Clarification to processing Key Usage values during CRL validation
draft-ietf-lamps-keyusage-crl-validation-03

Network Working Group                                         C. Bonnell
Internet-Draft                                            DigiCert, Inc.
Updates: 5280 (if approved)                           伊藤 忠彦 (T. Ito)
Intended status: Standards Track                         SECOM CO., LTD.
Expires: 22 May 2026                              大久保 智史 (T. Okubo)
                                            Penguin Securities Pte. Ltd.
                                                        18 November 2025

   Clarification to processing Key Usage values during CRL validation
              draft-ietf-lamps-keyusage-crl-validation-03

Abstract

   RFC 5280 defines the profile of X.509 certificates and certificate
   revocation lists (CRLs) for use in the Internet.  Section 4.2.1.3 of
   RFC 5280 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 RFC 5280 does not explicitly
   include a corresponding check for the presence of the keyUsage
   certificate extension.  This document updates RFC 5280 to require
   that check.

About This Document

   This note is to be removed before publishing as an RFC.

   The latest revision of this draft can be found at
   https://CBonnell.github.io/ietf-lamps-keyusage-crl-validation-
   clarification/draft-ietf-lamps-keyusage-crl-validation.html.  Status
   information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-lamps-keyusage-crl-
   validation/.

   Source for this draft and an issue tracker can be found at
   https://github.com/CBonnell/lamps-keyusage-crl-validation-
   clarification.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  The risk of trusting CRLs signed with non-certified keys  . .   3
   4.  Checking the presence of the keyUsage extension . . . . . . .   4
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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.

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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.

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   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

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      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>.

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   [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

   Corey Bonnell
   DigiCert, Inc.
   Email: corey.bonnell@digicert.com

   Tadahiko Ito
   SECOM CO., LTD.
   Email: tadahiko.ito.public@gmail.com

   Additional contact information:

      伊藤 忠彦
      SECOM CO., LTD.

   Tomofumi Okubo
   Penguin Securities Pte. Ltd.
   Email: tomofumi.okubo+ietf@gmail.com

   Additional contact information:

      大久保 智史
      Penguin Securities Pte. Ltd.

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