Secure Telephone Identity Credentials: Certificates
RFC 8226

Internet Engineering Task Force (IETF)                       J. Peterson
Request for Comments: 8226                                       Neustar
Category: Standards Track                                      S. Turner
ISSN: 2070-1721                                                    sn3rd
                                                           February 2018

          Secure Telephone Identity Credentials: Certificates

Abstract

   In order to prevent the impersonation of telephone numbers on the
   Internet, some kind of credential system needs to exist that
   cryptographically asserts authority over telephone numbers.  This
   document describes the use of certificates in establishing authority
   over telephone numbers, as a component of a broader architecture for
   managing telephone numbers as identities in protocols like SIP.

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/rfc8226.

Copyright Notice

   Copyright (c) 2018 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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1. Introduction ....................................................2
   2. Terminology .....................................................3
   3. Authority for Telephone Numbers in Certificates .................4
   4. Certificate Usage with STIR .....................................5
   5. Enrollment and Authorization Using the TN Authorization List ....6
      5.1. Constraints on Signing PASSporTs ...........................8
      5.2. Certificate Extension Scope and Structure ..................8
   6. Provisioning Private Keying Material ............................9
   7. Acquiring Credentials to Verify Signatures ......................9
   8. JWT Claim Constraints Syntax ...................................10
   9. TN Authorization List Syntax ...................................12
   10. Certificate Freshness and Revocation ..........................14
      10.1. Acquiring the TN List by Reference .......................15
   11. IANA Considerations ...........................................16
      11.1. ASN.1 Registrations ......................................16
      11.2. Media Type Registrations .................................16
   12. Security Considerations .......................................17
   13. References ....................................................18
      13.1. Normative References .....................................18
      13.2. Informative References ...................................20
   Appendix A. ASN.1 Module ..........................................21
   Acknowledgments ...................................................24
   Authors' Addresses ................................................24

1.  Introduction

   The Secure Telephone Identity Revisited (STIR) problem statement
   [RFC7340] identifies the primary enabler of robocalling, vishing
   (voicemail hacking), swatting, and related attacks as the capability
   to impersonate a calling party number.  The starkest examples of
   these attacks are cases where automated callees on the Public
   Switched Telephone Network (PSTN) rely on the calling number as a
   security measure -- for example, to access a voicemail system.
   Robocallers use impersonation as a means of obscuring identity.
   While robocallers can, in the ordinary PSTN, block (that is,
   withhold) their caller identity, callees are less likely to pick up
   calls from blocked identities; therefore, appearing to call from some
   number, any number, is preferable.  Robocallers, however, prefer not
   to call from a number that can trace back to the robocaller, and
   therefore they impersonate numbers that are not assigned to them.

   One of the most important components of a system to prevent
   impersonation is the implementation of credentials that identify the
   parties who control telephone numbers.  With these credentials,
   parties can assert that they are in fact authorized to use telephony
   numbers (TNs), and thus they distinguish themselves from

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   impersonators unable to present such credentials.  For that reason,
   the STIR threat model [RFC7375] stipulates that "The design of the
   credential system envisioned as a solution to these threats must, for
   example, limit the scope of the credentials issued to carriers or
   national authorities to those numbers that fall under their purview."
   This document describes credential systems for telephone numbers
   based on [X.509] version 3 certificates in accordance with [RFC5280].
   While telephone numbers have long been part of the X.509 standard
   (X.509 supports arbitrary naming attributes to be included in a
   certificate; the telephoneNumber attribute was defined in the 1988
   [X.520] specification), this document provides ways to determine
   authority more aligned with telephone network requirements, including
   extending X.509 with a Telephony Number Authorization List
   certificate extension, which binds certificates to asserted authority
   for particular telephone numbers or, potentially, telephone number
   blocks or ranges.

   In the STIR in-band architecture specified in [RFC8224], two basic
   types of entities need access to these credentials: authentication
   services and verification services (or verifiers).  An authentication
   service must be operated by an entity enrolled with the certification
   authority (CA) (see Section 5), whereas a verifier need only trust
   the trust anchor of the authority and also have a means to access and
   validate the public keys associated with these certificates.
   Although the guidance in this document is written with the STIR
   in-band architecture in mind, the credential system described in this
   document could be useful for other protocols that want to make use of
   certificates to assert authority over telephone numbers on the
   Internet.

   This document specifies only the credential syntax and semantics
   necessary to support this architecture.  It does not assume any
   particular CA or deployment environment.  We anticipate that some
   deployment experience will be necessary to determine optimal
   operational models.

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.

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3.  Authority for Telephone Numbers in Certificates

   At a high level, this specification details two non-exclusive
   approaches that can be employed to determine authority over telephone
   numbers with certificates.

