Re: [TLS] Cipher suite values to indicate TLS capability
Adam Langley <agl@google.com> Mon, 11 June 2012 15:29 UTC
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Date: Mon, 11 Jun 2012 11:29:44 -0400
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From: Adam Langley <agl@google.com>
To: Nikos Mavrogiannopoulos <nmav@gnutls.org>
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Subject: Re: [TLS] Cipher suite values to indicate TLS capability
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On Fri, Jun 8, 2012 at 6:08 PM, Nikos Mavrogiannopoulos <nmav@gnutls.org> wrote: > It may not be true, but what would you do if the SCSV check fails? > Wouldn't you warn the user? We would either go back to TLS, or return the original network error that caused the fallback. We're not going to try telling the user that they "may be under attack" or such. > Most probably he'd say that your browser doesn't work. Let's > try the other one (which doesn't support SCSV and thus wouldn't detect a > possible attack). That's always a possibility but the argument that users will try other browsers, while true, is also true of many other changes that we've made: rejecting tiny EDH groups, rejecting RSA keys < 1024-bits, rejecting MD4 and MD5 signatures. With each of these we have, undoubtedly, lost users to browsers that don't implement these things. But we have also motivated people to fix things and provided cover for other browsers to make the same changes. This tension between breaking things for the common good and keeping users happy always exists in large software and I know that this proposal, even for just SSLv3, will break users behind a certain vendor's firewall product. But that's a cost to be balanced rather than a fatal flaw (and I'm already taking to that vendor). > What if a new server comes out that the SCSV fails for some > random reason? Once we start to use the SCSV, we draw a line in the sand and it's much less likely that there will be compatibility problems in the future. > I agree that it helps to mitigate some problem, but my > concern is, does any gain obtained justify the introduced protocol > complexity? Well, the complexity decision depends substantially on an individual's preferences. I obviously believe that this is worthwhile: the SCSV is a relatively minor addition and the downgrade attack is real. But from my point of view the protocol is already subject to several of these warts [1] that the specified protocol doesn't include. This would be a rather obvious wart on the spec, even if it's not so big compared to my reality. [1] http://www.imperialviolet.org/2011/02/04/oppractices.html Cheers AGL
- Re: [TLS] Cipher suite values to indicate TLS cap… Geoffrey Keating
- [TLS] Cipher suite values to indicate TLS capabil… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Geoffrey Keating
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Geoffrey Keating
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Chris Richardson
- Re: [TLS] Cipher suite values to indicate TLS cap… Wan-Teh Chang
- Re: [TLS] Cipher suite values to indicate TLS cap… Yoav Nir
- Re: [TLS] Cipher suite values to indicate TLS cap… Yoav Nir
- Re: [TLS] Cipher suite values to indicate TLS cap… Nikos Mavrogiannopoulos
- Re: [TLS] Cipher suite values to indicate TLS cap… Geoffrey Keating
- Re: [TLS] Cipher suite values to indicate TLS cap… Chris Richardson
- Re: [TLS] Cipher suite values to indicate TLS cap… Tom Ritter
- Re: [TLS] Cipher suite values to indicate TLS cap… Nikos Mavrogiannopoulos
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Nikos Mavrogiannopoulos
- Re: [TLS] Cipher suite values to indicate TLS cap… Adam Langley
- Re: [TLS] Cipher suite values to indicate TLS cap… Martin Rex