Re: [TLS] TLS 1.3 wishlist - better certificate selection

Anders Rundgren <anders.rundgren.net@gmail.com> Sat, 07 December 2013 12:30 UTC

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Date: Sat, 07 Dec 2013 13:29:46 +0100
From: Anders Rundgren <anders.rundgren.net@gmail.com>
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To: Henry Story <henry.story@bblfish.net>
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Cc: TLS Mailing List <tls@ietf.org>
Subject: Re: [TLS] TLS 1.3 wishlist - better certificate selection
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On 2013-12-07 13:11, Henry Story wrote:
> 
> On 7 Dec 2013, at 12:08, Anders Rundgren <anders.rundgren.net@gmail.com> wrote:
> 
>> On 2013-12-07 11:15, Henry Story wrote:
>>>
>>> On 18 Sep 2013, at 20:45, Michael D'Errico <mike-list@pobox.com> wrote:
>>>
>>>> A "wish list" for an upcoming new version of TLS is available
>>>> online (I found it via a message on Twitter):
>>>>
>>>> https://www.ietf.org/proceedings/87/slides/slides-87-tls-5.pdf
>>>>
>>>> Is it appropriate to discuss this now, prior to rechartering?
>>>
>>> Hi,
>>>
>>>  One thing that would be nice would be some way of having more flexibility
>>> for the server to request a client certificate. In TLS 1.2 it seems
>>> the only way to do this is to use the certificate_authorities
>>> list. 
>>>
>>>  With WebID over TLS [1] a server may in fact be satisfied if the 
>>> Client certificate contains a WebID in the Subject Alternative Name.
>>> But this then leaves the question open as to how the server can transmit
>>> to the client it's ability to accept such certificates.
>>>
>>> The only solution currently available that I know of 
>>> would be to create a CA with DN such as
>>>
>>>   CN=WebID, O={}
>>>
>>> that every WebID enabled certificate would claim it is signed by (somewhere
>>> in the certificte chain)
>>>
>>> Then this could be passed by the server in the certificate_authorities
>>> list as specified in http://tools.ietf.org/html/rfc5246#section-7.4.6
>>>
>>>
>>> Some people are worried that this would require CAs to resign their root
>>> CAs with a WebID certificate in case they wanted to release WebID 
>>> certificates.
>>>
>>> Is there a better way to do this currently? Could there be a better 
>>> way to do it?
>>
>> I believe we had this discussion before, so here we go again :-)
>> Making TLS CCA (Client Certificate Authentication) useful for consumer authentication (be it to an enterprise, bank or social network), is IMO unrealistic, no matter how desirable it may be.
>>
>> Technical reasons:
>> http://webpki.org/papers/PKI/webauth.pdf#page=1
> 
> This thread is not about your ideas of what is wrong with TLS and
> why you think it can never be solved, but what can be done to improve
> one particular aspect that concerns me.

Well, the issue you bring up is pretty much related to TLS.

> 
> Btw. your article mixes up User Interface issues which can be sorted by 
> browser vendors UI team with protocol issues which I want to discuss here.
> ( Eg: logging out would be easy if the browser showed the client certificate
> used when logged in ).

There's no logout in TLS CCA.

Currently you need a separate non-standard IDP-like thing + browser-specific logout-junk
to "emulate" what any password-using site can do using out-of-the-box solutions.

Tomcat doesn't even permit unknown CAs like required by WebID.

Note it is not WebID that's bad, it is TLS CCA.

> 
> 
>>
>> TLS CCA is also at odds with the market:
>> http://www.forbes.com/sites/amadoudiallo/2013/11/30/google-wants-to-make-your-passwords-obsolete
>>
>> As an authentication solution for VPNs (and even more "secure box-2-box" communication), TLS CCA is great but that's not "le web".
> 
> WebID allows you to use X509 Certs for global authentication without needing to 
> pass through CAs. We are using this every day and building some very cool apps on it. 
> 
>>
>> Regarding the specific WebID issue, I would be cautious about assigning additional semantics to existing constructs.
>> X.509 offers many other ways of signalling a unique/non-standard use.
> 
> Thanks for this one paragraph that is somewhat closer to being relevant to this thread.
> 
> So clearly if one could have a better way for the server to notify the client which
> certificates it accepts, this would allow selection on other fields of the certificates.
> Perhaps something like a regexpression in the certificate_authorities list, or a
> way to express capabilities of certificates.

IMO the best would rather be to nuke TLS CCA as the foundation for WebID.  It is
much_ simpler (note, I didn't say simple...) establishing something based on app-level security.
With WebCrypto you have it today.  With U2F it gets ubiquitous.

TLS 1.3 is as all other versions only about security enhancements.

Cheers
Anders

> 
> 
>>
>> Cheers,
>> Anders
>>
>>>
>>>
>>> Henry
>>>
>>>
>>> [1] https://dvcs.w3.org/hg/WebID/raw-file/tip/spec/index.html
>>>
>>>
>>> Social Web Architect
>>> http://bblfish.net/
>>>
>>>
>>
> 
> Social Web Architect
> http://bblfish.net/
>