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OCSP Usage for Secure Telephone Identity Certificates

Document Type Active Internet-Draft (stir WG)
Authors Jon Peterson , Sean Turner
Last updated 2023-07-28
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Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               S. Turner
Expires: 29 January 2024                                           sn3rd
                                                            28 July 2023

         OCSP Usage for Secure Telephone Identity Certificates


   When certificates are used as credentials to attest the assignment or
   ownership of telephone numbers, some mechanism is required to convey
   certificate freshness to relying parties.  Certififcate Revocation
   Lists (CRLs) are commonly used for this purpose, but for certain
   classes of certificates, including delegate certificates conveying
   their scope of authority by-reference in Secure Telephone Identity
   Revisited (STIR) systems, they may not be aligned with the needs of
   relying parties.  This document specifies the use of the Online
   Certificate Status Protocol (OCSP) as a means of retrieving real-time
   status information about such certificates, defining new extensions
   to compensate for the dynamism of telephone number assignments.

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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 29 January 2024.

Copyright Notice

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview of Certificate Verification Methods  . . . . . . . .   3
   4.  Using OCSP with TN Authorization Lists  . . . . . . . . . . .   5
     4.1.  OCSP Extension Specification  . . . . . . . . . . . . . .   5
     4.2.  STIR Certification Authorities and OCSP . . . . . . . . .   7
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The STIR problem statement [RFC7340] discusses many attacks on the
   telephone network that are enabled by impersonation, including
   various forms of robocalling, voicemail hacking, and swatting.  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.  The STIR certificates [RFC8226]
   specification describes a credential system based on [X.509] version
   3 certificates in accordance with [RFC5280] for that purpose.  Those
   credentials can then be used by STIR authentication services
   [RFC8224] to sign PASSporT objects [RFC8225] carried in a SIP
   [RFC3261] request.

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   [RFC8226] specifies an extension to X.509 that defines a Telephony
   Number (TN) Authorization List that may be included by certificate
   authorities in certificates.  This extension provides additional
   information that relying parties can use when validating transactions
   with the certificate.  When a SIP request, for example, arrives at a
   terminating administrative domain, the calling number attested by the
   SIP request can be compared to the TN Authorization List of the
   certificate that signed the request to determine if the caller is
   authorized to use that calling number in SIP.

   No specific recommendation is made in [RFC8226] for a means of
   determining the freshness of certificates with a TN Authorization
   List.  Moreover, there is significant dynamism in telephone number
   assignment, and due to practices like number portability, information
   about number assignment can suddenly become stale.  This problem is
   especially pronounced when a TN Authorization List extension
   associates a large block of telephone numbers with a certificate, as
   relying parties need a way to learn if any one of those telephone
   numbers has been ported to a different administrative entity.  To
   facilitate this, [RFC8226] Section 10.1 specifies a way that the TN
   Authorization List can be shared by-reference in a certificate, via a
   URL in the Authority Information Access extension, so that a more
   dynamic list can be maintained without continually reissuing the
   certificate.  For very large and/or complex TN Authorization Lists,
   however, this could require relying parties to redownload the entire
   list virtually every time they process a call.  Moreover, some
   certificate holders may be reluctant to share the entire list of
   telephone numbers associated with a certificate in cases where a
   relying party only needs to know, effectively, whether a single
   number (the calling party number for a particular call) is in the
   scope of authority for a certificate or not.  This document explores
   approaches to real-time status information for such certificates, and
   recommends an approach.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Overview of Certificate Verification Methods

   For traditional certificate status information, there are three
   common certificate verification mechanisms employed by CAs:

   1.  Certificate Revocation Lists (CRLs) [RFC5280] (and [RFC6818])

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

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

   Verifiers relying on status information need a way to obtain it -
   that is, 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 attractive solution because they are supported by
   traditional web PKI environments.  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 relying party have all of
   the CRL partitions).  In practice, CRLs are widely used in STIR
   environments, often through a federated approach where a community of
   trusted CAs pool their CRLs for distribution from a central point.

   CAs in the STIR architecture thus have already implemented CRLs,
   largely for audit purposes rather than real-time status information.
   The need for these CRLs is not likely to go away, especially for the
   case of service providers whose certificates are based on Service
   Provider Codes (SPCs).  For delegate STIR certificates ([RFC9060]),
   however, especially those with TN Authorization Lists based on
   telephone numbers, OCSP may provide an important optimizations.
   Between the OCSP and SCVP, 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 telephone-number
   based certificates, based on [RFC6960], incorporating some (but not
   all) of the optimizations of [RFC5019].

   Like most PKIX-developed protocols, OCSP is extensible; OCSP supports
   request extensions (including sending multiple requests at once) and
   per-request extensions.  As the relying party in STIR is validating a
   PASSporT associated with a telephone call, it is unlikely that the
   verifier will request authorization checks on multiple telephone
   numbers in one request, so a per-request extension is what is needed.

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4.  Using OCSP with TN Authorization Lists

   Certificates compliant with this specification SHOULD include a URL
   [RFC3986] pointing to an OCSP service in the Authority Information
   Access (AIA) certificate extension, via the "id-ad-ocsp" accessMethod
   specified in [RFC5280].  This can appear in addition to, or as an
   alternative to, the "id-ad-stirTNList" accessMethod specified in
   [RFC8226].  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.

