Re: [TLS] Review of PR #209

Andrei Popov <Andrei.Popov@microsoft.com> Wed, 16 September 2015 18:11 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: Martin Thomson <martin.thomson@gmail.com>, Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Thread-Topic: [TLS] Review of PR #209
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Date: Wed, 16 Sep 2015 18:11:06 +0000
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Cc: "tls@ietf.org" <tls@ietf.org>
Subject: Re: [TLS] Review of PR #209
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Martin's point makes sense to me: applications that need to do authentication upfront can still do that immediately after the handshake.

Cheers,

Andrei

-----Original Message-----
From: Martin Thomson [mailto:martin.thomson@gmail.com] 
Sent: Wednesday, September 16, 2015 10:48 AM
To: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
Cc: Andrei Popov <Andrei.Popov@microsoft.com>om>; tls@ietf.org
Subject: Re: [TLS] Review of PR #209

On 16 September 2015 at 08:30, Ilari Liusvaara <ilari.liusvaara@elisanet.fi> wrote:
> Problem with pulling client auth out of the handshake is that it 
> complicates applications that can't change identities involved with 
> active connection.

Why would that be unsupported here?  The server can still send CertificateRequest immediately after its Finished.  That looks exactly like it does today, only the order has changed.

> As then the application needs to ensure that the authentication occurs 
> between TLS handshake and actually starting up the protocol.

I'm not sure that is necessarily a problem.  If the claim is that the authentication attests to everything prior to its appearance, then you have no problem.  I think that claim is reasonable, but I'm happy to discuss it.

>> 2. The client can send Certificate and CertificateVerify at any time 
>> application data is permitted, regardless of whether the server had 
>> previously sent CertificateRequest.
>
> CertificateRequest contains the permitted signature algorithms for the 
> PoP signature, which TLS library needs to verify before dumping the 
> certificate chain on application (which can then figure out things 
> like trust anchors).
>
> Without CertificateRequest, one has little idea what algorithms are 
> acceptable there.

Arguably, signature_algorithms covers that adequately.  Though I'll grant that certificate chain validation often happens in a separate component to the TLS stuff.

> Basically, one must clear the pipeline before changing identities, and 
> with protocols like HTTP/2, this is extremely expensive (seemingly 
> even more expensive than establishing a new connecion).

There's been some confusion about this with HTTP/2.  I think that if you adopt the position that authentication applies to the entire session - even retroactively - then the only remaining concern is the correlation one.  If you have three tabs to the site open where all of them are making requests and you see a certificate request, which one do you pop the dialog on?

> Also, it should sign the certificate, to avoid possible weirdness due 
> to signatures not properly binding the key (some schemes do, most 
> don't).

I agree with this.