X.509 Certificate Extended Key Usage (EKU) for Automation
draft-ietf-lamps-automation-keyusages-02

LAMPS Working Group                                         H. Brockhaus
Internet-Draft                                                   Siemens
Intended status: Standards Track                            D. Goltzsche
Expires: 11 July 2025                                   Siemens Mobility
                                                          7 January 2025

       X.509 Certificate Extended Key Usage (EKU) for Automation
                draft-ietf-lamps-automation-keyusages-02

Abstract

   RFC 5280 specifies several extended key purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  This document defines
   KeyPurposeIds for general-purpose and trust anchor configuration
   files, for software and firmware update packages, and for safety-
   critical communication to be included in the Extended Key Usage (EKU)
   extension of X.509 v3 public key certificates used by industrial
   automation and the ERJU System Pillar.

Status of This Memo

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   This Internet-Draft will expire on 11 July 2025.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   4
   3.  Extended Key Purpose for Automation . . . . . . . . . . . . .   5
   4.  Including the Extended Key Purpose in Certificates  . . . . .   6
   5.  Implications for a Certification Authority  . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  11
   Appendix B.  History of Changes . . . . . . . . . . . . . . . . .  12
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Automation hardware and software products will strategically be more
   safe and secure by fulfilling mandatory, generic system requirements
   related to cyber security driven by federal offices like the European
   Union Cyber Resilience Act [EU-CRA] governed by the European
   Commission and the High Representative of the Union for Foreign
   Affairs and Security Policy.  Automation products connected to the
   internet would bear the CE marking to indicate they comply.  Such
   regulation was announced in the 2020 EU Cybersecurity Strategy
   [EU-STRATEGY], and complements other legislation in this area,
   specifically the NIS2 Framework, Directive on measures for a high
   common level of cybersecurity across the Union [NIS2]. 2020 EU
   Cybersecurity Strategy suggests to implement and extend international
   standards such as the Security for industrial automation and control
   systems - Part 4-2: Technical security requirements for IACS
   components [IEC.62443-4-2] and the Industrial communication networks
   - Network and system security - Part 3-3: System security
   requirements and security levels [IEC.62443-3-3].  Automation
   hardware and software products of diverse vendors that are connected

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   on automation networks and the internet build a typical automation
   solution.  Harmonized attributes would allow transparency of security
   properties and interoperability for vendors in context of secure
   software and firmware updates, general-purpose configuration, trust
   anchor configuration, and secure safety communication.

   A concrete example for Automation is a Rail Automation system.  The
   Europe's Rail Joint Undertaking System Pillar [ERJU] will deliver a
   unified operational concept and a functional, safe, and secure system
   architecture alongside with system requirements for Rail Automation.
   The deliverables include due consideration of cyber security aspects
   based on the IEC 62443 series of standards, focused on the European
   railway network to which Directive 2016/797 - Interoperability of the
   rail system within the EU [Directive-2016_797] applies.

   The ERJU System Pillar Cyber Security Working Group makes use of PKIs
   to generate X.509 PKI certificates.  The certificates are used for
   the following purposes, among others:

   *  Validating signatures of general-purpose software configuration
      files.

   *  Validating signatures of trust anchor configuration files.

   *  Validating signatures of software and firmware update packages.

   *  Authenticating communication endpoints authorized for safety-
      critical communication.

   [RFC5280] specifies several key usage extensions, defined via
   KeyPurposeIds, for X.509 certificates.  Key usage extensions added to
   a certificate are meant to express intent as to the purpose of the
   named usage, for humans and for complying libraries.  In addition,
   the IANA registry "SMI Security for PKIX Extended Key Purpose"
   [RFC7299] contains additional KeyPurposeIds.  The use of the
   anyExtendedKeyUsage KeyPurposeId, as defined in Section 4.2.1.12 of
   [RFC5280], is generally considered a poor practice.  This is
   especially true for certificates, whether they are multi-purpose or
   single-purpose, within the context of ERJU System Pillar.