   The first approach is to leverage the existing subject of the
   certificate to ascertain that the holder of the certificate is
   authorized to claim authority over a telephone number.  The subject
   might be represented as a domain name in the subjectAltName, such as
   an "example.net" where that domain is known to relying parties as a
   carrier, or represented with other identifiers related to the
   operation of the telephone network, including Service Provider Codes
   (SPCs) such as Operating Company Numbers (OCNs) or Service Provider
   Identifiers (SPIDs) via the TN Authorization List specified in this
   document.  A relying party could then employ an external data set or
   service that determines whether or not a specific telephone number is
   under the authority of the carrier identified as the subject of the
   certificate and use that to ascertain whether or not the carrier
   should have authority over a telephone number.  Potentially, a
   certificate extension to convey the URI of such an information
   service trusted by the issuer of the certificate could be developed
   (though this specification does not propose one).  Alternatively,
   some relying parties could form bilateral or multilateral trust
   relationships with peer carriers, trusting one another's assertions
   just as telephone carriers in the Signaling System 7 (SS7) network
   today rely on transitive trust when displaying the calling party
   telephone number received through SS7 signaling.

   The second approach is to extend the syntax of certificates to
   include a new attribute, defined here as the TN Authorization List,
   which contains a list of telephone numbers defining the scope of
   authority of the certificate.  Relying parties, if they trust the
   issuer of the certificate as a source of authoritative information on
   telephone numbers, could therefore use the TN Authorization List
   instead of the subject of the certificate to make a decision about
   whether or not the signer has authority over a particular telephone
   number.  The TN Authorization List could be provided in one of two
   ways: as a literal value in the certificate or as a network service
   that allows relying parties to query in real time to determine that a
   telephone number is in the scope of a certificate.  Using the TN
   Authorization List rather than the certificate subject makes sense
   when, for example, for privacy reasons the certificate owner would
   prefer not to be identified, or in cases where the holder of the
   certificate does not participate in the sort of traditional carrier
   infrastructure that the first approach assumes.

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   The first approach requires little change to existing Public Key
   Infrastructure (PKI) certificates; for the second approach, we must
   define an appropriate enrollment and authorization process.  For the
   purposes of STIR, the over-the-wire format specified in [RFC8224]
   accommodates either of these approaches: the methods for
   canonicalizing, for signing, for identifying and accessing the
   certificate, and so on remain the same; it is only the verifier
   behavior and authorization decision that will change, depending on
   the approach to telephone number authority taken by the certificate.
   For that reason, the two approaches are not mutually exclusive, and
   in fact a certificate issued to a traditional telephone network
   service provider could contain a TN Authorization List or not, were
   it supported by the CA issuing the credential.  Regardless of which
   approach is used, certificates that assert authority over telephone
   numbers are subject to the ordinary operational procedures that
   govern certificate use per [RFC5280].  This means that verification
   services must be mindful of the need to ensure that they trust the
   trust anchor that issued the certificate and that they have some
   means to determine the freshness of the certificate (see Section 10).

4.  Certificate Usage with STIR

   [RFC8224], Section 7.4 requires that all credential systems used by
   STIR explain how they address the requirements enumerated below.
   Certificates as described in this document address the STIR
   requirements as follows:

   1.  The URI [RFC3986] schemes permitted in the SIP Identity header
       "info" parameter, as well as any special procedures required to
       dereference the URIs: while normative text is given below in
       Section 7, this mechanism permits the HTTP [RFC7230], CID
       (Content-ID) [RFC2392], and SIP URI schemes to appear in the
       "info" parameter.

   2.  Procedures required to extract keying material from the resources
       designated by the URI: implementations perform no special
       procedures beyond dereferencing the "info" URI.  See Section 7.

   3.  Procedures used by the verification service to determine the
       scope of the credential: this specification effectively proposes
       two methods, as outlined in Section 3: one where the subject (or,
       more properly, subjectAltName) of the certificate indicates the
       scope of authority through a domain name, and relying parties
       either trust the subject entirely or have some direct means of
       determining whether or not a number falls under a subject's
       authority; and another where an extension to the certificate as
       described in Section 9 identifies the scope of authority of the
       certificate.

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   4.  The cryptographic algorithms required to validate the
       credentials: for this specification, that means the signature
       algorithms used to sign certificates.  This specification
       REQUIRES that implementations support both the Elliptic Curve
       Digital Signature Algorithm (ECDSA) with the P-256 curve (see
       [DSS]) and RSA PKCS #1 v1.5 ("PKCS" stands for "Public-Key
       Cryptography Standards") (see [RFC8017], Section 8.2) for
       certificate signatures.  Implementers are advised that the latter
       algorithm is mandated only as a transitional mechanism, due to
       its widespread use in existing PKIs, but we anticipate that this
       mechanism will eventually be deprecated.

   5.  Finally, note that all certificates compliant with this
       specification:

       *  MUST provide cryptographic keying material sufficient to
          generate the ECDSA using P-256 and SHA-256 signatures
          necessary to support the ES256 hashed signatures required by
          PASSporT [RFC8225], which in turn follows the JSON Web Token
          (JWT) [RFC7519].

