Re: [TLS] Downgrade protection, fallbacks, and server time
Eric Rescorla <ekr@rtfm.com> Wed, 01 June 2016 22:43 UTC
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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 01 Jun 2016 15:43:07 -0700
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To: David Benjamin <davidben@chromium.org>
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Subject: Re: [TLS] Downgrade protection, fallbacks, and server time
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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. -Ekr P.S. The FALLBACK_SCSV isn't a substitute because it isn't signed by the server. 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 > > _______________________________________________ > TLS mailing list > TLS@ietf.org > https://www.ietf.org/mailman/listinfo/tls > >
- Re: [TLS] Downgrade protection, fallbacks, and se… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… Martin Rex
- [TLS] Downgrade protection, fallbacks, and server… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… Eric Rescorla
- Re: [TLS] Downgrade protection, fallbacks, and se… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… Eric Rescorla
- Re: [TLS] Downgrade protection, fallbacks, and se… Martin Thomson
- [TLS] no fallbacks please [was: Downgrade protect… Nikos Mavrogiannopoulos
- Re: [TLS] no fallbacks please [was: Downgrade pro… Yoav Nir
- Re: [TLS] Downgrade protection, fallbacks, and se… Hubert Kario
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] Downgrade protection, fallbacks, and se… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… Viktor Dukhovni
- Re: [TLS] no fallbacks please [was: Downgrade pro… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… Hubert Kario
- Re: [TLS] no fallbacks please [was: Downgrade pro… David Benjamin
- Re: [TLS] Downgrade protection, fallbacks, and se… David Benjamin
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] Downgrade protection, fallbacks, and se… Viktor Dukhovni
- Re: [TLS] no fallbacks please [was: Downgrade pro… Martin Thomson
- Re: [TLS] no fallbacks please [was: Downgrade pro… Dave Garrett
- Re: [TLS] no fallbacks please [was: Downgrade pro… Nikos Mavrogiannopoulos
- Re: [TLS] no fallbacks please [was: Downgrade pro… Ilari Liusvaara
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] no fallbacks please [was: Downgrade pro… Xiaoyin Liu
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] no fallbacks please [was: Downgrade pro… Eric Rescorla
- Re: [TLS] no fallbacks please [was: Downgrade pro… Andrei Popov
- Re: [TLS] no fallbacks please [was: Downgrade pro… Eric Rescorla
- Re: [TLS] no fallbacks please [was: Downgrade pro… Viktor Dukhovni
- Re: [TLS] no fallbacks please [was: Downgrade pro… David Benjamin
- Re: [TLS] no fallbacks please [was: Downgrade pro… Dave Garrett
- Re: [TLS] no fallbacks please [was: Downgrade pro… Bill Frantz
- Re: [TLS] Downgrade protection, fallbacks, and se… Yaron Sheffer
- Re: [TLS] Downgrade protection, fallbacks, and se… Stefan Winter
- Re: [TLS] no fallbacks please [was: Downgrade pro… Hubert Kario
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Peter Gutmann
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Peter Gutmann
- Re: [TLS] no fallbacks please [was: Downgrade pro… Dave Garrett
- Re: [TLS] no fallbacks please [was: Downgrade pro… Jeffrey Walton
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Peter Gutmann
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Kyle Rose
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Hubert Kario
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Yoav Nir
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Salz, Rich
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Yoav Nir
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Hubert Kario
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Yoav Nir
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … David Benjamin
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Andrei Popov
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Yuhong Bao
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Dave Garrett
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Hubert Kario
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Hubert Kario
- Re: [TLS] [FORGED] Re: no fallbacks please [was: … Nikos Mavrogiannopoulos
- Re: [TLS] no fallbacks please [was: Downgrade pro… Tony Arcieri