Internet Engineering Task Force (IETF)                        T. Reddy.K
Request for Comments: 9509                                      J. Ekman
Category: Standards Track                                          Nokia
ISSN: 2070-1721                                               D. Migault
                                                               Ericsson
                                                             March 2024


 X.509 Certificate Extended Key Usage (EKU) for 5G Network Functions

Abstract

  RFC 5280 specifies several extended key purpose identifiers
  (KeyPurposeIds) for X.509 certificates.  This document defines
  encrypting JSON objects in HTTP messages, using JSON Web Tokens
  (JWTs), and signing the OAuth 2.0 access tokens KeyPurposeIds for
  inclusion in the Extended Key Usage (EKU) extension of X.509 v3
  public key certificates used by Network Functions (NFs) for the 5G
  System.

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/rfc9509.

Copyright Notice

  Copyright (c) 2024 IETF Trust and the persons identified as the
  document authors.  All rights reserved.

  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 and restrictions with respect
  to this document.  Code Components extracted from this document must
  include Revised BSD License text as described in Section 4.e of the
  Trust Legal Provisions and are provided without warranty as described
  in the Revised BSD License.

Table of Contents

  1.  Introduction
  2.  Terminology
  3.  Extended Key Purpose for Network Functions
  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
  Acknowledgments
  Contributor
  Authors' Addresses

1.  Introduction

  The operators of 5G ("fifth generation") systems as defined by 3GPP
  make use of an internal PKI to generate X.509 PKI certificates for
  the Network Functions (NFs) (Section 6 of [TS23.501]) in a 5G System.
  The certificates are used for the following purposes:

  *  Client and Server certificates for NFs in 5G Core (5GC) Service
     Based Architecture (SBA) (see Section 6.1.3c of [TS33.310] and
     Section 6.7.2 of [TS29.500])

  *  Client Credentials Assertion (CCA) uses JSON Web Tokens (JWTs)
     [RFC7519] and is secured with digital signatures based on the JSON
     Web Signature (JWS) [RFC7515] (see Section 13.3.8.2 of [TS33.501],
     and Section 6.7.5 of [TS29.500]).

  *  Certificates for encrypting JSON objects in HTTP messages between
     Security Edge Protection Proxies (SEPPs) using JSON Web Encryption
     (JWE) [RFC7516] (see Section 13.2.4.4 of [TS33.501], Section 6.3.2
     of [TS33.210], Section 6.7.4 of [TS29.500], and Section 5.3.2.1 of
     [TS29.573]).

  *  Certificates for signing the OAuth 2.0 access tokens for service
     authorization to grant temporary access to resources provided by
     NF producers using JWS (see Section 13.4.1 of [TS33.501] and
     Section 6.7.3 of [TS29.500]).

  [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 publicly trusted certificates, whether they are
  multi-purpose or single-purpose, within the context of 5G Systems and
  the 5GC Service Based Architecture.

  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 a NF that generates the public/private keys and
  applies for a certificate to the operator certification authority
  could obtain a certificate that can be misused for tasks that this NF
  is not entitled to perform.  For example, a NF service consumer could
  potentially impersonate NF service producers using its certificate.
  Additionally, in cases where the certificate's purpose is intended
  for use by the NF service consumer as a client certificate, it's
  essential to ensure that the NF with this client certificate and the
  corresponding private key are not allowed to sign the Client
  Credentials Assertion (CCA).  When a NF service producer receives the
  signed CCA from the NF service consumer, the NF should only accept
  the token if the CCA 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 server (e.g., NF
  service producer), and the KeyPurposeId id-kp-clientAuth
  (Section 4.2.1.12 of [RFC5280]) can be used to identify that the
  certificate is for a client (e.g., NF service consumer).  However,
  there are currently no KeyPurposeIds for the other usages of
  certificates in a 5G System.  This document addresses the above
  problem by defining the EKU extension of X.509 public key
  certificates for signing the JWT Claims Set using JWS, encrypting
  JSON objects in HTTP messages using JWE, and signing the OAuth 2.0
  access tokens using JWS.

  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
  implementations.

  Although the specification focuses on a 5G use case, the standard
  KeyPurposeIds defined in this document can be used in other
  deployments.

2.  Terminology

  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.  Extended Key Purpose for Network Functions

  This specification defines the KeyPurposeIds id-kp-jwt, id-kp-
  httpContentEncrypt, and id-kp-oauthAccessTokenSigning and uses these,
  respectively, for: signing the JWT Claims Set of CCA using JWS,
  encrypting JSON objects in HTTP messages between Security Edge
  Protection Proxies (SEPPs) using JWE, and signing the OAuth 2.0
  access tokens for service authorization to grant temporary access to
  resources provided by NF producers using JWS.  As described in
  [RFC5280], "[i]f the [Extended Key Usage] extension is present, then
  the certificate MUST only be used for one of the purposes indicated."
  [RFC5280] also notes that "[i]f multiple [key] purposes are indicated
  the application need not recognize all purposes indicated, as long as
  the intended purpose is present."

  Network Functions that verify the signature of a CCA represented as a
  JWT, decrypt JSON objects in HTTP messages between Security Edge
  Protection Proxies (SEPPs) using JWE, or verify the signature of an
  OAuth 2.0 access tokens for service authorization to grant temporary
  access to resources provided by NF producers using JWS 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 calculation and/or to
  keyEncipherment for secret key encryption.

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 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-jwt, id-kp-httpContentEncrypt, and
  id-kp-oauthAccessTokenSigning in a certificate indicates that the
  public key encoded in the certificate has been certified for use in
  the following:

  1.  Validating the JWS Signature in JWT.  The distinction between JWS
      and JWE is determined by the Key Usage (KU) that is set to
      digitalSignature or nonRepudiation for JWS and keyEncipherment
      for JWE.

