Re: [TLS] TLS-OBC proposal

Nikos Mavrogiannopoulos <nmav@gnutls.org> Thu, 01 September 2011 11:49 UTC

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Subject: Re: [TLS] TLS-OBC proposal
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On 08/22/2011 07:53 PM, Dirk Balfanz wrote:
> Dear TLS-WG,
>
> I few weeks ago I presented a proposal for an "origin-bound certificates"
> TLS extension at IETF 81. It's much easier to understand what this extension
> is trying to accomplish when presented in a broader context of web
> authentication (which I tried to give in Quebec), so I put together a little
> web site that is supposed to do just that for those who couldn't be at the
> IETF meeting: http://browserauth.net (This took me a little while, hence the
> delay in sending out the I-D).
[...]
> All feedback is welcome, of course, but if you can't think of anything to
> comment on, I have a couple particular questions:

I like the proposal. As far as I understand you try to fix the usability 
problem of using multiple certificates for different sites. This is 
similar to an SSH-style authentication for users. As a side-effect 
pseudonymity can be achieved. Some minor comments.

* In http://www.browserauth.net/tls-client-authentication you mention 
the inability to log-out as one reason that TLS client authentication 
fails. However this is not addressed by OBC if I understand correctly. 
(to be honest I think that the inability to log-out is just a UI 
problem. TLS authentication does not need to imply log-in. You might 
have buttons login and logout that will log the user in and out even if 
TLS with authentication was used)

* Why you require a critical X.509 extension for that? (I also don't 
like the IA5String... why not UTF8?)

* After having read the proposal the name OBC was clear to me. However 
before reading it, the name didn't give any clue (to me) of the proposal.

> - in Section 2.1.1 we currently stipulate that the client include the origin
> of the origin-bound cert as part of the OBC X.509 extension. When we
> implemented this, we didn't really know what to do with that data on the
> server side. If the client also uses TLS-SNI, then we could imagine
> comparing the SNI and OBC origins in some manner on the server side, but
> there isn't really a vehicle to signal an error back to the client saying
> "your SNI and OBC origins don't match". Similarly, we could perhaps at some
> higher layer compare the HTTP "Host" header with the OBC origin, but then
> again - what do you do when they don't match?

I'd send a TLS alert or do nothing. What does this check add?

> Respond with a 400 at the HTTP
> layer? An alternative to the current language in the I-D would be to simply
> mark the cert as origin-bound, but without putting the origin itself into
> the cert. Any thoughts?

I don't like having the origin in the certificate as a critical extension.

regards,
Nikos