   If the purpose of the issued certificates is not restricted, i.e.,
   the type of operations for which a public key contained in the
   certificate can be used are not specified, those certificates could
   be used for another purpose than intended, increasing the risk of
   cross-protocol attacks.  Failure to ensure proper segregation of
   duties means that an application or system that generates the public/
   private keys and applies for a certificate to the operator
   certification authority could obtain a certificate that can be

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   misused for tasks that this application or system is not entitled to
   perform.  For example, management of trust anchor is a particularly
   critical task.  A device could potentially accept a trust anchor
   configuration file signed by a service that uses a certificate with
   no EKU or with the KeyPurposeId id-kp-codeSigning (Section 4.2.1.12
   of [RFC5280]) or id-kp-documentSigning [RFC9336].  A device should
   only accept trust anchor configuration files if the file is signed
   with a certificate that has been explicitly issued for this purpose.

   The KeyPurposeId id-kp-serverAuth (Section 4.2.1.12 of [RFC5280]) can
   be used to identify that the certificate is for a TLS WWW server, and
   the KeyPurposeId id-kp-clientAuth (Section 4.2.1.12 of [RFC5280]) can
   be used to identify that the certificate is for a TLS WWW client.
   However, there are currently no KeyPurposeIds for usage with X.509
   certificates for safety-critical communication.

   This document addresses the above problems by defining keyPurposeIds
   for the EKU extension of X.509 public key certificates.  These
   certificates are either used for signing files (general-purpose
   configuration and trust anchor configuration files, software and
   firmware update packages) or are used for safety-critical
   communication.

   Vendor-defined KeyPurposeIds used within a PKI governed by the vendor
   or a group of vendors typically do not pose interoperability
   concerns, as non-critical extensions can be safely ignored if
   unrecognized.  However, using or misusing KeyPurposeIds outside of
   their intended vendor-controlled environment can lead to
   interoperability issues.  Therefore, it is advisable not to rely on
   vendor-defined KeyPurposeIds.  Instead, the specification defines
   standard KeyPurposeIds to ensure interoperability across various
   vendors and industries.

   Although the specification focuses on use in industrial automation,
   the definitions are intentionally broad to allow the use of the
   KeyPurposeIds defined in this document in other deployments as well.
   Whether and how any of the KeyPurpose OIDs defined in this document
   must be described in more detail in the technical standards and
   certificate policies relevant to the industrial sector.

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.

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3.  Extended Key Purpose for Automation

   This specification defines the KeyPurposeIds id-kp-configSigning, id-
   kp-trustanchorSigning, id-kp-updateSigning, and id-kp-
   safetyCommunication and uses these, respectively, for: signing
   general-purpose or trust anchor configuration files, signing software
   or firmware update packages, or authenticating communication peers
   for safety-critical communication.  As described in Section 4.2.1.12
   of [RFC5280], "[i]f the [extended key usage] extension is present,
   then the certificate MUST only be used for one of the purposes
   indicated" and "[i]f multiple [key] purposes are indicated the
   application need not recognize all purposes indicated, as long as the
   intended purpose is present".

   None of the keyPurposeId's specified in this document are
   intrinsically mutually exclusive.  Instead, the acceptable
   combinations of those KeyPurposeId's with others specified in this
   document and with other KeyPurposeId's specified elsewhere are left
   to the technical standards of the respective area of application and
   the certificate policy of the respective PKI.  For example, a
   technical standard may specify: 'Different keys and certificate MUST
   be used for safety communication and for trust anchor updates, and a
   relying party MUST ignore the KeyPurposeId id-kp-trustanchorSigning
   if id-kp-safetyCommunication is one of the specified key purposes in
   a certificate.', and the certificate policy may specify: 'The id-kp-
   safetyCommunication KeyPuposeId SHOULD not be included in an issued
   certificate together with the KeyPurposeId id-kp-trustanchorSigning.'
   Technical standards and certificate policies of other area of
   application may specify other rules.  Further consideration on
   prohibiting combinations of KeyPurposeIds is described in the
   Security Considerations section of this document.

   Systems or applications that verify the signature of a general-
   purpose or trust anchor configuration file, the signature of a
   software or firmware update package, or the authentication of a
   communication peer for safety-critical communication SHOULD require
   that corresponding KeyPurposeIds be specified by the EKU extension.
   If the certificate requester knows the certificate users are mandated
   to use these KeyPurposeIds, it MUST enforce their inclusion.
   Additionally, such a certificate requester MUST ensure that the
   KeyUsage extension be set to digitalSignature or nonRepudiation (also
   designated as contentCommitment) for signature verification and if
   needed to keyEncipherment for secret key encryption and/or
   keyAgreement for key agreement.