       *  MUST support both ECDSA with P-256 and RSA PKCS #1 v1.5 for
          certificate signature verification.

   This document also includes additional certificate-related
   requirements:

   o  See Section 5.1 for requirements related to the JWT Claim
      Constraints certificate extension.

   o  See Section 7 for requirements related to relying parties
      acquiring credentials.

   o  See Sections 10 and 10.1 for requirements related to certificate
      freshness and the Authority Information Access (AIA) certificate
      extension.

5.  Enrollment and Authorization Using the TN Authorization List

   This document covers three models for enrollment when using the TN
   Authorization List extension.

   The first enrollment model is one where the CA acts in concert with
   national numbering authorities to issue credentials to those parties
   to whom numbers are assigned.  In the United States, for example,
   telephone number blocks are assigned to Local Exchange Carriers
   (LECs) by the North American Numbering Plan Administration (NANPA),
   who is in turn directed by the national regulator.  LECs may also

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   receive numbers in smaller allocations, through number pooling, or
   via an individual assignment through number portability.  LECs assign
   numbers to customers, who may be private individuals or organizations
   -- and organizations take responsibility for assigning numbers within
   their own enterprise.  This model requires top-down adoption of the
   model from regulators through to carriers.  Assignees of E.164
   numbering resources participating in this enrollment model should
   take appropriate steps to establish trust anchors.

   The second enrollment model is a bottom-up approach where a CA
   requires that an entity prove control by means of some sort of test
   that, as with certification authorities for web PKI, might either be
   (1) automated or (2) a manual administrative process.  As an example
   of an automated process, an authority might send a text message to a
   telephone number containing a URL (which might be dereferenced by the
   recipient) as a means of verifying that a user has control of a
   terminal corresponding to that number.  Checks of this form are
   frequently used in commercial systems today to validate telephone
   numbers provided by users.  This is comparable to existing enrollment
   systems used by some certificate authorities for issuing S/MIME
   credentials for email by verifying that the party applying for a
   credential receives mail at the email address in question.

   The third enrollment model is delegation: that is, the holder of a
   certificate (assigned by either of the two methods above) might
   delegate some or all of their authority to another party.  In some
   cases, multiple levels of delegation could occur: a LEC, for example,
   might delegate authority to a customer organization for a block of
   100 numbers used by an IP PBX, and the organization might in turn
   delegate authority for a particular number to an individual employee.
   This is analogous to delegation of organizational identities in
   traditional hierarchical PKIs who use the name constraints extension
   [RFC5280]; the root CA delegates names in sales to the sales
   department CA, names in development to the development CA, etc.  As
   lengthy certificate delegation chains are brittle, however, and can
   cause delays in the verification process, this document considers
   optimizations to reduce the complexity of verification.

   Future work might explore methods of partial delegation, where
   certificate holders delegate only part of their authority.  For
   example, individual assignees may want to delegate to a service
   authority for text messages associated with their telephone number
   but not for other functions.

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5.1.  Constraints on Signing PASSporTs

   The public key in the certificate is used to validate the signature
   on a JWT [RFC7519] that conforms to the conventions specified in
   PASSporT [RFC8225].  This specification supports constraints on the
   JWT claims, thereby allowing the CA to grant different permissions to
   certificate holders -- for example, those enrolled from
   proof-of-possession versus delegation.  A Certificate Policy (CP) and
   a Certification Practice Statement (CPS) [RFC3647] are produced as
   part of the normal PKI bootstrapping process (i.e., the CP is written
   first, and then the CA says how it conforms to the CP in the CPS).  A
   CA that wishes to place constraints on the JWT claims MUST include
   the JWT Claim Constraints certificate extension in issued
   certificates.  See Section 8 for information about the certificate
   extension.

5.2.  Certificate Extension Scope and Structure

   This specification places no limits on the number of telephone
   numbers that can be associated with any given certificate.  Some
   service providers may be assigned millions of numbers and may wish to
   have a single certificate that can be applied to signing for any one
   of those numbers.  Others may wish to compartmentalize authority over
   subsets of the numbers they control.

   Moreover, service providers may wish to have multiple certificates
   with the same scope of authority.  For example, a service provider
   with several regional gateway systems may want each system to be
   capable of signing for each of their numbers but not want to have
   each system share the same private key.

   The set of telephone numbers for which a particular certificate is
   valid is expressed in the certificate through a certificate
   extension; the certificate's extensibility mechanism is defined in
   [RFC5280], but the TN Authorization List extension is specified in
   this document.

