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Versions: (draft-campbell-oauth-resource-indicators) 00 01 02

OAuth Working Group                                          B. Campbell
Internet-Draft                                             Ping Identity
Intended status: Standards Track                              J. Bradley
Expires: April 22, 2019                                           Yubico
                                                           H. Tschofenig
                                                             Arm Limited
                                                        October 19, 2018


                   Resource Indicators for OAuth 2.0
                draft-ietf-oauth-resource-indicators-01

Abstract

   An extension to the OAuth 2.0 Authorization Framework defining
   request parameters that enable a client to explicitly signal to an
   authorization server about the location of the protected resource(s)
   to which it is requesting access.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 22, 2019.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of



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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
     1.1.  Requirements Notation and Conventions . . . . . . . . . .   3
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Resource Parameter  . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Authorization Request . . . . . . . . . . . . . . . . . .   5
     2.2.  Access Token Request  . . . . . . . . . . . . . . . . . .   6
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  OAuth Parameters Registration . . . . . . . . . . . . . .  10
     4.2.  OAuth Extensions Error Registration . . . . . . . . . . .  10
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  11
   Appendix B.  Document History . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   Several years of deployment and implementation experience with The
   OAuth 2.0 Authorization Framework [RFC6749] has uncovered a need, in
   some circumstances, for the client to explicitly signal to the
   authorization server where it intends to use the access token it is
   requesting.

   Knowing the protected resource (a.k.a. resource server, application,
   API, etc.) that will process the access token enables the
   authorization server to construct the token as necessary for that
   entity.  Properly encrypting the token (or content within the token)
   to a particular resource, for example, requires knowing which
   resource will receive and decrypt the token.  Furthermore, various
   resources oftentimes have different requirements with respect to the
   data contained in, or referenced by, the token and knowing the
   resource where the client intends to use the token allows the the
   authorization server to mint the token accordingly.

   Specific knowledge of the intended recipient(s) of the access token
   also helps facilitate improved security characteristics of the token
   itself.  Bearer tokens, currently the most commonly utilized type of
   OAuth access token, allow any party in possession of a token to get
   access to the associated resources.  To prevent misuse, several
   important security assumptions must hold, one of which is that an
   access token must only be valid for use at a specific protected



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   resource and for a specific scope of access.  Section 5.2 of
   [RFC6750], for example, prescribes including the token's intended
   recipients within the token to prevent token redirect.  When the
   authorization server is informed of the resource that will process
   the access token, it can restrict the intended audience of that token
   to the given resource such that the token cannot be used successfully
   at other resources.

   OAuth scope, from Section 3.3 of [RFC6749], is sometimes overloaded
   to convey the location or identity of the protected resource,
   however, doing so isn't always feasible or desirable.  Scope is
   typically about what access is being requested rather than where that
   access will be redeemed (e.g. "email", "admin:org", "user_photos",
   "channels:read", and "channels:write" are a small sample of scope
   values in use in the wild that convey only the type of access and not
   the location).

   In some circumstances and for some deployments, a means for the
   client to signal to the authorization server where it intends to use
   the access token it's requesting is important and useful.  A number
   of implementations and deployments of OAuth 2.0 have already employed
   proprietary parameters toward that end.  Going forward, this
   specification aspires to provide a standardized and interoperable
   alternative to the proprietary approaches.

1.1.  Requirements Notation and Conventions

   The key words "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.

1.2.  Terminology

   This specification uses the terms "access token", "refresh token",
   "authorization server", "resource server", "authorization endpoint",
   "authorization request", "authorization response", "token endpoint",
   "grant type", "access token request", "access token response", and
   "client" defined by The OAuth 2.0 Authorization Framework [RFC6749].

2.  Resource Parameter

   In requests to the authorization server, a client MAY indicate the
   protected resource (a.k.a. resource server, application, API, etc.)
   to which it is requesting access by including the following parameter
   in the request.




