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STIR                                                         J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               S. Turner
Expires: November 10, 2017                                         sn3rd
                                                             May 9, 2017


          Secure Telephone Identity Credentials: Certificates
                    draft-ietf-stir-certificates-14

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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 10, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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



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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

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

1.  Introduction

   The STIR problem statement [RFC7340] identifies the primary enabler
   of robocalling, vishing, 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
   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, and 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 which identify the
   parties who control telephone numbers.  With these credentials,
   parties can assert that they are in fact authorized to use telephony
   numbers, and thus distinguish themselves from impersonators unable to



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   present such credentials.  For that reason the STIR threat model
   [RFC7375] stipulates, "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 Telephone 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
   [I-D.ietf-stir-rfc4474bis], 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 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 RFC
   2119 [RFC2119].

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.



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   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 OCNs or 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 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 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.

   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
   [I-D.ietf-stir-rfc4474bis] accommodates either of these approaches:
   the methods for canonicalizing, 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



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   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

   [I-D.ietf-stir-rfc4474bis] 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 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.

   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 ECDSA with the P-256
       curve (see [DSS]) and RSA PKCS#1 v1.5 (see [RFC3447] Section 8.2)
       for certificate signatures.  Implementers are advised that RS256
       is mandated only as a transitional mechanism, due to its



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       widespread use in existing PKI, 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 [I-D.ietf-stir-passport], which in turn follows 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 certificate
      policies extension.

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

   o  See Section 10 and Section 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 Administrator (NANPA),
   who is in turn directed by the national regulator.  LECs may also
   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.




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   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,
   which, as with certification authorities for web PKI, might either be
   automated or 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
   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.

5.1.  Constraints on Signing PASSporTs

   The public key in the certificate is used to validate the signature
   on a JSON Web Token (JWT) [RFC7519] that conforms to the conventions
   specified in PASSporT [I-D.ietf-stir-passport].  This specification
   supports constraints on the JWT claims, which allows the CA to grant
   different permissions to certificate holders, for example those
   enrolled from proof-of-possession versus delegation.  A Certification
   Policy and a Certification Practice Statement [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



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   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
   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 [I-D.ietf-stir-rfc4474bis], they must hold a private key
   corresponding to a certificate with authority over the calling
   number.  [I-D.ietf-stir-rfc4474bis] 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



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   particular number which 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
   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
   [I-D.ietf-stir-rfc4474bis] necessarily support SIP, and consequently
   SIP itself can serve as a way to deliver certificates.
   [I-D.ietf-stir-rfc4474bis] provides an "info" parameter of the
   Identity header which contains a URI for the credential used to
   generate the Identity header; [I-D.ietf-stir-rfc4474bis] also
   requires documents which 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 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.

   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.



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   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
   [I-D.ietf-stir-passport] (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.

   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.






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   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 25 }

   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

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 Telephony Number (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].
   What follows is the syntax and semantics of the extension.

   The subjects of certificates containing the TN Authorization List
   extension are the administrative entities to whom numbers are



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   assigned or delegated.  In an end entity certificate, TN
   Authorization List indicates the TNs which the certificate has been
   authorized.  In a CA certificate, the TN Authorization List limits
   the set of TNs for certification paths that include this certificate.

   The Telephony Number (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

    -- Service Provider Codes 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#*"))

   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.  Service Provider Codes as described in this document are a
       generic term for the identifiers used to designate service
       providers in the telepohone networks today.  In North American
       context, these would include Operating Company Numbers (OCNs) as
       specified in [ATIS-0300251], related Service Provide Identifiers
       (SPIDs), or other similar identifiers for service providers.




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       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, or

   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
   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 securely download the list of numbers associated
   with a certificate, or to verify that a single number is under the
   authority of this certificate.  For more on this optimization, see
   Section 10.1.

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 signed 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., OCSP)

   The tradeoff between short lived certificates and using status
   information is that the former's burden is on the front end (i.e.,



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   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 recommndation for a
   means of determinate 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.

10.1.  Acquiring TN Lists 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 which
   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 list of TNs are
   authorized for use by the caller.

   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.  The document returned by dereferencing that
   URI will contain the complete TN Authorization List (see Section 9)
   for the certificate.

   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




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   up-to-date and complete cache for the numbers associated with crucial
   certificate holders could give an important boost to performance.

