Internet-Draft EKUs for NFs September 2023
Reddy, et al. Expires 23 March 2024 [Page]
Workgroup:
LAMPS WG
Internet-Draft:
draft-ietf-lamps-nf-eku-04
Published:
Intended Status:
Standards Track
Expires:
Authors:
T. Reddy
Nokia
J. Ekman
Nokia
D. Migault
Ericsson

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, 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 working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on 23 March 2024.

Table of Contents

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:

[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.

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."

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.
     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 of using Excluded KeyPurposeId and Permitted KeyPurposeId by an relying party to permit or prohibit combinations of KeyPurposeIds 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.

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.

Thanks to Paul Wouters, and Éric 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, , <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, , <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, , <https://www.rfc-editor.org/info/rfc7515>.
[RFC7516]
Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)", RFC 7516, DOI 10.17487/RFC7516, , <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, , <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, , <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

[RFC5246]
Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/RFC5246, , <https://www.rfc-editor.org/info/rfc5246>.
[RFC7299]
Housley, R., "Object Identifier Registry for the PKIX Working Group", RFC 7299, DOI 10.17487/RFC7299, , <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, , <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, , <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, , <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>.
[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
Jani Ekman
Nokia
Finland
Daniel Migault
Ericsson
Canada