Network Working Group                                        J. Peterson
Internet-Draft                                                   Neustar
Intended status: Informational                             June 12, 2017
Expires: December 14, 2017

                 PASSporT Extension for Diverted Calls


   This document extends PASSporT, which conveys cryptographically-
   signed information about the people involved in personal
   communications, to include an indication that a call has been
   diverted from its originally destination to a new one.  This
   information can greatly improve the decisions made by verification
   services in call forwarding scenarios.

Status of This Memo

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

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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.  PASSporT 'div' Claim  . . . . . . . . . . . . . . . . . . . .   3
   4.  Using 'div' in SIP  . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Authentication Service Behavior . . . . . . . . . . . . .   5
     4.2.  Verification Service Behavior . . . . . . . . . . . . . .   5
   5.  Extending 'div' . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   9.  Informative References  . . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   8

1.  Introduction

   PASSporT [I-D.ietf-stir-passport] is a token format based on JWT
   [RFC7519] for conveying cryptographically-signed information about
   the people involved in personal communications; it is used with STIR
   [I-D.ietf-stir-rfc4474bis] to convey a signed assertion of the
   identity of the participants in real-time communications established
   via a protocol like SIP.  This specification extends PASSporT to
   include an indication that a call has been diverted from its
   originally destination to a new one.

   Although the STIR problem statement [RFC7340] is focused on
   preventing the impersonation of the caller's identity, which is a
   common enabler for threats such as robocalling and voicemail hacking
   on the telephone network today, it also provides a signature over the
   called number as the authentication service sees it.  As
   [I-D.ietf-stir-rfc4474bis] Section 12.1 describes, this protection
   over the contents of the To header field is intended to prevent a
   class of cut-and-paste attacks.  If Alice calls Bob, for example, Bob
   might attempt to cut-and-paste the Identity header in Alice's INVITE
   into a new INVITE that Bob sends to Carol, and thus be able to fool
   Carol into thinking the call came from Alice and not Bob. With the
   signature over the To header, the INVITE Carol sees will clearly have
   been destined originally for Bob, and thus Carol can view the INVITE
   as suspect.

   However, as [I-D.ietf-stir-rfc4474bis] Section 12.1.1 points out, it
   is difficult for Carol to confirm or reject these suspicions based on
   the information she receives from the baseline PASSporT object.  The
   common "call forwarding" service serves as a good example of the fact

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   that the original called party number is not always the numebr to
   which a call is delivered.  The address in the To header field value
   of SIP requests is not supposed to change, accordingly to baseline
   [RFC3261], as it is the Request-URI that is supposed to be updated
   when a call is retargeted, but practically speaking some operational
   environments do alter the To header field.  There are a number of
   potential ways for intermediaries to indicate that such a forwarding
   operating has taken place.  The History-Info header field [RFC7044]
   was created to store the Request-URIs that are discarded by a call in
   transit.  The SIP Diversion header field [RFC5806], though historic,
   is still used for this purpose by some operators today.  Neither of
   these header fields provide any cryptographic assurance of secure
   redirection, and they can both capture minor syntactical changes in
   URIs that do not reflect a change to the actual target of a call.

   This specification therefore extends PASSporT with an explicit
   indication that original called number in PASSporT no longer reflects
   the destination to which a call is likely to be delivered.
   Verification services and the relying parties who make authorization
   decisions about communications may use this indication to confirm
   that a legitimate retargeting of the call has taken place, rather
   than a cut-and-paste attack.

2.  Terminology

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

3.  PASSporT 'div' Claim

   This specification defines a new JSON Web Token claim for "div" which
   indicates a previous destination for a call during its routing
   process.  When a retargeting entity receives a call signed with a
   PASSporT, it may act as an authentication service and create a new
   PASSporT containing the "div" claim to attach to the call (without
   removing the original PASSporT).  Note that a new PASSporT is only
   necessary when the cannical form of the "dest" identifier (per the
   canonicalization procedures in [I-D.ietf-stir-rfc4474bis] Section 8)
   changes due to this retargeting. "div" is typically populated with a
   destination address found in the "dest" field of PASSporT received by
   the retargeting entity.  These new PASSporT generated by retargeting
   entities MUST include the "div" PASSporT type, and an "x5u" field
   pointing to a credential that the retargeting entity controls.  The
   new PASSporT will look as follows:

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   { "typ":"passport",
     "x5u":"" }

   A PASSporT claims object containing "div" is populated with a
   modification of the original token before the call was retargeted: at
   a high level, the original identifier for the called party in the
   "dest" array will become the "div" claim in the new PASSporT.  If the
   "dest" array of the original PASSporT contains multiple identifiers,
   the retargeting entity MUST select only one them to occupy the "div"
   field in the new PASSporT. and in particular, it MUST select an
   identifier that is within the scope of the credential that the
   retargeting entity will specify in the "x5u" of the header (as
   described below).

