Internet Engineering Task Force (IETF) H. Brockhaus
Request for Comments: 9809 Siemens
Category: Standards Track D. Goltzsche
ISSN: 2070-1721 Siemens Mobility
June 2025
X.509 Certificate Extended Key Usage (EKU) for Configuration, Updates,
and Safety Safety-Critical Communication
Abstract
RFC 5280 defines the Extended Key Usage (EKU) extension and specifies
several extended key purpose identifiers (KeyPurposeIds) for use with
that extension in 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 EKU extension of X.509
v3 public key certificates.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9809.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Conventions and Definitions
3. Extended Key Purpose for Configuration Files, Update Packages,
and Safety Safety-Critical Communication
4. Including the Extended Key Purpose in Certificates
5. Implications for a Certification Authority
6. Security Considerations
7. Privacy Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. ASN.1 Module
Appendix B. Use Cases
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
Key purpose identifiers (KeyPurposeIds) added to the certificate's
EKU extension [RFC5280] are meant to express intent as to the purpose
of the named usage, for humans and complying libraries. A full list
of KeyPurposeIds is maintained in the IANA registry "SMI Security for
PKIX Extended Key Purpose" [SMI-PKIX-PURPOSE]. The use of the
anyExtendedKeyUsage KeyPurposeId, as defined in Section 4.2.1.12 of
[RFC5280], is generally considered a poor practice.
This document defines KeyPurposeIds for certificates that 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.
If the purpose of an issued certificate is not restricted, i.e., restricted (i.e., the
type of
operations for which a of the public key contained in the certificate can be used
in unintended ways, ways), the risk of cross-
application cross-application attacks is
increased. Failure to ensure adequate 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
(CA) could obtain a certificate that can be misused for tasks that
this application or system is not entitled to perform. For example,
management of trust anchors 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
KeyPurposeIds 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 verified 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 used either for signing files (general-purpose
configuration files, trust anchor configuration files, and software
and firmware update packages) or for safety-critical communication.
Vendor-defined KeyPurposeIds used within a PKI governed by vendors
typically do not pose interoperability concerns, as non-critical
extensions can be safely ignored if unrecognized. However, using
KeyPurposeIds outside of their intended vendor-controlled environment
or in ExtendedKeyUsage extensions that have been marked critical can
lead to interoperability issues. Therefore, it is advisable not to
rely on vendor-defined KeyPurposeIds. Instead, this specification
defines standard KeyPurposeIds to ensure interoperability across
various vendors and industries.
The definitions of these KeyPurposeIds are intentionally broad to
allow their use in different deployments even though they were
initially motivated by industrial automation and rail automation (see
Appendix B). The details for each deployment need to be described in
the relevant technical standards and certificate policies.
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.
This document uses terms defined in [RFC5280]. X.509 certificate
extensions are defined using ASN.1 [X.680] [X.690].
The term "safety-critical communication" refers to communication that
could, under certain conditions, lead to a state in which human life,
health, property, or the environment is endangered. For the
definition of "safety", see [NIST_Glossary] [NIST.SP.800-160] and
[ISO.IEC.IEEE_12207].
3. Extended Key Purpose for Configuration Files, Update Packages, and
Safety
Safety-Critical Communication
This specification defines the KeyPurposeIds id-kp-configSigning, id-
kp-trustAnchorConfigSigning, id-kp-updatePackageSigning, and id-kp-
safetyCommunication. These KeyPurposeIds are used, respectively, following KeyPurposeIds:
* id-kp-configSigning: Used for signing general-purpose
configuration files, files.
* id-kp-trustAnchorConfigSigning: Used for signing trust anchor
configuration files, files.
* id-kp-updatePackageSigning: Used for signing software or firmware
update packages,
and packages.
* id-kp-safetyCommunication: Used for 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 KeyPurposeIds specified in this document are
intrinsically mutually exclusive. Instead, the acceptable
combinations of those KeyPurposeIds with others specified in this
document and with other KeyPurposeIds specified elsewhere are left to
the technical standards of the respective application and the
certificate policy of the respective PKI. For example, a technical
standard may specify the following: "Different keys and certificates
must be used for safety safety-critical communication and for trust anchor
updates, and a relying party must ignore the KeyPurposeId id-kp-
trustAnchorConfigSigning if id-kp-safetyCommunication is one of the
specified key purposes in a certificate." For example, the
certificate policy may specify the following: "The id-kp-
safetyCommunication KeyPuposeId should not be included in an issued
certificate together with the KeyPurposeId id-kp-
trustAnchorConfigSigning." Technical standards and certificate
policies of different applications may specify other rules. Further
considerations on prohibiting combinations of KeyPurposeIds is
described in Section 6.