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4.  Including the Extended Key Purpose in Certificates

   [RFC5280] specifies the EKU X.509 certificate extension for use on
   end entity certificates.  The extension indicates one or more
   purposes for which the certified public key is valid.  The EKU
   extension can be used in conjunction with the Key Usage (KU)
   extension, which indicates the set of basic cryptographic operations
   for which the certified key may be used.  The EKU extension syntax is
   repeated here for convenience:

      ExtKeyUsageSyntax  ::=  SEQUENCE SIZE (1..MAX) OF KeyPurposeId

      KeyPurposeId  ::=  OBJECT IDENTIFIER

   As described in [RFC5280], the EKU extension may, at the option of
   the certificate issuer, be either critical or non-critical.  The
   inclusion of KeyPurposeIds id-kp-configSigning, id-kp-
   trustanchorSigning, id-kp-updateSigning, and id-kp-
   safetyCommunication in a certificate indicates that the public key
   encoded in the certificate has been certified for the following
   usages:

   *  id-kp-configSigning

      A public key contained in a certificate containing the
      KeyPurposeId id-kp-configSigning may be used for verifying
      signatures of general-purpose configuration files of various
      formats (for example XML, YAML, or JSON).  Configuration files are
      used to configure hardware or software.

   *  id-kp-trustanchorSigning

      A public key contained in a certificate containing the
      KeyPurposeId id-kp-trustanchorSigning may be used for verifying
      signatures of trust anchor configuration files of various formats
      (for example XML, YAML, or JSON).  Trust anchor configuration
      files are used to add or remove trust anchors to the trust store
      of a device.

   *  id-kp-updateSigning

      A public key contained in a certificate containing the
      KeyPurposeId id-kp-updateSigning may be used for verifying
      signatures of secure software or firmware update packages.  Update
      packages are used to install software (including bootloader,
      firmware, safety-related applications, and others) on systems.

   *  id-kp-safetyCommunication

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      A public key contained in a certificate containing the
      KeyPurposeId id-kp-safetyCommunication may be used to authenticate
      a communication peer for safety-critical communication based on
      TLS or other protocols.

      id-kp  OBJECT IDENTIFIER  ::=
          { iso(1) identified-organization(3) dod(6) internet(1)
            security(5) mechanisms(5) pkix(7) 3 }

      id-kp-configSigning        OBJECT IDENTIFIER ::= { id-kp TBD2 }
      id-kp-trustanchorSigning   OBJECT IDENTIFIER ::= { id-kp TBD3 }
      id-kp-updateSigning        OBJECT IDENTIFIER ::= { id-kp TBD4 }
      id-kp-safetyCommunication  OBJECT IDENTIFIER ::= { id-kp TBD5 }

5.  Implications for a Certification Authority

   The procedures and practices employed by a certification authority
   MUST ensure that the correct values for the EKU extension as well as
   the KU extension are inserted in each certificate that is issued.
   The inclusion of the id-kp-configSigning, id-kp-trustanchorSigning,
   id-kp-updateSigning, and id-kp-safetyCommunication KeyPurposeIds does
   not preclude the inclusion of other KeyPurposeIds.

6.  Security Considerations

   The Security Considerations of [RFC5280] are applicable to this
   document.  These extended key usage key purposes do not introduce new
   security risks but instead reduces existing security risks by
   providing the means to identify if the certificate is generated to
   verify the signature of a general-purpose or trust anchor
   configuration file, the signature of a software or firmware update
   package, or the authentication of a communication peer for safety-
   critical communication.

   To reduce the risk of specific cross-protocol attacks, the relying
   party or the relying party software may additionally prohibit use of
   specific combinations of KeyPurposeIds.  The procedure for allowing
   or disallowing combinations of KeyPurposeIds using excluded
   KeyPurposeId and permitted KeyPurposeId, as carried out by a relying
   party, is defined in Section 4 of [RFC9336].  The technical standards
   and certificate policies of the area of application should specify
   concrete requirements for excluded or permitted EKUs or their
   combinations.  An example of excluded KeyPurposeIds can be the
   presence of the anyExtendedKeyUsage KeyPurposeId.  Examples of
   allowed KeyPurposeIds combinations can be the presence of id-kp-
   safetyCommunication together with id-kp-clinetAuth or id-kp-
   serverAuth.