   The subjects of certificates containing the TN Authorization List
   extension are typically the administrative entities to whom numbers
   are assigned or delegated.  For example, a LEC might hold a
   certificate for a range of telephone numbers.  In some cases, the
   organization or individual issued such a certificate may not want to
   associate themselves with a certificate; for example, a private
   individual with a certificate for a single telephone number might not
   want to distribute that certificate publicly if every verifier
   immediately knew their name.  The certification authorities issuing
   certificates with the TN Authorization List extensions may, in

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   accordance with their policies, obscure the identity of the subject,
   though mechanisms for doing so are outside the scope of this
   document.

6.  Provisioning Private Keying Material

   In order for authentication services to sign calls via the procedures
   described in [RFC8224], they must hold a private key corresponding to
   a certificate with authority over the calling number.  [RFC8224]
   does not require that any particular entity in a SIP deployment
   architecture sign requests, only that it be an entity with an
   appropriate private key; the authentication service role may be
   instantiated by any entity in a SIP network.  For a certificate
   granting authority only over a particular number that has been issued
   to an end user, for example, an end-user device might hold the
   private key and generate the signature.  In the case of a service
   provider with authority over large blocks of numbers, an intermediary
   might hold the private key and sign calls.

   The specification RECOMMENDS distribution of private keys through
   PKCS #8 objects signed by a trusted entity -- for example, through
   the Cryptographic Message Syntax (CMS) package specified in
   [RFC5958].

7.  Acquiring Credentials to Verify Signatures

   This specification documents multiple ways that a verifier can gain
   access to the credentials needed to verify a request.  As the
   validity of certificates does not depend on the method of their
   acquisition, there is no need to standardize any single mechanism for
   this purpose.  All entities that comply with [RFC8224] necessarily
   support SIP, and consequently SIP itself can serve as a way to
   deliver certificates.  [RFC8224] provides an "info" parameter of the
   Identity header; this parameter contains a URI for the credential
   used to generate the Identity header.  [RFC8224] also requires that
   documents that define credential systems list the URI schemes that
   may be present in the "info" parameter.  For implementations
   compliant with this specification, three URI schemes are REQUIRED:
   the CID URI, the SIP URI, and the HTTP URI.

   The simplest way for a verifier to acquire the certificate needed to
   verify a signature is for the certificate to be conveyed in a
   SIP request along with the signature itself.  In SIP, for example, a
   certificate could be carried in a multipart MIME body [RFC2046], and
   the URI in the Identity header "info" parameter could specify that
   body with a CID URI [RFC2392].  However, in many environments this
   is not feasible due to message size restrictions or lack of necessary
   support for multipart MIME.

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   The Identity header "info" parameter in a SIP request may contain a
   URI that the verifier dereferences.  Implementations of this
   specification are REQUIRED to support the use of SIP for this
   function (via the SUBSCRIBE/NOTIFY mechanism) as well as HTTP and
   HTTPS.

   Note well that as an optimization, a verifier may have access to a
   service, a cache, or other local store that grants access to
   certificates for a particular telephone number.  However, there may
   be multiple valid certificates that can sign a call setup request for
   a telephone number, and as a consequence, there needs to be some
   discriminator that the signer uses to identify their credentials.
   The Identity header "info" parameter itself can serve as such a
   discriminator, provided implementations use that parameter as a key
   when accessing certificates from caches or other sources.

8.  JWT Claim Constraints Syntax

   Certificate subjects are limited to specific values for PASSporT
   claims with the JWT Claim Constraints certificate extension; issuers
   permit all claims by omitting the JWT Claim Constraints certificate
   extension from the certificate's extension field [RFC5280].  The
   extension is non-critical, applicable only to end-entity
   certificates, and defined with ASN.1 [X.680] [X.681] [X.682] [X.683]
   later in this section.  The syntax of the claims is given in
   PASSporT; specifying new claims follows the procedures in [RFC8225],
   Section 8.3.

   This certificate extension is optional, but if present, it constrains
   the claims that authentication services may include in the PASSporT
   objects they sign.  Constraints are applied by issuers and enforced
   by verifiers when validating PASSporT claims as follows:

   1.  mustInclude indicates claims that MUST appear in the PASSporT in
       addition to iat, orig, and dest.  The baseline claims of PASSporT
       ("iat", "orig", and "dest") are considered to be permitted by
       default and SHOULD NOT be included.  If mustInclude is absent,
       iat, orig, and dest MUST appear in the PASSporT.

   2.  permittedValues indicates that if the claim name is present, the
       claim MUST contain one of the listed values.

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   Consider two examples with a PASSporT claim called "confidence" with
   values "low", "medium", and "high":

   o  If a CA issues to an authentication service a certificate that
      contains the mustInclude JWTClaimName "confidence", then an
      authentication service MUST include the "confidence" claim in all
      PASSporTs it generates; a verification service will treat as
      invalid any PASSporT it receives with a PASSporT claim that
      does not include the "confidence" claim.

   o  If a CA issues to an authentication service a certificate that
      contains the permittedValues JWTClaimName "confidence" and a
      permitted "high" value, then an authentication service will treat
      as invalid any PASSporT it receives with a PASSporT claim that
      does not include the "confidence" claim with a "high" value.

