Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               S. Turner
Expires: September 22, 2016                                        Sn3rd
                                                          March 21, 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 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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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 22, 2016.

Copyright Notice

   Copyright (c) 2016 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
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   ( 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.  Enrollment and Authorization using the TN Authorization List    5
     4.1.  Certificate Extension Scope and Structure . . . . . . . .   6
   5.  Provisioning Private Keying Material  . . . . . . . . . . . .   7
   6.  Acquiring Credentials to Verify Signatures  . . . . . . . . .   7
   7.  Verifying Certificate Scope with TN Authorization List  . . .   8
   8.  Certificate Freshness and Revocation  . . . . . . . . . . . .  10
     8.1.  Choosing a Verification Method  . . . . . . . . . . . . .  10
     8.2.  Using OCSP with TN Auth List  . . . . . . . . . . . . . .  11
       8.2.1.  OCSP Extension Specification  . . . . . . . . . . . .  12
     8.3.  Acquiring TN Lists By Reference . . . . . . . . . . . . .  13
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   12. Informative References  . . . . . . . . . . . . . . . . . . .  15
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

1.  Introduction

   The STIR problem statement [I-D.ietf-stir-problem-statement]
   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 that they are in fact authorized to use telephony
   numbers, and thus distinguish themselves from impersonators unable to
   present such credentials.  This document describes credential systems
   for telephone numbers based on X.509 version 3 certificates in

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   accordance with [RFC5280].  While telephone numbers have long been a
   part of the X.509 standard, this document extends X.509 with a
   Telephone Number Authorization List which binds certificates to
   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 (see
   Section 4), whereas a verifier need only trust the root certificate
   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
   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 certification authority or deployment environment.  We
   anticipate that some deployment experience will be necessary to
   determine optimal operational models.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as
   described in RFC 2119 [RFC2119] and RFC 6919 [RFC6919].

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 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

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   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
   the first approach assumes.

   The first approach requires little change to existing 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
   the certification authority issuing the credential.  Regardless of
   which approaches 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 root certificate authority that issued the certificate, and

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   that they have some means to determine the freshness of the
   certificate (see Section 8).

4.  Enrollment and Authorization using the TN Authorization List

   This document assumes a threefold model for certificate enrollment
   when using the TN Authorization List extension.

   The first enrollment model is one where the certification authority
   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.

   The second enrollment model is a bottom-up approach where a
   certification authority 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

   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 Public Key Infrastructures (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

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   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

   [TBD] Future versions of this specification may address adding a
   level of assurance indication to certificates to differentiate those
   enrolled from proof-of-possession versus delegation.

   [TBD] Future versions of this specification may also discuss 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.

4.1.  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 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,

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   though mechanisms for doing so are outside the scope of this

5.  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 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].

6.  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 certificates.
   [I-D.ietf-stir-rfc4474bis] provides an "info" parameter of the
   Identity header which contains a URI where the credential used to
   generate the Identity header, and 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.

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   More commonly, the Identity header "info" parameter in a SIP request
   may contain a URI that the verifier dereferences with a network call.
   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 [RFC7030].

   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.  Verifiers

7.  Verifying Certificate Scope with TN Authorization List

   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 particular is to allow a verifier to
   ascertain when the certification authority has designed that the
   subject has authority over a particular telephone number or number

   The Telephony Number (TN) Authorization List certificate extension is
   identified by the following object identifier:

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

   The TN Authorization List certificate extension has the following

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

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

      TNEntry ::= CHOICE {

         spid  ServiceProviderIdentifierList,

         range 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"))

   [TBD- do we really need to do IA5String?  The alternative would be
   UTF8String, e.g.: UTF8String (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) 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
   3) a single telephone number can be listed.

   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

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   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
   future version of this specification.

8.  Certificate Freshness and Revocation

   Regardless of which of the approaches in Section 3 is followed for
   using certificates, some sort of 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 6), and then would need to either:

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

      Rely on status information from the authority

   The tradeoff between short lived certificates and using status
   information is 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
   performing enrollment for each call is more of an impact that using
   status information.  This document therefore recommends relying on
   status information.

8.1.  Choosing a Verification Method

   There are three common certificate verification mechanisms employed
   by CAs:

      Certificate Revocation Lists (CRLs) [RFC5280]

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

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      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 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 for
   validating the freshness of certificates, based on [RFC6960],
   incorporating some (but not all) of the optimizations of [RFC5019].

8.2.  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].  Baseline OCSP however supports
   only three possible response values: good, revoked, or unknown.  With
   some extension, OCSP would not indicate whether the certificate is
   authorized for a particular telephone number that the verifier is

   [TBD] What would happen in the unknown case?  Can we profile OCSP
   usage so that unknown is never returned for our extension?

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

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      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.

   [TBD] HVE OCSP requires SHA-1 be used as the hash algorithm,
   we're6960 obviously going to change this to be SHA-256.

   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.

8.2.1.  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
   requestExtensions in requests.  It may also appear in the
   responseExtensions.  When an OCSP server includes a number in the
   responseExtensions, 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 requestExtensions, then the server is not

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   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

   Note that 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
   these extensions in a baseline OCSP environment with some HVE
   optimizations.  [More TBD]

   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.

8.3.  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

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   URI to ascertain whether the list of TNs authorized for use by the

   HTTPS is the most obvious candidate for a protocol to be used for
   fetching the list of telephone number 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 7)
   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.

9.  Acknowledgments

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

10.  IANA Considerations

   This document makes use of object identifiers for the TN Certificate
   Extension defined in Section 7, TN-HVE OCSP extension in
   Section 8.2.1, and the TN by reference AIA access descriptor defined
   in Section 8.3.  It therefore requests that the IANA make the
   following assignments:

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

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

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

11.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see RFC 3280 [RFC3280], in
   particular its Security Considerations.

12.  Informative References

              Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              draft-ietf-stir-problem-statement-05 (work in progress),
              May 2014.

              Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-07
              (work in progress), February 2016.

              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements", draft-peterson-sipping-
              retarget-00 (work in progress), February 2005.

   [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,

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000,

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   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              DOI 10.17487/RFC3263, June 2002,

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              DOI 10.17487/RFC3280, April 2002,

   [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,

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

   [RFC6919]  Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
              for Use in RFCs to Indicate Requirement Levels", RFC 6919,
              DOI 10.17487/RFC6919, April 2013,

   [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,

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   [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,

   [RFC7299]  Housley, R., "Object Identifier Registry for the PKIX
              Working Group", RFC 7299, DOI 10.17487/RFC7299, July 2014,

   [X.680]    USC/Information Sciences Institute, "Information
              Technology - Abstract Syntax Notation One.", ITU-T X.680,
              ISO/IEC 8824-1:2002, 2002.

   [X.681]    USC/Information Sciences Institute, "Information
              Technology - Abstract Syntax Notation One: Information
              Object Specification", ITU-T X.681, ISO/IEC 8824-2:2002,

   [X.682]    USC/Information Sciences Institute, "Information
              Technology - Abstract Syntax Notation One: Constraint
              Specification", ITU-T X.682, ISO/IEC 8824-3:2002, 2002.

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

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].


Authors' Addresses

   Jon Peterson
   Neustar, Inc.


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   Sean Turner


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