11.  IANA Considerations

   This document makes use of object identifiers for the TN Certificate
   Extension defined in Section 9, the TN by reference AIA access
   descriptor defined in Section 10.1, and the ASN.1 module identifier
   defined in Appendix A.  It therefore requests that the IANA make the
   following assignments:

   o  JWT Claim Constraints Certificate Extension in the SMI Security
      for PKIX Certificate Extension registry:

      http://www.iana.org/assignments/smi-numbers/smi-numbers.xhtml#smi-
      numbers-1.3.6.1.5.5.7.1

   o  TN Certificate Extension in the SMI Security for PKIX Certificate
      Extension registry: http://www.iana.org/assignments/smi-numbers/
      smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.1

   o  TNS by reference access descriptor in the SMI Security for PKIX
      Access Descriptor registry: http://www.iana.org/assignments/smi-
      numbers/smi-numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.48

   o  The TN ASN.1 module in SMI Security for PKIX Module Identifier
      registry: http://www.iana.org/assignments/smi-numbers/smi-
      numbers.xhtml#smi-numbers-1.3.6.1.5.5.7.0

12.  Security Considerations

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

13.  Acknowledgments

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

14.  References







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14.1.  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,
              version 4", NIST FIPS PUB 186-4, 2013.

   [I-D.ietf-stir-passport]
              Wendt, C. and J. Peterson, "Personal Assertion Token
              (PASSporT)", draft-ietf-stir-passport-11 (work in
              progress), February 2017.

   [I-D.ietf-stir-rfc4474bis]
              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-16
              (work in progress), February 2017.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://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,
              <http://www.rfc-editor.org/info/rfc2392>.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, DOI 10.17487/RFC3447, February
              2003, <http://www.rfc-editor.org/info/rfc3447>.

   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc5280>.






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   [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,
              <http://www.rfc-editor.org/info/rfc5912>.

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7230>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://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,
              <http://www.rfc-editor.org/info/rfc7519>.

   [X.509]    ITU-T Recommendation X.509 | ISO/IEC 9594-8, "Information
              technology - Open Systems Interconnection - The Directory:
              Public-key and attribute certificate frameworks", 2012.

   [X.680]    ITU-T Recommendation X.680 | ISO/IEC 8824-1, "Information
              Technology - Abstract Syntax Notation One: Specification
              of basic notation", 2015.

   [X.681]    ITU-T Recommendation X.681 | ISO/IEC 8824-2, "Information
              Technology - Abstract Syntax Notation One: Information
              Object Specification", 2015.

   [X.682]    ITU-T Recommendation X.682 | ISO/IEC 8824-2, "Information
              Technology - Abstract Syntax Notation One: Constraint
              Specification", 2015.

   [X.683]    ITU-T Recommendation X.683 | ISO/IEC 8824-3, "Information
              Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications", 2015.

14.2.  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,
              <http://www.rfc-editor.org/info/rfc2046>.



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   [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,
              <http://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,
              <http://www.rfc-editor.org/info/rfc7340>.

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

   [X.520]    ITU-T Recommendation X.520 | ISO/IEC 9594-6, "Information
              technology - Open Systems Interconnection - The Directory:
              Selected Attribute Types", 2012.

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] through [X.683].

   The modules defined in this document are compatible with the most
   current ASN.1 specification published in 2015 (see [X.680], [X.681],
   [X.682], [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(88) }

   DEFINITIONS EXPLICIT TAGS ::= BEGIN

   IMPORTS

   id-ad, id-pe
   FROM PKIX1Explicit-2009  -- From [RFC5912]
     { 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



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   FROM PKIX-CommonTypes-2009  -- From [RFC5912]
     { iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkixCommon-02(57) }

   ;

   --
   -- JWT Claim Constraints Certificate Extension
   --

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

   id-pe-JWTClaimConstraints OBJECT IDENTIFIER ::= { id-pe 25 }

   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

   --
   -- Telephone Number Authorization List Certificate Extension
   --

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

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




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

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

   ServiceProviderCode ::= IA5String

   -- Service Provider Codes 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

Authors' Addresses

   Jon Peterson
   Neustar, Inc.

   Email: jon.peterson@neustar.biz


   Sean Turner
   sn3rd

   Email: sean@sn3rd.com













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