Re: [pkix] How do we differentiate authentic servers from proxies performing TLS interception?

Tom Ritter <tom@ritter.vg> Tue, 17 November 2015 03:08 UTC

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From: Tom Ritter <tom@ritter.vg>
Date: Mon, 16 Nov 2015 22:07:52 -0500
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Subject: Re: [pkix] How do we differentiate authentic servers from proxies performing TLS interception?
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>From some of the emails, it sounds like the goal to differentiate a CA
that is going to authenticate some specific set of DNS names (a
private CA that is operating publicly) from a CA that is going to
authenticate potentially all DNS names (interception).  What is the
actual difference between those?

Obviously bad guys can lie. And putting a bit into a certificate does
not magically inform users of the risks they are taking on when they
install a CA.

It's my understanding (and I could be slightly wrong in the details
but I think I'm right in the generality) that Brazil requires you to
install a CA to do some sort of citizenship operation (get a passport?
pay taxes?).

I think the security win here is NOT to put a bit in the certificate
that says "I state I won't do interception" - but leave it perfectly
capable of doing so if it was malicious or compromised... but rather
to put a feature in clients that asks after installation: "Do you
trust [Brazilian CA] to authenticate [taxes.br]?   [Yes, and I trust
it for the entire Internet] [No] [Yes]"  This drives towards marking a
CA for what the user wants to trust it as.

In theory this can be done with Name Constraints, and I like those and
hope they'd be used in conjunction - but in practice constraining
certificates ahead of time in this way is difficult. Telling someone
"Nope, I can't issue a certificate for taxes.brazil.com" doesn't work
out well.

-tom