   The new target for the call selected by the retargeting entity
   becomes the value of the "dest" array of the new PASSporT.  The
   "orig" value MUST be copied into the new PASSporT from the original
   PASSporT received by the retargeting entity; the regargeting entity
   SHOULD retain the "iat" value from the original PASSporT, though if
   in the underlying signaling protocol (e.g.  SIP) the retargeting
   entity changes the date and time information in the retargeted
   request, the new PASSporT should reflect that.  No other extension
   claims should be copied from the original PASSporT to the "div"

   So, for an original PASSporT of the form:

      { "orig":{"tn":"12155551212"},
        "iat":1443208345 }

   If the retargeting entity is changing the target from 12155551213 to
   12155551214, the new PASSporT with "div" would look as follows:

      { "orig":{"tn":"12155551212"},
        "div":{"tn":"121555551213"} }

   After the PASSporT header and claims have been constructed, their
   signature is generated per the guidance in [I-D.ietf-stir-passport] -
   except for the credential required to sign it.  While in the ordinary
   construction of a PASSporT, the credential used to sign will have
   authority over the identity in the "orig" claim (for example, a
   certificate with authority over the telephone number in "orig" per
   [I-D.ietf-stir-certificates]), for all PASSporTs using the "div" type

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   the signature MUST be created with a credential with authority over
   the identity present in the "div" claim.  So for the example above,
   where the original "dest" is "12155551213", the signer of the new
   PASSporT object MUST have authority over that telephone number, and
   need not have any authority over the telephone number present in the
   "orig" claim.

4.  Using 'div' in SIP

   This section specifies SIP-specific usage for the "div" PASSporT type
   and its handling in the SIP Identity header field "ppt" parameter
   value.  Other using protocols of PASSporT may define behvaior
   specific to their use of the "div" claim.

4.1.  Authentication Service Behavior

   An authentication service only adds an Identity header field
   containing the "div" PASSporT type to an SIP request that already
   contains at least one Identity header field; it MUST NOT add a "div"
   request to an INVITE that contains no other Identity headers fields.
   When adding an Identity header with a PASSporT object containing a
   "div" claim, SIP authentication services MUST also add a "ppt"
   parameter to that Identity header with a value of "div".  The
   resulting compact form Identity header might look as follows:

  Identity: ..sv5CTo05KqpSmtHt3dcEiO/1CWTSZtnG3iV+1nmurLXV/HmtyNS7Ltrg9dlxkWzo
      pPqOg1uXndzHbG7mR6Rl9BnUhHufVRbp51Mn3w0gfUs=; \

   A SIP authentication service typically will derive the new value of
   "dest" from a new Request-URI that is set for the SIP request before
   it is forwarded.  Older values of the Request-URI may appear in
   headers like Diversion or History-Info; this document specifies no
   specific interaction between the "div" mechanism and those SIP
   headers.  Note as well that because PASSporT operates on
   canonicalized telephone numbers and normalized URIs, many smaller
   changes to the syntax of identifiers that might be captured by other
   mechanisms (like History-Info) that record regargeting will likely
   not require a "div" PASSporT.

4.2.  Verification Service Behavior

   [I-D.ietf-stir-rfc4474bis] Section 6.2 Step 5 requires that
   specifications defining "ppt" values describe any additional verifier
   behavior.  The behavior specified for the "div" value of "ppt" is as

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   In order to use the "div" extension, a verificiation service needs to
   inspect all of the valid Identity headers associated with a request,
   as an Identity header containing "div" necessary refers to an earlier
   PASSporT associated with the request.  In particular, the
   verification service must find an earlier PASSporT associated with
   the call that contains a "dest" claim with a value equivalent to the
   "div" claim in the current PASSporT.  It is possible that this
   earlier PASSporT will also contain a "div", and that it will in turn
   chain to a still earlier PASSporT stored in a different Identity
   header.  Ultimately, by looking at this chain of transformations and
   validating the associated signatures, the verification service will
   be able to ascertain that the appropriate parties were responsible
   for the retargeting of the call to its ultimate destination; this can
   help the verification service to determine that original PASSporT in
   the call was not simply used in a cut-and-paste attack.  This will
   help relying parties to make any associated authorization decisions
   in terms of how the call will be treated - though those policies are
   outside the scope of this document.

5.  Extending 'div'

   Past experience has shown that there may be additional information
   about the motivation for retargeting that relying parties might
   consider when making authorization decisions about a call, see for
   example the "reason" associated with the SIP Diversion header field
   [RFC5806].  Future extensions to this specification might incorporate
   reasons into "div".

6.  Acknowledgments

   We would like to thank YOU for contributions to this problem
   statement and framework.

7.  IANA Considerations

   This specification requests that the IANA add a new claim to the JSON
   Web Token Claims registry as defined in [RFC7519].

   Claim Name: "div"

   Claim Description: New Target of a Call

   Change Controller: IESG

   Specification Document(s): [RFCThis]

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8.  Security Considerations

   This specification describes a security feature, and is primarily
   concerned with increasing security when calls are forwarded.
   Including information about how calls were retargeted during the
   routing process can reveal to downstream entities particulars of the
   policies used to route calls through the network.  But including this
   information about forwarding is at the discretion of the retargeting
   entity, so if there is a requirement to keep the original called
   number confidential, no PASSporT should be created for that
   retargeting - the only consequence will be that downstream entities
   will have less confidence that the PASSporT was meant to be
   associated with this call.

9.  Informative References

              Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", draft-ietf-stir-
              certificates-14 (work in progress), May 2017.

              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,

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

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   [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,

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

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

Author's Address

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


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