Re: [TLS] Review of PR #209

Martin Thomson <martin.thomson@gmail.com> Tue, 15 September 2015 22:26 UTC

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Date: Tue, 15 Sep 2015 15:25:59 -0700
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From: Martin Thomson <martin.thomson@gmail.com>
To: Andrei Popov <Andrei.Popov@microsoft.com>
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Subject: Re: [TLS] Review of PR #209
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On 15 September 2015 at 15:03, Andrei Popov <Andrei.Popov@microsoft.com> wrote:
>> That is, how does the server identify whether this is unilateral or in response to its own request?
>
> The model I'm thinking of is where the server receives a request from the client, determines that the request requires authentication, then queries session state to see whether a suitable client credential is available.
> If such client credential is not available, the server sends CertificateRequest. In this model, it does not matter whether the client volunteered a credential or responded to the server's request.

I'm concerned that this produces an indeterminate state on the server.
Say that the server receives a Certificate after it sends
CertificateRequest.  What if that Certificate doesn't conform to the
request in some way: was the Certificate just a unilateral offer that
was sent before the client received the CertificateRequest, is the
client unable to understand the CertificateRequest, or is the client
in error?  Depending on which of these is really the case, the server
is unable to decide whether it should continue awaiting a Certificate
or not.

It's not a huge issue, but I'd be happier if we nailed this sort of thing down.

>> How does a client determine if the NewSessionTicket that it receives includes its authentication? That is, how can a client know whether a resumed session will need a certificate or not?  (I'm not sure about this one, but the first thought that occurs is that the server could include an indicator in the NewSessionTicket message.)
>
> I'd say the client should send the latest ticket it has, assume that it has all session context including client identity, and the server will request creds if this is not the case.

That doesn't work for clients that send credentials without prompting.

>> What value do you see in having a spontaneous NewSessionTicket messages?  Is this just a case of not wanting to bind it more formally to something that the client sends?
>
> The reason I want to allow NewSessionTicket messages in mid-stream is to allow the server to include newly obtained client creds in the session state. If the server wants to do this, it can send NewSessionTicket after processing the client's CertificateVerify.

Would you be OK with saying that NewSessionTicket can only be sent in
response to CertificateVerify or Finished?