Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               S. Turner
Expires: January 9, March 13, 2017                                            sn3rd
                                                            July 8,
                                                       September 9, 2016

          Secure Telephone Identity Credentials: Certificates


   In order to prevent the impersonation of telephone numbers on the
   Internet, some kind of credential system needs to exist that
   cryptographically proves 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

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   This Internet-Draft will expire on January 9, March 13, 2017.

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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.  Levels Of Assurance . . . . . . . . . . . . . . . . . . .   7
     5.2.  Certificate Extension Scope and Structure . . . . . . . .   8
   6.  Provisioning Private Keying Material  . . . . . . . . . . . .   8
   7.  Acquiring Credentials to Verify Signatures  . . . . . . . . .   9
   8.  Verifying Certificate Scope with  TN Authorization List Syntax  . . . . . . . . . . . . . . . .  10
   9.  Certificate Freshness and Revocation  . . . . . . . . . . . .  11  12
     9.1.  Choosing a Verification Method  . . . . . . . . . . . . .  12
     9.2.  Using OCSP with TN Auth List  . . . . . . . . . . . . . .  13
       9.2.1.  OCSP Extension Specification  . . . . . . . . . . . .  13  14
     9.3.  Acquiring TN Lists By Reference . . . . . . . . . . . . .  15  16
   10. Acknowledgments . . IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16  17
   11. IANA Security Considerations . . . . . . . . . . . . . . . . . . .  18
   12. Acknowledgments . .  16
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  17 . .  18
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  20  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22  23

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 calling 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 prove assert that they are in fact authorized to use telephony
   numbers, and thus distinguish themselves from impersonators unable to
   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 a part of the X.509 standard, 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 prove 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",
   "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.

   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 "" 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
   Identifiers (SPIDs) could serve as a subject as well.  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 taht 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
   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, depending on 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 9).

4.  Certificate Usage with STIR

   [I-D.ietf-stir-rfc4474bis] Section 7.4 requires that all credential
   systems used
   for signing by STIR explain how they address the Identity header requirements
   enumerated below.  Certificates as described in SIP detail this document address
   the following: STIR requirements as follows:

   1.  The URI 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, 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 8 identifies the scope of authority of the

   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 [RFC4754]) 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 widespread use in existing PKI, but we anticipate that
       this mechanism will eventually be deprecated.

   5.  Finally, note that all certificates compliant with this

       *  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

   o  See Section 5.1 for requirements related to the certificate
      policies extension.

   o  See Section 7 for requirements related to the TN Query certificate

   o  See Section 9.2 and Section 9.3 for requirements related to the
      Authority Information Access (AIA) certificate extension.

   o  See Section 9.2.1 for requirements related to the authority key
      identifier and subject key identifier certificate extensions.

5.  Enrollment and Authorization using the TN Authorization List

   This document assumes a threefold model covers three models for certificate 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.

   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 versions of this specification may also discuss 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.  Levels Of Assurance

   This specification supports different level of assurance (LOA).  The
   LOA indications, which are Object Identifiers (OIDs) included in the
   certificate's certificate policies extension [RFC5280], allow CAs to
   differentiate 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 CAs
   say how they conform to the CP in the CPS).  OIDs are used to
   reference the CP and if the CA wishes it can also include a reference
   to the CPS with the certificate policies extension.  CAs that wish to
   express different LOAs MUST include the certificate policies
   extension in issued certificates.  See Section 11 for additional
   information about the LOA registry.

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 is capable of 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

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.  This specification  [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
   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 acquire deliver certificates.
   [I-D.ietf-stir-rfc4474bis] provides an "info" parameter of the
   Identity header which contains a URI where for the credential used to
   generate the Identity header, and header; [I-D.ietf-stir-rfc4474bis] also
   requires documents which define credential systems to 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.

   More commonly, the

   The Identity header "info" parameter in a SIP request may contain a
   URI that the verifier dereferences with a network call. 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, via
   the Enrollment over Secure Transport mechanisms described in RFC 7030

   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.  Verifying Certificate Scope with  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 designed designated that the subject has
   authority over a particular telephone number or number range.  The
   Telephony Number (TN) Authorization List certificate extension is
   included in the Certificate's extension field [RFC5280].  The
   extension is defined with ASN.1, defined in [X.680] through [X.683].
   What follows is the syntax and semantics of the extension.

