Handling of Identity Header Errors for Secure Telephone Identity Revisited (STIR)
draft-ietf-stir-identity-header-errors-handling-08

STIR Working Group                                              C. Wendt
Internet-Draft                                                Somos Inc.
Intended status: Standards Track                        25 February 2023
Expires: 29 August 2023

                Identity Header Errors Handling for STIR
           draft-ietf-stir-identity-header-errors-handling-08

Abstract

   This document extends STIR and the Authenticated Identity Management
   in the Session Initiation Protocol (SIP) error handling procedures to
   include the mapping of verification failure reasons to STIR defined
   4xx codes so the failure reason of an Identity header field can be
   conveyed to the upstream authentication service when local policy
   dictates that the call should continue in the presence of a
   verification failure.  This document also defines procedures that
   enable a failure reason to be mapped to a specific Identity header
   field for scenarios that use multiple Identity header fields where
   some may have errors and others may not and the handling of those
   situations is defined.

Status of This Memo

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   This Internet-Draft will expire on 29 August 2023.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Reason header field protocol "STIR" . . . . . . . . . . . . .   3
   4.  Use of provisional response to signal errors without
           terminating the call  . . . . . . . . . . . . . . . . . .   3
   5.  Handling of a verification error when there are multiple
           Identity header fields  . . . . . . . . . . . . . . . . .   4
   6.  Handling multiple verification errors . . . . . . . . . . . .   5
   7.  Removal of the Reason header field by Authentication
           Service . . . . . . . . . . . . . . . . . . . . . . . . .   5
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .   7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   The STIR framework as described in [RFC7340] is an authentication
   framework for asserting a telephone number or URI based identity
   using a digital signature and certificate based framework as
   described in [RFC8225] and [RFC8226] respectively.  [RFC8224]
   describes the use of the STIR framework in the SIP protocol [RFC3261]
   and defines both the authentication service that creates a PASSporT,
   defined in [RFC8225], and delivers it in an Identity header field and
   the verification service that correspondingly verifies the PASSporT
   and embedded originating identity.

   This document is concerned with errors in validating PASSporTs and
   Identity header fields and how they are communicated in special cases
   and defines a solution to help address the potential issue of
   multiple Identity header fields and the plurality of potential
   verification errors.  Additionally, it addresses the issue of the
   current 4xx error response and that when there is a verification
   error, the call is terminated.  In some deployments, it may be the
   case that the policy for handling errors dictates that calls should
   continue even if there is a verification error.  In many cases of,
   for example, inadvertent or operational errors that do not represent

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   any identity falsification type of attempt, the policy of continuing
   the call even though the identity is not verified, may be the
   preferred policy.  In these cases, the authentication service should
   still be notified of the error so that corrective action can be taken
   to fix any issues.  This specification will discuss the use of the
   Reason header field in subsequent provisional (1xx) responses in
   order to deliver the error back to the authentication service or
   other SIP path network equipment responsible for error handling.

   For the handling of multiple Identity header fields and the potential
   situation that some of the Identity header fields in a call may pass
   verification but others may have errors, this document defines the
   method of adding an identifier so that the authentication service can
   uniquely identify which Identity header field is being referred to in
   the case of an error.

2.  Terminology

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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.  Reason header field protocol "STIR"

   This document defines a new Reason header field [RFC3326] protocol
   "STIR" for STIR applications using SIP as defined in [RFC8224].  The
   use of "STIR" as a reason header field protocol with the [RFC8224]
   defined error cause codes allows the use of multiple Reason header
   fields defined in [RFC3326] and updated in
   [I-D.ietf-sipcore-multiple-reasons].  Any provisional SIP Response
   message or final response message, with the exception of a 100
   (Trying), MAY contain one or more Reason header fields with a STIR
   related cause code defined in [RFC8224] or future specifications.
   The use of multiple Reason header field is discussed in more detail
   later in the document.

