Re: [TLS] Further TLS 1.3 deployment updates

[Hubert Kario <[e-mail]>]

On Thursday, 13 December 2018 18:04:12 CET David Benjamin wrote:
> On Thu, Dec 13, 2018 at 10:54 AM Hubert Kario <[e-mail]> wrote:
> > On Wednesday, 12 December 2018 23:21:43 CET David Benjamin wrote:
> > > Hi folks,
> > > 
> > > We have one more update for you all on TLS 1.3 deployment issues. Over
> > > the
> > > course of deploying TLS 1.3 to Google servers, we found that JDK 11
> > > unfortunately implemented TLS 1.3 incorrectly. On resumption, it fails
> > > to
> > > send the SNI extension. This means that the first connection from a JDK
> > > 11
> > > client will work, but subsequent ones fail.
> > > https://bugs.openjdk.java.net/browse/JDK-8211806
> > > 
> > > It appears this will be fixed in JDK 11.0.2, which is not yet released.
> > > In
> > > the meantime, we have sadly had to detect JDK 11 clients and disable TLS
> > > 1.3 for them. This, in turn, raises a problem with the downgrade signal
> > > in
> > > ServerHello.random. JDK 11 does implement that downgrade signal, so the
> > > workaround cannot send it. However, the signal is not effective for
> > > other
> > > clients unless all TLS 1.2 ServerHellos are marked.
> > > 
> > > To salvage this for now, we've introduced a second value, generated
> > > 
> > > randomly:
> > >     0xed, 0xbf, 0xb4, 0xa8, 0xc2, 0x47, 0x10, 0xff
> > > 
> > > When Google servers detect JDK 11 and disable TLS 1.3 to work around
> > > this
> > > issue, they will use that value in ServerHello.random instead of the
> > > standard 0x44, 0x4f, 0x57, 0x4e, 0x47, 0x52, 0x44, 0x01. Future versions
> > > of
> > > Chrome will treat the new value as an alias of the standard one. Other
> > > clients may wish to do the same, but please properly test your TLS 1.3
> > > implementation first.
> > 
> > there is now a server test script in tlsfuzzer for standard downgrade
> > sentinel:
> > 
> > https://github.com/tomato42/tlsfuzzer/blob/master/scripts/test-downgrade-p
> > rotection.py
> > 
> > example of usage: https://github.com/tomato42/tlsfuzzer/pull/479/files
> 
> That's not the problematic direction here. If someone ships a TLS 1.3
> *client* which enforces the downgrade sentinel, it is important that the
> TLS 1.3 implementation not contain show-stopping bugs. The reason JDK 11's
> problem impacts the downgrade sentinel is because JDK 11 lacks a working
> client TLS 1.3 implementation, but it insists it has one by way of
> enforcing the signal on the client.

I know, but if people start fiddling with downgrade signal, they should verify 
that it works correctly in general case — I was replying to the last sentence 
only.
-- 
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