Clarification to processing Key Usage values during CRL validation

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Table of Contents

1. Introduction

[RFC5280] defines the profile of X.509 certificates and certificate revocation lists (CRLs) for use in the Internet. This profile requires that certificates which certify keys for signing CRLs contain the keyUsage extension with the cRLSign bit asserted. Additionally, [RFC5280] defines steps for the validation of CRLs. While there is a requirement for CRL validators to verify that the cRLSign bit is asserted in the keyUsage extension of the CRL issuer's certificate, there is no requirement for validators to verify the presence of the key usage extension itself. The lack of such a requirement may manifest in an issue in some Public Key Infrastructures where a CRL issuer who has been certified by a Certification Authority to issue CRLs on its behalf can sign CRLs using a key that has not been certified for signing CRLs.¶

Section 3 describes the issue in detail.¶

Section 4 describes the amended CRL validation algorithm that remediates the issue.¶

2. Conventions and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

3. The risk of signing CRLs with non-certified keys

In some Public Key Infrastructures, entities are delegated by Certification Authorities to issue CRLs. CRLs whose scope encompasses certificates that have not been issued by the CRL issuer are known as "indirect CRLs".¶

Certification Authorities delegate the issuance of CRLs to other entities by issuing to the entity a certificate that asserts the cRLSign bit in the keyUsage extension. The Certification Authority will then issue certificates that fall within the scope of the indirect CRL by including the crlDistributionPoints extension and specifying the distinguished name ("DN") of the CRL issuer in the cRLIssuer field of the corresponding distribution point.¶

The CRL issuer issues CRLs that assert the indirectCRL boolean within the issuingDistributionPoint extension.¶

Applications which consume CRLs follow the validation algorithm as specified in Section 6.3 of [RFC5280]. In particular, Section 6.3.3 contains the following step for CRL validation:¶

• (f) Obtain and validate the certification path for the issuer of the complete CRL. The trust anchor for the certification path MUST be the same as the trust anchor used to validate the target certificate. If a keyUsage extension is present in the CRL issuer's certificate, verify that the cRLSign bit is set.¶

Notably, there is no requirement for certificate-consuming applications to verify the presence of the keyUsage extension itself.¶

Additionally, the certificate profile in [RFC5280] does not require the inclusion of the keyUsage extension in a certificate if the certified public key is not used for verifying the signatures of other certificates or CRLs. Section 4.2.1.3 of [RFC5280] says:¶

• Conforming CAs MUST include this extension in certificates that contain public keys that are used to validate digital signatures on other public key certificates or CRLs.¶

The allowance for the issuance of certificates without the keyUsage extension and the lack of a check for the inclusion of the keyUsage extension during CRL verification can manifest in a security issue. A concrete example is described below.¶

1. The Certification Authority issues an end-entity CRL issuer certificate to subject X that certifies key A for signing CRLs by explicitly including the keyUsage extension and asserting the cRLSign bit in accordance with Section 4.2.1.3 of [RFC5280].¶

2. The Certification Authority issues one or more certificates that include the crlDistributionPoints extension with the DN for subject X included in the cRLIssuer field. This indicates that the CRL-based revocation information for these certificates will be provided by subject X.¶

3. The Certification Authority issues an end-entity certificate to subject X that certifies key B. This certificate contains no key usage extension, as the certified key is not intended to be used for signing CRLs and could be a “mundane” certificate of any type (e.g., S/MIME, document signing certificate where the corresponding private key is stored on the filesystem of the secretary’s laptop, etc.).¶

4. subject X signs a CRL using key B and publishes the CRL at the distributionPoint specified in the crlDistributionPoints extension of the certificates issued in step 2.¶

5. Relying parties download the CRL published in step 4. The CRL validates successfully according to Section 6.3.3 of [RFC5280], as the CRL issuer DN matches, and the check for the presence of the cRLSign bit in the keyUsage extension is skipped because the keyUsage extension is absent.¶

4. Checking the presence of the keyUsage extension

To remediate the security issue described in Section 3, this document specifies the following amendment to step (f) of the CRL algorithm as found in Section 6.3.3 of [RFC5280].¶

OLD:¶

• (f) Obtain and validate the certification path for the issuer of the complete CRL. The trust anchor for the certification path MUST be the same as the trust anchor used to validate the target certificate. If a keyUsage extension is present in the CRL issuer's certificate, verify that the cRLSign bit is set.¶

NEW:¶

• (f) Obtain and validate the certification path for the issuer of the complete CRL. The trust anchor for the certification path MUST be the same as the trust anchor used to validate the target certificate. Verify that the keyUsage extension is present in the CRL issuer's certificate and verify that the cRLSign bit is set.¶

5. Security Considerations

If a Certification Authority has issued certificates to be used for CRL verification but do not include the keyUsage extension in accordance with Section 4.2.1.3 of [RFC5280], then relying party applications that have implemented the modified verification algorithm as specified in this document will be unable to verify CRLs issued by the CRL issuer in question.¶

It is strongly RECOMMENDED that Certification Authorities include the keyUsage extension in certificates to be used for CRL verification to ensure that there are no interoperability issues where updated applications are unable to verify CRLs.¶

If it is not possible to update the profile of CRL issuer certificates, then the policy management authority of the affected Public Key Infrastructure SHOULD update the subject naming requirements to ensure that certificates to be used for different purposes contain unique DNs.¶

6. IANA Considerations

This document has no IANA actions.¶

7. Normative References

[RFC2119]

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/rfc/rfc2119>.

[RFC5280]

Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/rfc/rfc5280>.

[RFC8174]

Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

Acknowledgments

TODO acknowledge.¶

Authors' Addresses