[TLS] Interaction between cookies and middlebox compat mode

Matt Caswell <matt@openssl.org> Wed, 27 December 2017 19:17 UTC

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From: Matt Caswell <matt@openssl.org>
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Date: Wed, 27 Dec 2017 19:17:20 +0000
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Subject: [TLS] Interaction between cookies and middlebox compat mode
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Consider the scenario where a server is operating statelessly (i.e.
using the cookie extension) and a client is operating in middlebox
compat mode.

In that case the client sends an initial ClientHello and receives a
ServerHello(HRR) back with a cookie in it. Before it sends its second
ClientHello it first sends a dummy CCS.

>From the server perspective it is operating statelessly so until it gets
a ClientHello with a valid cookie in it, any message it receives is
considered the "first" one. Therefore, because the server has forgotten
about the initial interaction with the client, the first message it sees
is a CCS.

Draft-22 says this:

  "An implementation may receive an unencrypted record of type
   change_cipher_spec consisting of the single byte value 0x01 at any
   time during the handshake and MUST simply drop it without further
   processing."

Does "any time during the handshake" include the very first message it
receives? If so then this has implications for servers that accept
connections from TLSv1.2 (and below) clients (regardless of whether the
server is stateless or not), i.e. an incoming connection that starts
with a CCS and then goes on to negotiate TLSv1.2 would be accepted which
seems odd.

If it doesn't mean that then a stateless server will receive a CCS as
the first message and not know how to handle it.

Maybe there are different rules for handling CCS for stateless servers?
Or perhaps middlebox compat mode should never be used in a scenario
where a stateless server is being used (e.g. QUIC). Either way it seems
that some clarification of the wording would help.

Matt