Re: [TLS] SNI and Resumption/0-RTT (Martin Rex) Fri, 21 October 2016 13:52 UTC

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To: Andrei Popov <>
Date: Fri, 21 Oct 2016 15:52:06 +0200 (CEST)
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From: (Martin Rex)
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Subject: Re: [TLS] SNI and Resumption/0-RTT
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Andrei Popov wrote:
> Perhaps it's OK to resume a session with a different SNI if in this
> session the server has proved an identity that matches the new SNI.
> In order to enforce this, the server would have to cache (or save in
> the ticket) a list of identities it presented in each resumable session?

The current wording in rfc6066 may be slightly confusing about what is
acutally important and why.

The server ought to perform a full handshake whenever the full handshake
will result in selection & use of a _different_ TLS server certificate
than what was used for the original full handshake on a session resumption.
This is a direct consequence of the principle of least surprise.

This is also the most backwards-compatible behaviour when upgrading the
server from a does-not-support-SNI to a supports-SNI state/implementation.

You do *NOT* want to have session caching interfere with the
server certificate that a client gets to see, because that would
essentially result in not-quite-deterministic server behaviour.

Sometimes there may be bugs in client-side session caching,
and clients proposing the wrong session for resumption, and the server
doing a full handshake results in interoperable, deterministic and
secure behaviour.