Systems or applications that verify the signature of a general-
purpose configuration file 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 for signature verification, to keyEncipherment for
public key encryption, and keyAgreement for key agreement.
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-
trustAnchorConfigSigning, id-kp-updatePackageSigning, 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 (e.g., XML, YAML, or JSON). Configuration files are used
to configure hardware or software.
* id-kp-trustAnchorConfigSigning
A public key contained in a certificate containing the
KeyPurposeId id-kp-trustAnchorConfigSigning may be used for
verifying signatures of trust anchor configuration files of
various formats (e.g., 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-updatePackageSigning
A public key contained in a certificate containing the
KeyPurposeId id-kp-updatePackageSigning may be used for verifying
signatures of 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
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 41 }
id-kp-trustAnchorConfigSigning OBJECT IDENTIFIER ::= { id-kp 42 }
id-kp-updatePackageSigning OBJECT IDENTIFIER ::= { id-kp 43 }
id-kp-safetyCommunication OBJECT IDENTIFIER ::= { id-kp 44 }
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 and the KU
extension are inserted in each certificate that is issued. The
inclusion of the id-kp-configSigning, id-kp-trustAnchorConfigSigning,
id-kp-updatePackageSigning, 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 EKU key purposes do not introduce new security risks
but instead reduce existing security risks by providing the means to
identify if a 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 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 application should explicitly enumerate requirements
for excluded or permitted KeyPurposeIds or their combinations. It is
out of scope of this document to enumerate those, but 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-
clientAuth or id-kp-serverAuth.
7. Privacy Considerations
In some protocols (e.g., TLS 1.2 [RFC5246]), certificates are
exchanged in the clear. In other protocols (e.g., TLS 1.3
[RFC8446]), 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, which may reveal private
information of the certificate subject.
8. IANA Considerations
IANA has registered the following ASN.1 [X.680] module OID in the
"SMI Security for PKIX Module Identifier" registry [SMI-PKIX-MOD].
This OID is defined in Appendix A.
+=========+=============================+===========+
| Decimal | Description | Reference |
+=========+=============================+===========+
| 117 | id-mod-config-update-sc-eku | RFC 9809 |
+---------+-----------------------------+-----------+
Table 1
IANA has also registered the following OIDs in the "SMI Security for
PKIX Extended Key Purpose" registry [SMI-PKIX-PURPOSE]. These OIDs
are defined in Section 4.
+=========+================================+===========+
| Decimal | Description | Reference |
+=========+================================+===========+
| 41 | id-kp-configSigning | RFC 9809 |
+---------+--------------------------------+-----------+
| 42 | id-kp-trustAnchorConfigSigning | RFC 9809 |
+---------+--------------------------------+-----------+
| 43 | id-kp-updatePackageSigning | RFC 9809 |
+---------+--------------------------------+-----------+
| 44 | id-kp-safetyCommunication | RFC 9809 |
+---------+--------------------------------+-----------+
Table 2
9. References
9.1. 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/info/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/info/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/info/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.680-202102-I/en>.
[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.690-202102-I/en>.
9.2. 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/info/rfc5246>.
[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/info/rfc8446>.
[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/info/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/info/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/info/rfc9509>.
[Directive-2016_797]
European Parliament, Council of the European Union,
"Directive (EU) 2016/797 of the European Parliament and of
the Council of 11 May 2016 on the interoperability of the
rail system within the European Union", May 2020,
<https://eur-lex.europa.eu/eli/dir/2016/797/2020-05-28>.
[ERJU] Europe's Rail Joint Undertaking, "Shared Cybersecurity
Services Specification - SP-SEC-ServSpec - V1.0", February
2025, <https://rail-research.europa.eu/wp-
content/uploads/2025/03/ERJU-SP-Cybersecurity-
Specifications-V1.0.zip>.