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   resource
      Indicates the location of the target service or resource where
      access is being requested.  Its value MUST be an absolute URI, as
      specified by Section 4.3 of [RFC3986], which MAY include a query
      component but MUST NOT include a fragment component.  Multiple
      "resource" parameters MAY be used to indicate that the requested
      token is intended to be used at multiple resources.

   The parameter value indicates the location of a protected resource,
   typically as an https URL, where the client is requesting access.
   This enables the authorization server to apply policy as appropriate
   for the resource, such as determining the type and content of tokens
   to be issued, if and how tokens are encrypted, and applying
   appropriate audience restrictions.

   The client SHOULD provide the most specific URI that it can for the
   complete API or set of resources it intends to access.  In practice a
   client will know a base URI for the application or resource that it
   interacts with, which is appropriate to use as the value of the
   "resource" parameter.  The client SHOULD use the base URI of the API
   as the "resource" parameter value unless specific knowledge of the
   resource dictates otherwise.  For example, the value
   "https://api.example.com/" would be used for a resource that is the
   exclusive application on that host, however, if the resource is one
   of many applications on that host, something like
   "https://api.example.com/app/" would be used as a more specific
   value.  Another example, for an API like SCIM [RFC7644] that has
   multiple endpoints such as "https://apps.example.com/scim/Users",
   "https://apps.example.com/scim/Groups", and
   "https://apps.example.com/scim/Schemas" The client would use
   "https://apps.example.com/scim/" as the resource so that the issued
   access token is valid for all the endpoints of the SCIM API.

   The following error code is provided for an authorization server to
   indicate problems with the requested resource(s) in response to an
   authorization request or access token request.  And can also be used
   to inform the client that it has requested an invalid combination of
   resource and scope.

   invalid_target
      The requested resource is invalid, unknown, or malformed.

   The authorization server SHOULD audience restrict issued access
   tokens to the resource(s) indicated by the "resource" parameter.
   Audience restrictions can be communicated in JSON Web Tokens
   [RFC7519] with the "aud" claim and the top-level member of the same
   name provides the audience restriction information in a Token
   Introspection [RFC7662] response.  The authorization server may use



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   the exact "resource" value as the audience or it may map from that
   value to a more general URI or abstract identifier for the given
   resource.

2.1.  Authorization Request

   When the "resource" parameter is used in an authorization request to
   the authorization endpoint, it indicates the location of the
   protected resource(s) to which access is being requested.  When an
   access token will be returned directly from the authorization
   endpoint via the implicit flow (Section 4.2 of OAuth 2.0 [RFC6749]),
   the requested resource is applicable to that access token.  In the
   code flow (Section 4.1 of OAuth 2.0 [RFC6749]) where an an
   intermediate representation of the authorization grant (the
   authorization code) is returned from the authorization endpoint, the
   requested resource is applicable to the full authorization grant.

   For authorization requests sent as a JWTs, such as when using JWT
   Secured Authorization Request [I-D.ietf-oauth-jwsreq], a single
   "resource" parameter value is represented as a JSON string while
   multiple values are represented as an array of strings.

   If the client omits the "resource" parameter when requesting
   authorization, the authorization server MAY process the request with
   no specific resource or by using a pre-defined default resource
   value.  Alternatively, the authorization server MAY require clients
   to specify the resource(s) they intend to access and MAY fail
   requests that omit the parameter with an "invalid_target" error.  The
   authorization server might use this data to inform the user about the
   resources the client is going to access on her behalf, to meet policy
   decision (e.g. refuse the request due to unknown resources), and
   determine the set of resources that can be used in subsequent access
   token requests.

   If the authorization server fails to parse the provided value(s) or
   does not consider the resource(s) acceptable, it should reject the
   request with an an error response using the error code
   "invalid_target" as the value of the "error" parameter and can
   provide additional information regarding the reasons for the error
   using the "error_description" and/or "error_uri" parameters.

   An example of an authorization request where the client tells the
   authorization server that it wants an access token for use at
   "https://api.example.com/app/" is shown in Figure 1 below (extra line
   breaks and indentation are for display purposes only).






