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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:¶

• 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.¶

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.¶

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.¶

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:¶

Authors' Addresses