Re: Review of draft-saintandre-tls-server-id-check

[Peter Saint-Andre <[e-mail]>]

On 9/13/10 12:39 PM, Stefan Santesson wrote:
> Peter,
> 
> On 10-09-13 6:08 PM, "Peter Saint-Andre" <[e-mail]> wrote:
>>
>> Hi Shumon,
>>
>> As I see it, this I-D is attempting to capture best current practices
>> regarding the issuance and checking of certificates containing
>> application server identities. Do we have evidence that any existing
>> certification authorities issue certificates containing both an SRVname
>> for the source domain (e.g., example.com) and dNSName for the target
>> domain (e.g., apphosting.example.net)? Do we have evidence that any
>> existing application clients perform such checks? If not, I would
>> consider such complications to be out of scope for this I-D.
>>
>> That said, we need to be aware that if such usage arises in the future,
>> someone might write a document that updates or obsoletes this I-D; in
>> fact the present authors very much expect that such documents will
>> emerge after the Internet community (specifically certification
>> authorities, application service providers, and application client
>> developers) have gained more experience with PKIX certificates in the
>> context of various application technologies.
>>
>> Peter
> 
> I would like to turn the question around and ask why this specification need
> to have an opinion on whether a relying party feels he have to check both
> host name and service?

Stop right there. :) I sense a possible source of confusion. What do you
mean by "host name" and what do you mean by "service"?

In this I-D, we talk about "DNS domain name" and "service type", which
map quite well to _Service.Name from RFC 4985: the DNS domain name is
the "source domain" provided by the user or configured into the client
(e.g., "example.com") and the "service type" is a given application
protocol that could be serviced by the source domain (e.g., "IMAP").

This I-D is attempting to gently nudge people in the direction of
checking both the DNS domain name and the service type. IMHO this is
consistent with considering the SRVName and uniformResourceIdentifier
subjectAltName entries as more tightly scoped than dNSName or CN, and
therefore as potentially more "secure" in some sense (the subject might
want to limit use of a particular certificate to only the service type
identified in the SRVName or uniformResourceIdentifier).

If by "host name" you mean "target domain" as defined in the I-D (and
mapping to "Target" from RFC 2782) then we have more to discuss.

> I'm not against describing the typical case, as long as this specification
> does not imply that a relying party that has a reason to check two name
> types is doing something wrong.

That is not the intent of this I-D, however that would be functionality
over and above what this I-D defines.

> I have no extremely good examples of practical implementation here but
> checking both host name and service seems like both extremely easy and good
> practice.

With respect to revisions to this I-D, the lack of good examples
troubles me because we have been trying to abstract from common usage,
not to define guidelines for use cases that have not yet been defined,
implemented, and deployed.

Given that you would prefer to leave the door open to more advanced
checking rules, I think you would object to this text in Section 4.3:

   Once the client has constructed its list of reference identifiers and
   has received the server's presented identifiers in the form of a PKIX
   certificate, the client checks its reference identifiers against the
   presented identifiers for the purpose of finding a match.  It does so
   by seeking a match and stopping the search if any presented
   identifier matches one of its reference identifiers.  The search
   fails if the client exhausts its list of reference identifiers
   without finding a match.

You are saying that it is not necessarily appropriate to stop the search
once a single match is found, because the client might be configured to
look for multiple matches (e.g., a match against both the source domain
and the target domain). Would you like to suggest text that covers such
a case? Here is a possible rewrite of Section 4.3 that might address
your concern.

###

4.3.  Seeking a Match

   Once the client has constructed its list of reference identifiers and
   has received the server's presented identifiers in the form of a PKIX
   certificate, the client checks its reference identifiers against the
   presented identifiers for the purpose of finding a match.  The search
   fails if the client exhausts its list of reference identifiers
   without finding a match.  The search succeeds if any presented
   identifier matches one of the reference identifiers, at which point
   the client SHOULD stop the search.

      Implementation Note: A client might be configured to perform
      multiple searches, i.e., to match more than one reference
      identifier; although such behavior is not forbidden by this
      document, rules for matching multiple reference identifiers are a
      matter for implementation or future specification.

      Security Note: A client MUST NOT seek a match for a reference
      identifier of CN-ID if the presented identifiers include an
      SRV-ID, URI-ID, DNS-ID, or any application-specific subjectAltName
      entry types supported by the client.

   Detailed comparison rules for finding a match are provided in the
   following sections.


###

Peter

-- 
Peter Saint-Andre
https://stpeter.im/