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X.509 Certificate Extended Key Usage (EKU) for 5G Network Functions
draft-ietf-lamps-nf-eku-05

Document Type Active Internet-Draft (lamps WG)
Authors Tirumaleswar Reddy.K , Jani Ekman , Daniel Migault
Last updated 2024-01-22 (Latest revision 2023-09-22)
Replaces draft-reddy-lamps-jose-eku
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IANA IANA review state IANA OK - Actions Needed
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IANA expert review comments The expert has approved the SMI Security for PKIX Extended Key Purpose and the SMI Security for PKIX Module Identifier registrations.
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Details
draft-ietf-lamps-nf-eku-05
LAMPS WG                                                        T. Reddy
Internet-Draft                                                  J. Ekman
Intended status: Standards Track                                   Nokia
Expires: 25 March 2024                                        D. Migault
                                                                Ericsson
                                                       22 September 2023

  X.509 Certificate Extended Key Usage (EKU) for 5G Network Functions
                       draft-ietf-lamps-nf-eku-05

Abstract

   RFC 5280 specifies several extended key purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  This document defines
   encrypting JSON objects in HTTP messages, JSON Web Token (JWT) 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 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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   This Internet-Draft will expire on 25 March 2024.

Copyright Notice

   Copyright (c) 2023 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
   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Extended Key Purpose for Network Functions  . . . . . . . . .   4
   4.  Including the Extended Key Purpose in Certificates  . . . . .   5
   5.  Implications for a Certification Authority  . . . . . . . . .   6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   7
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     11.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     11.2.  Informative References . . . . . . . . . . . . . . . . .   8
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 5GC Service Based
      Architecture (see Section 6.1.3c of [TS33.310])

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

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

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   *  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]).

   [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 5G Core 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 which generates the public/private keys and
   applies for a certificate to the operator CA, could obtain a
   certificate which 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 is 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 5G System.  This document addresses the above problem
   by defining the Extended Key Usage (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.

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   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, id-kp-oauthAccessTokenSigning and uses these for
   respectively 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."

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   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
   the specification of corresponding KeyPurposeIds by the Extended Key
   Usage (EKU) extension.  If the certificate requester knows the
   certificate users are mandated to use these KeyPurposeIds, it MUST
   enforce their inclusion.  Additionally, such 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 Extended Key Usage (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 KeyPurposeId 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 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 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.

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        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 TBD1 }
   id-kp-httpContentEncrypt OBJECT IDENTIFIER ::= { id-kp TBD2 }
   id-kp-oauthAccessTokenSigning OBJECT IDENTIFIER ::= { id-kp TBD3 }

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 means
   to identify if the certificate is generated to sign the JWT Claims
   Set, signing the OAuth 2.0 access tokens using JWS or to encrypt 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
   KeyPurposeId include the presence of the anyExtendedKeyUsage
   KeyPurposeId or the complete absence of the EKU extension in a
   certificate.  Examples of Permitted KeyPurposeId include the presence
   of id-kp-jwt, id-kp-httpContentEncrypt or id-kp-
   oauthAccessTokenSigning KeyPurposeId.

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 EKU extension can help an
   attacker to monitor the Certificate Transparency logs [RFC9162] to
   identify the purpose of the certificate.

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8.  IANA Considerations

   IANA is 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 |
   +=========+===============================+============+
   | TBD1    | id-kp-jwt                     | This-RFC   |
   +---------+-------------------------------+------------+
   | TBD2    | id-kp-httpContentEncrypt      | This-RFC   |
   +---------+-------------------------------+------------+
   | TBD3    | id-kp-oauthAccessTokenSigning | This-RFC   |
   +---------+-------------------------------+------------+

                             Figure 1: Table 1

   IANA is also 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 |
   +=========+==========================+============+
   | TBD4    | id-mod-nf-eku            | This-RFC   |
   +---------+--------------------------+------------+

                             Figure 2: Table 2

9.  Contributors

   The following individuals have contributed to this document:

         German Peinado
         Nokia

         Email: german.peinado@nokia.com

10.  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.

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

11.  References

11.1.  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>.

11.2.  Informative References

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   [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] "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; System
              architecture for the 5G System (5GS); Stage 2 (Release
              18), 3GPP TS 23.501 V18.0.0 Dec 2022,",
              <https://www.3gpp.org/ftp/Specs/
              archive/23_series/23.501/23501-i00.zip>.

   [TS33.210] "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects;Network
              Domain Security (NDS); IP network layer security (Release
              17), 3GPP TS 33.210 V17.1.0 Sept 2022,",
              <https://www.3gpp.org/ftp/Specs/
              archive/33_series/33.210/33210-h10.zip>.

   [TS33.310] "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; Network
              Domain Security (NDS); Authentication Framework (AF)
              (Release 17), 3GPP 33.310 V17.4.0, Sept 2022,",
              <https://www.3gpp.org/ftp/Specs/
              archive/33_series/33.310/33310-h40.zip>.

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   [TS33.501] "3rd Generation Partnership Project; Technical
              Specification Group Services and System Aspects; Security
              architecture and procedures for 5G system (Release 17), ,
              3GPP TS:33.501 V17.7.0, Sept 2022,",
              <https://www.3gpp.org/ftp/Specs/
              archive/33_series/33.501/33501-h70.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 (TBD4) }

   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 TBD1 }
   id-kp-httpContentEncrypt OBJECT IDENTIFIER ::= { id-kp TBD2 }
   id-kp-oauthAccessTokenSigning OBJECT IDENTIFIER ::= { id-kp TBD3 }

   END
   <CODE ENDS>

Authors' Addresses

   Tirumaleswar Reddy
   Nokia
   India
   Email: kondtir@gmail.com

   Jani Ekman
   Nokia
   Finland
   Email: jani.ekman@nokia.com

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   Daniel Migault
   Ericsson
   Canada
   Email: daniel.migault@ericsson.com

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