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. Section 4.2.1.3 of [RFC5280] requires CRL issuer certificates to contain the keyUsage extension with the cRLSign bit asserted. However, the CRL validation algorithm specified in Section 6.3 of [RFC5280] does not explicitly include a corresponding check for the presence of the keyUsage certificate extension. This document updates [RFC5280] to require that check.¶

Section 3 describes the security concern that motivates this update.¶

Section 4 updates the CRL validation algorithm to resolve this concern.¶

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 trusting CRLs signed with non-certified keys

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

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.¶

This step does not explicitly specify a check for the presence of the keyUsage extension itself.¶

Similarly, 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.¶

Certification Authorities can 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 sign 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 signs CRLs that assert the indirectCRL boolean within the issuingDistributionPoint extension.¶

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 signs 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 signs 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 signs 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 signed 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. If the version of the CRL issuer’s certificate is version 3 (v3), then verify that the keyUsage extension is present and verify that the cRLSign bit is set.¶

This change ensures that the CRL issuer's key is certified for CRL signing. However, this check is not performed if the CRL issuer's key is certified using a version 1 (v1) or version 2 (v2) X.509 certificate, as these versions do not have an extensions field where the key usage extension can be included.¶

5. Security Considerations

If a Certification Authority has signed 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 signed 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

The authors would like to thank the participants on the LAMPS Working Group mailing list for their insightful feedback and comments. In particular, the authors extend sincere appreciation to Carl Wallace, David Hook, Deb Cooley, John Gray, Michael St. Johns, Mike Ounsworth, Russ Housley, Serge Mister, and Tomas Gustavsson for their reviews and suggestions, which greatly improved the quality of this document.¶

Authors' Addresses