Re: [certid] Review of draft-saintandre-tls-server-id-check

[Shumon Huque <[e-mail]>]

On Thu, Sep 09, 2010 at 12:59:29AM +0200, Stefan Santesson wrote:
> Peter,
> 
> I don't see the problem with accepting a host names provided by DNS SRV
> record as the reference identity.
> 
> Could you elaborate the threat?
> 
> Example:
> 
> I ask the DNS for the host providing smtp services for example.com
> I get back that the following hosts are available
> 
> smtp1.example.com, and;
> smtp2.example.com
> 
> I contact the first one using a TSL connection and receives a server
> certificate with the SRVName _smtp.example.com and the dNSName
> smtp1.example.com
> 
> The certificate confirms that the host in fact is smtp1.example.com and that
> it is authorized to provide smtp services for the domain example.com.
> 
> That is all you need. The host name from the DNS server was setting you on
> the track but is not considered trusted. What you trust is the names in the
> certificate.

This is a more complicated example than the current draft
addresses.

In your example, the client is verifying "a combination of 
identifiers" (SRVName and dNSName) in the certificate. This
seems like a reasonable thing to do, but this is not what
most clients do today (I'd be happy to be corrected about
that). Typically, they consider a match successful once they've
found a single acceptable reference identifier. In that case,
you can't simply use a reference identifier that contains
a DNS mapped identifier unless you've obtained it an authenticated
(or statically configured) manner.

-- 
Shumon Huque
University of Pennsylvania.