



Internet Engineering Task Force (IETF)                        R. Housley
Request for Comments: 9310                                Vigil Security
Category: Standards Track                                      S. Turner
ISSN: 2070-1721                                                    sn3rd
                                                       J. Preuß Mattsson
                                                              D. Migault
                                                                Ericsson
                                                            January 2023


       X.509 Certificate Extension for 5G Network Function Types

Abstract

   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.

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/rfc9310.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Network Function Types Certificate Extension
   4.  ASN.1 Module
   5.  Security Considerations
   6.  Privacy Considerations
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  NFType Strings
   Appendix B.  Example Certificate Containing a NFTypes Extension
   Acknowledgements
   Authors' Addresses

1.  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 [TS29.510], 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 [RFC5280] 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.

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.  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 [RFC5280].  The NFTypes certificate extension MUST be
   identified by the following object identifier:

     id-pe-nftype  OBJECT IDENTIFIER ::=
        { iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanisms(5) pkix(7) id-pe(1) 34 }

   This extension MUST NOT be marked critical.

   The NFTypes extension MUST have the following syntax:

     NFTypes ::= SEQUENCE SIZE (1..MAX) OF NFType

     NFType ::= IA5String (SIZE (1..32))

   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.

4.  ASN.1 Module

   This section provides an ASN.1 Module [X.680] for the NFTypes
   certificate extension, and it follows the conventions established in
   [RFC5912] and [RFC6268].

   <CODE BEGINS>
     NFTypeCertExtn
       { iso(1) identified-organization(3) dod(6) internet(1)
         security(5) mechanisms(5) pkix(7) id-mod(0)
         id-mod-nftype(106) }

     DEFINITIONS IMPLICIT TAGS ::=
     BEGIN

     IMPORTS
       EXTENSION
       FROM PKIX-CommonTypes-2009  -- RFC 5912
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) id-mod(0)
           id-mod-pkixCommon-02(57) } ;


     -- NFTypes Certificate Extension

     ext-NFType EXTENSION ::= {
       SYNTAX NFTypes
       IDENTIFIED BY id-pe-nftype }

     -- NFTypes Certificate Extension OID

     id-pe-nftype  OBJECT IDENTIFIER ::=
        { iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanisms(5) pkix(7) id-pe(1) 34 }

     -- NFTypes Certificate Extension Syntax

     NFTypes ::= SEQUENCE SIZE (1..MAX) OF NFType

     NFType ::= IA5String (SIZE (1..32))

     END
   <CODE ENDS>

5.  Security Considerations

   The security considerations of [RFC5280] 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 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.

6.  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
   NFTypes certificate extension can help an observer determine which
   systems are of most interest based on the plaintext certificate
   transmission.

7.  IANA Considerations

   For the NFTypes certificate extension defined in Section 3, 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 Section 4, 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).

8.  References

8.1.  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>.

   [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>.

   [TS29.510] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; 5G System;
              Network Function Repository Services; Stage 3 (Release
              17)", 3GPP TS:29.510 V17.8.0, December 2022,
              <https://www.3gpp.org/ftp/Specs/
              archive/29_series/29.510/29510-h80.zip>.

   [TS33.310] 3rd Generation Partnership Project, "Technical
              Specification Group Services and System Aspects; Network
              Domain Security (NDS); Authentication Framework (AF)
              (Release 17)", 3GPP TS:33.310 V17.5.0, December 2022,
              <https://www.3gpp.org/ftp/Specs/
              archive/33_series/33.310/33310-h50.zip>.

   [X.680]    ITU-T, "Information technology -- Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
              <https://www.itu.int/rec/T-REC-X.680>.

8.2.  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>.

   [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
              Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
              DOI 10.17487/RFC5912, June 2010,
              <https://www.rfc-editor.org/info/rfc5912>.

   [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
              for the Cryptographic Message Syntax (CMS) and the Public
              Key Infrastructure Using X.509 (PKIX)", RFC 6268,
              DOI 10.17487/RFC6268, July 2011,
              <https://www.rfc-editor.org/info/rfc6268>.

   [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>.

   [TS29.571] 3rd Generation Partnership Project, "Technical
              Specification Group Core Network and Terminals; 5G System;
              Common Data Types for Service Based Interfaces; Stage 3
              (Release 17)", 3GPP TS:29.571 V17.8.0, December 2022,
              <https://www.3gpp.org/ftp/Specs/
              archive/29_series/29.571/29571-h80.zip>.

Appendix A.  NFType Strings

   Table 6.1.6.3.3-1 of [TS29.510] 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.

