Re: [TLS] Commentary on the client authentication presentation slides

Yoav Nir <ynir.ietf@gmail.com> Mon, 10 August 2015 19:54 UTC

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From: Yoav Nir <ynir.ietf@gmail.com>
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Date: Mon, 10 Aug 2015 22:53:22 +0300
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To: Andrei Popov <Andrei.Popov@microsoft.com>
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Subject: Re: [TLS] Commentary on the client authentication presentation slides
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> On Aug 10, 2015, at 10:04 PM, Andrei Popov <Andrei.Popov@microsoft.com> wrote:
> 
>> Ideally a solution would work with HTTP/1 over TLS 1.3, HTTP/2 over TLS 1.3, HTTP/2 over TLS 1.2, and for completeness HTTP/1 over TLS 1.2.
> Correct, anything less than this will create deployment problems.
> 
>> I’d like to point out that I am very interested in this use case, but I’m not sure that this is the solution.
> I'm open to alternatives that fix the broken use case. 

One solution would be to move the authentication for this use case (and perhaps for HTTPS in general) from the transport layer to either HTTP as an authentication method or to the application (through some standardized exchange of JSON objects).

Another option is to allow renegotiation and your mechanism in HTTP/2 with some change of behavior that eliminates the race condition. For example:
 1. The server receives a request for a resource that requires a client certificate
 2. The server sends a new HTTP message (or code) that says that client certificate authentication is required.
 3. The client sends a new HTTP control frame with the highest number of resource that it has requested.
 4. The server finishes serving all resources that don’t need authentication with a number below what the client sent
 5. The client does not initiate any new requests.
 6. When the server is done, it initiates renegotiation or the new mechanism
 7. After renegotiation, the client can resume sending requests, all of which are authenticated.

The details may be different, but something like this can bring determinism to the process.

>> We still get a race condition where several requests might be answered before, after or during authentication depending on the timing of the TLS handshake message vs the HTTP messages.
> The idea is that before answering a request that requires client auth, the server checks if a client cred is available. If there is no suitable client cred available, the request is blocked until the client authenticates. This does not necessarily have to block other requests that do not require client auth.

This is a somewhat contrived example, but consider an object in the web page that says “Hello, guest” when no credential is available, and “Hello, Andrei” when a credential is available.  If the page has optional authentication and you have presented the certificate, then depending on timing you might still see “Hello, guest” on the page.

Yoav