  2.  Encrypting JSON objects in HTTP messages (for example, encrypting
      the content-encryption key (CEK) with the recipient's public key
      using the RSAES-OAEP algorithm to produce the JWE Encrypted Key).
      KU is set to keyEncipherment.

  3.  Signing OAuth 2.0 access tokens.  In this case, KU is set to
      digitalSignature or nonRepudiation.

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

  id-kp-jwt OBJECT IDENTIFIER ::= { id-kp 37 }
  id-kp-httpContentEncrypt OBJECT IDENTIFIER ::= { id-kp 38 }
  id-kp-oauthAccessTokenSigning OBJECT IDENTIFIER ::= { id-kp 39 }

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-jwt, id-kp-httpContentEncrypt, and id-kp-
  oauthAccessTokenSigning KeyPurposeIds does not preclude the inclusion
  of other KeyPurposeIds.

6.  Security Considerations

  The Security Considerations of [RFC5280] are applicable to this
  document.  This extended key purpose does not introduce new security
  risks but instead reduces existing security risks by providing the
  means to identify if the certificate is generated to sign the JWT
  Claims Set, signing the OAuth 2.0 access tokens using JWS, or
  encrypting the CEK in JWE for encrypting JSON objects in HTTP
  messages.

  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].  Examples of Excluded
  KeyPurposeIds include the presence of the anyExtendedKeyUsage
  KeyPurposeId or the complete absence of the EKU extension in a
  certificate.  Examples of Permitted KeyPurposeIds include the
  presence of id-kp-jwt, id-kp-httpContentEncrypt, or id-kp-
  oauthAccessTokenSigning KeyPurposeIds.

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 has registered 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 |
  +=========+===============================+============+
  | 37      | id-kp-jwt                     | RFC 9509   |
  +---------+-------------------------------+------------+
  | 38      | id-kp-httpContentEncrypt      | RFC 9509   |
  +---------+-------------------------------+------------+
  | 39      | id-kp-oauthAccessTokenSigning | RFC 9509   |
  +---------+-------------------------------+------------+

                          Table 1

  IANA has registered 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 |
  +=========+===============+============+
  | 108     | id-mod-nf-eku | RFC 9509   |
  +---------+---------------+------------+

                  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>.

  [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
             Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
             2015, <https://www.rfc-editor.org/info/rfc7515>.

  [RFC7516]  Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
             RFC 7516, DOI 10.17487/RFC7516, May 2015,
             <https://www.rfc-editor.org/info/rfc7516>.

  [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
             (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
             <https://www.rfc-editor.org/info/rfc7519>.

  [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>.

  [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>.

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>.

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

  [TS23.501] 3GPP, "System architecture for the 5G System (5GS)",
             Release 18.4.0, 3GPP TS 23.501, December 2023,
             <https://www.3gpp.org/ftp/Specs/
             archive/23_series/23.501/23501-i40.zip>.

  [TS29.500] 3GPP, "5G System; Technical Realization of Service Based
             Architecture; Stage 3", Release 18.4.0, 3GPP TS 29.500,
             December 2023, <https://www.3gpp.org/ftp/Specs/
             archive/29_series/29.500/29500-i40.zip>.

  [TS29.573] 3GPP, "5G System; Public Land Mobile Network (PLMN)
             Interconnection; Stage 3", Release 18.5.0, 3GPP TS 29.573,
             December 2023, <https://www.3gpp.org/ftp/Specs/
             archive/29_series/29.573/29573-i50.zip>.

  [TS33.210] 3GPP, "Network Domain Security (NDS); IP network layer
             security", Release 17.1.0, 3GPP TS 33.210, September 2022,
             <https://www.3gpp.org/ftp/Specs/
             archive/33_series/33.210/33210-h10.zip>.

  [TS33.310] 3GPP, "Network Domain Security (NDS); Authentication
             Framework (AF)", Release 18.2.0, 3GPP TS 33.310, December
             2023, <https://www.3gpp.org/ftp/Specs/
             archive/33_series/33.310/33310-i20.zip>.

  [TS33.501] 3GPP, "Security architecture and procedures for 5G
             system", Release 18.4.0, 3GPP TS 33.501, December 2023,
             <https://www.3gpp.org/ftp/Specs/
             archive/33_series/33.501/33501-i40.zip>.

Appendix A.  ASN.1 Module

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

  <CODE BEGINS>
  NF-EKU
    { iso(1) identified-organization(3) dod(6) internet(1)
    security(5) mechanisms(5) pkix(7) id-mod(0)
    id-mod-nf-eku (108) }

  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-jwt OBJECT IDENTIFIER ::= { id-kp 37 }
  id-kp-httpContentEncrypt OBJECT IDENTIFIER ::= { id-kp 38 }
  id-kp-oauthAccessTokenSigning OBJECT IDENTIFIER ::= { id-kp 39 }

  END
  <CODE ENDS>

Acknowledgments

  We would like to thank Corey Bonnell, Ilari Liusvaara, Carl Wallace,
  and Russ Housley for their useful feedback.  Thanks to Yoav Nir for
  the secdir review, Elwyn Davies for the genart review, and Benson
  Muite for the intdir review.

  Thanks to Paul Wouters, Lars Eggert, and Éric Vyncke for the IESG
  review.

Contributor

  The following individual has contributed to this document:

  German Peinado
  Nokia
  Email: [email protected]


Authors' Addresses

  Tirumaleswar Reddy.K
  Nokia
  India
  Email: [email protected]


  Jani Ekman
  Nokia
  Finland
  Email: [email protected]


  Daniel Migault
  Ericsson
  Canada
  Email: [email protected]

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