[ERJU-web] Europe's Rail Joint Undertaking, "Europe's Rail Joint
Undertaking - System Pillar",
<https://rail-research.europa.eu/system_pillar/>.
[EU-CRA] European Commission, "Proposal for a REGULATION OF THE
EUROPEAN PARLIAMENT AND OF THE COUNCIL "Regulation (EU) 2024/2847 of the
European Parliament and of the Council of 23 October 2024
on horizontal cybersecurity requirements for products with
digital elements and amending Regulation Regulations (EU) 2019/1020",
September 2022, <https://digital-
strategy.ec.europa.eu/en/library/cyber-resilience-act>. No 168/2013
and (EU) 2019/1020 and Directive (EU) 2020/1828 (Cyber
Resilience Act)", November 2024,
<https://eur-lex.europa.eu/eli/reg/2024/2847/oj>.
[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>.
[NIST_Glossary]
[NIST.SP.800-160]
Ross, R., Winstead, M., and M. McEvilley, "Engineering
Trustworthy Secure Systems", NIST CSRC, "safety",
<https://csrc.nist.gov/glossary/term/safety>. SP 800-160v1r1,
DOI 10.6028/NIST.SP.800-160v1r1, November 2022,
<https://doi.org/10.6028/NIST.SP.800-160v1r1>.
[ISO.IEC.IEEE_12207]
ISO/IEC/IEEE, "Systems and software engineering - Software
life cycle processes", ISO/IEC/IEEE 12207:2017, November
2017, <https://www.iso.org/standard/63712.html>.
[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>.
[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>.
[CE-marking]
European Commission, "CE marking", <https://single-market-
economy.ec.europa.eu/single-market/ce-marking_en>.
[SMI-PKIX-PURPOSE]
IANA, "SMI Security for PKIX Extended Key Purpose",
<https://www.iana.org/assignments/smi-numbers>.
[SMI-PKIX-MOD]
IANA, "SMI Security for PKIX Module Identifier",
<https://www.iana.org/assignments/smi-numbers>.
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-config-update-sc-eku (117) }
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 41 }
id-kp-trustAnchorConfigSigning OBJECT IDENTIFIER ::= { id-kp 42 }
id-kp-updatePackageSigning OBJECT IDENTIFIER ::= { id-kp 43 }
id-kp-safetyCommunication OBJECT IDENTIFIER ::= { id-kp 44 }
END
<CODE ENDS>
Appendix B. Use Cases
These use cases are only for informational purposes.
Automation hardware and software products strive to become more safe
and secure by fulfilling mandatory, generic system requirements
related to cybersecurity, e.g., 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 and sold in the EU after 2027 must bear the so-called "CE
marking" [CE-marking] to indicate that they comply. comply with the EU-CRA.
Such regulation was announced in the 2020 EU Cybersecurity Strategy
[EU-STRATEGY] and complements other legislation in this area, like
the NIS2 Framework, Directive directive on measures for a high common level of cybersecurity
for network and information systems (NIS) across the European Union
[NIS2].
The 2020 EU Cybersecurity Strategy [EU-STRATEGY] suggests
implementing and extending international standards such as "Security
for industrial automation and control systems - Part 4-2: Technical
security requirements for IACS components"
[IEC.62443-4-2] (IACS
refers to Industrial Automation and Control System) and "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 on automation
networks and the Internet can be used to build common automation
solutions. Standardized attributes would allow transparency of
security properties and interoperability for vendors in the context
of software and firmware updates, general-
purpose general-purpose configuration,
trust anchor configuration, and safety safety-critical communication.
A concrete example for automation is a rail automation system. The
Europe's Rail web page [ERJU-web] states:
| The System Pillar brings rail sector representatives under a
| single coordination body. To achieve this, the System Pillar will
| deliver a unified operational concept and a functional, safe and
| secure system architecture, with due consideration of cyber-
| security aspects, focused on the European railway network to which
| Directive 2016/797 applies (i.e. the heavy rail network) as well
| as associated specifications and/or standards.
See [Directive-2016_797]. For details about the System Pillar, see
[ERJU].
Acknowledgments
We would like to thank the authors of [RFC9336] and [RFC9509] for
their excellent template.
We also thank all reviewers of this document for their valuable
feedback.
Contributors
Szofia Fazekas-Zisch
Siemens AG
Breslauer Str. 5
90766 Fuerth
Germany
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