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7.  Privacy Considerations

   In some security protocols, such as TLS 1.2 [RFC5246], certificates
   are exchanged in the clear.  In other security protocols, such as TLS
   1.3 [RFC8446], the certificates are encrypted.  The inclusion of the
   EKU extension can help an observer determine the purpose of the
   certificate.  In addition, if the certificate is issued by a public
   certification authority, the inclusion of an EKU extension can help
   an attacker to monitor the Certificate Transparency logs [RFC9162] to
   identify the purpose of the certificate.

8.  IANA Considerations

   IANA is requested to register the following ASN.1 [X.680] module OID
   in the "SMI Security for PKIX Module Identifier" registry
   (1.3.6.1.5.5.7.0).  This OID is defined in Appendix A.

             +=========+=======================+============+
             | Decimal | Description           | References |
             +=========+=======================+============+
             | TBD1    | id-mod-automation-eku | This-RFC   |
             +---------+-----------------------+------------+

                                 Table 1

   IANA is also requested to register the following OIDs in the "SMI
   Security for PKIX Extended Key Purpose" registry (1.3.6.1.5.5.7.3).
   These OIDs are defined in Section 4.

           +=========+===========================+============+
           | Decimal | Description               | References |
           +=========+===========================+============+
           | TBD2    | id-kp-configSigning       | This-RFC   |
           +---------+---------------------------+------------+
           | TBD3    | id-kp-trustanchorSigning  | This-RFC   |
           +---------+---------------------------+------------+
           | TBD4    | id-kp-updateSigning       | This-RFC   |
           +---------+---------------------------+------------+
           | TBD5    | id-kp-safetyCommunication | This-RFC   |
           +---------+---------------------------+------------+

                                 Table 2

9.  Acknowledgments

   We would like to thank the authors of [RFC9336] and [RFC9509] for
   their excellent template.

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   We also thank all reviewers of this document for their valuable
   feedback.

10.  References

10.1.  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>.

   [X.680]    ITU-T, "Information Technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680 , February 2021,
              <https://www.itu.int/rec/T-REC.X.680>.

   [X.690]    ITU-T, "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690 , February 2021,
              <https://www.itu.int/rec/T-REC.X.690>.

10.2.  Informative References

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/rfc/rfc5246>.

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
              <https://www.rfc-editor.org/rfc/rfc7299>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

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   [RFC9162]  Laurie, B., Messeri, E., and R. Stradling, "Certificate
              Transparency Version 2.0", RFC 9162, DOI 10.17487/RFC9162,
              December 2021, <https://www.rfc-editor.org/rfc/rfc9162>.

   [RFC9336]  Ito, T., Okubo, T., and S. Turner, "X.509 Certificate
              General-Purpose Extended Key Usage (EKU) for Document
              Signing", RFC 9336, DOI 10.17487/RFC9336, December 2022,
              <https://www.rfc-editor.org/rfc/rfc9336>.

   [RFC9509]  Reddy.K, T., Ekman, J., and D. Migault, "X.509 Certificate
              Extended Key Usage (EKU) for 5G Network Functions",
              RFC 9509, DOI 10.17487/RFC9509, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9509>.

   [Directive-2016_797]
              European Parliament, Council of the European Union,
              "Directive 2016/797 - Interoperability of the rail system
              within the EU", May 2020,
              <https://eur-lex.europa.eu/eli/dir/2016/797/2020-05-28>.

   [ERJU]     Europe's Rail Joint Undertaking, "SP-Cybersecurity-
              SharedCybersecurityServices - Review 3 Final Draft Specs
              (V0.90)", September 2024, <https://rail-
              research.europa.eu/wp-content/uploads/2023/10/
              ERJU_SP_CyberSecurity_Review3_Files.zip>.

   [EU-CRA]   European Commission, "Proposal for a REGULATION OF THE
              EUROPEAN PARLIAMENT AND OF THE COUCIL on horizontal
              cybersecurity requirements for products with digital
              elements and amending Regulation (EU) 2019/1020",
              September 2022, <https://digital-
              strategy.ec.europa.eu/en/library/cyber-resilience-act>.