4.  Use of provisional response to signal errors without terminating the
    call

   In cases where local policy dictates that a call should continue
   regardless of any verification errors that may have occured,
   including 4XX errors described in [RFC8224] Section 6.2.2, then the
   verification service MUST NOT send the 4XX as a response, but rather
   include the error response code and reason phrase in a Reason header
   field, defined in [RFC3326], in the next provisional or final
   responses sent to the authentication service.

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   Example Reason header field:

   Reason: STIR ;cause=436 ;text="Bad Identity Info"

5.  Handling of a verification error when there are multiple Identity
    header fields

   In cases where a SIP message includes multiple Identity header fields
   and one of those Identity header fields has an error, the
   verification service MUST include the error response code and reason
   phrase associated with the error in a Reason header field, defined in
   [RFC3326], in the next provisional or final responses sent to the
   authentication service.  The reason cause in the Reason header field
   MUST represent the error that occurred when verifying the contents of
   the Identity header field.  For a SIP INVITE containing multiple
   Identity header fields, the "ppi" parameter for the Reason header
   field is RECOMMENDED.  As defined in [RFC8224], the STIR error codes
   used in responses are based on an error associated with a specific
   identity header field representing a single error occurring with the
   verification and processing of that identity header field.  The
   association of a "ppi" parameter with a Reason header field using
   "STIR" protocol MUST only identify a single cause code in the context
   of a call dialog defined in [RFC8224] or in future documents defining
   STIR related errors.  The associated PASSporT object can be included
   either in full form or in compact form, where only the signature of
   the PASSporT is included with two periods as a prefix as defined in
   [RFC8225] Section 7 to identify the reported Identity header field
   with an error.  Compact form is the recommended form as full form may
   include information that could have privacy or security implications
   in some call scenarios as discussed in Section 9.

   Example Reason header field with full form PASSporT:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi= \
   "eyJhbGciOiJFUzI1NiIsInR5cCI6InBhc3Nwb3J0IiwieDV1I \
   joiaHR0cHM6Ly9jZXJ0LmV4YW1wbGUub3JnL3Bhc3Nwb3J0LmNlciJ9.eyJ \
   kZXN0Ijp7InVyaSI6WyJzaXA6YWxpY2VAZXhhbXBsZS5jb20iXX0sImlhdC \
   I6IjE0NDMyMDgzNDUiLCJvcmlnIjp7InRuIjoiMTIxNTU1NTEyMTIifX0.r \
   q3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
   ojNCpTzO3QfPOlckGaS6hEck7w"

   Example Reason header field with compact form PASSporT:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi= \
   "..rq3pjT1akEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYs \
   ojNCpTzO3QfPOlckGaS6hEck7w"

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6.  Handling multiple verification errors

   If there are multiple Identity header field verification errors being
   reported the verification service MUST include a corresponding number
   of Reason header fields per error.  These Reason header fields should
   include a "ppi" parameters including the full or compact form of the
   PASSporT with cause and text parameters identifying each error.  As
   mentioned previously, the potential use of multiple Reason header
   fields defined in [RFC3326] is updated in
   [I-D.ietf-sipcore-multiple-reasons] allowing multiple Reason header
   fields with the same protocol value.  For this specification, "STIR"
   should be used for any STIR error defined in [RFC8224] or future
   specifications.

   Example Reason header fields for two identity info errors:

   Reason: STIR ;cause=436 ;text="Bad Identity Info" ;ppi=     \
   "..rq3pjT1hoRwakEGjHCnWSwUnshd0-zJ6F1VOgFWSjHBr8Qjpjlk-cpFY \
   pFYsojNCpTzO3QfPOlckGaS6hEck7w"

   Reason: STIR ;cause=438 ;text="Invalid Identity Header" ;ppi=  \
   "..rJ6F1VOgFWSjHBr8Qjpjlk-cpFYpFYsq3pjT1hoRwakEGjHCnWSwUnsh \
   d0-zckGaS6hEck7wojNCpTzO3QfPOl"

7.  Removal of the Reason header field by Authentication Service

   When an Authentication Service [RFC8224] receives the Reason header
   field with a PASSporT it generated as part of an Identity header
   field and the authentication of a call, it should first follow local
   policy to recognize and acknowledge the error (e.g. perform
   operational actions like logging or alarming), but then MUST remove
   the identified Reason header field to avoid the PASSporT information
   from going upstream to a UAC or UAS that may not be authorized to see
   claim information contained in the PASSporT for privacy or other
   reasons.

