STIR Working Group                                           J. Peterson
Internet-Draft                                                   NeuStar
Intended status: Standards Track                               S. Turner
Expires: September 25, 2015                                         IECA
                                                          March 24, 2015

          Secure Telephone Identity Credentials: Certificates


   In order to prove ownership of telephone numbers on the Internet,
   some kind of public infrastructure needs to exist that binds
   cryptographic keys to authority over telephone numbers.  This
   document describes a certificate-based credential system for
   telephone numbers, which could be used as a part of a broader
   architecture for managing telephone numbers as identities in
   protocols like SIP.

Status of This Memo

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   This Internet-Draft will expire on September 22, 2015.

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   document authors.  All rights reserved.

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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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Enrollment and Authorization . . . . . . . . . . . . . . . . .  3
     3.1.  Certificate Scope and Structure  . . . . . . . . . . . . .  4
     3.2.  Provisioning Private Keying Material . . . . . . . . . . .  5
   4.  Acquiring Credentials to Verify Signatures . . . . . . . . . .  5
     4.1.  Verifying Certificate Scope  . . . . . . . . . . . . . . .  6
     4.2.  Certificate Freshness and Revocation . . . . . . . . . . .  7
   5.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   8.  Informative References . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12

1.  Introduction

   As is discussed in the STIR problem statement [13], the primary
   enabler of robocalling, vishing, swatting and related attacks is the
   capability to impersonate a calling party number.  The starkest
   examples of these attacks are cases where automated callees on the
   Public Switched Telephone Network (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 an authority responsible for issuing credentials to
   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 credentials.  This document describes a credential system for
   telephone numbers based on X.509 version 3 certificates in accordance

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   with [7].  While telephone numbers have long been a part of the X.509
   standard, the certificates described in this document may contain
   telephone number blocks or ranges, and accordingly it uses an
   alternate syntax.

   In the STIR in-band architecture, two basic types of entities need
   access to these credentials: authentication services, and
   verification services (or verifiers); see [15].  An authentication
   service must be operated by an entity enrolled with the certification
   authority (see Section 3), whereas a verifier need only trust the
   root certificate of the authority, and have a means to acquire and
   validate certificates.

   This document attempts to specify only the basic elements necessary
   for this architecture.  Only through deployment experience will it be
   possible to decide directions for future work.

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 [1] and RFC 6919 [2].

3.  Enrollment and Authorization

   This document assumes a threefold model for certificate enrollment.

   The first enrollment model is one where the certification authority
   (CA) acts in concert with national numbering authorities to issue
   credentials to those parties to whom numbers are assigned.  In the
   United States, for example, telephone number blocks are assigned to
   Local Exchange Carriers (LECs) by the North American Numbering Plan
   Administrator (NANPA), who is in turn directed by the national
   regulator.  LECs may also receive numbers in smaller allocations,
   through number pooling, or via an individual assignment through
   number portability.  LECs assign numbers to customers, who may be
   private individuals or organizations - and organizations take
   responsibility for assigning numbers within their own enterprise.

   The second enrollment model is one where a certification authority
   requires that an entity prove control by means of some sort of test.
   For example, an authority might send a text message to a telephone
   number containing a URL (which might be deferenced 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

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   used by some certificate authorities for issuing S/MIME credentials
   for email by verifying that the party applying for a credential
   receives mail at the email address in question.

   The third enrollment model is delegation: that is, the holder of a
   certificate (assigned by either of the two methods above) may
   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 customer organization for a block of 100
   numbers, 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 [3]; the root CA delegates names in sales to the sales
   department CA, names in development to the development CA, etc.  As
   lengthy certificate delegation chains are brittle, however, and can
   cause delays in the verification process, this document considers
   optimizations to reduce the complexity of verification.

   [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.

3.1.  Certificate Scope and Structure

   The subjects of telephone number certificates are the administrative
   entities to whom numbers are assigned or delegated.  For example, a
   LEC might hold a certificate for a range of telephone numbers.  [TBD
   - what if the subject is considered a privacy leak?]

   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.

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   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
   [7] but the telephone number authorization extension is defined in
   this document.

3.2.  Provisioning Private Keying Material

   In order for authentication services to sign calls via the procedures
   described in [15], they must possess a private key corresponding to a
   certificate with authority over the calling number.  This
   specification does not require that any particular entity 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 [8].

4.  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 circumstances of
   their acquistion, there is no need to standardize any single
   mechanism for this purpose.  All entities that comply with [15]
   necessarily support SIP, and consequently SIP itself can serve as a
   way to acquire certificates.  This specific does allow delivery
   through alternate means as well.

   The simplest way for a verifier to acquire the certificate needed to
   verify a signature is for the certificate be conveyed along with the
   signature itself.  In SIP, for example, a certificate could be
   carried in a multipart MIME body [9], and the URI in the Identity-
   Info header could specify that body with a CID URI [10].  However, in
   many environments this is not feasible due to message size
   restrictions or lack of necessary support for multipart MIME.

   Alternatively, the Identity-Info header of 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

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   well as HTTP, via the Enrollment over Secure Transport mechanisms
   described in RFC 7030 [11].