   [EU-STRATEGY]
              European Commission, "The EU's Cybersecurity Strategy for
              the Digital Decade", December 2020, <https://digital-
              strategy.ec.europa.eu/en/library/eus-cybersecurity-
              strategy-digital-decade-0>.

   [NIS2]     European Commission, "Directive (EU) 2022/2555 of the
              European Parliament and of the Council", December 2024,
              <https://digital-strategy.ec.europa.eu/en/policies/
              nis2-directive>.

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   [IEC.62443-4-2]
              IEC, "Security for industrial automation and control
              systems - Part 4-2: Technical security requirements for
              IACS components", IEC 62443-4-2:2019 , February 2019,
              <https://webstore.iec.ch/publication/34421>.

   [IEC.62443-3-3]
              IEC, "Industrial communication networks - Network and
              system security - Part 3-3: System security requirements
              and security levels", IEC 62443-3-3:2013 , August 2013,
              <https://webstore.iec.ch/publication/7033>.

Appendix A.  ASN.1 Module

   The following module adheres to ASN.1 specifications [X.680] and
   [X.690].

   <CODE BEGINS>

   Automation-EKU
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) id-mod(0)
       id-mod-eu-automation-eku (TBD1) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- OID Arc

   id-kp OBJECT IDENTIFIER ::=
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) kp(3) }

   -- Extended Key Usage Values

   id-kp-configSigning        OBJECT IDENTIFIER ::= { id-kp TBD2 }
   id-kp-trustanchorSigning   OBJECT IDENTIFIER ::= { id-kp TBD3 }
   id-kp-updateSigning        OBJECT IDENTIFIER ::= { id-kp TBD4 }
   id-kp-safetyCommunication  OBJECT IDENTIFIER ::= { id-kp TBD5 }

   END

   <CODE ENDS>

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Appendix B.  History of Changes

   [RFC Editor: Please remove this appendix in the release version of
   the document.]

   Changes from 01 -> 02:

   *  Updates Sections 1, 3, and 6 addressing last call comments (see
      "WG Last Call for draft-ietf-lamps-automation-keyusages-01")

   Changes from 01 -> 02:

   *  Implemented the changes requested during WGLC

   Changes from 00 -> 01:

   *  Fixed some minor nids and wording issues

   draft-ietf-lamps-automation-keyusages version 00:

   *  Updated document and filename after WG adoption

   Changes from 00 -> 01:

   *  Updated last paragraph of Section 1 addressing WG adoption
      comments by Rich and Russ

   *  Updated name and OID of ASN.1 module

   draft-brockhaus-lamps-automation-keyusages version 00:

   *  Broadened the scope to general automation use case and use ERJU as
      an example.

   *  Fixed some nits reported.

   draft-brockhaus-lamps-eu-rail-keyusages version 00:

   *  Initial version of the document following best practices from RFC
      9336 and RFC 9509

Contributors

   Szofia Fazekas-Zisch
   Siemens AG Digital Industries Factory Automation
   Breslauer Str. 5
   90766 Fuerth
   Germany

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   Email: szofia.fazekas-zisch@siemens.com
   URI:   https://www.siemens.com

   Baptiste Fouques
   Alstom
   Email: baptiste.fouques@alstomgroup.com

   Daniel Gutierrez Orta
   CAF Signalling
   Email: daniel.gutierrez@cafsignalling.com

   Martin Weller
   Hitachi Rail
   Email: martin.weller@urbanandmainlines.com

   Nicolas Poyet
   SNCF
   Email: nicolas.poyet@sncf.fr

Authors' Addresses

   Hendrik Brockhaus
   Siemens
   Werner-von-Siemens-Strasse 1
   80333 Munich
   Germany
   Email: hendrik.brockhaus@siemens.com
   URI:   https://www.siemens.com

   David Goltzsche
   Siemens Mobility
   Ackerstrasse 22
   38126 Braunschweig
   Germany
   Email: david.goltzsche@siemens.com
   URI:   https://www.mobility.siemens.com

Brockhaus & Goltzsche     Expires 11 July 2025                 [Page 13]