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[TLS] Authenticating incompatible protocols

Martin Thomson <mt@lowentropy.net> Tue, 14 July 2020 01:21 UTC

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Date: Tue, 14 Jul 2020 11:21:24 +1000
From: Martin Thomson <mt@lowentropy.net>
To: tls@ietf.org
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Subject: [TLS] Authenticating incompatible protocols
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The work in DNSOP on the SVCB record raised a few awkward questions about the potential for downgrade attacks.  Where protocols aren't compatible -- that is, A is not compatible with B if you can't attempt A and negotiate B -- you don't get downgrade protection.  ALPN only really protects against downgrades with compatible protocols.

With QUIC, and increasing diversity of protocol usage across TLS and DTLS, there are more opportunities for incompatible protocols to be used.

I've done a quick writeup of something that might work:

https://datatracker.ietf.org/doc/draft-thomson-tls-snip/
https://martinthomson.github.io/snip/draft-thomson-tls-snip.html

Thoughts would be appreciated.

As a footnote: this makes some assumptions about the way that ALPN is used.  That is, this relies on the same ALPN not being used in incompatible protocols.  The ALPN registry already lists one counterexample in stun.turn [RFC7443] which can be used over both DTLS and TLS.  I personally think that was a mistake, but I know that others disagree.

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