   The JWT Claim Constraints certificate extension is identified by the
   following object identifier (OID), which is defined under the id-pe
   OID arc defined in [RFC5280] and managed by IANA (see Section 11):

     id-pe-JWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 27 }

   The JWT Claim Constraints certificate extension has the following
   syntax:

     JWTClaimConstraints ::= SEQUENCE {
       mustInclude [0] JWTClaimNames OPTIONAL,
         -- The listed claim names MUST appear in the PASSporT
         -- in addition to iat, orig, and dest.  If absent, iat, orig,
         -- and dest MUST appear in the PASSporT.
       permittedValues [1] JWTClaimPermittedValuesList OPTIONAL }
         -- If the claim name is present, the claim MUST contain one of
         -- the listed values.
     ( WITH COMPONENTS { ..., mustInclude PRESENT } |
       WITH COMPONENTS { ..., permittedValues PRESENT } )

     JWTClaimPermittedValuesList ::= SEQUENCE SIZE (1..MAX) OF
                                       JWTClaimPermittedValues

     JWTClaimPermittedValues ::= SEQUENCE {
       claim  JWTClaimName,
       permitted  SEQUENCE SIZE (1..MAX) OF UTF8String }

     JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName

     JWTClaimName ::= IA5String

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9.  TN Authorization List Syntax

   The subjects of certificates containing the TN Authorization List
   extension are the administrative entities to whom numbers are
   assigned or delegated.  When a verifier is validating a caller's
   identity, local policy always determines the circumstances under
   which any particular subject may be trusted, but the purpose of the
   TN Authorization List extension in particular is to allow a verifier
   to ascertain when the CA has designated that the subject has
   authority over a particular telephone number or number range.  The
   non-critical TN Authorization List certificate extension is included
   in the certificate's extension field [RFC5280].  The extension is
   defined with ASN.1 [X.680] [X.681] [X.682] [X.683].  The syntax and
   semantics of the extension are as follows.

   The subjects of certificates containing the TN Authorization List
   extension are the administrative entities to whom numbers are
   assigned or delegated.  In an end-entity certificate, the TN
   Authorization List indicates the TNs that it has authorized.  In a CA
   certificate, the TN Authorization List limits the set of TNs for
   certification paths that include this certificate.

   The TN Authorization List certificate extension is identified by the
   following object identifier (OID), which is defined under the id-pe
   OID arc defined in [RFC5280] and managed by IANA (see Section 11):

     id-pe-TNAuthList OBJECT IDENTIFIER ::= { id-pe 26 }

   The TN Authorization List certificate extension has the following
   syntax:

    TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNEntry

    TNEntry ::= CHOICE {
      spc   [0] ServiceProviderCode,
      range [1] TelephoneNumberRange,
      one   [2] TelephoneNumber
      }

    ServiceProviderCode ::= IA5String

    -- SPCs may be OCNs, various SPIDs, or other SP identifiers
    -- from the telephone network.

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    TelephoneNumberRange ::= SEQUENCE {
      start TelephoneNumber,
      count INTEGER (2..MAX),
      ...
      }

    TelephoneNumber ::= IA5String (SIZE (1..15)) (FROM ("0123456789#*"))

   The TN Authorization List certificate extension indicates the
   authorized phone numbers for the call setup signer.  It indicates one
   or more blocks of telephone number entries that have been authorized
   for use by the call setup signer.  There are three ways to identify
   the block:

   1.  SPCs as described in this document are a generic term for the
       identifiers used to designate service providers in telephone
       networks today.  In North American context, these would include
       OCNs as specified in [ATIS-0300251], related SPIDs, or other
       similar identifiers for service providers.  SPCs can be used to
       indirectly name all of the telephone numbers associated with that
       identifier for a service provider.

   2.  Telephone numbers can be listed in a range (in the
       TelephoneNumberRange format), which consists of a starting
       telephone number and then an integer count of numbers within the
       range, where the valid boundaries of ranges may vary according to
       national policies.  The count field is only applicable to start
       fields whose values do not include "*" or "#" (i.e., a
       TelephoneNumber that does not include "*" or "#").  count
       MUST NOT make the number increase in length (i.e., a
       TelephoneNumberRange with TelephoneNumber=10 and count=91 is
       invalid); formally, given the inputs count and TelephoneNumber of
       length D, TelephoneNumber + count MUST be less than 10^D.

   3.  A single telephone number can be listed (as a TelephoneNumber).

   Note that because large-scale service providers may want to associate
   many numbers, possibly millions of numbers, with a particular
   certificate, optimizations are required for those cases to prevent
   the certificate size from becoming unmanageable.  In these cases, the
   TN Authorization List may be given by reference rather than by value,
   through the presence of a separate certificate extension that permits
   verifiers to either (1) securely download the list of numbers
   associated with a certificate or (2) verify that a single number is
   under the authority of this certificate.  For more on this
   optimization, see Section 10.1.

