Re: [TLS] Verifying X.509 Certificate Chains out of order

Simon Josefsson <simon@josefsson.org> Thu, 16 October 2008 06:41 UTC

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From: Simon Josefsson <simon@josefsson.org>
To: pgut001@cs.auckland.ac.nz (Peter Gutmann)
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Date: Thu, 16 Oct 2008 08:41:55 +0200
In-Reply-To: <E1KqMBg-0006c8-WA@wintermute01.cs.auckland.ac.nz> (Peter Gutmann's message of "Thu, 16 Oct 2008 19:18:20 +1300")
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Subject: Re: [TLS] Verifying X.509 Certificate Chains out of order
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pgut001@cs.auckland.ac.nz (Peter Gutmann) writes:

> So the tradeoff made was to significantly negatively impact usability in
> exchange for addressing a perceived privacy threat, specifically the fact that
> if I connect to a site that (for some reason) decides that it doesn't want to
> use traditional browser cookies or cache cookies or web bugs or Flash cookies
> or a million other ways of tracking users (including SSL session cache
> identifiers in the specific case of SSL) then they can now find out that I'm
> /C=US/O=Verisign/OU=Class 1 CA/OU=No liability accepted/CN=The Jolly Green
> Giant/email=qwertyuiop@hotmail.com.  Maybe I'm missing something here, but
> this seems to be a case of doing something that significantly negatively
> affects security usability (and therefore actual real security) in order to
> address an imaginary issue that only a geek could dream up.  Is there some
> other issue here that I'm missing?

Well, as a geek, I can dream up other issues: probing clients which bank
they are using, for those banks that use client-TLS, or other similar
probing.  Just add many https link for small images on your site to
https server that send the bank's CA that you are interested in as a
trusted root, and watch clients send back their end-entity bank
certificate.

However, I believe highly in usability, and don't think the privacy
attack above even comes close to warrant a poor user interface.

If someone really wants to solve this privacy problem, add a certificate
extension that tells browsers to never announce a particular end-entity
certificate except to particular hosts, and make browsers support it.  I
suspect you'll have trouble convincing everyone to implement the
feature, and the IETF to standardize it, because people will question
whether the privacy problem is a serious problem.

/Simon
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