   The Telephony Number (TN) Authorization List certificate extension is
   identified by the following object identifier: identifier (OID), which is defined
   under the id-ce OID arc defined in [RFC5280] and managed by IANA (see
   Section 10):

     id-ce-TNAuthList OBJECT IDENTIFIER ::= { id-ce TBD }

   The TN Authorization List certificate extension has the following

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

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

     TNEntry ::= CHOICE {
       spid  [0] ServiceProviderIdentifierList,
       range [1] TelephoneNumberRange,
       one       E164Number }

     ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
                                         OCTET STRING

     -- When all three are present: SPID, Alt SPID, and Last Alt SPID

     TelephoneNumberRange ::= SEQUENCE {
       start E164Number,
       count INTEGER }

     E164Number ::= 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.  A Service Provider Identifier (SPID) (SPID, also known as an Operating
       Company Number (OCN) or Carrier Identification Code (CIC), as
       specified in [ATIS-0300050]) can be used to indirectly name all
       of the telephone numbers associated with that service provider,

   2.  Telephone numbers can be listed in a range, and range (in the
       TelephoneNumberRange format), or

   3.  A single telephone number can be listed. listed (as an E164Number).

   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.  This optimization will be detailed in is left for future version of this specification.

9.  Certificate Freshness and Revocation

   Regardless of which of the approaches in Section 3 is followed for
   using certificates, some sort of a certificate verification mechanism is required.
   However, the traditional problem of certificate freshness gains a new
   wrinkle when using the TN Authorization List extension, 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 the 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, see
        Section 9.2)

   The tradeoff 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 it is assumed that
   generating a short-lived certificate for each all, and consequently
   performing enrollment for each call call, is more of an impact that using than
   acquiring status information.  This document therefore recommends
   relying on status information.

9.1.  Choosing a Verification Method

   There are three common certificate verification mechanisms employed
   by CAs:

   1.  Certificate Revocation Lists (CRLs) [RFC5280]

   2.  Online Certificate Status Protocol (OCSP) [RFC6960], and

   3.  Server-based Certificate Validation Protocol (SCVP) [RFC5055].

   When relying on status information, the verifier needs to obtain the
   status information - but before that can happen, the verifier needs
   to know where to locate it.  Placing the location of the status
   information in the certificate makes the certificate larger, but it
   eases the client workload.  The CRL Distribution Point certificate
   extension includes the location of the CRL and the Authority
   Information Access certificate extension includes the location of
   OCSP and/or SCVP servers; both of these extensions are defined in
   [RFC5280].  In all cases, the status information location is provided
   in the form of an URI.

   CRLs are an obviously attractive solution because they are supported
   by every CA.  CRLs have a reputation of being quite large (10s of
   MBytes), because CAs maintain and issue one monolithic CRL with all
   of their revoked certificates, but CRLs do support a variety of
   mechanisms to scope the size of the CRLs based on revocation reasons
   (e.g., key compromise vs CA compromise), user certificates only, and
   CA certificates only as well as just operationally deciding to keep
   the CRLs small.  However, scoping the CRL introduces other issues
   (i.e., does the RP have all of the CRL partitions).

   CAs in the STIR architecture will likely all create CRLs for audit
   purposes, but it NOT RECOMMENDED that they be relying relied upon for status
   information.  Instead, one of the two "online" options is preferred.
   Between the two, OCSP is much more widely deployed and this document
   therefore recommends the use of OCSP in high-volume environments
   (HVE) for validating the freshness of certificates, based on
   [RFC6960], incorporating some (but not all) of the optimizations of
   [RFC5019].  CRLs MUST be signed with the same algorithm as the

9.2.  Using OCSP with TN Auth List

   Certificates compliant with this specification therefore SHOULD
   include a URL pointing to an OCSP service in the Authority
   Information Access (AIA) certificate extension, via the "id-ad-ocsp"
   accessMethod specified in [RFC5280].  It is RECOMMENDED that entities
   that issue certificates with the Telephone Number Authorization List
   certificate extension run an OCSP server for this purpose.  Baseline
   OCSP however supports only three possible response values: good,
   revoked, or unknown.  Without some extension, OCSP would not indicate
   whether the certificate is authorized for a particular telephone
   number that the verifier is validating.

