Re: [OPSEC] [TLS] Call For Adoption: draft-wang-opsec-tls-proxy-bp

Eric Rescorla <ekr@rtfm.com> Tue, 28 July 2020 11:13 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 28 Jul 2020 04:12:55 -0700
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To: Martin Thomson <mt@lowentropy.net>
Cc: Ron Bonica <rbonica=40juniper.net@dmarc.ietf.org>, OPSEC <opsec@ietf.org>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [OPSEC] [TLS] Call For Adoption: draft-wang-opsec-tls-proxy-bp
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On Tue, Jul 28, 2020 at 12:26 AM Martin Thomson <mt@lowentropy.net> wrote:

> The following text from Section 5.3 is deeply problematic:
>
>    A decryption policy decision MAY be made based on the server
>    certificate or other trustworthy parameters.  To verify possession of
>    private keys that are associated with a particular server
>    certificate, the proxy SHOULD complete an out-of-band TLS handshake
>    with the same TLS server IP address and TCP port as targeted by the
>    TLS client.
>
> It is possible that the authors misunderstand how TLS works, but this
> check won't work.  Not only because TLS 1.3 encrypts information, but
> because this is only necessary if the proxy forwards a ClientHello from the
> client to the server.


In addition, this check is susceptible to trivial forwarding attack in
which the server in question forwards the data to the true server.

-Ekr