Re: [TLS] TLS Next Proto negotiation

Adam Langley <agl@google.com> Thu, 21 July 2011 22:11 UTC

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Date: Thu, 21 Jul 2011 18:11:08 -0400
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From: Adam Langley <agl@google.com>
To: Marsh Ray <marsh@extendedsubset.com>
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Subject: Re: [TLS] TLS Next Proto negotiation
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On Thu, Jul 21, 2011 at 6:06 PM, Marsh Ray <marsh@extendedsubset.com>; wrote:
> The server needs to see the client's NextProtocol message to know what
> protocol will be in use. But when a session is resumed, the server sends the
> Finished message before the client. Some app protocols have the server send
> the first application data, e.g., "username: ". If such an app protocol is a
> possibility, doesn't the use of NPN introduce an extra round-trip delay in
> case of session resumption?

Yes, NPN precludes the possibility of a server-side False Start.

> Also, since the semantics of NPN apply explicitly to the connection (rather
> than the session or connection state), perhaps the use of NPN ought to be
> contingent on the successful negotiation of RFC 5746 Renegotiation
> Indication. Multiplexing multiple protocols from same server port are likely
> to provide additional opportunities for an attacker to put the server in a
> state where it is willing to accept a renegotiating ClientHello. It will
> also provide more ways for an attacker to have the server echo back chosen
> plaintext to be interpreted as HTTP response headers (or other protocols
> could be attacked).

That sounds quite sensible to me. I'm happy to include such a requirement.


Cheers

AGL