Re: [Trans] Precertificate format

Stephen Kent <> Thu, 11 September 2014 18:20 UTC

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Date: Thu, 11 Sep 2014 14:15:16 -0400
From: Stephen Kent <>
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Subject: Re: [Trans] Precertificate format
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> ...
> I managed to miss that proposal. I've found it now.
> There seems to be a flaw: if I'm an evil CA wishing to issue an evil
> certificate, I simply log a precert, minus serial, get an SCT*, log a
> certificate containing that SCT*, which I then revoke when requested
> to do so,
> In order to attack a user with the evil certificate, I simply issue a
> second copy with a different serial, containing the original SCT*, and
> the certificate works. Yes, the discrepancy should be discovered in
> audit, but that is a significantly weaker protection than we get if
> the serial is included in the pre-certificate.
I agree that the attack you describe would work, but it needs to be
evaluated in the overall context of how CT works in the case of several
different types of attack scenarios. The threat model and attack model text
I just submitted provides a first cut at describing such scenarios. Once we
get agreement on that model, let's revisit the question of whether the 
you noted above is significant relative to other residual vulnerabilities.
> Also this adds quite a lot of complexity in order to allow what
> appears to be, so far, an entirely theoretical use case.
I do know that when VeriSign used the Safekeyper HSM to issue all of its 
(which it did for several years), it would have been impossible to 
generate a
pre-cert and matching final cert. So, the concern I raise would have 
been a show
stopper for them in that time frame. I guess it depends on how one defines a
"theoretical use case" :-)

Separately, the pre-cert model, requires a CA to issue two certs with 
the same
serial number, which is a bad security practice. I think it makes sense to
re-consider forcing CAs to behave this way.