Re: [TLS] Broken browser behaviour with SCADA TLS

Peter Gutmann <pgut001@cs.auckland.ac.nz> Thu, 05 July 2018 12:05 UTC

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From: Peter Gutmann <pgut001@cs.auckland.ac.nz>
To: Ilari Liusvaara <ilariliusvaara@welho.com>
CC: "<tls@ietf.org>" <tls@ietf.org>
Thread-Topic: [TLS] Broken browser behaviour with SCADA TLS
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Date: Thu, 5 Jul 2018 12:05:01 +0000
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Subject: Re: [TLS] Broken browser behaviour with SCADA TLS
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Ilari Liusvaara <ilariliusvaara@welho.com>; writes:

>1) Advertise DHE, accept weak groups. Vulernable to LOGJAM.

Only if the weak groups you accept are 512 bits.

A bigger question is, why does Chrome think it's up to it to decide whether
the device user's choice of key size is appropriate or not?  As I mentioned
earlier, in many cases 1024-bit keys are not only perfectly fine but more than
enough.  In fact given the hardness of the DLP they're probably OK in
virtually all cases as long as everyone doesn't share the same group2 prime.

The crazy thing is that although Chrome rejects a connection to a PFS,
relatively safe (via the DLP's hardness) 1024-bit DHE server, it's perfectly
happy connecting to a far less safe (both in terms of factorability and use of
pure RSA) 1024-bit RSA server.  

This is "security" done backwards!

Peter.