All,

I would like to answer a question here about why the "DRAFT Policy about 
CRLRevocation Reason Codes for TLS End-Entity Certificates 
<https://docs.google.com/document/d/1ESakR4MiwyENyuLefyH2wG8rYbtnmG1xeSYvDNpS-EI/edit?usp=sharing>"
 
bans the other reason codes.

In the draft policy, these reason codes are allowed:

   - keyCompromise (RFC 5280 Reason Flag #1)
   - affiliationChanged (RFC 5280 Reason Flag #3)
   - superseded (RFC 5280 Reason Flag #4)
   - cessationOfOperation (RFC 5280 Reason Flag #5)
   - privilegeWithdrawn ((RFC 5280 Reason Flag #9)

And the following reason codes are banned for TLS end-entity certificates. 
Meaning that if revocation is for one of the following, then the reasonCode 
extension MUST NOT be provided for that entry in the CRL.

   - unspecified (0) 
   - cACompromise  (2)
   - certificateHold (6)
   - "-- value 7 is not used" 
   - removeFromCRL  (8)
   - aACompromise  (10) 

These banned reason codes are either already banned by the BRs or they are 
not applicable to end-entity TLS certificates. Below is a detailed 
explanation for each of them.

unspecified (0)
Section 7.2.2 of the BRs says: “The CRLReason indicated MUST NOT be 
unspecified (0). If the reason for revocation is unspecified, CAs MUST omit 
reasonCode entry extension”

cACompromise  (2)
This reason code is used when revoking an intermediate certificate.
When an intermediate certificate is revoked and added to OneCRL all 
certificates signed by that intermediate certificate are also treated as 
revoked.
https://www.ccadb.org/policy#4-intermediate-certificates says: "If an 
intermediate certificate is revoked, the CCADB must be updated to mark it 
as revoked, giving the reason why, within 24 hours for a security incident, 
and within 7 days for any other reason."
Section 4.1 of Mozilla's Root Store Policy says: "If the revocation of an 
intermediate certificate chaining up to a root in Mozilla’s root program is 
due to a security concern, as well as performing the actions defined in the 
CCADB Policy, a security bug must be filed in Bugzilla 
<https://bugzilla.mozilla.org/enter_bug.cgi?product=NSS&component=CA%20Certificate%20Compliance&groups=crypto-core-security>
."

certificateHold (6)
Section 7.2.2 of the BRs says: “ If a CRL entry is for a Certificate 
subject to these Requirements, the CRLReason MUST NOT be certificateHold 
(6).”

removeFromCRL  (8)
Section 4.10.1 of the BRs says: “Revocation entries on a CRL or OCSP 
Response MUST NOT be removed until after the Expiry Date of the revoked 
Certificate.”

aACompromise  (10) 
Not applicable to TLS certificates. aACompromise is used for attribute 
certificates when aspects of the attribute authority (AA) have been 
compromised.

I hope this detailed explanation is helpful.

Thanks,
Kathleen


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