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

Eric Rescorla <ekr@rtfm.com> Thu, 31 December 2015 00:30 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 30 Dec 2015 19:30:13 -0500
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To: Watson Ladd <watsonbladd@gmail.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 Wed, Dec 30, 2015 at 7:23 PM, Watson Ladd <watsonbladd@gmail.com> wrote:

>
> 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.
>
Note that session-hash does this, so it seems best to use session-hash if
you want
to adopt this strategy.

-Ekr


> >
> >
> > -Ilari
> >
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