Internet Engineering Task Force (IETF) T. Reddy.K
Request for Comments: 9509 J. Ekman
Category: Standards Track Nokia
ISSN: 2070-1721 D. Migault
Ericsson
March 2024
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, 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
https://www.rfc-editor.org/info/rfc9509.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction
2. Terminology
3. Extended Key Purpose for Network Functions
4. Including the Extended Key Purpose in Certificates
5. Implications for a Certification Authority
6. Security Considerations
7. Privacy Considerations
8. IANA Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. ASN.1 Module
Acknowledgments
Contributor
Authors' Addresses
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.
3. 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
[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
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.
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.
+=========+===============================+============+
| Decimal | Description | References |
+=========+===============================+============+
| 37 | id-kp-jwt | RFC 9509 |
+---------+-------------------------------+------------+
| 38 | id-kp-httpContentEncrypt | RFC 9509 |
+---------+-------------------------------+------------+
| 39 | id-kp-oauthAccessTokenSigning | RFC 9509 |
+---------+-------------------------------+------------+
Table 1
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.
+=========+===============+============+
| Decimal | Description | References |
+=========+===============+============+
| 108 | id-mod-nf-eku | RFC 9509 |
+---------+---------------+------------+
Table 2
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<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, May 2008,
<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, May
2015, <https://www.rfc-editor.org/info/rfc7515>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<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, May 2015,
<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,
May 2017, <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>.
9.2. Informative References
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC7299] Housley, R., "Object Identifier Registry for the PKIX
Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,
<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, August 2018,
<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,
December 2021, <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, December 2022,
<https://www.rfc-editor.org/info/rfc9336>.
[TS23.501] 3GPP, "System architecture for the 5G System (5GS)",
Release 18.4.0, 3GPP TS 23.501, December 2023,
<https://www.3gpp.org/ftp/Specs/
archive/23_series/23.501/23501-i40.zip>.
[TS29.500] 3GPP, "5G System; Technical Realization of Service Based
Architecture; Stage 3", Release 18.4.0, 3GPP TS 29.500,
December 2023, <https://www.3gpp.org/ftp/Specs/
archive/29_series/29.500/29500-i40.zip>.
[TS29.573] 3GPP, "5G System; Public Land Mobile Network (PLMN)
Interconnection; Stage 3", Release 18.5.0, 3GPP TS 29.573,
December 2023, <https://www.3gpp.org/ftp/Specs/
archive/29_series/29.573/29573-i50.zip>.
[TS33.210] 3GPP, "Network Domain Security (NDS); IP network layer
security", Release 17.1.0, 3GPP TS 33.210, September 2022,
<https://www.3gpp.org/ftp/Specs/
archive/33_series/33.210/33210-h10.zip>.
[TS33.310] 3GPP, "Network Domain Security (NDS); Authentication
Framework (AF)", Release 18.2.0, 3GPP TS 33.310, December
2023, <https://www.3gpp.org/ftp/Specs/
archive/33_series/33.310/33310-i20.zip>.
[TS33.501] 3GPP, "Security architecture and procedures for 5G
system", Release 18.4.0, 3GPP TS 33.501, December 2023,
<https://www.3gpp.org/ftp/Specs/
archive/33_series/33.501/33501-i40.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 (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:
German Peinado
Nokia
Email: german.peinado@nokia.com
Authors' Addresses
Tirumaleswar Reddy.K
Nokia
India
Email: kondtir@gmail.com
Jani Ekman
Nokia
Finland
Email: jani.ekman@nokia.com