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     GET /as/authorization.oauth2?response_type=token
        &client_id=example-client
        &state=XzZaJlcwYew1u0QBrRv_Gw
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
        &resource=https%3A%2F%2Fapi.example.com%2Fapp%2F HTTP/1.1
     Host: authorization-server.example.com

               Figure 1: Implicit Flow Authorization Request

   Below in Figure 2 is an example of an authorization request using the
   "code" response type where the the client is requesting access to the
   resource owner's contacts and calendar data at
   "https://cal.example.com/" and "https://contacts.example.com/" (extra
   line breaks and indentation are for display purposes only).

     GET /as/authorization.oauth2?response_type=code
        &client_id=s6BhdRkqt3
        &state=tNwzQ87pC6llebpmac_IDeeq-mCR2wLDYljHUZUAWuI
        &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
        &scope=calendar%20contacts
        &resource=https%3A%2F%2Fcal.example.com%2F
        &resource=https%3A%2F%2Fcontacts.example.com%2F HTTP/1.1
     Host: authorization-server.example.com

                 Figure 2: Code Flow Authorization Request

2.2.  Access Token Request

   When the "resource" parameter is used on an access token request made
   to the token endpoint, for all grant types, it indicates the location
   of the target service or protected resource where the client intends
   to use the requested access token.

   The resource value(s) that are acceptable to an authorization server
   in fulfilling an access token request are at its sole discretion
   based on local policy or configuration.  In the case of a
   "refresh_token" or "authorization_code" grant type request, such
   policy may limit the acceptable resources to those that were
   originally granted by the resource owner or a subset thereof.  In the
   "authorization_code" case where the requested resources are a subset
   of the set of resources originally granted, the authorization server
   will issue an access token based on that subset of requested
   resources while any refresh token that is returned is bound to the
   full original grant.

   When requesting a token, the client can indicate the desired target
   service(s) where it intends to use that token by way of the
   "resource" parameter and can indicate the desired scope of the



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   requested token using the "scope" parameter.  The semantics of such a
   request are that the client is asking for a token with the requested
   scope that is usable at all the requested target services.
   Effectively, the requested access rights of the token are the
   cartesian product of all the scopes at all the target services.  To
   the extent possible, when issuing access tokens, the authorization
   server should adapt the scope value associated with an access token
   to the value the respective resource is able to process and needs to
   know.  This further improves privacy as scope values give an
   indication of what services the resource owner uses and it improves
   security as scope values may contain confidential data.  As specified
   in Section 5.1 of [RFC6749], the authorization server must indicate
   the access token's effective scope to the client in the "scope"
   response parameter value when it differs from the scope requested by
   the client.

   Following from the code flow authorization request shown in Figure 2,
   the below examples show an "authorization_code" grant type access
   token request and response where the client tells the authorization
   server that it wants the access token for use at
   "https://cal.example.com/" (extra line breaks and indentation are for
   display purposes only).

     POST /as/token.oauth2 HTTP/1.1
     Host: authorization-server.example.com
     Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
     Content-Type: application/x-www-form-urlencoded

     grant_type=authorization_code
     &redirect_uri=https%3A%2F%2Fclient%2Eexample%2Eorg%2Fcb
     &code=10esc29BWC2qZB0acc9v8zAv9ltc2pko105tQauZ
     &resource=https%3A%2F%2Fcal.example.com%2F

                      Figure 3: Access Token Request

















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      HTTP/1.1 200 OK
      Content-Type: application/json
      Cache-Control: no-cache, no-store

      {
         "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
          JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
          iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgwMCwic2NvcGUiOiJjYWxlbmRhciIs
          ImF1ZCI6Imh0dHBzOi8vY2FsLmV4YW1wbGUuY29tLyJ9.nNWJ2dXSxaDRdMUK
          lzs-cYIj8MDoM6Gy7pf_sKrLGsAFf1C2bDhB60DQfW1DZL5npdko1_Mmk5sUf
          zkiQNVpYw",
         "token_type":"Bearer",
         "expires_in":3600,
         "refresh_token":"4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2",
         "scope":"calendar"
      }

                      Figure 4: Access Token Response

   A subsequent access token request, using the refresh token, where the
   client tells the authorization server that it wants an access token
   for use at "https://contacts.example.com/" is shown in Figure 5 below
   with the response shown in Figure 6 (extra line breaks and
   indentation are for display purposes only).

