Re: [TLS] Another IRINA bug in TLS

mrex@sap.com (Martin Rex) Wed, 20 May 2015 19:43 UTC

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To: Tom Ritter <tom@ritter.vg>
Date: Wed, 20 May 2015 21:43:25 +0200 (CEST)
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From: mrex@sap.com (Martin Rex)
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Subject: Re: [TLS] Another IRINA bug in TLS
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Tom Ritter wrote:
> On 20 May 2015 at 09:05, Watson Ladd <watsonbladd@gmail.com>; wrote:
>> https://weakdh.org/
>>
>> Transcript hashing will solve this problem.
> 
> Will it?  (Or at least, I'm not sure where you intend to include the
> transcript hash, in which case - maybe.)
> 
> I think the issue is the same as the Section 4.4 here:
> https://www.schneier.com/paper-ssl-revised.pdf - the underlying cause
> being that the ephemeral parameters' signature doesn't include enough
> context.   In Schneier's, the algorithm needed to be included (it
> still isn't, in TLS 1.2 - we just (hope) we have stricter checking on
> the data in implementations).

Correct.  For some unclear reason, the original signature over the
ServerKeyExchange handshake message was ill-designed, in that it
does not cover all prior handshake messages (ClientHello,ServerHello,
ServerCertificate), but only the ClientHello.random & ServerHello.random
besides the Servers parameters.


Now the issue of weak ephemeral Diffie-Helman keys has been raised
here before (14-May-2007):

  http://www.ietf.org/mail-archive/web/tls/current/msg01647.html

and a few other occasions.  The last time that I remember the behaviour
of the installed base (not refusing short or bogus DH parameters)
being described as a problem was in the description of the 
Triple-Handshake-Attack (return of renego).

-Martin