Nokia

India kondtir@gmail.com

Nokia

Finland jani.ekman@nokia.com

Ericsson

Canada daniel.migault@ericsson.com

LAMPS WG 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.

The Operators of 5G systems make use of an internal PKI to generate X.509 PKI certificates for the Network Functions (NFs) 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 ) Client Credentials Assertion (CCA) is JSON Web Tokens (JWT) and is secured with digital signatures based on JSON Web Signature (JWS) (see Section 13.3.8.2 of ). Certificates for encrypting JSON objects in HTTP messages between Security Edge Protection Proxies (SEPPs) using JSON Web Encryption (JWE) (Section 13.2.4.4 of ) and Section 6.3.2 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 ) specifies several extended key purpose identifiers (EKU), defined via KeyPurposeIds, for X.509 certificates. In addition, the IANA repository "SMI Security for PKIX Extended Key Purpose" 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). 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 ) 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 ) can be used to identify that the certificate is for a client (e.g., NF service consumer). However, there is currently no KeyPurposeIds for the other usages of certificates in 5G System. This document defines 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 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.

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.

This specification defines the KeyPurposeIds id-kp-jwt, id-kp-httpContentEncrypt, id-kp-oauthAccessTokenSigning 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 , "[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." Applications verifying the signature of a Client Credentials Assertion (CCA) represented as JWT, decrypting JSON objects in HTTP messages between Security Edge Protection Proxies (SEPPs) using JWE or verifying the signature of an OAuth 2.0 access tokens for service authorization to grant temporary access to resources provided by NF producers using JWS MAY require corresponding KeyPurposeIds be specified by the EKU extention. In addition, such application MUST require the keyUsage extension be set to digitalSignature or nonRepudiation (also designated as contentCommitment) for the signature calculation and/or to keyEncipherment for encryption of the secret key.

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:

As described in , the EKU extension may, at the option of the certificate issuer, be either critical or non-critical. 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 to be used for validating the the JWS Signature in JWT, encrypting JSON objects in HTTP messages (for example, encrypt the CEK with the recipient's public key using the RSAES-OAEP algorithm to produce the JWE Encrypted Key) and signing the OAuth 2.0 access tokens. The distinction between JWS and JWE is performed via the KU that is set to digitalSignature or nonRepudiation for JWS and keyEncipherment for JWE.

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.

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 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 . 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.

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 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 to identify the purpose of the certificate.

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.

Table 1

IANA is also requested to register 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 .

Table 2

The following individuals have contributed to this document:

We would like to thank Corey Bonnell, Ilari Liusvaara, Carl Wallace and Russ Housley for their useful feedback.

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 (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 ]]>