Re: [quicwg/base-drafts] Prohibit TLS 1.3 middlebox compatibility mode (#3595)

Marten Seemann <notifications@github.com> Thu, 23 April 2020 07:30 UTC

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From: Marten Seemann <notifications@github.com>
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Subject: Re: [quicwg/base-drafts] Prohibit TLS 1.3 middlebox compatibility mode (#3595)
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@marten-seemann commented on this pull request.



> @@ -1631,6 +1632,21 @@ PROTOCOL_VIOLATION.
 As a result, EndOfEarlyData does not appear in the TLS handshake transcript.
 
 
+## Prohibit TLS Middlebox Compatibility Mode {#compat-mode}
+
+Appendix D.4 of {{!TLS13}} describes an alteration to the TLS 1.3 handshake as
+a workaround for bugs in some middleboxes. The TLS 1.3 middlebox compatibility
+mode involves setting the legacy_session_id field to a 32-byte value in the
+ClientHello and ServerHello, then sending a change_cipher_spec record. Both
+field and record carry no semantic content and are ignored.
+
+This mode has no use in QUIC as it only applies to middleboxes that interfere
+with TLS over TCP. A client MUST NOT request the use of the TLS 1.3
+compatibility mode. A server MUST treat the receipt of a TLS ClientHello that
+requests the use of the TLS 1.3 middlebox compatibility mode as a connection
+error of type PROTOCOL_VIOLATION.

> In your record layer, when you receive the handshake traffic secret from the handshake layer, you inject CCS depending on the length of SH.legacy_session_id that has been retained. At the moment, there's nothing that has to be done at the handshake layer on the server-side.

Assume that you're not validating the empty `legacy_session_id` on the server side, how would you prevent the injection of CCS then?

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