To make it obvious (I thought it was): I agree, and think we need to make that 
fact more widely known.

From: Eric Rescorla <e...@rtfm.com>
Date: Thursday, April 29, 2021 at 2:24 PM
To: Rich Salz <rs...@akamai.com>
Cc: Martin Thomson <m...@lowentropy.net>, "dns-priv...@ietf.org" 
<dns-priv...@ietf.org>, "tls@ietf.org" <tls@ietf.org>
Subject: Re: [dns-privacy] [TLS] Martin Duke's No Objection on 
draft-ietf-dprive-xfr-over-tls-11: (with COMMENT)

Probably not, but I agree with MT.

The general idea here is that any given protocol trace should only be 
interpretable in one way. So, either you need the interior protocol to be 
self-describing or you need to separate the domains with ALPN. I don't believe 
that either the IP ACL or mTLS addresses this issue, and in fact arguably mTLS 
makes the problem worse because it provides authenticated protocol traces which 
might be usable for cross-protocol attacks.

-Ekr


On Thu, Apr 29, 2021 at 7:26 AM Salz, Rich 
<rsalz=40akamai....@dmarc.ietf.org<mailto:40akamai....@dmarc.ietf.org>> wrote:
>    No new protocol should use TLS without ALPN.  It only opens space for 
> cross-protocol attacks.  Did the working group consider this possibility in 
> their discussions?

I don't believe that message has been made as public as it should be.

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