       "5G_DDNMF"        "LMF"             "PKMF"
       "5G_EIR"          "MBSF"            "SCEF"
       "AANF"            "MBSTF"           "SCP"
       "ADRF"            "MB_SMF"          "SCSAS"
       "AF"              "MB_UPF"          "SCSCF"
       "AMF"             "MFAF"            "SEPP"
       "AUSF"            "MME"             "SMF"
       "BSF"             "MNPF"            "SMSF"
       "CBCF"            "N3IWF"           "SMS_GMSC"
       "CEF"             "NEF"             "SMS_IWMSC"
       "CHF"             "NRF"             "SOR_AF"
       "DCCF"            "NSACF"           "SPAF"
       "DRA"             "NSSAAF"          "TSCTSF"
       "EASDF"           "NSSF"            "UCMF"
       "GBA_BSF"         "NSWOF"           "UDM"
       "GMLC"            "NWDAF"           "UDR"
       "HSS"             "PANF"            "UDSF"
       "ICSCF"           "PCF"             "UPF"
       "IMS_AS"          "PCSCF"

Appendix B.  Example Certificate Containing a NFTypes Extension

   The example certificate conforms to the certificate profile in
   Table 6.1.3c.3-1 of [TS33.310].  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 [TS29.571].

   -----BEGIN CERTIFICATE-----
   MIIC0DCCAlagAwIBAgIUDD5o44zEdfSghT2hMK+P/EjGHlowCgYIKoZIzj0EAwMw
   FTETMBEGA1UECgwKRXhhbXBsZSBDQTAeFw0yMjExMjkxODE0NThaFw0yMzExMjkx
   ODE0NThaMDkxCzAJBgNVBAYTAlVTMSowKAYDVQQKEyE1Z2MubW5jNDAwLm1jYzMx
   MS4zZ3BwbmV0d29yay5vcmcwdjAQBgcqhkjOPQIBBgUrgQQAIgNiAATJ6IFHI683
   q/JJjsJUfEiRFqGQ6uKDGJ0oqDP6wEhRAuvyEyz5pgRmz/7Mze1+s1qcnPU9mo1v
   rIW9rjKhb/Hm8H9TPvnMQwCRCtKvCD90MkWvc/G8qyCBpCms3zNOJOijggFBMIIB
   PTATBggrBgEFBQcBIgQHMAUWA0FNRjAXBgNVHSAEEDAOMAwGCmCGSAFlAwIBMDAw
   DgYDVR0PAQH/BAQDAgeAMBMGA1UdJQQMMAoGCCsGAQUFBwMCMB0GA1UdDgQWBBRM
   Z5KgwYlYn885mKID55ZcEznIBzAfBgNVHSMEGDAWgBSIf6IE6QtqjXR2+p/xCtRh
   4PqzNTAxBgNVHR8EKjAoMCagJKAihiBodHRwOi8vZXhhbXBsZS5jb20vZXhhbXBs
   ZWNhLmNybDB1BgNVHREBAf8EazBpgjhhbWYxLmNsdXN0ZXIxLm5ldDIuYW1mLjVn
   Yy5tbmM0MDAubWNjMzExLjNncHBuZXR3b3JrLm9yZ4YtdXJuOnV1aWQ6ZjgxZDRm
   YWUtN2RlYy0xMWQwLWE3NjUtMDBhMGM5MWU2YmY2MAoGCCqGSM49BAMDA2gAMGUC
   MEtQEut9kelkiMIMR+QzkSNGIuR30Lr23ftarLi9wMp3ZRIJYQgaAWc6gmf3MVAp
   7QIxAKMoYAtw5srkNjE+Zg6CqEkf9f2banFltRuPbTp4B0Xraz5z/jn3NDPM9ata
   SHUxOQ==
   -----END CERTIFICATE-----

   The following shows the example certificate.  The values on the left
   are the ASN.1 tag (in hexadecimal) and the length (in decimal).

