Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                               S. Turner
Expires: September 14, 2017                                        sn3rd
                                                          March 13, 2017

         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.  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

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   This Internet-Draft will expire on September 14, 2017.

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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Certificate Verification Methods  . . . . . . . . . . . . . .   3
     3.1.  Using OCSP with TN Auth List  . . . . . . . . . . . . . .   4
       3.1.1.  OCSP Extension Specification  . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   7.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  ASN.1 Module . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

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
   [I-D.ietf-stir-certificates] 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 [I-D.ietf-stir-rfc4474bis] to sign
   PASSporT objects [I-D.ietf-stir-passport] carried in a SIP [RFC3261]

   The STIR certificates document 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

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   However, 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.

   No specific recommendation is made in the STIR certificates document
   for a means of determining the freshness of certificates with a TN
   Authorization List.  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 RFC
   2119 [RFC2119].

3.  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])

   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 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

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   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 probably not for real-time status information.  Any
   such CRLs used MUST be signed with the same algorithm as the
   certificate.  We thus anticipate that 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].

3.1.  Using OCSP with TN Auth List

   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].  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, that mechanism can be reused 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 delay, which is something to consider because it will add

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   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

3.1.1.  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 10 }

     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 Section 2 of

   o  Clients MUST include the OCSP TNQuery extension in requests'

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   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  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].

   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.  IANA Considerations

   This document makes use of object identifiers for the TN-HVE OCSP
   extension in Section 3.1.1 and the ASN.1 module identifier defined in
   Appendix A.  It therefore requests that the IANA make the following

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

5.  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.

   Another 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 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 generic SIP capability to carry OCSP staples that
   is outside the scope of this document.

6.  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].

7.  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.

8.  References

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8.1.  Normative References

              Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", draft-ietf-stir-
              certificates-11 (work in progress), October 2016.

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

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

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

   [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, <>.

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   [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,

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

   [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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8.2.  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].

   [TO DO: this ASN.1 module is a stub and needs to be redone!]

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  TN-Module-2016-2 {
  iso(1) identified-organization(3) dod(6) internet(1)
  security(5) mechanisms(5) pkix(7) id-mod(0)
  id-mod-tn-module(88) }


  id-ad, id-ad-ocsp, id-pe                               -- 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) }

  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) }


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

  -- Telephone Number Query OCSP Extension

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

  TNQuery ::= E164Number

  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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