]> Vigil Security, LLC

Herndon, VA US housley@vigilsec.com

sn3rd

Washington, DC US sean@sn3rd.com

Ericsson

Kista Sweden john.mattsson@ericsson.com

Ericsson

Saint Laurent, QC Canada daniel.migault@ericsson.com

Security Internet-Draft This document specifies the certificate extension for including Network Function Types (NFTypes) for the 5G System in X.509v3 public key certificates as profiled in RFC 5280.

Introduction The 3rd Generation Partnership Project (3GPP) has specified several Network Functions (NFs) as part of the service-based architecture within the 5G System. There are 49 NF Types defined in for 3GPP Release 17; they are listed in Table 6.1.6.3.3-1 of , and each NF type is identified by a short ASCII string. Operators of 5G systems make use of an internal PKI to identify interface instances in the NFs in a 5G system. X.509v3 public key certificates are used, and the primary function of a certificate is to bind a public key to the identity of an entity that holds the corresponding private key, known as the certificate subject. The certificate subject and the subjectAltName certificate extension can be used to support identity-based access control decisions. This document specifies the NFTypes certificate extension to support role-based access control decisions by providing a list of NF Types associated with the certificate subject. The NFTypes certificate extension can be used by operators of 5G systems or later.

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.

Network Functions Certificate Extension This section specifies the NFTypes certificate extension, which provides a list of NF Types associated with the certificate subject. The NFTypes certificate extension MAY be included in public key certificates . The NFTypes certificate extension MUST be identified by the following object identifier: This extension MUST NOT be marked critical. The NFTypes extension MUST have the following syntax: The NFTypes MUST contain at least one NFType. The NFTypes MUST NOT contain the same NFType more than once. Each NFType MUST contain only an ASCII string; however, the string MUST NOT include control characters (values 0 through 31), the space character (value 32), or the delete character (value 127). Each NFType MUST contain at least one ASCII character and MUST NOT contain more than 32 ASCII characters. The NFType uses the IA5String type to permit inclusion of the underscore character ('_'), which is not part of the PrintableString character set.

ASN.1 Module This section provides an ASN.1 module for the NFTypes certificate extension, and it follows the conventions established in and .

Security Considerations The Security Considerations of are applicable to this document. Some of the ASCII strings that specify the NF Types are standard. See Appendix A for values defined in 3GPP. Additionally, an operator MAY assign its own NF Types for use in their own network. Since the NF Type is used for role-based access control decisions, an operator-assigned NF Type MUST NOT overlap with a value already defined in the commonly defined set. Use of the same ASCII string by two different operators for different roles could lead to confusion or incorrect access control decisions. The mechanism for an operator to determine whether an ASCII string associated with a NF Type is unique across operators is outside the scope of this document. The certificate extension supports many different forms of role-based access control to support the diversity of activities that NFs are trusted to perform in the overall system. Different levels of confidence that the NFTypes are proper assigned might be needed to contribute to the overall security of the 5G system. For example, more confidence might be needed to make access control decisions related to a scarce resource than implementation of filtering policies. As a result, different operators might have different trust models for NFTypes certificate extension.

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 NFType certificate extension can help an observer determine which systems are of most interest based on the plaintext certificate transmission.

IANA Considerations For the NFType certificate extension in , IANA is requested to assign an object identifier (OID) for the certificate extension. The OID for the certificate extension should be allocated in the "SMI Security for PKIX Certificate Extension" registry (1.3.6.1.5.5.7.1). For the ASN.1 Module in , IANA is requested to assign an object identifier (OID) for the module identifier. The OID for the module should be allocated in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

Acknowledgements Many thanks to Ben Smeets, Michael Li, Tim Hollebeek, and Roman Danyliw for their review, comments, and assistance.

References Normative References 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] ITU-T 3rd Generation Partnership Project 3rd Generation Partnership Project 3rd Generation Partnership Project 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. 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. 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] The Public Key Infrastructure using X.509 (PKIX) certificate format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates those ASN.1 modules to conform to the 2002 version of ASN.1. There are no bits-on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. The Cryptographic Message Syntax (CMS) format, and many associated formats, are expressed using ASN.1. The current ASN.1 modules conform to the 1988 version of ASN.1. This document updates some auxiliary ASN.1 modules to conform to the 2008 version of ASN.1; the 1988 ASN.1 modules remain the normative version. There are no bits- on-the-wire changes to any of the formats; this is simply a change to the syntax. This document is not an Internet Standards Track specification; it is published for informational purposes. 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. 3rd Generation Partnership Project

NFType Strings Table 6.1.6.3.3-1 of defines the ASCII strings for the NF Types specified in 3GPP documents, which are listed below in alphabetical order. This list is not exhaustive.

Example Certificate Containing a NFTypes Extension The example certificate conformes to certificate profile in Table 6.1.3c.3-1 of . In addition, the NFTypes certificate is included with only one NFType, and it is "AMF". The SubjectAltName certificate extension contains a fully qualified domain names (FQDN) and a uniformResourceIdentifier, which carries the NF Instance ID as specified in Clause 5.3.2 of . The following shows the example certificate. The values on the left are the ASN.1 tag (in hexadecimal) and the length (in decimal).