   At a high level, there are two ways that a client might pose this
   authorization question:

      For this certificate, is the following number currently in its
      scope of validity?
      What are all the telephone numbers associated with this
      certificate, or this certificate subject?

   Only the former lends itself to piggybacking on the OCSP status
   mechanism; since the verifier is already asking an authority about
   the certificate's status, why not reuse that mechanism, instead of
   creating a new service that requires additional round trips?  Like
   most PKIX-developed protocols, OCSP is extensible; OCSP supports
   request extensions (including sending multiple requests at once) and
   per-request extensions.  It seems unlikely that the verifier will be
   requesting authorization checks on multiple telephone numbers in one
   request, so a per-request extension is what is needed.

   The requirement to consult OCSP in real time results in a network
   round-trip time of day, which is something to consider because it
   will add to the call setup time.  OCSP server implementations
   commonly pre-generate responses, and to speed up HTTPS connections,
   servers often provide OCSP responses for each certificate in their
   hierarchy.  If possible, both of these OCSP concepts should be
   adopted for use with STIR.

9.2.1.  OCSP Extension Specification

   The extension mechanism for OCSP follows X.509 v3 certificate
   extensions, and thus requires an OID, a criticality flag, and ASN.1
   syntax as defined by the OID.  The criticality specified here is
   optional: per [RFC6960] Section 4.4, support for all OCSP extensions
   is optional.  If the OCSP server does not understand the requested
   extension, it will still provide the baseline validation of the
   certificate itself.  Moreover, in practical STIR deployments, the
   issuer of the certificate will set the accessLocation for the OCSP
   AIA extension to point to an OCSP service that supports this
   extension, so the risk of interoperability failure due to lack of
   support for this extension is minimal.

   The OCSP TNQuery extension is included as one of the request's
   singleRequestExtensions.  It may also appear in the response's
   singleExtensions.  When an OCSP server includes a number in the
   response's singleExtensions, this informs the client that the
   certificate is still valid for the number that appears in the TNQuery
   extension field.  If the TNQuery is absent from a response to a query
   containing a TNQuery in its singleRequestExtension, then the server
   is not able to validate that the number is still in the scope of
   authority of the certificate.

     id-pkix-ocsp-stir-tn  OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }

     TNQuery ::= E164Number

   The HVE OCSP profile [RFC5019] prohibits the use of per-request
   extensions.  As it is anticipated that STIR will use OCSP in a high-
   volume environment, many of the optimizations recommended by HVE are
   desirable for the STIR environment.  This document therefore uses the
   HVE optimizations augmented as follows:

   o  Implementations MUST use SHA-256 as the hashing algorithm for the
      CertID.issuerNameHash and the CertID.issuerKeyHash values.  That
      is CertID.hashAlgorithm is id-sha256 [RFC4055] and the values are
      truncated to 160-bits as specified Option 1 in Sectin Section 2 of as per

   o  Clients MUST include the OCSP TNQuery extension in requests'

   o  Servers MUST include the OCSP TNQuery extension in responses'

   o  Servers SHOULD return responses that would otherwise have been
      "unknown" as "not good" (i.e., return only "good" and "not good"

   o  Clients MUST treat returned "unknown" responses as "not good".

   o  If the server uses ResponderID, it MUST generate the KeyHash using
      SHA-256 and truncate the value to 160-bits as specified in Option
      1 in Section 2 of [RFC7093].

   o  Implementations MUST support ECDSA using P-256 and SHA-256.  Note
      that [RFC6960] requires RSA with SHA-256 be supported.

   o  There is no requirement to support SHA-1, RSA with SHA-1, or DSA
      with SHA-1.

   OCSP responses MUST be signed using the same algorithm as the
   certificate being checked.

   To facilitate matching the authority key identifier values found in
   CA certificates with the KeyHash used in the OCSP response,
   certificates compliant with this specification MUST generate
   authority key identifiers and subject key identifers identifiers using the
   SHA-256 and truncate the value to 160-bits as specified in Option 1
   in Section 2 of [RFC7093].