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10.  Certificate Freshness and Revocation

   Regardless of which of the approaches in Section 3 is followed for
   using certificates, a certificate verification mechanism is required.
   However, the traditional problem of certificate freshness gains a new
   wrinkle when using the TN Authorization List extension with telephone
   numbers or number ranges (as opposed to SPCs), because verifiers must
   establish not only that a certificate remains valid but also that the
   certificate's scope contains the telephone number that the verifier
   is validating.  Dynamic changes to number assignments can occur due
   to number portability, for example.  So, even if a verifier has a
   valid cached certificate for a telephone number (or a range
   containing the number), the verifier must determine that the entity
   that created the PASSporT, which includes a digital signature, is
   still a proper authority for that number.

   To verify the status of such a certificate, the verifier needs to
   acquire the certificate if necessary (via the methods described in
   Section 7) and then would need to either:

   a.  Rely on short-lived certificates and not check the certificate's
       status, or

   b.  Rely on status information from the authority (e.g., the Online
       Certificate Status Protocol (OCSP)).

   The trade-off between short-lived certificates and using status
   information is that the former's burden is on the front end (i.e.,
   enrollment) and the latter's burden is on the back end (i.e.,
   verification).  Both impact call setup time, but some approaches to
   generating a short-lived certificate, like requiring one for each
   call, would incur a greater operational cost than acquiring status
   information.  This document makes no particular recommendation for a
   means of determining certificate freshness for STIR, as this requires
   further study and implementation experience.  Acquiring online status
   information for certificates has the potential to disclose private
   information [RFC7258] if proper precautions are not taken.  Future
   specifications that define certificate freshness mechanisms for STIR
   MUST note any such risks and provide countermeasures where possible.

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10.1.  Acquiring the TN List by Reference

   One alternative to checking certificate status for a particular
   telephone number is simply acquiring the TN Authorization List by
   reference, that is, through dereferencing a URL in the certificate,
   rather than including the value of the TN Authorization List in the
   certificate itself.

   Acquiring a list of the telephone numbers associated with a
   certificate or its subject lends itself to an application-layer
   query/response interaction outside of certificate status, one that
   could be initiated through a separate URI included in the
   certificate.  The AIA extension (see [RFC5280]) supports such a
   mechanism: it designates an OID to identify the accessMethod and an
   accessLocation, which would most likely be a URI.  A verifier would
   then follow the URI to ascertain whether the TNs in the list are
   authorized for use by the caller.  As with the certificate extension
   defined in Section 9, a URI dereferenced from an end-entity
   certificate will indicate the TNs that the caller has been
   authorized.  Verifiers MUST support the AIA extension, and the
   dereferenced URI from a CA certificate limits the set of TNs for
   certification paths that include this certificate.

   HTTPS is the most obvious candidate for a protocol to be used for
   fetching the list of telephone numbers associated with a particular
   certificate.  This document defines a new AIA accessMethod, called
   "id-ad-stirTNList", which uses the following AIA OID:

     id-ad-stirTNList  OBJECT IDENTIFIER ::= { id-ad 14 }

   When the "id-ad-stirTNList" accessMethod is used, the accessLocation
   MUST be an HTTPS URI.  Dereferencing the URI will return the complete
   DER-encoded TN Authorization List (see Section 9) for the certificate
   with a Content-Type of application/tnauthlist (see Section 11.2).

   Delivering the entire list of telephone numbers associated with a
   particular certificate will divulge to STIR verifiers information
   about telephone numbers other than the one associated with the
   particular call that the verifier is checking.  In some environments,
   where STIR verifiers handle a high volume of calls, maintaining an
   up-to-date and complete cache for the numbers associated with crucial
   certificate holders could give an important boost to performance.

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11.  IANA Considerations

11.1.  ASN.1 Registrations

   This document makes use of object identifiers for the TN certificate
   extension defined in Section 9, the "TN List by reference" AIA access
   descriptor defined in Section 10.1, and the ASN.1 module identifier
   defined in Appendix A.  Therefore, per this document, IANA has made
   the following assignments, as shown on
   <https://www.iana.org/assignments/smi-numbers>:

   o  TN Authorization List certificate extension in the "SMI Security
      for PKIX Certificate Extension" (1.3.6.1.5.5.7.1) registry:

      26  id-pe-TNAuthList

   o  JWT Claim Constraints certificate extension in the "SMI Security
      for PKIX Certificate Extension" (1.3.6.1.5.5.7.1) registry:

      27  id-pe-JWTClaimConstraints

   o  TN List by reference access descriptor in the "SMI Security for
      PKIX Access Descriptor" (1.3.6.1.5.5.7.48) registry:

