Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Standards Track                          March 11, 2019
Expires: September 12, 2019

                      STIR Certificate Delegation


   The Secure Telephone Identity Revisited (STIR) certificate profile
   provides a way to attest authority over telephone nunbers and related
   identifiers for the purpose of preventing telephone number spoofing.
   This specification details how that authority can be delegated from a
   parent certificate to a subordinate certificate, in cases where
   service providers grant credentials to enterprises or other customers
   capable of signing calls with STIR.

Status of This Memo

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   This Internet-Draft will expire on September 12, 2019.

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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.  Delegation of STIR Certificates . . . . . . . . . . . . . . .   3
     3.1.  Authentication Services Signing with Delegate
           Certificates  . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Verification Service Behavior for Delegate Certificate
           Signatures  . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Acquiring Certificate Chains in STIR  . . . . . . . . . . . .   5
   5.  ACME and Delegation . . . . . . . . . . . . . . . . . . . . .   5
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   The STIR problem statement [RFC7340] reviews the difficulties facing
   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 SIP [RFC3261]

   [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 PASSporT to determine if the caller is
   authorized to use that calling number.

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   Initial deployment of [RFC8226] has focused on the use of Service
   Provider Codes (SPCs) to attest the scope of authority of a
   certificate.  Typically, these codes are internal telephone network
   identifiers such as the Operating Company Numbers (OCNs) assigned to
   carriers in the United States.  Allocations at finer levels of
   granularity, to blocks of telephone numbers or even to individual
   numbers, are also desirable for enterprise use cases.  [RFC8226] gave
   an overview of a certificate enrollment model based on "delegation,"
   whereby the holder of certificate might allocate a subset of that
   certificates authority to another party.  This specification details
   how delegation of authority works for STIR certificates.

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.  Delegation of STIR Certificates

   STIR delegate certificates are certificates containing a TNAuthList
   object that have been signed with the private key of a parent
   certificate that itself contains a TNAuthList object.  The parent
   certificate needs to have its CA boolean set to "true", indicating
   that that it can sign certificates.  Every STIR delegate certificate
   identifies its parent certificate with a standard [RFC5280] Authority
   Key Identifier.

   The authority bestowed on the holder of the delegate certificate by
   the parent certificate is recorded in the delegate certificate's
   TNAuthList.  Because STIR certificates use the TNAuthList object
   rather than the Subject Name for indicating the scope of their
   authority, traditional [RFC5280] name constraints are not directly
   applicable to STIR.  In a manner similar to the RPKI [RFC6480]
   "encompassing" semantics, each delegate certificate must have a
   TNAuthList scope that is equal to or a subset of its parent
   certificate's scope: it must be "encompassed."  For example, a parent
   certificate with a TNAuthList that attested authority for the
   numbering range +1-212-555-1000 through 1999 could issue a
   certificate to one delegate attesting authority for the range
   +1-212-555-1500 through 1599, and to another delegate a certificate
   for the individual number +1-212-555-1824.

   Delegate certificates may themselves be issued with the CA boolean
   set to "true" so that they can serve as parent certificates to
   further delegates; effectively, this delegate certificate is a cross-
   certificate, as its issuer is not the same as its subject.  In the

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   STIR ecosystem, certification authority certificates may be used to
   sign PASSporTs; this removes the need for creating a redundant end-
   entity certificate with an identical TNAuthList to its parent, though
   if for operational or security reasons certificate holders wish to do
   so, they may.

   Parent certificates may have a TNAuthList containing one or more
   SPCs, one or more telephone number ranges, or both.  Delegations from
   a parent certificate that contains only SPCs to a delegate
   certificate containing a telephone number or number range are
   permitted.  Ascertaining whether or not a given telephone number
   belongs to the service provider identified by an SPC requires access
   to industry numbering databases that are outside the scope of this
   specification; entities that are constructing a certificate path who
   have access to those resources can validate those delegations.

3.1.  Authentication Services Signing with Delegate Certificates

   Authentication service behavior for delegate certificates is little
   changed from baseline STIR behavior.  The same checks are performed
   by the authentication service, comparing the calling party number
   attested in call signaling with the scope of the authority of the
   signing certificate.  Authentication services SHOULD NOT use a
   delegate certificate without validating that its scope of authority
   is encompassed by that of its parent certificate, and if that
   certificate in turn has its own parent, the entire certificate path
   should be validated.

   Note that authentication services creating a PASSporT for a call
   signed with a delegate certificate MUST provide an "x5u" link
   corresponding to the entire certificate chain, rather than just the
   delegate certificate used to sign the call, as described in
   Section 4.

3.2.  Verification Service Behavior for Delegate Certificate Signatures

   The responsibility of a verification service validating PASSporTs
   signed with delegate certificates, while largely following baseline
   [RFC8224] and [RFC8225], requires some additional procedures.  When
   the verification service dereferences the "x5u" parameter, it will
   acquire a certificate list rather than a single certificate.  It MUST
   then validate all of the credentials in the list, identifying the
   parent certificate for each delegate through its AKID object.