   30 720: SEQUENCE {
   30 598:  SEQUENCE {
   A0   3:   [0] {
   02   1:    INTEGER 2
         :     }
   02  20:   INTEGER
         :    0C 3E 68 E3 8C C4 75 F4 A0 85 3D A1 30 AF 8F FC
         :    48 C6 1E 5A
   30  10:   SEQUENCE {
   06   8:    OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
         :     }
   30  21:   SEQUENCE {
   31  19:    SET {
   30  17:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER organizationName (2 5 4 10)
   0C  10:      UTF8String 'Example CA'
         :       }
         :      }
         :     }
   30  30:   SEQUENCE {
   17  13:    UTCTime 29/11/2022 18:14:58 GMT
   17  13:    UTCTime 29/11/2023 18:14:58 GMT
         :     }
   30  57:   SEQUENCE {
   31  11:    SET {
   30   9:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER countryName (2 5 4 6)
   13   2:      PrintableString 'US'
         :       }
         :      }
   31  42:    SET {
   30  40:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER organizationName (2 5 4 10)
   13  33:      PrintableString '5gc.mnc400.mcc311.3gppnetwork.org'
         :       }
         :      }
         :     }
   30 118:   SEQUENCE {
   30  16:    SEQUENCE {
   06   7:     OBJECT IDENTIFIER ecPublicKey (1 2 840 10045 2 1)
   06   5:     OBJECT IDENTIFIER secp384r1 (1 3 132 0 34)
         :      }
   03  98:    BIT STRING
         :     04 C9 E8 81 47 23 AF 37 AB F2 49 8E C2 54 7C 48
         :     91 16 A1 90 EA E2 83 18 9D 28 A8 33 FA C0 48 51
         :     02 EB F2 13 2C F9 A6 04 66 CF FE CC CD ED 7E B3
         :     5A 9C 9C F5 3D 9A 8D 6F AC 85 BD AE 32 A1 6F F1
         :     E6 F0 7F 53 3E F9 CC 43 00 91 0A D2 AF 08 3F 74
         :     32 45 AF 73 F1 BC AB 20 81 A4 29 AC DF 33 4E 24
         :     E8
         :     }
   A3 321:   [3] {
   30 317:    SEQUENCE {
   30  19:     SEQUENCE {
   06   8:      OBJECT IDENTIFIER nfTypes (1 3 6 1 5 5 7 1 34)
   04   7:      OCTET STRING, encapsulates {
   30   5:       SEQUENCE {
   16   3:        IA5String 'AMF'
         :         }
         :        }
         :       }
   30  23:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER certificatePolicies (2 5 29 32)
   04  16:      OCTET STRING, encapsulates {
   30  14:       SEQUENCE {
   30  12:        SEQUENCE {
   06  10:         OBJECT IDENTIFIER '2 16 840 1 101 3 2 1 48 48'
         :          }
         :         }
         :        }
         :       }
   30  14:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER keyUsage (2 5 29 15)
   01   1:      BOOLEAN TRUE
   04   4:      OCTET STRING, encapsulates {
   03   2:       BIT STRING 7 unused bits
         :        '1'B (bit 0)
         :        }
         :       }
   30  19:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER extKeyUsage (2 5 29 37)
   04  12:      OCTET STRING, encapsulates {
   30  10:       SEQUENCE {
   06   8:        OBJECT IDENTIFIER clientAuth (1 3 6 1 5 5 7 3 2)
         :         }
         :        }
         :       }
   30  29:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER subjectKeyIdentifier (2 5 29 14)
   04  22:      OCTET STRING, encapsulates {
   04  20:       OCTET STRING
         :        4C 67 92 A0 C1 89 58 9F CF 39 98 A2 03 E7 96 5C
         :        13 39 C8 07
         :        }
         :       }
   30  31:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER authorityKeyIdentifier (2 5 29 35)
   04  24:      OCTET STRING, encapsulates {
   30  22:       SEQUENCE {
   80  20:        [0]
         :        88 7F A2 04 E9 0B 6A 8D 74 76 FA 9F F1 0A D4 61
         :        E0 FA B3 35
         :         }
         :        }
         :       }
   30  49:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER cRLDistributionPoints (2 5 29 31)
   04  42:      OCTET STRING, encapsulates {
   30  40:       SEQUENCE {
   30  38:        SEQUENCE {
   A0  36:         [0] {
   A0  34:          [0] {
   86  32:           [6] 'http://example.com/exampleca.crl'
         :            }
         :           }
         :          }
         :         }
         :        }
         :       }
   30 117:     SEQUENCE {
   06   3:      OBJECT IDENTIFIER subjectAltName (2 5 29 17)
   01   1:      BOOLEAN TRUE
   04 107:      OCTET STRING, encapsulates {
   30 105:       SEQUENCE {
   82  56:        [2]
         :         'amf1.cluster1.net2.amf.5gc.mnc400.mcc311.3gppnet'
         :         'work.org'
   86  45:        [6]
         :         'urn:uuid:f81d4fae-7dec-11d0-a765-00a0c91e6bf6'
         :         }
         :        }
         :       }
         :      }
         :     }
         :    }
   30  10:  SEQUENCE {
   06   8:   OBJECT IDENTIFIER ecdsaWithSHA384 (1 2 840 10045 4 3 3)
         :    }
   03 104:  BIT STRING, encapsulates {
   30 101:   SEQUENCE {
   02  48:    INTEGER
         :     4B 50 12 EB 7D 91 E9 64 88 C2 0C 47 E4 33 91 23
         :     46 22 E4 77 D0 BA F6 DD FB 5A AC B8 BD C0 CA 77
         :     65 12 09 61 08 1A 01 67 3A 82 67 F7 31 50 29 ED
   02  49:    INTEGER
         :     00 A3 28 60 0B 70 E6 CA E4 36 31 3E 66 0E 82 A8
         :     49 1F F5 FD 9B 6A 71 65 B5 1B 8F 6D 3A 78 07 45
         :     EB 6B 3E 73 FE 39 F7 34 33 CC F5 AB 5A 48 75 31
         :     39
         :     }
         :    }
         :   }

Acknowledgements

   Many thanks to Ben Smeets, Michael Li, Tim Hollebeek, Roman Danyliw,
   Bernie Volz, and Éric Vyncke for their review, comments, and
   assistance.

Authors' Addresses

   Russ Housley
   Vigil Security, LLC
   Herndon, VA
   United States of America
   Email: housley@vigilsec.com


   Sean Turner
   sn3rd
   Washington, DC
   United States of America
   Email: sean@sn3rd.com


   John Preuß Mattsson
   Ericsson
   Kista
   Sweden
   Email: john.mattsson@ericsson.com


   Daniel Migault
   Ericsson
   Saint Laurent, QC
   Canada
   Email: daniel.migault@ericsson.com