      14  id-ad-stirTNList

   o  The TN ASN.1 module in the "SMI Security for PKIX Module
      Identifier" (1.3.6.1.5.5.7.0) registry:

      89  id-mod-tn-module

11.2.  Media Type Registrations

   Type name: application
    Subtype name: tnauthlist
    Required parameters: None
    Optional parameters: None
    Encoding considerations: Binary
    Security considerations:  See Section 12 of RFC 8226
    Interoperability considerations:
       The TN Authorization List inside this media type MUST be
       DER-encoded TNAuthorizationList.
    Published specification: RFC 8226
    Applications that use this media type:
       Issuers and relying parties of secure telephone identity
       certificates, to limit the subject's authority to a
       particular telephone number or telephone number range.
    Fragment identifier considerations: None

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    Additional information:
       Deprecated alias names for this type: None
       Magic number(s): None
       File extension(s): None
       Macintosh File Type Code(s): None
    Person & email address to contact for further information:
       Jon Peterson <jon.peterson@team.neustar>
    Intended usage: COMMON
    Restrictions on usage: None
    Author: Sean Turner <sean@sn3rd.com>
    Change controller: The IESG <iesg@ietf.org>

12.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see [RFC5280], in particular its
   Security Considerations section.

   If a certification authority issues a certificate attesting authority
   over many telephone numbers, the TNAuthList element can divulge to
   relying parties extraneous telephone numbers associated with the
   certificate that have no bearing on any given call in progress.  The
   potential privacy risk can be exacerbated by the use of AIA, as
   described in Section 10.1, to link many thousands of numbers to a
   single certificate.  Even an SPC in a certificate can be used to link
   a certificate to a particular carrier and, with access to industry
   databases, potentially the set of numbers associated with that SPC.
   While these practices may not cause concern in some environments, in
   other scenarios alternative approaches could minimize the data
   revealed to relying parties.  For example, a service provider with
   authority over a large block of numbers could generate short-lived
   certificates for individual TNs that are not so easily linked to the
   service provider or any other numbers that the service provider
   controls.  Optimizations to facilitate acquiring short-lived
   certificates are a potential area of future work for STIR.

   The TN Authorization List returned through a dereferenced URI is
   served over HTTPS; the TN Authorization List is therefore protected
   in transit.  But, the TN Authorization List served is not a signed
   object and therefore the server is trusted to faithfully return the
   TN Authorization List provided to it by the list generator.

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13.  References

13.1.  Normative References

   [ATIS-0300251]
              ATIS Recommendation 0300251, "Codes for Identification of
              Service Providers for Information Exchange", 2007.

   [DSS]      National Institute of Standards and Technology, U.S.
              Department of Commerce, "Digital Signature Standard
              (DSS)", NIST FIPS PUB 186-4, DOI 10.6028/NIST.FIPS.186-4,
              July 2013, <http://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.186-4.pdf>.

   [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>.

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,
              <https://www.rfc-editor.org/info/rfc2392>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [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>.

   [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>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <https://www.rfc-editor.org/info/rfc5958>.

   [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

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   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258,
              May 2014, <https://www.rfc-editor.org/info/rfc7258>.

   [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>.

   [RFC8017]  Moriarty, K., Ed., Kaliski, B., Jonsson, J., and A. Rusch,
              "PKCS #1: RSA Cryptography Specifications Version 2.2",
              RFC 8017, DOI 10.17487/RFC8017, November 2016,
              <https://www.rfc-editor.org/info/rfc8017>.

   [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>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [X.509]    International Telecommunication Union, "Information
              technology - Open Systems Interconnection - The Directory:
              Public-key and attribute certificate frameworks", ITU-T
              Recommendation X.509, ISO/IEC 9594-8, October 2016,
              <https://www.itu.int/rec/T-REC-X.509>.

   [X.680]    International Telecommunication Union, "Information
              Technology - Abstract Syntax Notation One (ASN.1):
              Specification of basic notation", ITU-T Recommendation
              X.680, ISO/IEC 8824-1, August 2015,
              <https://www.itu.int/rec/T-REC-X.680>.

   [X.681]    International Telecommunication Union, "Information
              Technology - Abstract Syntax Notation One (ASN.1):
              Information object specification", ITU-T Recommendation
              X.681, ISO/IEC 8824-2, August 2015,
              <https://www.itu.int/rec/T-REC-X.681>.

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   [X.682]    International Telecommunication Union, "Information
              Technology - Abstract Syntax Notation One (ASN.1):
              Constraint specification", ITU-T Recommendation
              X.682, ISO/IEC 8824-3, August 2015,
              <https://www.itu.int/rec/T-REC-X.682>.