   While ordinarily, relying parties have significant latitude in path
   construction when validating a certificate chain, STIR assumes a more
   rigid hierarchical subordination model, rather than one where relying
   parties may want to derive their own chains to particular trust

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   anchors.  If the certificate chain acquired from the "x5u" element of
   a PASSporT does not lead to an anchor that the verification service
   trusts, it treats the validation no differently than it would when a
   non-delegated certificate was issued by an untrust root; in SIP, it
   MAY return a 437 "Unsupported Credential" response if the call should
   be failed for lack of a valid Identity header.

4.  Acquiring Certificate Chains in STIR

   PASSporT [RFC8225] uses the "x5u" element to convey the URL where
   verification services can acquire the certificate used to sign a
   PASSporT.  This value is mirrored by the "info" parameter of the
   Identity header when a PASSporT is conveyed via SIP.  Commonly, this
   is an HTTPS URI.

   When a STIR delegate certificate is used to sign a PASSporT, the
   "x5u" element in the PASSporT will contain a URI indicating where a
   certificate list is available.  That list will be a concatenation of
   PEM encoded certificates of the type "application/pem-certificate-
   chain" defined in [I-D.ietf-acme-acme].  The list begins with the
   certificate used to sign the PASSporT, followed by its parent, and
   then any subsequent grandparents, great-grandparents, and so on.  The
   ordering MUST conform to the AKID/SKID order chain encoded in the
   certs themselves.  Note that ACME requires the first element in a
   pem-certificate-chain to be an end-entity certificate; STIR relaxes
   this requirement, as CA certificates are permitted to sign PASSporTs,
   so the first element in a pem-certificate-chain used for STIR MAY be
   a CA certificate.

5.  ACME and Delegation

   STIR deployments commonly use ACME [I-D.ietf-acme-acme] for
   certificate acquisition, and it is anticipated that delegate
   certificates as well will be acquired through an ACME interface.  An
   entity that wishes to acquire a certificate from a particular CA will
   request an Authority Token [I-D.ietf-acme-authority-token] from the
   parent with the desired TNAuthList
   [I-D.ietf-acme-authority-token-tnauthlist] object.  Note that if the
   client wishes to do further subdelegation of its own, it should
   request a token with the "ca" Authority Token flag set.

   The entity then presents that Authority Token to a certificate
   authority to acquire a STIR delegate certificate.  ACME returns an
   "application/pem-certificate-chain" object with suitable for
   publishing as an HTTPS resource for retrieval with the PASSporT "x5u"
   mechanism as discussed in Section 4.  If the CSR presented to the
   ACME server is for a certificate with the CA boolean set to "true",
   then the ACME server makes a policy decision to determine whether or

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   not it is appropriate to issue that certificate to the requesting
   entity.  In most ACME cases, that policy decision will be made based
   on the "ca" flag in the Authority Token.

6.  IANA Considerations

   This document contains no actions for the IANA.

7.  Privacy Considerations


8.  Security Considerations

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

9.  Acknowledgments

   We would like to thank Richard Barnes, Chris Wendt, Dave Hancock,
   Russ Housley, and Sean Turner for key input to the discussions
   leading to this document.

10.  References

10.1.  Normative References

              Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", draft-ietf-acme-acme-18 (work in progress),
              December 2018.

              Peterson, J., Barnes, M., Hancock, D., and C. Wendt, "ACME
              Challenges Using an Authority Token", draft-ietf-acme-
              authority-token-01 (work in progress), October 2018.

              Wendt, C., Hancock, D., Barnes, M., and J. Peterson,
              "TNAuthList profile of ACME Authority Token", draft-ietf-
              acme-authority-token-tnauthlist-01 (work in progress),
              October 2018.

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   [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,

   [RFC7044]  Barnes, M., Audet, F., Schubert, S., van Elburg, J., and
              C. Holmberg, "An Extension to the Session Initiation
              Protocol (SIP) for Request History Information", RFC 7044,
              DOI 10.17487/RFC7044, February 2014,

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

   [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,

10.2.  Informative References

              ATIS Recommendation 0300251, "Codes for Identification of
              Service Providers for Information Exchange", 2007.

   [DSS]      National Institute of Standards and Technology, U.S.
              Department of Commerce | NIST FIPS PUB 186-4, "Digital
              Signature Standard, version 4", 2013.

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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,

   [RFC5806]  Levy, S. and M. Mohali, Ed., "Diversion Indication in
              SIP", RFC 5806, DOI 10.17487/RFC5806, March 2010,

   [RFC6480]  Lepinski, M. and S. Kent, "An Infrastructure to Support
              Secure Internet Routing", RFC 6480, DOI 10.17487/RFC6480,
              February 2012, <>.

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

   [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.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]    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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Author's Address

   Jon Peterson
   Neustar, Inc.


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