Re: [TLS] CCS and key reset and renegotiation

Watson Ladd <watsonbladd@gmail.com> Thu, 05 June 2014 16:02 UTC

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Date: Thu, 5 Jun 2014 09:01:41 -0700
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From: Watson Ladd <watsonbladd@gmail.com>
To: Martin Thomson <martin.thomson@gmail.com>
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Cc: tls@ietf.org
Subject: Re: [TLS] CCS and key reset and renegotiation
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On Jun 5, 2014 8:53 AM, "Martin Thomson" <martin.thomson@gmail.com> wrote:
>
> On 5 June 2014 08:41, Salz, Rich <rsalz@akamai.com> wrote:
> >> I don't see why the incompetence of implementors should govern our
> >> decisions. If something cannot be implemented correctly it must be
removed,
> >> but why is rekeying such a thing?
> >
> > Because the line between “often get it wrong” and “cannot be
implemented” is
> > often a very thin one and it’s better to be cautious and safe, then
> > pedantically correct and usually broken.
>
>
> I tend to agree with Watson here.  This is a problem that happens
> during the initial handshake only.  Maybe we can design the handshake
> to ensure that CCS cannot be abused for TLS 1.3.  But I don't see how
> this vulnerability extends to subsequent handshakes or rekeying
> exchanges.

We did design that part of the handshake correctly provided you read the
example handshake as dictating the possible flows: that's why only OpenSSL
is vulnerable.

The spec needs a state machine.