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Re: [TLS] Interaction between session resumption and negotiated protocol version

Brian Smith <brian@briansmith.org> Fri, 03 April 2015 23:52 UTC

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Date: Fri, 03 Apr 2015 13:52:54 -1000
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From: Brian Smith <brian@briansmith.org>
To: Dave Garrett <davemgarrett@gmail.com>
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Subject: Re: [TLS] Interaction between session resumption and negotiated protocol version
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Dave Garrett <davemgarrett@gmail.com> wrote:
> TLS 1.2 has:
>
> Whenever a client already knows the highest protocol version known to
> a server (for example, when resuming a session), it SHOULD initiate
> the connection in that native protocol.
>
> TLS 1.3 currently has:
>
> A client resuming a session SHOULD initiate the connection using the
> version that was previously negotiated.

These are both bad. The best way for the client to behave is to
calculate ClientHello.client_version independently of the session
cache.

For any full handshake, the client_version should be the best version
the client supports. Since the server is allowed to respond to any
ClientHello by initiating a full handshake (e.g. because it blew away
its session cache), ClientHello.client_version shouldn't be different
for ClientHellos that include a session ID or session ticket.

Cheers,
Brian

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