   A verifier can however have access to a service that grants access to
   certificates for a particular telephone number.  Note however that
   there may be multiple valid certificates that can sign a call setup
   request for a telephone number, and that as a consequence, there
   needs to be some discriminator that the signer uses to identify their
   credentials.  The Identity-Info header itself can serve as such a

4.1.  Verifying Certificate Scope

   The subjects of these certificates 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
   for the purpose of validating a caller's identity, this certificate
   extension establishes whether or not a signer is authorized to sign
   for a particular number.

   The telephone 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

      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"))

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   [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
   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.

4.2.  Certificate Freshness and Revocation

   The problem of certificate freshness gains a new wrinkle in the
   telephone number context, 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 the
   certificate, which is included with the call, and they need to:

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

    o Rely on status information from the authority; there are three
      common mechanisms employed by CAs:

      * Certificate Revocation Lists (CRLs) [7],
      * Online Certificate Status Protocol (OCSP) [RFC6560], and
      * Server-based Certificate Validation Protocol (SCVP) [RFC5055].

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   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.

   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
   [7].  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 issue one 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.  Scoping the
   CRL though introduces other issues (i.e., does the RP have all of the
   CRL partitions).  CAs in this system 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
   recommended.  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
   [12].  Note that OCSP responses have three possible values: good,
   revoked, or unknown.

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

   [TBD] What would happen in the unknown case?

   The wrinkle here is that OCSP only provides status information it
   does not indicate whether the certificate's is authorized for the
   telephone number that the verifier is validating.  There's two ways
   to ask the authorization question:

    o For this certificate, is the following number currently in its
      scope of validity?

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    o What are the numbers associated with this certificate?

   The former seems to lend itself to piggybacking on the status
   mechanism; since the verifier is already asking an authority about
   the certificate's status why not use 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 (OCSP supports sending multiple requests at once)
   and per-request extensions. It seems unlikely that the verifier will
   be requesting authorization checks on multiple callers in one request
   so a per-request extension is what is needed.  But, support for any
   particular extension is optional and the HVE OCSP profile [12]
   prohibits the use of per-request extensions so there is some
   additional work required to modify existing OCSP responders.

   The extension mechanism itself is fairly straightforward and it's
   based on the X.509 v3 certificate extensions: an OID, a criticality
   flag, and ASN.1 syntax as defined by the OID.  The OID would be
   registered in the IANA PKIX arc, the criticality would likely be set
   to critical (i.e., if the OCSP responder doesn't understand the
   extension stop processing), and the syntax can be anything we desire.
    Applying the KISS principle, the syntax could simply be the TN being
   asserted by caller.  The responder could then determine whether the
   TN asserted in the OCSP per-request extension is still authorized for
   the certificate referred to in the certificate request field; the
   reference is a tuple of hash algorithm, issuer name hash, issuer key
   hash, and serial number.

   The second option seems more like a query response type of
   interaction and could be initiated through a URI included in the
   certificate.  Luckily, the AIA extension supports such a mechanism;
   it's an OID to identify the "access method" and an "access location",
   which would most most likely be a URI.  The verifier would then
   follow the URI to ascertain whether the list of TNs authorized for
   use by the caller.  There are obviously some privacy considerations
   with this approach.

   The need to check the authorizations in another round-trip is also
   something to consider because it will add to the call setup time.
   OCSP implementations commonly pre-generate responses and to speed up
   HTTPS connections the server provides OCSP responses for each
   certificate in their hierarchy.  If possible, both of these OCSP
   concepts should be adopted.

   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.  While not all possible
   categories of verifiers could implement such behavior, some sort of

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   event-driven notification of certificate status is another potential
   subject of future work.

5.  Acknowledgments

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

6.  IANA Considerations

   This memo includes no request to IANA at this time.  If we define an
   OCSP extension or AIA access method then we'll need an OID from the

7.  Security Considerations

   This document is entirely about security.  For further information on
   certificate security and practices, see RFC 3280 [5], in particular
   its Security Considerations.

8.  Informative References

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

   [2]        Barnes, R., Kent, S., and E. Rescorla, "Further Key Words
              for Use in RFCs to Indicate Requirement Levels", RFC 6919,
              April 1 2013.

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

   [4]        Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263, June

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

   [6]        Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

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   [7]        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, May 2008.

   [8]        Turner, S., "Asymmetric Key Packages", RFC 5958, August

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

   [10]       Levinson, E., "Content-ID and Message-ID Uniform Resource
              Locators", RFC 2392, August 1998.

   [11]       Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
              Secure Transport", RFC 7030, October 2013.

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

   [13]       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.

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

   [15]       Peterson, J., Jennings, C., and E. Rescorla,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", draft-ietf-stir-rfc4474bis-02
              (work in progress), October 2014.

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Authors' Addresses

   Jon Peterson
   Neustar, Inc.
   1800 Sutter St Suite 570
   Concord, CA  94520


   Sean Turner
   IECA, Inc.
   3057 Nutley Street, Suite 106
   Farifax, VA  22031


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