Re: [TLS] I-D Action: draft-ietf-tls-chacha20-poly1305-00.txt

Simon Josefsson <simon@josefsson.org> Tue, 16 June 2015 08:08 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: Jeffrey Walton <noloader@gmail.com>
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Date: Tue, 16 Jun 2015 10:08:30 +0200
In-Reply-To: <CAH8yC8=wUEpqrcCUdoYwTGbsjB25FUvHbj=zvXgo+5f4fmkt3Q@mail.gmail.com> (Jeffrey Walton's message of "Mon, 15 Jun 2015 19:46:01 -0400")
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Subject: Re: [TLS] I-D Action: draft-ietf-tls-chacha20-poly1305-00.txt
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Jeffrey Walton <noloader@gmail.com> writes:

>>>> What is the use-case?
>>>
>>> 0.   Authentication other than via certificate-based PKI.  Establish
>>>      anon TLS, and channel-bind the TLS-unique via GSSAPI or some
>>>      other authentication method.
>>>
>>> 1.  Unauthenticated opportunistic TLS.
>>>
>>>     * Server performs no unnecessary signature operations,
>>>       since the client can't verify the signature anyway.
>>>       (More precisely the client can't verify the authenticity
>>>       of the server keys, so it can only determine that somebody
>>>       signed the handshake, but no idea whether it is the intended
>>>       server).
>>
>> Both those use-cases can be achieved by choosing, say, ECDHE_ECDSA and
>> not verify the signature, right?
>>
> That's a violation of the engineering requirements :) If you don't
> need server authenticity, then you don't ask for it. If you ask for
> it, then you have to validate it.

It may appear that way -- but implementing support for ANON ciphers is
more work than NOT implementing support for certificate validation.
Supporting ANON in standards, implementation, testing/QA etc carry a
cost.  We should strive towards offsetting costs with use-cases that can
motivate the costs.

> I know I'm splitting hairs, but I would reject that kind of check-in.
> I fear a proliferation of
> https://crypto.stanford.edu/~dabo/pubs/abstracts/ssl-client-bugs.html.
> It also creates a few extra rules, and we have to teach them to
> developers, QA and auditors.

Negotiating these ciphers and not verifying the signature is already
widely deployed (e.g., SMTP STARTTLS), so there is nothing new that
needs to be done.

/Simon