   [X.683]    International Telecommunication Union, "Information
              Technology - Abstract Syntax Notation One (ASN.1):
              Parameterization of ASN.1 specifications", ITU-T
              Recommendation X.683, ISO/IEC 8824-4, August 2015,
              <https://www.itu.int/rec/T-REC-X.683>.

13.2.  Informative References

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,
              <https://www.rfc-editor.org/info/rfc2046>.

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              DOI 10.17487/RFC3647, November 2003,
              <https://www.rfc-editor.org/info/rfc3647>.

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,
              <https://www.rfc-editor.org/info/rfc7375>.

   [X.520]    International Telecommunication Union, "Information
              technology - Open Systems Interconnection - The Directory:
              Selected attribute types", ITU-T Recommendation
              X.520, ISO/IEC 9594-6, October 2016,
              <https://www.itu.int/rec/T-REC-X.520>.

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Appendix A.  ASN.1 Module

   This appendix provides the normative ASN.1 [X.680] definitions for
   the structures described in this specification using ASN.1, as
   defined in [X.680], [X.681], [X.682], and [X.683].

   The modules defined in this document are compatible with the most
   current ASN.1 specifications published in 2015 (see [X.680], [X.681],
   [X.682], and [X.683]).  None of the newly defined tokens in the 2008
   ASN.1 (DATE, DATE-TIME, DURATION, NOT-A-NUMBER, OID-IRI,
   RELATIVE-OID-IRI, TIME, TIME-OF-DAY) are currently used in any of the
   ASN.1 specifications referred to here.

   This ASN.1 module imports ASN.1 from [RFC5912].

    TN-Module-2016
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-tn-module(89) }

    DEFINITIONS EXPLICIT TAGS ::= BEGIN

    IMPORTS

    id-ad, id-pe
    FROM PKIX1Explicit-2009  -- From RFC 5912
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }

    EXTENSION
    FROM PKIX-CommonTypes-2009  -- From 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) }

    ;

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    --
    -- JWT Claim Constraints Certificate Extension
    --

    ext-jwtClaimConstraints EXTENSION  ::= {
      SYNTAX JWTClaimConstraints IDENTIFIED BY id-pe-JWTClaimConstraints
      }

    id-pe-JWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 27 }

    JWTClaimConstraints ::= SEQUENCE {
      mustInclude [0] JWTClaimNames OPTIONAL,
        -- The listed claim names MUST appear in the PASSporT
        -- in addition to iat, orig, and dest.  If absent, iat, orig,
        -- and dest MUST appear in the PASSporT.
      permittedValues [1] JWTClaimPermittedValuesList OPTIONAL }
        -- If the claim name is present, the claim MUST contain one of
        -- the listed values.
    ( WITH COMPONENTS { ..., mustInclude PRESENT } |
      WITH COMPONENTS { ..., permittedValues PRESENT } )

    JWTClaimPermittedValuesList ::= SEQUENCE SIZE (1..MAX) Of
                                      JWTClaimPermittedValues

    JWTClaimPermittedValues ::= SEQUENCE {
      claim  JWTClaimName,
      permitted  SEQUENCE SIZE (1..MAX) OF UTF8String }

    JWTClaimNames ::= SEQUENCE SIZE (1..MAX) OF JWTClaimName

    JWTClaimName ::= IA5String

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    --
    -- Telephony Number Authorization List Certificate Extension
    --

    ext-tnAuthList  EXTENSION  ::= {
      SYNTAX TNAuthorizationList IDENTIFIED BY id-pe-TNAuthList
      }

    id-pe-TNAuthList OBJECT IDENTIFIER ::= { id-pe 26 }

    TNAuthorizationList ::= SEQUENCE SIZE (1..MAX) OF TNEntry

    TNEntry ::= CHOICE {
      spc    [0] ServiceProviderCode,
      range  [1] TelephoneNumberRange,
      one    [2] TelephoneNumber
      }

    ServiceProviderCode ::= IA5String

    -- SPCs may be OCNs, various SPIDs, or other SP identifiers
    -- from the telephone network.

    TelephoneNumberRange ::= SEQUENCE {
      start TelephoneNumber,
      count INTEGER (2..MAX),
      ...
      }

    TelephoneNumber ::= IA5String (SIZE (1..15)) (FROM ("0123456789#*"))

    -- TN Access Descriptor

    id-ad-stirTNList OBJECT IDENTIFIER ::= { id-ad 14 }

    END

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Acknowledgments

   Anders Kristensen, Russ Housley, Brian Rosen, Cullen Jennings, Dave
   Crocker, Tony Rutkowski, John Braunberger, Eric Rescorla, and Martin
   Thomson provided key input to the discussions leading to this
   document.  Russ Housley provided some direct assistance and text
   surrounding the ASN.1 module.

Authors' Addresses

   Jon Peterson
   Neustar, Inc.

   Email: jon.peterson@neustar.biz

   Sean Turner
   sn3rd

   Email: sean@sn3rd.com

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