[cabfpub] CT Precertificates and the BRs

Rob Stradling rob.stradling at comodo.com
Tue Jan 7 12:14:11 UTC 2014

I've changed my mind.  I no longer think that a CT Precertificate (with 
the same Issuer Name/Key and Serial Number as the corresponding SSL 
Certificate) is currently illegal under the BRs.

The current scope of the BRs is "Certificates intended to be used for 
authenticating servers accessible through the Internet".  A CT 
Precertificate is only intended to be used to verify that the CA and the 
CT Log(s) are doing CT stuff correctly.  It's the corresponding SSL 
Certificate that is intended to be used for authenticating server(s).

As we continue to work on drafting new text to fix the loopholes in the 
Scope of the BRs, let's make sure that CT Precertificates remain legal!

On 17/12/13 13:18, Rob Stradling wrote:
> RFC6962 (Certificate Transparency) permits a Precertificate to be signed
> by the same CA Name/Key that signs the corresponding Certificate, and
> for the Precertificate and Certificate to share the same Serial Number.
> However, BRs Appendix B (4) says:
>      "All other fields and extensions MUST be set in accordance with RFC
>       5280."
> Although the title of Appendix B is "Certificate Extensions", I think
> "fields and extensions" must surely imply that "fields" are the
> non-extension parts of a certificate (such as the serial number).
> And since certificate serial numbers are not explicitly mentioned in
> Appendix B, I have to conclude that certificate serial numbers "MUST be
> set in accordance with RFC 5280".
> RFC 5280 Section says:
>      "The serial number...MUST be unique for each certificate issued by a
>       given CA (i.e., the issuer name and serial number identify a unique
>       certificate)".
> It seems that the practice of using the same CA Name/Key to sign both a
> Precertificate and Certificate is currently _illegal_ under the BRs.
> RFC6962 also permits a Precertificate to be signed by a subordinate
> Precertificate Signing Certificate.  This approach doesn't violate
> RFC5280 or the BRs, but some CAs will want to avoid the burden of
> managing a Precertificate Signing Certificate for every subordinate CA
> they operate.  So, Ben Laurie and I have been working on some other
> possible solutions, but our preferred outcome would be for both of the
> Precertificate signing options in RFC6962 to be made legal.
> Therefore, I would like to propose updating Appendix B of the BRs so
> that CAs are permitted to sign a Precertificate and a Certificate
> (sharing the same serial number) using the same CA Name/Key.
> Would anybody have a problem with that?

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

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