8.  IANA Considerations

   This document requests the definition of a new protocol value (and
   associated protocol cause) to be registered by the IANA into the
   "Reason Protocols" sub-registry under
   http://www.iana.org/assignments/sip-parameters as follows:

   Protocol Value   Protocol Cause            Reference
   --------------   ---------------           -----------
   STIR             STIR Error code           RFC 8224

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   This document also requests the definition of a new header field
   parameter name to be registered by IANA into the Header Field
   Parameters and Parameter Values sub-registry under
   https://www.iana.org/assignments/sip-parameters as follows:

   Header Field   Parameter Name   Predefined Values  Reference
   ------------   --------------   -----------------  ---------
   Reason         ppi               No                RFC THIS

9.  Security Considerations

   This specification discusses the use of a PASSporT as an identifier
   for cases where there are multiple identity header field errors
   occuring as part of the Reason header field response.  For some call
   scenarios (e.g. diversion based call flows) the signer of the
   PASSporT(s) may not be the first hop initiator of the call.  In those
   cases, there may be some security or privacy concerns associated with
   PASSporT information that is passed upstream beyond the
   authentication service that originally signed the PASSporT(s) in the
   resulting error Reason header field.  This specification states the
   authentication service MUST remove the Reason header field with the
   PASSporT to protect the security (e.g. use of potentially still fresh
   PASSporT for replay attacks) and privacy of any potential information
   that could be passed beyond the authentication service response back
   in the direction of the call initiator.  While this specification
   allows for both full and compact form of the PASSporT to be used as
   the error identifier, use of the compact form is RECOMMENDED to avoid
   the potential exposure of call information contained in the full form
   of the PASSporT.

10.  References

10.1.  Normative References

   [I-D.ietf-sipcore-multiple-reasons]
              Sparks, R., "Multiple SIP Reason Header Field Values",
              Work in Progress, Internet-Draft, draft-ietf-sipcore-
              multiple-reasons-01, 23 August 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-sipcore-
              multiple-reasons-01>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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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,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3326]  Schulzrinne, H., Oran, D., and G. Camarillo, "The Reason
              Header Field for the Session Initiation Protocol (SIP)",
              RFC 3326, DOI 10.17487/RFC3326, December 2002,
              <https://www.rfc-editor.org/info/rfc3326>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [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,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8225]  Wendt, C. and J. Peterson, "PASSporT: Personal Assertion
              Token", RFC 8225, DOI 10.17487/RFC8225, February 2018,
              <https://www.rfc-editor.org/info/rfc8225>.

   [RFC8226]  Peterson, J. and S. Turner, "Secure Telephone Identity
              Credentials: Certificates", RFC 8226,
              DOI 10.17487/RFC8226, February 2018,
              <https://www.rfc-editor.org/info/rfc8226>.

10.2.  Informative References

   [RFC7340]  Peterson, J., Schulzrinne, H., and H. Tschofenig, "Secure
              Telephone Identity Problem Statement and Requirements",
              RFC 7340, DOI 10.17487/RFC7340, September 2014,
              <https://www.rfc-editor.org/info/rfc7340>.

Appendix A.  Acknowledgements

   The author would like to thank David Hancock for help to identify
   these error scenarios and additionally Jon Peterson, Roman Shpount,
   Robert Sparks, Christer Holmberg and others in the STIR working group
   for their helpful feedback and discussion.

Author's Address

   Chris Wendt
   Somos Inc.

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   Email: chris-ietf@chriswendt.net

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