Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Dave Garrett]

The idea of using an empty extension as an indicator really isn't fundamentally 
different from what I'm suggesting here. We'd just have an arbitrary set of new 
version indicators mixed in with extensions instead of inside a new generalized 
basket. My replacement was (again, it's over a year old) designed to be a 
general purpose long-term solution that could handle 1.3, 1.4, draft versions, 
experiments, etc, without special-casing. I'm not fundamentally against just 
adding a TLS 1.3 version indicator extension and freezing the old version 
number to 1.2. It just feels a little more hacky to me, though in the 
short-term it's simpler.

With respect to the concern of version numbers being moved to a non-fixed 
position, we could just require that the new version list extension be first or 
last in the extensions list. Being required to be last would also permanently 
mitigate the known issue of some buggy servers choking with an empty extension 
last. Conversely, with an empty extension indicator for each 1.3+ version, we'd 
probably want to require that to be first in the list to avoid that bug. 
(servers would of course still have to parse as an extension as not all clients 
will be sending 1.3+, so it's not reliably placed like the current hello 
version)


Dave


On Friday, June 03, 2016 02:19:52 pm David Benjamin wrote:
> I think I could be convinced in either direction right now.
> 
> It is definitely ugly and more of a nuisance to implement. The alternative
> is a fallback and then painfully driving it out later. We've done that
> before and we have FALLBACK_SCSV and the server_random trick now.
> 
> At the same time, I am rather bored of this fallback game. We can actually
> avoid the intolerance problem with this mechanism. Suppose we used empty
> indicator extensions, one for each new version. Then as long as servers
> tolerate unknown extensions, we'll be fine. So far this has not rusted yet,
> and we can defend it from rust by having clients send random fake
> extensions out of a range of values we burn for this purpose[*] (or private
> use area). If one extension with a list of values, we can do something
> similar.
> 
> (Strictly speaking, the version already does not appear at a fixed position
> because a ClientHello may be pathologically fragmented. OpenSSL even had
> CVE-2014-3511 here. I believe the master branch no longer has a sniff-based
> version negotiation. BoringSSL hasn't for a while now. But rejecting such
> pathologically fragmented ClientHellos is reasonable and OpenSSL 1.0.x does
> it now.)
> 
> David
> 
> [*] I'm planning on writing something up here soon.
> 
> On Fri, Jun 3, 2016 at 1:40 PM Viktor Dukhovni 
> wrote:
> 
> > On Fri, Jun 03, 2016 at 06:39:58AM -0700, Eric Rescorla wrote:
> > > My opinion on this hasn't really changed since the last time. This seems
> > > like it's more complicated and it's not clear to me why it won't lead to
> > > exactly the same version intolerance problem in future.
> >
> > Doing version negotiation through extensions would be a major
> > implementation burden.  At present the client version appears early
> > in the ClientHello at a fixed position in the packet, and the server
> > can quickly grab the version, compute the highest shared version
> > and branch to the protocol implementation for that version to parse
> > the rest of the ClientHello.
> >
> > Putting the client version in an extension dramatically complicates
> > server-side processing.  So my view is that this would not be
> > progress.  This is IMNSHO even less likely to interoperate than
> > what we have now.

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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[David Benjamin]

I think I could be convinced in either direction right now.

It is definitely ugly and more of a nuisance to implement. The alternative
is a fallback and then painfully driving it out later. We've done that
before and we have FALLBACK_SCSV and the server_random trick now.

At the same time, I am rather bored of this fallback game. We can actually
avoid the intolerance problem with this mechanism. Suppose we used empty
indicator extensions, one for each new version. Then as long as servers
tolerate unknown extensions, we'll be fine. So far this has not rusted yet,
and we can defend it from rust by having clients send random fake
extensions out of a range of values we burn for this purpose[*] (or private
use area). If one extension with a list of values, we can do something
similar.

(Strictly speaking, the version already does not appear at a fixed position
because a ClientHello may be pathologically fragmented. OpenSSL even had
CVE-2014-3511 here. I believe the master branch no longer has a sniff-based
version negotiation. BoringSSL hasn't for a while now. But rejecting such
pathologically fragmented ClientHellos is reasonable and OpenSSL 1.0.x does
it now.)

David

[*] I'm planning on writing something up here soon.

On Fri, Jun 3, 2016 at 1:40 PM Viktor Dukhovni 
wrote:

> On Fri, Jun 03, 2016 at 06:39:58AM -0700, Eric Rescorla wrote:
>
> > My opinion on this hasn't really changed since the last time. This seems
> > like it's more complicated and it's not clear to me why it won't lead to
> > exactly the same version intolerance problem in future.
>
> Doing version negotiation through extensions would be a major
> implementation burden.  At present the client version appears early
> in the ClientHello at a fixed position in the packet, and the server
> can quickly grab the version, compute the highest shared version
> and branch to the protocol implementation for that version to parse
> the rest of the ClientHello.
>
> Putting the client version in an extension dramatically complicates
> server-side processing.  So my view is that this would not be
> progress.  This is IMNSHO even less likely to interoperate than
> what we have now.
>
> --
> Viktor.
>
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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Viktor Dukhovni]

On Fri, Jun 03, 2016 at 06:39:58AM -0700, Eric Rescorla wrote:

> My opinion on this hasn't really changed since the last time. This seems
> like it's more complicated and it's not clear to me why it won't lead to
> exactly the same version intolerance problem in future.

