Re: [TLS] Proposal for detecting fraudulent certificates
Martin Rex <mrex@sap.com> Mon, 26 September 2011 16:03 UTC
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From: Martin Rex <mrex@sap.com>
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To: fweimer@bfk.de
Date: Mon, 26 Sep 2011 18:05:57 +0200
In-Reply-To: <8262kfcr6q.fsf@mid.bfk.de> from "Florian Weimer" at Sep 26, 11 02:29:49 pm
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Subject: Re: [TLS] Proposal for detecting fraudulent certificates
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Florian Weimer wrote: > > I have submitted draft-weimer-tls-previous-certificate-00, which intents > to facilitate detection of fraudulent certificates used in the wild: > > <http://tools.ietf.org/html/draft-weimer-tls-previous-certificate-00> > > The basic idea is to use leaks from mobile clients moving between > networks with and without a clear path to the server. The previous > server certificate chain is included in the client hello, so the server > receives it when the client transitions to a network with a clear path. > > This draft prompted my previous question about extension size limits. > Unfortunately, that issue makes this very simple idea somewhat > complicated, but I tried to add a fairly straightforward workaround. Having the client send _only_ the servers certificate and not the entire certificate path should IMO be sufficient for the purpose you outline. I don't know why you think the 64 KByte TLS extension size limit would be a problem. A non-marginal fraction of the installed base of TLS will puke when encountering a TLS handshake message that is split/fragmented across TLS record boundaries (which have a limit of 16 KByte). The currently most likely situation where this occurs (and the only one that I've seen in the wild so far) are the CertificateRequest handshake message contains a large number (60+) of certification_authorities. Usually, the server tries to stuff the server response (ServerHello,Certificate, CertificateRequest,ServerHelloDone) into one single 16 KByte SSL Record, so even fitting the Server's certificate chain alone should not normally be a problem. But I don't see how this extension could realistically work in a sane fashion. The TLS implementation in TLS servers is not normally keeping a history of previous TLS Server certificates which it ever used (and might not have a persistence of its own for such a purpose), usually not even across process restarts, and having TLS backends of a Web-Server-Farm or hot-backups keep such a history in a consistent fashion is even more unlikely. And end users would probably prefer their TLS clients to tell them right away when a server credential changes unexpectedly, rather than a TLS server telling them, after they come home from a several weeks trip through some contry YYY, that their last several week of connect(s) had all been MITMd... -Martin
- [TLS] Proposal for detecting fraudulent certifica… Florian Weimer
- Re: [TLS] Proposal for detecting fraudulent certi… Martin Rex
- Re: [TLS] Proposal for detecting fraudulent certi… Florian Weimer
- Re: [TLS] Proposal for detecting fraudulent certi… Max Pritikin
- Re: [TLS] Proposal for detecting fraudulent certi… Martin Rex
- Re: [TLS] Proposal for detecting fraudulent certi… Florian Weimer