Re: [TLS] Adoption call for draft-davidben-tls13-pkcs1

Hubert Kario <hkario@redhat.com> Mon, 21 October 2019 16:13 UTC

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From: Hubert Kario <hkario@redhat.com>
To: David Benjamin <davidben@chromium.org>
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Date: Mon, 21 Oct 2019 18:13:10 +0200
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Subject: Re: [TLS] Adoption call for draft-davidben-tls13-pkcs1
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On Monday, 21 October 2019 17:43:52 CEST David Benjamin wrote:
> On Mon, Oct 21, 2019 at 9:42 AM Hubert Kario <hkario@redhat.com>; wrote:
> > On Friday, 18 October 2019 20:44:03 CEST Christopher Wood wrote:
> > > This email starts a call for adoption of draft-davidben-tls13-pkcs1-00,
> > > 
> > > which can be found here:
> > >    https://tools.ietf.org/html/draft-davidben-tls13-pkcs1-00
> > > 
> > > It will run until November 1, 2019. Please indicate whether or not you
> > 
> > would
> > 
> > > like to see this draft adopted and whether you will review and provide
> > > feedback on it going forward.
> > 
> > Yes, requiring RSA-PSS causes interoperability issues with smartcards that
> > don't implement this 16 year old algorithm. But being able to say "if
> > you're
> > using TLS 1.3 that means you are not using legacy crypto" has non
> > insignificant value too.
> > 
> > This document erodes that.
> 
> The document goes into the rationale here under Security Considerations.
> I'm unhappy about this too, but our experience is that devices without PSS
> support are fairly common in client certificates. The negotiation order
> means that accounting for such devices effectively means servers hold back
> TLS 1.3 for *all* their clients, not just those that are affected.
> Additionally, even if one could get the negotiation order correct, TLS 1.3
> fixes a serious privacy leak with client certificates. Keeping those
> clients on TLS 1.2 means they continue to leak their identity over the
> network.

yes, I've read it.

1. I don't expect that heterogeneous deployments of clients (where some 
   smartcards can do RSA-PSS and some can't) will be a significant portion, 
   let alone the majority, of deployments.
2. servers that do use client certificates for authentication are not 
   generally deployed for the public, on the internet. Those are closed 
   systems, so impact for publicly facing servers is minimal if they need to 
   downgrade to TLS 1.2.
3. while client certificate authentication /may/ leak client certificates in 
   plaintext in TLS 1.2, by sending them in initial handshake, is not the only 
   configuration that provides certificate based authentication. Adding client 
   certificates through renegotiation does not leak them and is fairly common 
   deployment model.
 
> To mitigate the second-order effects, the document intentionally makes the
> code points client-only (the above motivations don't apply for server
> keys), as well as allocating separate code points from the existing PKCS#1
> values. If a client or server wishes to not use[*] PKCS#1 signatures in TLS
> 1.3, it doesn't need to advertise/accept those code points. TLS libraries
> probably should also disable them by default.

yes, that's a very good part of this i-d (I will suggest making the language 
around it more strict than it is now, if the i-d is adopted), but the problem 
I have is that they are allowed at all, as that means the legacy hardware is 
given another decade if not two to get replaced

Having configurations that need to downgrade to TLS 1.2 for interoperability 
is a good thing because it raises obvious red flags during audits. Algorithms 
advertised by server in CertificateVerify are not an obvious red flag.
 
> Given all that, I think adding code points for deployments that need them
> is the right tradeoff.
> 
> [*] PKCS#1 signatures in certificates and the downgrade-sensitivity of the
> TLS 1.2 signature aside.
> 
> > So I'm against adoption of this draft by the WG.
> > 
> > If it is adopted, I will review and provide feedback on it.
> > --
> > 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_______________________________________________
> > TLS mailing list
> > TLS@ietf.org
> > https://www.ietf.org/mailman/listinfo/tls


-- 
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