     POST /as/token.oauth2 HTTP/1.1
     Host: authorization-server.example.com
     Authorization: Basic czZCaGRSa3F0Mzpoc3FFelFsVW9IQUU5cHg0RlNyNHlJ
     Content-Type: application/x-www-form-urlencoded

     grant_type=refresh_token
     &refresh_token=4LTC8lb0acc6Oy4esc1Nk9BWC0imAwH7kic16BDC2
     &resource=https%3A%2F%2Fcontacts.example.com%2Fapp%2F

                      Figure 5: Access Token Request
















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      HTTP/1.1 200 OK
      Content-Type: application/json
      Cache-Control: no-cache, no-store

      {
         "access_token":"eyJhbGciOiJFUzI1NiIsImtpZCI6Ijc3In0.eyJpc3MiOi
          JodHRwOi8vYXV0aG9yaXphdGlvbi1zZXJ2ZXIuZXhhbXBsZS5jb20iLCJzdWI
          iOiJfX2JfYyIsImV4cCI6MTU4ODQyMDgyNiwic2NvcGUiOiJjb250YWN0cyIs
          ImF1ZCI6Imh0dHBzOi8vY29udGFjdHMuZXhhbXBsZS5jb20vIn0.5f4yhqazc
          OSlJw4y94KPeWNEFQqj2cfeO8x4hr3YbHtIl3nQXnBMw5wREY5O1YbZED-GfH
          UowfmtNaA5EikYAw",
         "token_type":"Bearer",
         "expires_in":3600,
         "scope":"contacts"
      }

                      Figure 6: Access Token Response

3.  Security Considerations

   An access token that is audience restricted to a protected resource
   that obtains that token legitimately cannot be used to access
   resources on behalf of the resource owner at other protected
   resources.  The "resource" parameter enables a client to indicate the
   protected resources where the requested access token will be used,
   which in turn enables the authorization server to apply the
   appropriate audience restrictions to the token.

   Some servers may host user content or be multi-tenant.  In order to
   avoid attacks that might confuse a client into sending an access
   token to a resource that is user controlled or is owned by a
   different tenant, it is important to use a specific resource URI
   including a path component.  This will cause any access token issued
   for accessing the user controlled resource to have a invalid audience
   if replayed against the legitimate resource API.

   Although multiple occurrences of the "resource" parameter may be
   included in a request, using only a single "resource" parameter is
   encouraged.  A bearer token that has multiple intended recipients
   (audiences) can be used by any one of those recipients at any other.
   Thus, a high degree of trust between the involved parties is needed
   when using access tokens with multiple audiences.  Furthermore an
   authorization server may be unwilling or unable to fulfill a token
   request with multiple resources.







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4.  IANA Considerations

4.1.  OAuth Parameters Registration

   This specification registers the following value in the IANA "OAuth
   Parameters" registry [IANA.OAuth.Parameters] established by
   [RFC6749].

   o  Parameter name: resource
   o  Parameter usage location: authorization request, token request
      [[TODO: draft-ietf-oauth-token-exchange will have already
      registered this for 'token request' and this draft has a more
      generalized usage and needs to somehow either update that
      registration or do a partial registration and reference]]
   o  Change controller: IESG
   o  Specification document(s): [[ this specification ]]

4.2.  OAuth Extensions Error Registration

   This specification registers the following error in the IANA "OAuth
   Extensions Error Registry" [IANA.OAuth.Parameters] established by
   [RFC6749].

   o  Error name: invalid_target
   o  Error usage location: implicit grant error response, token error
      response [[TODO: draft-ietf-oauth-token-exchange will have already
      registered this for 'token error response' and this draft has a
      more generalized usage and needs to somehow either update that
      registration or do a partial registration and reference]]
   o  Related protocol extension: resource parameter
   o  Change controller: IESG
   o  Specification document(s): [[ this specification ]]

5.  References

5.1.  Normative References

   [IANA.OAuth.Parameters]
              IANA, "OAuth Parameters",
              <http://www.iana.org/assignments/oauth-parameters>.

