Re: [TLS] Downgrade protection, fallbacks, and server time

David Benjamin <davidben@chromium.org> Wed, 29 June 2016 20:51 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Wed, 29 Jun 2016 20:51:04 +0000
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To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] Downgrade protection, fallbacks, and server time
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On Wed, Jun 1, 2016 at 6:57 PM Eric Rescorla <ekr@rtfm.com>; wrote:

> On Wed, Jun 1, 2016 at 3:53 PM, David Benjamin <davidben@chromium.org>;
> wrote:
>
>> On Wed, Jun 1, 2016 at 6:43 PM Eric Rescorla <ekr@rtfm.com>; wrote:
>>
>>> 2% is actually pretty good, but I agree that we're going to need
>>> fallback.
>>>
>>> I'd be fine with moving the 8 bytes to the end, but I wonder if it would
>>> be better to
>>> instead have the *client* indicate its max version and the server check.
>>> That would
>>> have the advantage that it would leave more of the server random intact.
>>>
>>
>> That sounds reasonable to me. I'd probably suggest still putting it at
>> the end since there's no reason to risk things, but I'm not aware of
>> anything that uses the client timestamp. (None of the clients I maintain
>> send it.)
>>
>
> Putting it at the end of the client gives the client options, so that
> would be fine.
>

To get this thread back on the original point, I put together a PR here for
the minimal change which just moves it to the end of server_random. This
would be sufficient to fix the problem which prevents today's 1.2 servers
from implementing 1.3.

https://github.com/tlswg/tls13-spec/pull/508

I don't particular care whether it's moved to the client or not. If you
think that's better, I'm happy to change it or defer to a different PR. (I
wasn't sure exactly what construction you were thinking for the
client_random version.)

David


>
>
>>
>>> -Ekr
>>>
>>> P.S. The FALLBACK_SCSV isn't a substitute because it isn't signed by the
>>> server.
>>>
>>
>> Yeah, FALLBACK_SCSV only gives us TLS 1.0-1.2-level downgrade protection
>> (otherwise the version fallback even loses that one). I only mean in terms
>> of mitigating the additional damage done by the fallback, not replacing the
>> entire mechanism.
>>
>> (Although, do we actually get the stronger protection if the client
>> accepts plain RSA key exchange? I've never been very clear on that.
>> Realistically, clients will be accepting plain RSA for a long while.)
>>
>
> Yes, that's correct. I don't believe we have a good plan for plain RSA.
>
> -Ekr
>
>
>> David
>>
>>
>>>
>>> On Wed, Jun 1, 2016 at 3:29 PM, David Benjamin <davidben@chromium.org>;
>>> wrote:
>>>
>>>> In case folks hoped we could bump the ClientHello version without those
>>>> dreaded browser fallbacks, I have bad news. :-( 1.3 intolerance very much
>>>> exists. (Maybe we should just give up on ClientHello.version and use an
>>>> extension? Extensions have rusted less.)
>>>>
>>>> I picked a large list of top sites and tried connecting to them. Just
>>>> under 2% of them could handle a TLS 1.2 ClientHello, but not the same
>>>> ClientHello with the version switched to 1.3 (no other changes, so I
>>>> haven't tested a real 1.3 ClientHello). They're not unknown hosts either. I
>>>> expect if you probe any top site list, you'll very quickly find some.
>>>>
>>>> Fortunately, the ServerHello.random trick fixes this. But it currently
>>>> clobbers the old server timestamp which tlsdate[1] uses for clock
>>>> synchronization. There really should be a less silly secure timestamping
>>>> protocol, but, today, tlsdate has users. Any servers which expect to be a
>>>> tlsdate target can't honor this MUST without tlsdate gone.
>>>>
>>>> (FALLBACK_SCSV works fine as well, but I gather people prefer the
>>>> ServerHello.random one and would be unhappy having to implement both in
>>>> clients with a fallback because not all servers do the latter.)
>>>>
>>>> I mentioned this before, but rather than clobbering the first 8 bytes,
>>>> the last 8 bytes seems as reasonable and avoids this unnecessary
>>>> complication to TLS 1.3 deployment. If folks agree now that the
>>>> fallback's resurrection is more certain, I'm happy to put together a PR.
>>>>
>>>> David
>>>>
>>>> [1] https://github.com/ioerror/tlsdate
>>>>
>>>> _______________________________________________
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>>>> https://www.ietf.org/mailman/listinfo/tls
>>>>
>>>>
>>>