X.509 Certificate Extended Key Usage (EKU) for Javascript Object Signing and Encryption

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Table of Contents

1. Introduction

[RFC5280] specifies several extended key purpose identifiers (KeyPurposeIds) for X.509 certificates. In addition, the IANA repository "SMI Security for PKIX Extended Key Purpose" [RFC7299] includes a number of KeyPurposeIds. While usage of the anyExtendedKeyUsage KeyPurposeId is bad practice - especially but not only for for publicly trusted certificates (multi-purpose or single-purpose) - there are no extended key purpose identifiers explicitly assigned for JSON Web Signature (JWS) [RFC7515] JSON Web Encryption (JWE) [RFC7516] or their CBOR Object Signing and Encryption (COSE) [RFC9052] counterparts defined here as CBOR Object Web Signature (CWS) and CBOR Object Web Encryption (CWE). JSON Web Signature (JWS) (resp. CBOR Object Web Signature (CWS)) represents content secured with digital signatures or Message Authentication Codes (MACs) using JSON-based [RFC7159] data structures (resp. [RFC9052]). The JWS and CWS cryptographic mechanisms provide integrity protection for an arbitrary sequence of octets. JSON Web Encryption (JWE) (resp. CBOR Object Web Encryption (CWE)) represents encrypted content using JSON-based data structures. The JWE and CWE cryptographic mechanisms encrypt and provide integrity protection for an arbitrary sequence of octets. The cryptographic algorithms and identifiers to be used with the JSON Web Signature (JWS) and JSON Web Encryption (JWE) (resp. CWS, CWE) are defined in [RFC7518] (resp. [RFC9052]).¶

Network Functions (NFs) as part of the service-based architecture within the 5G System [TS23.501]. The Operators of 5G systems make use of an internal PKI to generate X.509 PKI certificates for the NFs in a 5G system. The certificates are used for the following purposes:¶

• Client and Server certificates for NFs in 5GC Service Based Architecture (Section 6.1.3c of [TS33.310])¶
• Certificates for signing Client Credentials Assertion (CCA) tokens using JWS (Section 13.3.8.2 of [TS33.501])¶
• Certificates for encrypting JSON objects in HTTP messages between Security Edge Protection Proxies (SEPPs) using JWE (Section 13.2.4.4 of [TS33.501]) and Section 6.3.2 of [TS33.210])¶
• Certificates for signing the OAuth 2.0 access tokens for service authorization to grant temporary access to resources provided by NF producers using JWS (Section 13.4.1 of [TS33.501])¶

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, violating the CA policies, and increasing the risk of cross-protocol attacks. Failure to ensure proper segregation of duties means that a NF who 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 impersonate NF service producers using its certificate. Another example, if the purpose of the certificate is for the NF service consumer is to use it as a client certificate, the NF with this client certificate and corresponding private key must not be allowed to sign the CCA. When a NF service producer receives the signed CCA from the NF service consumer, the NF would accept the token even if CCA is signed with a certificate not 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 is no KeyPurposeIds to identify whether the certificate can be used to generate JWS, JWE, CWS or CWE.¶

Vendor-defined KeyPurposeIds that are used in a PKI governed by the vendor or a group of vendors pose no interoperability concern. Appropriating, or misappropriating as the case may be, KeyPurposeIds for use outside of their originally intended vendor or group of vendors controlled environment can introduce problems, the impact of which is difficult to determine. Therefore, it is not favorable to use vendor-defined KeyPurposeIds for JWS, JWE, CWS and CWE in deployments that are not governed by the vendor.¶

This document defines extended key purpose identifiers for JWS, JWE, CWS and CWE.¶

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.¶

CBOR Object Web Signature (CWS): Signing Object of a CBOR Object Signing and Encryption (COSE) defined in [RFC9052] Section 4.¶

CBOR Object Web Encryption (CWE): Encryption Object of a CBOR Object Signing and Encryption (COSE) defined in [RFC9052] Section 4.¶

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 are inserted in each certificate that is issued. Unless certificates are governed by a vendor-specific PKI, certificates that indicate usage for JWS, JWE, CWS or CWE MAY include the id-kp-jws, id-kp-jwe, id-kp-cws or id-kp-cwe KeyPurposeId. The inclusion of the id-kp-jws, id-kp-jwe, id-kp-cws or id-kp-cwe 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 process JWS or CWS signature or toencrypt the CEK in JWE or CWE.¶

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 JWS, JWE, CWS or CWE 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 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). This OID is defined in Section 4.¶

             +=========+=======================+============+
             | Decimal | Description           | References |
             +=========+=======================+============+
             | TBD1    | id-kp-jws             | This-RFC   |
             +---------+-----------------------+------------+
             | TBD2    | id-kp-jwe             | This-RFC   |
             +---------+-----------------------+------------+
             | TBD3    | id-kp-cws             | This-RFC   |
             +---------+-----------------------+------------+
             | TBD4    | id-kp-cwe             | This-RFC   |
             +---------+-----------------------+------------+

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 |
               +=========+==========================+============+
               | TBD5    | id-mod-jose-and-cose-eku | This-RFC   |
               +---------+--------------------------+------------+

9. Contributors

The following individuals have contributed to this document:¶

      German Peinado
      Nokia

      Email: german.peinado@nokia.com

10. Acknowledgments

TODO¶

Appendix A. ASN.1 Module

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

<CODE BEGINS>

JOSE-EKU
  { iso(1) identified-organization(3) dod(6) internet(1)
  security(5) mechanisms(5) pkix(7) id-mod(0)
  id-mod-jose-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-jws OBJECT IDENTIFIER ::= { id-kp TBD1 }
id-kp-jwe OBJECT IDENTIFIER ::= { id-kp TBD2 }
id-kp-cws OBJECT IDENTIFIER ::= { id-kp TBD3 }
id-kp-cwe OBJECT IDENTIFIER ::= { id-kp TBD4 }

END


<CODE ENDS>

Authors' Addresses