Re: [TLS] Ensuring consistent strength across certificate, ECDHE, cipher, and MAC

Hubert Kario <hkario@redhat.com> Wed, 23 March 2016 12:12 UTC

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From: Hubert Kario <hkario@redhat.com>
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Date: Wed, 23 Mar 2016 12:31:20 +0100
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Subject: Re: [TLS] Ensuring consistent strength across certificate, ECDHE, cipher, and MAC
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On Wednesday 23 March 2016 00:38:13 Timothy Jackson wrote:
> I’ve noted that many (most?) TLS implementations choose their ECDHE
> curves seemingly without regard to the cipher suite strength. Thus,
> they'll select an AES256 cipher suite (e.g.
> TLS_ECDHE_ECDSA_WITH_AES256_SHA384), but then generate an ECDHE key
> on the P256 curve. This seems odd to me, since the P256 curve
> obviously has a lower security strength than AES256. This seems
> important issue to resolve because most products (and even TLS
> libraries) do not allow the administrator to configure the available
> ECDHE curves, only the cipher suites. Thus, ECDHE may be invisibly
> undermining the security of your TLS connection.

In my experience, many (12%) servers simply ignore the list of curves 
advertised by client and use the P-256 curve always.

Some (58%) check if it was advertised and fallback to non-ECDHE if P-256 
is not advertised.

> Is this an intentional choice by this group for some reason that I
> haven’t realized yet?

IMHO, it's just a result of limitation in the libraries - i.e. closer to 
"a bug" than "design choice"
 
> How would this group feel about a proposal to address this by
> specifying in the 1.3 specification that implementations must ensure
> that the strength of the certificate must be >= strength of ECDHE/DHE
> >= strength of the cipher? Perhaps an equivalency rule for the MAC
> might also be in order? Apologies if this is already resolved
> somewhere in the draft RFC. I looked but didn’t find it.

Recommendation? maybe. Requirement? unneeded if not harmful, for the 
points already raised
 
> For what it’s worth, I’ve noticed OpenSSL and other implementations
> trying to address this by creating a “Suite B Mode”, but that seems
> to address a specific case but leave the generic case unresolved.

Only the new OpenSSL actually can use more than one curve on the server 
side...

-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 99/71, 612 45, Brno, Czech Republic