Doing version negotiation through extensions would be a major
implementation burden.  At present the client version appears early
in the ClientHello at a fixed position in the packet, and the server
can quickly grab the version, compute the highest shared version
and branch to the protocol implementation for that version to parse
the rest of the ClientHello.

Putting the client version in an extension dramatically complicates
server-side processing.  So my view is that this would not be
progress.  This is IMNSHO even less likely to interoperate than
what we have now.

-- 
Viktor.

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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Eric Rescorla]

That's a clearer version of what I was trying to say.

-Ekr


On Fri, Jun 3, 2016 at 10:28 AM, Andrei Popov 
wrote:

> It’s not that the existing version negotiation mechanism doesn’t work; the
> problem is that implementers got it wrong. Similarly, implementers can mess
> up the new negotiation mechanism. Especially since we have to support it at
> the same time as the old one.
>
>
>
> Cheers,
>
>
>
> Andrei
>
>
>
> *From:* TLS [mailto:tls-boun...@ietf.org] *On Behalf Of *Eric Rescorla
> *Sent:* Friday, June 3, 2016 6:40 AM
> *To:* Dave Garrett 
> *Cc:* tls@ietf.org
> *Subject:* Re: [TLS] no fallbacks please [was: Downgrade protection,
> fallbacks, and server time]
>
>
>
> My opinion on this hasn't really changed since the last time. This seems
> like it's more complicated and it's not clear to me why it won't lead to
> exactly the same version intolerance problem in future.
>
>
>
> -Ekr
>
>
>
>
>
> On Thu, Jun 2, 2016 at 9:17 PM, Dave Garrett 
> wrote:
>
> Allrighty then; time to dust off and rebase an old changeset I was
> fiddling with last year on this topic:
>
> https://github.com/davegarrett/tls13-spec/commit/058ff1518508b094b8c9f1bd4096be9393f20076
> (I cleaned up a bit when rebasing, but it probably needs some work; was
> just a WIP branch, never a PR)
>
> This was the result of prior discussions on-list about TLS version
> intolerance. The gist of the proposal:
> 1) Freeze all the various version number fields.
> 2) Send a list of all supported versions in an extension. (version IDs
> converted to 16-bit ints instead of 8-bit pairs)
> 3) Use short (1 or 2 value, based on hello version) predefined lists for
> hellos from old clients not sending the extension.
> 4) Compare lists to find highest overlap, avoiding guesswork or problems
> with noncontinuous lists.
> 5) Forget the old mess of version intolerance existed.
>
> Do we want to consider scrapping the old version negotiation method again?
>
>
> Dave
>
>
>
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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Andrei Popov]

It’s not that the existing version negotiation mechanism doesn’t work; the 
problem is that implementers got it wrong. Similarly, implementers can mess up 
the new negotiation mechanism. Especially since we have to support it at the 
same time as the old one.

Cheers,

Andrei

From: TLS [mailto:tls-boun...@ietf.org] On Behalf Of Eric Rescorla
Sent: Friday, June 3, 2016 6:40 AM
To: Dave Garrett 
Cc: tls@ietf.org
Subject: Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, 
and server time]

My opinion on this hasn't really changed since the last time. This seems like 
it's more complicated and it's not clear to me why it won't lead to exactly the 
same version intolerance problem in future.

-Ekr


On Thu, Jun 2, 2016 at 9:17 PM, Dave Garrett 
> wrote:
Allrighty then; time to dust off and rebase an old changeset I was fiddling 
with last year on this topic:
https://github.com/davegarrett/tls13-spec/commit/058ff1518508b094b8c9f1bd4096be9393f20076
(I cleaned up a bit when rebasing, but it probably needs some work; was just a 
WIP branch, never a PR)

This was the result of prior discussions on-list about TLS version intolerance. 
The gist of the proposal:
1) Freeze all the various version number fields.
2) Send a list of all supported versions in an extension. (version IDs 
converted to 16-bit ints instead of 8-bit pairs)
3) Use short (1 or 2 value, based on hello version) predefined lists for hellos 
from old clients not sending the extension.
4) Compare lists to find highest overlap, avoiding guesswork or problems with 
noncontinuous lists.
5) Forget the old mess of version intolerance existed.

Do we want to consider scrapping the old version negotiation method again?


Dave

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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Eric Rescorla]

My opinion on this hasn't really changed since the last time. This seems
like it's more complicated and it's not clear to me why it won't lead to
exactly the same version intolerance problem in future.