   Ideally, once a certificate has been acquired by a verifier, some
   sort of asynchronous mechanism could notify and update the verifier
   if the scope of the certificate changes so that verifiers could
   implement a cache.  While not all possible categories of verifiers
   could implement such behavior, some sort of event-driven notification
   of certificate status is another potential subject of future work.
   One potential direction is that a future SIP SUBSCRIBE/NOTIFY-based
   accessMethod for AIA might be defined (which would also be applicable
   to the method described in the following section) by some future

   Strategies for stapling OCSP [RFC6961] have become common in some web
   PKI environments as an optimization which allows web servers to send
   up-to-date certificate status information acquired from OCSP to
   clients as TLS is negotiated.  A similar mechanism could be
   implemented for SIP requests, in which the authentication service
   adds status information for its certificate to the SIP request, which
   would save the verifier the trouble of performing the OCSP dip
   itself.  Especially for high-volume authentication and verification
   services, this could result in significant performance improvements.
   This is left as an optimization for future work.

9.3.  Acquiring TN Lists By Reference

   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 OCSP, 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 number numbers associated with a particular
   certificate.  This document defines a new AIA accessMethod, called
   "id-ad-stir-tn", which uses the following AIA OID:

     id-ad-stir-tn  OBJECT IDENTIFIER ::= { id-ad TBD }

   When the "id-ad-stir-tn" 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 8)
   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
   up-to-date and complete cache for the numbers associated with crucial
   certificate holders could give an important boost to performance.

10.  Acknowledgments

   Russ Housley, Brian Rosen, Cullen Jennings and Eric Rescorla provided
   key input to the discussions leading to this document.

11.  IANA Considerations

   This document makes use of object identifiers for the TN Certificate
   Extension defined in Section 8, TN-HVE OCSP extension in
   Section 9.2.1, the TN by reference AIA access descriptor defined in
   Section 9.3, and the ASN.1 module identifier defined in Appendix A.
   It therefore requests that the IANA make the following assignments:

   o  TN Certificate Extension in the SMI Security for PKIX Certificate
      Extension registry:

   o  TN-HVE OCSP extension in the SMI Security for PKIX Online
      Certificate Status Protocol (OCSP) registry:

   o  TNS by reference access descriptor in the SMI Security for PKIX
      Access Descriptor registry:

   o  The TN ASN.1 module in SMI Security for PKIX Module Identifier

   This document also makes use of the Level of Assurance (LoA) Profiles
   registry defined in [RFC6711] because as is stated in RFC 6711: "Use
   of the registry by protocols other than SAML is encouraged."  IANA is
   requested to creae the STIR Levels of Assurance (LOA) sub-registry in
   the Level of Assurance (LoA) Profile registry.  Instead of
   registering a SAML Context Class, the Certificate Policy's Object
   Identifier representing the LOA is included in the registry.  An
   example registration is as follows:



   1.  Name of requester: J.  Random User

   2.  Email address of requester:

   3.  Organization of requester: Example Trust Frameworks LLP

   4.  Requested registration:


      Name  foo-loa-1

      Informational URL

      Certificate Policy Object Identifier:

      NOTE: Do not register this example.  The OID is purposely invalid.

   Experts are expected to ensure the reference CP includes the OID
   being registered.


11.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see [RFC5280], in particular its
   Security Considerations.  For OCSP-related security considerations
   see [RFC6960] and [RFC5019]

12.  Acknowledgments

   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.

13.  References

              ATIS Recommendation 0300050, "Carrier Identification Code
              (CIC) Assignment Guidelines", 2012.

              Wendt, C. and J. Peterson, "Persona Assertion Token",
              draft-ietf-stir-passport-07 (work in progress), June September

              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-09 draft-ietf-stir-rfc4474bis-11
              (work in progress), May August 2016.

   [RFC2046]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Two: Media Types", RFC 2046,
              DOI 10.17487/RFC2046, November 1996,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2392]  Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, DOI 10.17487/RFC2392, August 1998,

   [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, <>.

   [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,

   [RFC4055]  Schaad, J., Kaliski, B., and R. Housley, "Additional
              Algorithms and Identifiers for RSA Cryptography for use in
              the Internet X.509 Public Key Infrastructure Certificate
              and Certificate Revocation List (CRL) Profile", RFC 4055,
              DOI 10.17487/RFC4055, June 2005,

   [RFC4754]  Fu, D. and J. Solinas, "IKE and IKEv2 Authentication Using
              the Elliptic Curve Digital Signature Algorithm (ECDSA)",
              RFC 4754, DOI 10.17487/RFC4754, January 2007,

   [RFC5019]  Deacon, A. and R. Hurst, "The Lightweight Online
              Certificate Status Protocol (OCSP) Profile for High-Volume
              Environments", RFC 5019, DOI 10.17487/RFC5019, September
              2007, <>.

