Re: [TLS] raising ceiling vs. floor (was: New Version Notification for draft-moriarty-tls-oldversions-diediedie-00.txt)

[Hubert Kario <[e-mail]>]

On Tuesday, 10 July 2018 16:03:24 CEST Eric Rescorla wrote:
> On Tue, Jul 10, 2018 at 6:38 AM, Hubert Kario <[e-mail]> wrote:
> > On Tuesday, 10 July 2018 06:17:56 CEST Viktor Dukhovni wrote:
> > > On Tue, Jul 10, 2018 at 08:56:14AM +1000, Martin Thomson wrote:
> > > > Is there any reason why we wouldn't also consider deprecating cipher
> > > > suites we don't like?  For instance, RFC 5246 mandates the
> > > > implementation of TLS_RSA_WITH_AES_128_CBC_SHA, which we can probably
> > > > agree isn't ideal for several reasons.
> > > 
> > > Is the objection primarily to AES-128-CBC or to RSA key exchange?
> > > With EtM there's AFAIK/IMHO not much wrong with AES-128-CBC, it
> > > outperforms AES-256-CBC, and the various CBC issues are resolved
> > > via EtM.
> > > 
> > > > The ECDHE suites with AES-GCM
> > > > are widely available, perhaps widely enough that we might consider a
> > > > stronger move and update 5246 to modern suites.
> > > 
> > > More generally, as noted in RFC7435, you get more security by raising
> > > the ceiling than by raising the floor.  Breaking the ability to
> > > communicate with legacy systems may feel satisfying, but does not
> > > generally improve the security of the up-to-date systems, barring
> > > downgrade issues in the protocol.
> > 
> > The github version of the document points out that the security of TLS 1.2
> > downgrade protection to TLS 1.1 or TLS 1.0 depends on SHA-1.
> 
> Well, yes and no. If you allow static RSA, then yes.  If you require
> (EC)DHE, then the anti-downgrade measures in the TLS 1.3 random values are
> intended to protect against downgrade even if SHA-1 is compromised (because
> the randoms are signed).

but randoms in TLS 1.0 and TLS 1.1 are signed (effectively) with SHA-1...

-- 
Regards,
Hubert Kario
Senior Quality Engineer, QE BaseOS Security team
Web: www.cz.redhat.com
Red Hat Czech s.r.o., Purkyňova 115, 612 00  Brno, Czech Republic