   Consulting OCSP in real time results in a network round-trip delay,
   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.

4.1.  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 carries the telephone number for which
   the query is being performed, typically the telephone number in the
   "orig" field of a PASSporT being validated.  The TNQuery extension
   may also appear in the response's singleExtensions; when an OCSP
   server includes a telephone 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.

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     id-pkix-ocsp-stir-tn  OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }

     TNQuery ::= TelephoneNumber

   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:

   *  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 Section 2 of

   *  Clients MUST include the OCSP TNQuery extension in requests'

   *  Servers MUST include the OCSP TNQuery extension in responses'

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

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

   *  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].

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

   *  This removes the 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 identifiers using the
   SHA-256 and truncate the value to 160-bits as specified in Option 1
   in Section 2 of [RFC7093].

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   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

4.2.  STIR Certification Authorities and OCSP

   In a STIR deployment, certification authorities will typically be the
   entities that operate OCSP servers.  Ultimately, the OCSP response
   MUST be signed by a CA in the certification chain of the end entitiy
   certificate that signed the PASSporT being verified.  In the case of
   multilevel certificate delegation (i.e.  [RFC9060]), this means the
   OCSP response may be signed by any of the parent "encompassing"
   certificates of the end entity delegate certificate in question.

5.  IANA Considerations

   This document makes use of object identifiers for the TN-HVE OCSP
   extension in Section 4.1 and the ASN.1 module identifier defined in
   Appendix A.  It therefore requests that the IANA make the following

   TN-OCSP-Module-2016 OID in the SMI Security for PKIX Module
   Identifier registry:
   smi- numbers.xhtml#smi-numbers-

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

6.  Privacy Considerations

   Querying for real-time status information about certificates can
   allow parties monitoring communications to gather information about
   relying parties and the originators of communications.
   Unfortunately, the TNQuery extension adds a new field that could
   potentailly be monitored by OCSP eavesdroppers: the calling telephone
   number provides a specific piece of additional data about the
   originator of communications.  Using OCSP over TLS is one potential
   countermeasure to this threat, as described in [RFC6960]
   Appendix A.1.

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   Preventing eavesdropping reduces on potential privacy leak, though of
   course using OCSP reveals to the OCSP service (likely acting for the
   certification authority) the verification service where calls from a
   given telephone number are terminating.  Bear in mind that STIR
   assumes that verification services use HTTPS to acquire certificates
   (by referencing the "x5u" field of the PASSporT) already, so some
   connection between the verification service and a certificate
   repository (likely acting for the certification authority or
   authentication service) is unavoidable.  This OCSP extension further
   reveals the calling telephone number as it arrives at the
   verification service to the OCSP service.

   One way to mitigate leaking information about relying parties is to
   use OCSP stapling.  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 PASSporT or 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
   furthermore result in significant performance improvements.  This
   would however require work on a capability to carry OCSP staples that
   is outside the scope of this document.

7.  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].

8.  Acknowledgments

   Stephen Farrell provided key input to the discussions leading to this
   document.  Russ Housley provided some direct assistance and text
   surrounding the ASN.1 module.

9.  References

9.1.  Normative References

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

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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,

   [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,

   [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,

   [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, <>.

   [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,

   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
              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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   [RFC7093]  Turner, S., Kent, S., and J. Manger, "Additional Methods
              for Generating Key Identifiers Values", RFC 7093,
              DOI 10.17487/RFC7093, December 2013,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,

   [RFC9060]  Peterson, J., "Secure Telephone Identity Revisited (STIR)
              Certificate Delegation", RFC 9060, DOI 10.17487/RFC9060,
              September 2021, <>.

   [X.509]    ITU-T Recommendation X.509 (10/2012) | 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 (08/2015) | ISO/IEC 8824-1,
              "Information Technology - Abstract Syntax Notation One:
              Specification of basic notation".

   [X.681]    ITU-T Recommendation X.681 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Information Object Specification".

   [X.682]    ITU-T Recommendation X.682 (08/2015) | ISO/IEC 8824-2,
              "Information Technology - Abstract Syntax Notation One:
              Constraint Specification".

   [X.683]    ITU-T Recommendation X.683 (08/2015) | ISO/IEC 8824-3,
              "Information Technology - Abstract Syntax Notation One:
              Parameterization of ASN.1 Specifications".

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9.2.  Informative References

   [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,

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

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

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] and [RFC8226].

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    { iso(1) identified-organization(3) dod(6) internet(1)
      security(5) mechanisms(5) pkix(7) id-mod(0)
      id-mod-tn-ocsp-module-2023(TBD) }



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

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

    FROM TN-Module-2016  -- From RFC 8226
      { iso(1) identified-organization(3) dod(6) internet(1) security(5)
        mechanisms(5) pkix(7) id-mod(0) id-mod-tn-module(89) }

  id-pkix-ocsp OBJECT IDENTIFIER ::= id-ad-ocsp

  -- Telephone Number Query OCSP Extension

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

  TNQuery ::= TelephoneNumber

  id-pkix-ocsp-stir-tn OBJECT IDENTIFIER ::= { id-pkix-ocsp 10 }


Authors' Addresses

   Jon Peterson
   Neustar, Inc.

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   Sean Turner

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