]> Vigil Security, LLC

Herndon VA United States of America housley@vigilsec.com

sn3rd

Washington DC United States of America sean@sn3rd.com

Ericsson

Kista Sweden john.mattsson@ericsson.com

Ericsson

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

Security lamps Digital Certificate This document specifies the certificate extension for including Network Function Types (NFTypes) for the 5G System in X.509 v3 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 56 NF Types defined 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.509 v3 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 Function Types 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. 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 NFTypes MUST NOT contain the same NFType more than once. If the NFTypes contain more than one NFType, the NFTypes MUST appear in ascending lexicographic order using the ASCII values. 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 for values defined in 3GPP Release 17. 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 were properly 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 the 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 the NFTypes certificate extension can help an observer determine which systems are of most interest based on the plaintext certificate transmission.

IANA Considerations For the NFTypes certificate extension defined in , IANA has assigned an object identifier (OID) for the certificate extension. The OID for the certificate extension has been allocated in the "SMI Security for PKIX Certificate Extension" registry (1.3.6.1.5.5.7.1). For the ASN.1 Module defined in , IANA has assigned an OID for the module identifier. The OID for the module has been allocated in the "SMI Security for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

References Normative References ITU-T ITU-T Recommendation X.680 ISO/IEC 8824-1:2021 3rd Generation Partnership Project 3GPP TS:29.510 V17.8.0 3rd Generation Partnership Project 3GPP TS:33.310 V17.5.0 Informative References 3rd Generation Partnership Project 3GPP TS:29.571 V17.8.0

NFType Strings Table 6.1.6.3.3-1 of defines the ASCII strings for the NF Types specified in 3GPP documents; these enumeration values in 3GPP Release 17 are listed below in ascending lexicographic order. This list is not exhaustive.

Example Certificate Containing a NFTypes Extension The example certificate conforms to the 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 name (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).

Acknowledgements Many thanks to , , , , , and for their review, comments, and assistance.