Nokia

India kondtir@gmail.com

Nokia

Finland jani.ekman@nokia.com

Ericsson

Canada daniel.migault@ericsson.com

sec lamps 3GPP 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 .

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Table of Contents

• .  Introduction
• .  Terminology
• .  Extended Key Purpose for Network Functions
• .  Including the Extended Key Purpose in Certificates
• .  Implications for a Certification Authority
• .  Security Considerations
• .  Privacy Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• .  ASN.1 Module
• Acknowledgments
• Contributor
• Authors' Addresses

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 ) 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 and Section 6.7.2 of )
• Client Credentials Assertion (CCA) uses JSON Web Tokens (JWTs) and is secured with digital signatures based on the JSON Web Signature (JWS) (see Section 13.3.8.2 of , and Section 6.7.5 of ).
• Certificates for encrypting JSON objects in HTTP messages between Security Edge Protection Proxies (SEPPs) using JSON Web Encryption (JWE) (see Section 13.2.4.4 of , Section 6.3.2 of , Section 6.7.4 of , and Section 5.3.2.1 of ).
• 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 and Section 6.7.3 of ).

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" contains additional KeyPurposeIds. The use of the anyExtendedKeyUsage KeyPurposeId, as defined in , 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 () can be used to identify that the certificate is for a server (e.g., NF service producer), and the KeyPurposeId id-kp-clientAuth () 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.

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 when, and only when, they appear in all capitals, as shown here.

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 , "[i]f the [Extended Key Usage] extension is present, then the certificate MUST only be used for one of the purposes indicated." 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.

Including the Extended Key Purpose in Certificates 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 , 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 }

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.

Security Considerations The Security Considerations of 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 . 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.

Privacy Considerations In some security protocols, such as TLS 1.2 , certificates are exchanged in the clear. In other security protocols, such as TLS 1.3 , 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 to identify the purpose of the certificate.

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 .

Decimal	Description	References
37	id-kp-jwt	RFC 9509
38	id-kp-httpContentEncrypt	RFC 9509
39	id-kp-oauthAccessTokenSigning	RFC 9509

IANA has registered the following ASN.1 module OID in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0). This OID is defined in .

Decimal	Description	References
108	id-mod-nf-eku	RFC 9509

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] JSON Web Signature (JWS) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and an IANA registry defined by that specification. Related encryption capabilities are described in the separate JSON Web Encryption (JWE) specification. JSON Web Encryption (JWE) represents encrypted content using JSON-based data structures. Cryptographic algorithms and identifiers for use with this specification are described in the separate JSON Web Algorithms (JWA) specification and IANA registries defined by that specification. Related digital signature and Message Authentication Code (MAC) capabilities are described in the separate JSON Web Signature (JWS) specification. JSON Web Token (JWT) is a compact, URL-safe means of representing claims to be transferred between two parties. The claims in a JWT are encoded as a JSON object that is used as the payload of a JSON Web Signature (JWS) structure or as the plaintext of a JSON Web Encryption (JWE) structure, enabling the claims to be digitally signed or integrity protected with a Message Authentication Code (MAC) and/or encrypted. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. ITU-T ITU-T Informative References This document specifies Version 1.2 of the Transport Layer Security (TLS) protocol. The TLS protocol provides communications security over the Internet. The protocol allows client/server applications to communicate in a way that is designed to prevent eavesdropping, tampering, or message forgery. [STANDARDS-TRACK] When the Public-Key Infrastructure using X.509 (PKIX) Working Group was chartered, an object identifier arc was allocated by IANA for use by that working group. This document describes the object identifiers that were assigned in that arc, returns control of that arc to IANA, and establishes IANA allocation policies for any future assignments within that arc. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. RFC 5280 specifies several extended key purpose identifiers (KeyPurposeIds) for X.509 certificates. This document defines a general-purpose Document-Signing KeyPurposeId for inclusion in the Extended Key Usage (EKU) extension of X.509 public key certificates. Document-Signing applications may require that the EKU extension be present and that a Document-Signing KeyPurposeId be indicated in order for the certificate to be acceptable to that Document-Signing application. 3GPP 3GPP 3GPP 3GPP 3GPP 3GPP

ASN.1 Module The following module adheres to ASN.1 specifications and . 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

Acknowledgments We would like to thank , , , and for their useful feedback. Thanks to for the secdir review, for the genart review, and for the intdir review. Thanks to , , and for the IESG review.

Contributor The following individual has contributed to this document: Nokia

german.peinado@nokia.com

Authors' Addresses Nokia

India kondtir@gmail.com

Nokia

Finland jani.ekman@nokia.com

Ericsson

Canada daniel.migault@ericsson.com