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

[Stefan Santesson <[e-mail]>]

Peter,
Comments in line;


On 10-09-13 9:16 PM, "Peter Saint-Andre" <[e-mail]> wrote:

> 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"?
> 
Sorry for sloppy use of words.

With host name I mean here the actual DNS name of the host, which might be
host1.example.com (dNSName)

By service I mean the service under a given domain, which for the same host
might be _xmpp.example.com (SRVName)

Under the current rules, using this example I read it that the following
apply:

- If you are just checking the SRVName you will not learn the legitimate
host DNS name. So a certificate issued to host2.example.com will be accepted
even if you intended to contact host1.example.com (even if that information
is in the cert).

- If you just check the dNSName, you will miss the fact that you talk to the
desiganted ldap server and not the xmpp server (even if that information is
in the cert).


> 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.
> 

This sounds better to me.

If I'm not totally wrong in my example above I also think it would be good
with a security note stating that an SRVName may not provide the full host
DNS name, and if it is important to verify the host DNS name, you must
verify the dNSName in addition to what else you are checking.

/Stefan