Re: [Trans] Precertificate format

Rob Stradling <> Mon, 15 September 2014 09:32 UTC

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Date: Mon, 15 Sep 2014 10:32:06 +0100
From: Rob Stradling <>
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Subject: Re: [Trans] Precertificate format
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On 12/09/14 20:26, Erwann Abalea wrote:
> Nice idea, solves the RFC5280 concerns, and doesn't require a completely
> new data structure.
> IIUC, what you propose is that the PreCert is a CMS (RFC5652) with a
> signedData content-type, for which the data is the TBSCertificate
> (name-redacted or not, no necessary poison extension). The SignerInfo
> refers to the PreCert issuer (CA or dedicated issuer, same as now).
> It can only work if the log signs the content (=TBSCertificate) and not
> the whole CMS, thus ignoring the PreCert issuer signature. Leaving that
> signature aside isn't more risky than it is now because it's already the
> case (the log removes the poison extension before signing the resulting
> certificate, right?).

Yes.  The log removes the poison extension, and (if a Precertificate 
Signing Certificate was used) it also changes the issuer name and AKI to 
match those of the final certificate's issuing CA.  This behaviour can 
remain the same.

I agree that using CMS to sign the Precert TBSCertificate is a good 
solution to the duplicated certificate serial number problem.  :-)

> If the log signs the whole CMS, the SCT covers information unknown to
> the browser at connection time (content of SignerInfo, maybe additional
> attributes --authenticated or not--, digest algorithm chosen by the
> PreCert issuer, etc).
> 2014-09-12 20:44 GMT+02:00 Jeremy Rowley <
> <>>:
>     Why not use a TBSCertificate from RFC 5280 with no modifications
>     from the final certificate (no poison extension) and sign it with a
>     PKCS7 signature instead of a RFC 5280 signature?  By doing this you
>     are not creating a valid certificate so you are not technically
>     breaking RFC 5280 (re-using serial numbers) and it couldn't be used
>     as a certificate even if some software incorrectly ignored the
>     poison extension.
>     Jeremy
>     -----Original Message-----
>     From: Trans [
>     <>] On Behalf Of Stephen Kent
>     Sent: Thursday, September 11, 2014 12:15 PM
>     To: <>
>     Subject: Re: [Trans] Precertificate format
>     Ben,
>      > ...
>      > 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 vulnerability 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 certs (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.
>     Steve
>     _______________________________________________
>     Trans mailing list
> <>
>     _______________________________________________
>     Trans mailing list
> <>
> --
> Erwann.
> _______________________________________________
> Trans mailing list

Rob Stradling
Senior Research & Development Scientist
COMODO - Creating Trust Online
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