   [RFC5055]  Freeman, T., Housley, R., Malpani, A., Cooper, D., and W.
              Polk, "Server-Based Certificate Validation Protocol
              (SCVP)", RFC 5055, DOI 10.17487/RFC5055, December 2007,

   [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,

   [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,

   [RFC5958]  Turner, S., "Asymmetric Key Packages", RFC 5958,
              DOI 10.17487/RFC5958, August 2010,

   [RFC6711]  Johansson, L., "An IANA Registry for Level of Assurance
              (LoA) Profiles", RFC 6711, DOI 10.17487/RFC6711, August
              2012, <>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

   [RFC7093]  Turner, S., Kent, S., and J. Manger, "Additional Methods
              for Generating Key Identifiers Values", RFC 7093,
              DOI 10.17487/RFC7093, December 2013,

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,

   [RFC7375]  Peterson, J., "Secure Telephone Identity Threat Model",
              RFC 7375, DOI 10.17487/RFC7375, October 2014,

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [X.509]    ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-8,
              "Information technology - Open Systems Interconnection -
              The Directory: Public-key and attribute certificate
              frameworks", 2012.

   [X.520]    ITU-T Recommendation X.520 (10/2012) | ISO/IEC 9594-6,
              "Information technology - Open Systems Interconnection -
              The Directory: Selected Attribute Types", 2012.

   [X.680]    USC/Information Sciences Institute,    ITU-T Recommendation X.680 (08/2015) | ISO/IEC 8824-1,
              "Information Technology - Abstract Syntax Notation One.", One:
              Specification of basic notation".

   [X.681]    ITU-T X.680, Recommendation X.681 (08/2015) | ISO/IEC 8824-1:2002, 2002.

   [X.681]    USC/Information Sciences Institute, 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Information Object Specification", Specification".

   [X.682]    ITU-T X.681, Recommendation X.682 (08/2015) | ISO/IEC 8824-2:2002,

   [X.682]    USC/Information Sciences Institute, 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Specification", Specification".

   [X.683]    ITU-T X.682, Recommendation X.683 (08/2015) | ISO/IEC 8824-3:2002, 2002.

   [X.683]    USC/Information Sciences Institute, 8824-3,
              "Information Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications", ITU-T X.683,
              ISO/IEC 8824-4:2002, 2002. Specifications".

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

      id-ad, id-ad-ocsp                               -- From [RFC5912]
      FROM PKIX1Explicit-2009 {
        iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-explicit-02(51) }

     id-ce                                            -- From [RFC5912]
     FROM PKIX1Implicit-2009 {
       iso(1) identified-organization(3) dod(6) internet(1) security(5)
       mechanisms(5) pkix(7) id-mod(0) id-mod-pkix1-implicit-02(59) }

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


     -- TN Entry Certificate Extension

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

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

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

     TNEntry ::= CHOICE {
       spid   [0] ServiceProviderIdentifierList,
       range  [1] TelephoneNumberRange,
       one        E164Number }

     ServiceProviderIdentifierList ::= SEQUENCE SIZE (1..3) OF
                                         OCTET STRING

     -- When all three are present: SPID, Alt SPID, and Last Alt SPID

     TelephoneNumberRange ::= SEQUENCE {
       start E164Number,
       count INTEGER }

     E164Number ::= IA5String (SIZE (1..15)) (FROM ("0123456789"))

     -- TN OCSP Extension

     re-ocsp-tn-query  EXTENSION ::= {
       SYNTAX TNQuery IDENTIFIED BY id-pkix-ocsp-stir-tn }

     TNQuery ::= E164Number

     -- TN Access Descriptor

     id-ad-stir-tn          OBJECT IDENTIFIER ::= { id-ad TBD }

     --  Object Identifiers

     id-pkix-ocsp           OBJECT IDENTIFIER ::= id-ad-ocsp
     id-ce-TNAuthList       OBJECT IDENTIFIER ::= { id-ce TBD }
     id-pkix-ocsp-stir-tn   OBJECT IDENTIFIER ::= { id-pkix-ocsp TBD }


Authors' Addresses

   Jon Peterson
   Neustar, Inc.


   Sean Turner