN should start with an identifier that 
>>> essentially identifies the administrative domain in which the names 
>>> are issued, e.g. \DOMAIN=ESNET.NET, followed by a 
>>> \CN=abbf16d0-3b5f-11da-8cd6-0800200c9a66
>>> In that way, a CA could be restraint to issue random names within a 
>>> certain domain.
>>>     
>>
>>
>> Here's the subject name I had from Thawte:
>> E = helm at fionn.es.net, CN = Michael Helm
>>
>> That's it.
>>
>> The E= was just for my convenience.  I could create other certificates
>> with a different E= attribute if I needed to.
>>
>> Name collisions by themselves - so what?  I have the same
>> name on my driver's license and on my library card.  Nobody
>> gets worked up over that.  What I think you want, is
>> to make sure that same name string isn't certified to
>> two different people.  But we don't have technical means
>> guarantee this. Even the current name constraints / signing policy
>> scheme cannot prevent this, it can only make it a little more
>> difficult.
>>
>> You can eliminate most "legitimate" collisions by including
>> some link to the issuer in any authentication determination.
>> That's the administrative domain.
>>
>> You find some CA issues duplicate DN's from other domains?
>> Don't use them.   In any event, having an issuer field will limit what 
>> damage they can do.
>>
>> You find some collision?   You don't like it?  Take it up with the 
>> CA's that
>> did it.  They are highly motivated not to have this problem.
>>
>> Why is this such a huge problem?  I have never understood the amount 
>> of time & energy spent on it in our community.  I sure wish we didn't 
>> have
>> the current signing policy file scheme.
>>
>>   
> 
> 

-- 

*****************************************************************
David W. Chadwick, BSc PhD
Professor of Information Systems Security
The Computing Laboratory, University of Kent, Canterbury, CT2 7NF
Tel: +44 1227 82 3221
Fax +44 1227 762 811
Mobile: +44 77 96 44 7184
Email: D.W.Chadwick at kent.ac.uk
Home Page: http://www.cs.kent.ac.uk/people/staff/dwc8/index.html
Research Web site: http://sec.cs.kent.ac.uk
Entrust key validation string: MLJ9-DU5T-HV8J
PGP Key ID is 0xBC238DE5

*****************************************************************