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 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.

that is the downgrade issue in the protocol

https://github.com/sftcd/tls-oldversions-diediedie/blob/
bd6bdc37ec258094f1e1010fba19e8763f2beaee/draft-moriarty-tls-oldversions-
diediedie.txt#L142-L145
-- 
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