[Cscwg-public] CRL Revocation Date Clarification Pre-Ballot

Tue Sep 21 15:02:25 UTC 2021

Hi Dimitris,

> If the CA already knows that the Subscriber Private Key was compromised and malicious code was signed at a certain date in the past, we should allow the CRL to contain that agreed revocation date without requiring a first CRL and then a "subsequent" CRL entry.

Agreed, that is the intent. In addition to the case you mentioned, I also am attempting to address the case where a certificate is revoked (and CRLs/OCSP responses have already been published denoting the revoked status) but later it is discovered that the certificate should be considered to be invalid starting from a different time. In this case, the CA should be able to change the revocationDate in subsequently published CRLs/OCSP responses to convey the new date.

I tweaked the wording as follows (made some changes to the existing third paragraph of 13.2.1 as well the new fourth paragraph):

“A Certificate MAY have a one-to-one relationship or one-to-many relationship with the signed Code. Regardless, revocation of a Certificate may invalidate the Code Signatures on all signed Code, some of which could be perfectly sound. Because of this, the CA MAY specify the time at which the Certificate is first considered to be invalid in the revocationDate field of a CRL entry or the revocationTime field of a OCSP response to time-bind the set of software affected by the revocation, and software should continue to treat objects containing a timestamp dated before the revocation date as valid.

If a Code Signing Certificate previously has been revoked, and the CA later becomes aware of a more appropriate revocation date, then the CA MAY use that revocation date in subsequent CRL entries and OCSP responses for that Code Signing Certificate.

Effective 2022-02-01, if the CA includes the Invalidity Date CRL entry extension in a CRL entry for a Code Signing Certificate, then the time encoded in the Invalidity Date CRL extension SHALL be equal to the time encoded in the revocationDate field of the CRL entry.”

Does this revised wording help to clarify?

Thanks,

Corey

From: Dimitris Zacharopoulos (HARICA) <dzacharo at harica.gr> 
Sent: Tuesday, September 21, 2021 2:01 AM
To: Corey Bonnell <Corey.Bonnell at digicert.com>; cscwg-public at cabforum.org
Subject: Re: [Cscwg-public] CRL Revocation Date Clarification Pre-Ballot

Hi Corey,

If the CA already knows that the Subscriber Private Key was compromised and malicious code was signed at a certain date in the past, we should allow the CRL to contain that agreed revocation date without requiring a first CRL and then a "subsequent" CRL entry.

Thanks,
Dimitris.

On 20/9/2021 7:52 μ.μ., Corey Bonnell via Cscwg-public wrote:

Hello,

As discussed last week, it would be valuable to ensure that there is clarity regarding how revocation/invalidity dates are encoded in CRLs so that relying party software can make the correct trust decisions regarding compromised code. Attached is a small change to 13.2.1 to reflect that the revocationDate CRL entry field shall be used to denote when a certificate is invalid. The proposed language allows for the Invalidity Date CRL entry extension to continue to appear, but the time encoded in it must be the same as the revocationDate for the entry. I don’t believe this causes issues with Windows CRL processing, please let me know if it does and I’ll remove the provision.

For reference, here are the two proposed paragraphs to be added to 13.2.1:

If a Code Signing Certificate is revoked, and the CA later becomes aware of a more appropriate revocation date, then the CA MAY use that revocation date in subsequent CRL entries and OCSP responses for that Code Signing Certificate.

Effective 2022-02-01, if the CA includes the Invalidity Date CRL entry extension in a CRL entry for a Code Signing Certificate, then the time encoded in the Invalidity Date CRL extension SHALL be equal to the time encoded in the revocationDate field of the CRL entry.

Given that the revocation date is potentially security sensitive, I think it’s worthwhile to get this clarified prior to the RFC 3647/Pandoc effort. In addition to comments/questions on the proposed language, we’re looking for two endorsers.

Thanks,

Corey

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