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Re: [TLS] Triple Handshake Fix.

Nico Williams <nico@cryptonector.com> Wed, 30 April 2014 22:09 UTC

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Date: Wed, 30 Apr 2014 17:09:54 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Karthik Bhargavan <karthik.bhargavan@gmail.com>
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Cc: "tls@ietf.org" <tls@ietf.org>, Alfredo Pironti <alfredo@pironti.eu>
Subject: Re: [TLS] Triple Handshake Fix.
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On Wed, Apr 30, 2014 at 8:25 AM, Karthik Bhargavan
<karthik.bhargavan@gmail.com> wrote:
> On Wed, Apr 30, 2014 at 2:15 PM, Bodo Moeller <bmoeller@acm.org> wrote
>> Thanks, but haven't we seen that this alone doesn't actually fix the
>> problem?  I think we'll need a requirement for clients and servers
>> implementing this spec to do one of the following, because an attacker can't
>> be expected to opt in to extended master secrets in its handshakes:
>>
>> (1) If the current session does not use an extended master secret, don't
>> allow renegotiation.
>>
>> (2) If a session does not use an extended master secret, don't allow it to
>> be resumed.

Why not both?

> I agree that the first approach is probably more palatable for legacy
> applications. We should require no-renegotiation, and strongly recommend
> that application not use channel bindings or exported TLS keys (or PRFs) in
> this case.

That's difficult.  It requires first-class APIs for getting CB, which
many libraries lack.

Nico
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