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

Ilari Liusvaara <ilariliusvaara@welho.com> Thu, 31 December 2015 21:47 UTC

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Date: Thu, 31 Dec 2015 23:47:05 +0200
From: Ilari Liusvaara <ilariliusvaara@welho.com>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] draft-ietf-tls-curve25519-01: Is public key validation necessary or helpful?
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On Thu, Dec 31, 2015 at 12:55:09PM -0800, Eric Rescorla wrote:
> On Thu, Dec 31, 2015 at 12:49 PM, Ilari Liusvaara <ilariliusvaara@welho.com>
> wrote:
> 
> > On Thu, Dec 31, 2015 at 12:23:50PM -0800, Eric Rescorla wrote:
> > > On Thu, Dec 31, 2015 at 12:20 PM, Ilari Liusvaara <
> > ilariliusvaara@welho.com>
> > > wrote:
> > >
> > > 2. Implementations which only do new algorithms can mandate EMS and not
> > > implement old derivation at all, provided we make that a rule here.
> >
> > Well, the EMS spec already says that endpoints SHOULD abort if
> > extension is not present if they don't want to interop with
> > legacy implementations.
> 
> 
> Correct. But as you say, that's a check that you can omit. However, if you
> only
> implement the EMS derivation than (unless I'm missing something) even if you
> omit the check this should just result in a handshake failure.

Yes, if you don't want to interop with legacy implementations, don't
implement the legacy key derivation.

Unfortunately, I think I figured out that client requiring ECDHE
even with only NIST curves with full checking (which is contributory
if that is even possible) does _not_ prevent THS attacks.

So, if the protocol run on TLS does not resist THS attacks anyway
somehow, you MUST require EMS.



-Ilari