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Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?

Watson Ladd <watsonbladd@gmail.com> Thu, 31 December 2015 00:23 UTC

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Date: Wed, 30 Dec 2015 19:23:12 -0500
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From: Watson Ladd <watsonbladd@gmail.com>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Cc: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>, "<tls@ietf.org>" <tls@ietf.org>
Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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On Dec 30, 2015 7:08 PM, "Ilari Liusvaara" <ilariliusvaara@welho.com> wrote:
>
> On Thu, Dec 31, 2015 at 09:55:10AM +1100, Martin Thomson wrote:
> > On 30 December 2015 at 22:16, Ilari Liusvaara <ilariliusvaara@welho.com>
wrote:
> > >> Would it make sense to have session hash as a requirement in TLS
> > >> 1.2 when you want to use Curve25519?
> > >
> > > I don't think that is reasonable.
> >
> > I think that is entirely reasonable.  TLS 1.2 relies on contributory
> > behaviour.  25519 doesn't provide that unless you do some extra
> > checking that we know many implementations don't do.
> >
> > I'd be OK with either requiring session hash, some checking of values,
> > or both.  Otherwise we create a situation where the shared secret can
> > be forced by an attacker.
>
> The draft already has the checks.
>
> I also think I figured out a way to truly force contributory behaviour
> without any checks:
>
> It is a bit nasty hack: Throw the exchange keys into the PMS, expanding
> it from 32/56 bytes to 96/168 bytes.

Why not hash the public values into the result of the key exchange? I don't
want security to depend on omittable checks.
>
>
> -Ilari
>
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