Re: [dns-privacy] [TLS] Martin Duke's No Objection on draft-ietf-dprive-xfr-over-tls-11: (with COMMENT)

Eric Rescorla <ekr@rtfm.com> Thu, 29 April 2021 19:04 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Thu, 29 Apr 2021 12:03:31 -0700
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To: Stephen Farrell <stephen.farrell@cs.tcd.ie>
Cc: "Salz, Rich" <rsalz=40akamai.com@dmarc.ietf.org>, "dns-privacy@ietf.org" <dns-privacy@ietf.org>, "tls@ietf.org" <tls@ietf.org>
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Subject: Re: [dns-privacy] [TLS] Martin Duke's No Objection on draft-ietf-dprive-xfr-over-tls-11: (with COMMENT)
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On Thu, Apr 29, 2021 at 11:38 AM Stephen Farrell <stephen.farrell@cs.tcd.ie>
wrote:

>
>
> On 29/04/2021 19:28, Salz, Rich wrote:
> > To make it obvious (I thought it was): I agree, and think we need to
> > make that fact more widely known.
>
> I think I agree but seems like ECH may add a subtlety - maybe
> what we need to promote is the idea that new protocols should
> define new ALPN strings, but also that intermediaries can't
> depend on those to route connections as the inner and outer
> ALPN values can be independent in the case of ECH (use of
> which might not that visible to the application if a library
> were to default to use of ECH where possible).
>

Correct. The purpose of ALPN in this context is to avoid cross-protocol
attacks on the endpoints. Reliance on them by intermediaries is difficult
absent some fairly strong assumptions about the endpoints.

-Ekr


> Cheers,
> S.
>
> >
> > From: Eric Rescorla <ekr@rtfm.com> Date: Thursday, April 29, 2021 at
> > 2:24 PM To: Rich Salz <rsalz@akamai.com> Cc: Martin Thomson
> > <mt@lowentropy.net>et>, "dns-privacy@ietf.org" <dns-privacy@ietf.org>rg>,
> > "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.com@dmarc.ietf.org<mailto:40akamai.com@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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