Re: [Trans] Precertificate format

Ben Laurie <benl@google.com> Thu, 11 September 2014 15:53 UTC

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References: <540DFA75.2040000@gmail.com> <540E0E90.1070208@bbn.com> <540E28FD.7050809@gmail.com> <540ECD3A.4040704@primekey.se> <540F4598.5010505@bbn.com> <CABrd9SSg5=wuierLoqAU00pMHxgGx+=ai5mHv4u5t6zm43yDWg@mail.gmail.com> <5410779A.20209@bbn.com> <CABrd9STnjqDBF4-5ABJ86M_d0bwRyjRNjRW6Hnj9UpeYC7Xz9A@mail.gmail.com> <5411BDE4.1060508@bbn.com>
Date: Thu, 11 Sep 2014 16:53:06 +0100
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From: Ben Laurie <benl@google.com>
To: Stephen Kent <kent@bbn.com>
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Subject: Re: [Trans] Precertificate format
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On 11 September 2014 16:21, Stephen Kent <kent@bbn.com> wrote:
> Ben,
>>
>> Until there is such a mechanism, omitting serial numbers makes it hard
>> (or impossible?) for anyone to take effective action on violations
>> discovered via CT. So, CT has to require serial numbers until then.
>> This language allows that to happen.
>
> I think we're in agreement, which I why I proposed an alternative
> mechanism to log serial numbers, without requiring a CA to have
> to assign them prior to final cert issuance.

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.

Also this adds quite a lot of complexity in order to allow what
appears to be, so far, an entirely theoretical use case.