Re: [TLS] Ala Carte Cipher suites - was: DSA should die

Brian Sniffen <bsniffen@akamai.com> Tue, 07 April 2015 23:25 UTC

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From: Brian Sniffen <bsniffen@akamai.com>
To: Tony Arcieri <bascule@gmail.com>, "Salz\, Rich" <rsalz@akamai.com>
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Date: Tue, 07 Apr 2015 19:25:34 -0400
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Subject: Re: [TLS] Ala Carte Cipher suites - was: DSA should die
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> As myself and many others have described, we're essentially being
> asked to compute the combinatorial explosion of different ciphersuite
> configurations by hand.  Guess what happens when you do that? People
> make mistakes.

Lots of them, and terrible mistakes.  For example, the OpenSSL
configuration language encourages selections like:

HIGH
TLSv1:SSLv3:-EDH:-ADH:-NULL:@STRENGTH
TLSv1:SSLv3:HIGH:-SSLv2:-MEDIUM:-LOW

all of which admit terrible choices.  I think this is because those look
like short, easily comprehensible commands---but aren't.

> Seems like a huge win to me. So what's the problem from an implementer
> perspective besides "it'd be hard"?

But that's not the wire format.  That's the best attempt of implementors
to provide a better configuration format than just listing everything
you want.  After a lot of trying for better, I am persuaded that the
right answer is to explicitly list exactly what you want, or to
reference first-order named sets of what you want---that set algebra on
cipher selections is more dangerous.  The best advice we can give now
could be expressed to OpenSSL as 'EECDH+AESGCM', but it's not much more
work to say
ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES128-GCM-SHA256,
and at least that looks like something you should read carefully and
make sure you didn't swap a : and a -.

-Brian

-- 
Brian Sniffen