   [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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   [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,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/info/rfc6749>.

   [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>.

5.2.  Informative References

   [I-D.ietf-oauth-jwsreq]
              Sakimura, N. and J. Bradley, "The OAuth 2.0 Authorization
              Framework: JWT Secured Authorization Request (JAR)",
              draft-ietf-oauth-jwsreq-16 (work in progress), April 2018.

   [RFC6750]  Jones, M. and D. Hardt, "The OAuth 2.0 Authorization
              Framework: Bearer Token Usage", RFC 6750,
              DOI 10.17487/RFC6750, October 2012,
              <https://www.rfc-editor.org/info/rfc6750>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,
              <https://www.rfc-editor.org/info/rfc7519>.

   [RFC7644]  Hunt, P., Ed., Grizzle, K., Ansari, M., Wahlstroem, E.,
              and C. Mortimore, "System for Cross-domain Identity
              Management: Protocol", RFC 7644, DOI 10.17487/RFC7644,
              September 2015, <https://www.rfc-editor.org/info/rfc7644>.

   [RFC7662]  Richer, J., Ed., "OAuth 2.0 Token Introspection",
              RFC 7662, DOI 10.17487/RFC7662, October 2015,
              <https://www.rfc-editor.org/info/rfc7662>.

Appendix A.  Acknowledgements

   This specification was developed within the OAuth Working Group under
   the chairmanship of Hannes Tschofenig and Rifaat Shekh-Yusef with
   Eric Rescorla and Benjamin Kaduk serving as Security Area Directors.
   Additionally, the following individuals contributed ideas, feedback,
   and wording that helped shape this specification:

   Sergey Beryozkin, William Denniss, Vladimir Dzhuvinov, George
   Fletcher, Dick Hardt, Phil Hunt, Michael Jones, Torsten Lodderstedt,



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   Anthony Nadalin, Justin Richer, Nat Sakimura, Filip Skokan, and Hans
   Zandbelt.

Appendix B.  Document History

   [[ to be removed by the RFC Editor before publication as an RFC ]]

   draft-ietf-oauth-resource-indicators-01

   o  Significant rework of the main section of the document attempting
      to clarify a number of things that came up at, around and after
      IETF 102 and the call for adoption.
   o  Change the "invalid_resource" error to "invalid_target" to align
      with draft-ietf-oauth-token-exchange, which has some overlap in
      functionality.
   o  Allow the "resource" parameter value to have a query component
      (aligning with draft-ietf-oauth-token-exchange).
   o  Moved the Security Considerations section to before the IANA
      Considerations.
   o  Other editorial updates.
   o  Rework the Acknowledgements section.
   o  Use RFC 8174 boilerplate.

   draft-ietf-oauth-resource-indicators-00

   o  First version of the working group document.  A replica of draft-
      campbell-oauth-resource-indicators-02.

   draft-campbell-oauth-resource-indicators-02

   o  No changes.

   draft-campbell-oauth-resource-indicators-01

   o  Move Hannes Tschofenig, who wrote https://tools.ietf.org/html/
      draft-tschofenig-oauth-audience in '13, from Acknowledgements to
      Authors.
   o  Added IANA Considerations to register the "resource" parameter and
      "invalid_resource" error code.

   draft-campbell-oauth-resource-indicators-00

   o  Initial draft to define a resource parameter for OAuth 2.0.








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Authors' Addresses

   Brian Campbell
   Ping Identity

   Email: brian.d.campbell@gmail.com


   John Bradley
   Yubico

   Email: ve7jtb@ve7jtb.com


   Hannes Tschofenig
   Arm Limited

   Email: hannes.tschofenig@gmx.net

































Campbell, et al.         Expires April 22, 2019                [Page 13]


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