-Ekr


On Thu, Jun 2, 2016 at 9:17 PM, Dave Garrett  wrote:

> Allrighty then; time to dust off and rebase an old changeset I was
> fiddling with last year on this topic:
>
> https://github.com/davegarrett/tls13-spec/commit/058ff1518508b094b8c9f1bd4096be9393f20076
> (I cleaned up a bit when rebasing, but it probably needs some work; was
> just a WIP branch, never a PR)
>
> This was the result of prior discussions on-list about TLS version
> intolerance. The gist of the proposal:
> 1) Freeze all the various version number fields.
> 2) Send a list of all supported versions in an extension. (version IDs
> converted to 16-bit ints instead of 8-bit pairs)
> 3) Use short (1 or 2 value, based on hello version) predefined lists for
> hellos from old clients not sending the extension.
> 4) Compare lists to find highest overlap, avoiding guesswork or problems
> with noncontinuous lists.
> 5) Forget the old mess of version intolerance existed.
>
> Do we want to consider scrapping the old version negotiation method again?
>
>
> Dave
>
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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Hubert Kario]

On Friday 03 June 2016 07:39:06 Xiaoyin Liu wrote:
> > Date: Fri, 3 Jun 2016 11:33:54 +0300
> > From: ilariliusva...@welho.com
> > To: tls@ietf.org
> > Subject: Re: [TLS] no fallbacks please [was: Downgrade protection,
> > fallbacks, and server time]> 
> > On Fri, Jun 03, 2016 at 08:37:34AM +0200, Nikos Mavrogiannopoulos 
wrote:
> > > A simpler proposal is:
> > > Consider TLS 1.3 as a feature, and negotiate it using an empty
> > > extension. If the extension is present a server assumes TLS 1.3.
> > 
> > Well, AFAIK, in current editor's draft, key_share or pre_shared_key
> > is always present and none are meaningful in TLS.1.2.
> 
> But they cannot be used to distinguish TLS 1.3 with any future
> versions, if these two extensions still exist in TLS 1.4, 1.5, ... .

TLSv1.4 and TLSv1.5 can introduce their own extensions, empty ones in 
worst case
-- 
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

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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Xiaoyin Liu]

> Date: Fri, 3 Jun 2016 11:33:54 +0300
> From: ilariliusva...@welho.com
> To: tls@ietf.org
> Subject: Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, 
> and server time]
> 
> On Fri, Jun 03, 2016 at 08:37:34AM +0200, Nikos Mavrogiannopoulos wrote:
> 
> > A simpler proposal is:
> > Consider TLS 1.3 as a feature, and negotiate it using an empty
> > extension. If the extension is present a server assumes TLS 1.3.
> 
> Well, AFAIK, in current editor's draft, key_share or pre_shared_key
> is always present and none are meaningful in TLS.1.2.
But they cannot be used to distinguish TLS 1.3 with any future versions, if 
these two extensions still exist in TLS 1.4, 1.5, ... .
Best,Xiaoyin
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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Hubert Kario]

On Friday 03 June 2016 08:37:34 Nikos Mavrogiannopoulos wrote:
> A simpler proposal is:
> Consider TLS 1.3 as a feature, and negotiate it using an empty
> extension. If the extension is present a server assumes TLS 1.3.

If anything, it should be this.

Extension with version negotiation introduced because version 
negotiation is commonly gotten wrong doesn't look like a solution to 
me...

That being said, I would prefer the solution to be a compliance test 
suite that checks if servers do handle correctly future versions, future 
extensions and future ciphersuites correctly.
-- 
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

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Re: [TLS] no fallbacks please [was: Downgrade protection, fallbacks, and server time]

[Nikos Mavrogiannopoulos]

On Fri, 2016-06-03 at 00:17 -0400, Dave Garrett wrote:
> Allrighty then; time to dust off and rebase an old changeset I was
> fiddling with last year on this topic:
> https://github.com/davegarrett/tls13-spec/commit/058ff1518508b094b8c9
> f1bd4096be9393f20076
> (I cleaned up a bit when rebasing, but it probably needs some work;
> was just a WIP branch, never a PR)
> This was the result of prior discussions on-list about TLS version
> intolerance. The gist of the proposal:
> 1) Freeze all the various version number fields.
> 2) Send a list of all supported versions in an extension. (version
> IDs converted to 16-bit ints instead of 8-bit pairs)
> 3) Use short (1 or 2 value, based on hello version) predefined lists
> for hellos from old clients not sending the extension.
> 4) Compare lists to find highest overlap, avoiding guesswork or
> problems with noncontinuous lists.
> 5) Forget the old mess of version intolerance existed.

I had originally similar thoughts, but doesn't that introduce a 4th
version negotiation mechanism? We already have the current version
negotiation mechanism(1), the extension mechanism(2), the ciphersuite
negotiation(3), and we are getting a new version negotiation mechanism
based on extensions(4).

Note that (1),(2) and (3) aren't getting away if we introduce (4). The
code will stay in implementations for more than a decade.

A simpler proposal is:
Consider TLS 1.3 as a feature, and negotiate it using an empty
extension. If the extension is present a server assumes TLS 1.3.

regards,
Nikos

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