Grant Negotiation and Authorization Protocol Resource Server Connections
draft-ietf-gnap-resource-servers-00

GNAP                                                      J. Richer, Ed.
Internet-Draft                                       Bespoke Engineering
Intended status: Standards Track                              A. Parecki
Expires: 30 October 2021                                            Okta
                                                              F. Imbault
                                                                acert.io
                                                           28 April 2021

Grant Negotiation and Authorization Protocol Resource Server Connections
                  draft-ietf-gnap-resource-servers-00

Abstract

   GNAP defines a mechanism for delegating authorization to a piece of
   software, and conveying that delegation to the software.  This
   extension defines methods for resource servers (RS) to communicate
   with authorization servers (AS) in an interoperable fashion.

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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   Drafts is at https://datatracker.ietf.org/drafts/current/.

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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 30 October 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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
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   extracted from this document must 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

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Access Token Formats  . . . . . . . . . . . . . . . . . . . .   3
   3.  Resource-Server-Facing API  . . . . . . . . . . . . . . . . .   4
     3.1.  RS-facing AS Discovery  . . . . . . . . . . . . . . . . .   4
     3.2.  Protecting RS requests to the AS  . . . . . . . . . . . .   4
     3.3.  Token Introspection . . . . . . . . . . . . . . . . . . .   5
     3.4.  Registering a Resource Handle . . . . . . . . . . . . . .   6
   4.  Deriving a downstream token . . . . . . . . . . . . . . . . .   8
   5.  Requesting Resources With Insufficient Access . . . . . . . .  10
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  11
   Appendix A.  Document History . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   The core GNAP protocol does not define any one specific mechanism for
   the resource server (RS) to communicate with the authorization server
   (AS), allowing the connection between these components to be solved
   orthogonally to the core protocol's concerns.  For example, the RS
   and AS roles could be fulfilled by the same piece of software with
   common storage, obviating the need for any connecting protocol.
   However, it is often desirable to have the RS and AS communicate at
   runtime for a variety of purposes, including allowing the RS to
   validate and understand the rights and privileges associated with a
   grant of access represented by an access token issued by (AS), or
   negotiating the capabilities of either party.  These types of
   connections are particularly useful for connecting an AS and RS from
   different vendors, allowing interoperable distributed deployments of
   GNAP-protected systems.

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   This specification defines several means for a RS and AS to
   communicate these aspects with each other, including structured
   access tokens and RS-facing APIs.  This specification also discusses
   methods for an RS to derive a downstream token for calling another
   chained RS as well as a client-facing discovery mechanism that can be
   used to bootstrap the GNAP process when the client instance does not
   know which AS protects a given RS.

   The means of the authorization server issuing the access token to the
   client instance and the means of the client instance presenting the
   access token to the resource server are the subject of the GNAP core
   protocol specification [I-D.ietf-gnap-core-protocol].

1.1.  Terminology

   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.

   This document contains non-normative examples of partial and complete
   HTTP messages, JSON structures, URLs, query components, keys, and
   other elements.  Some examples use a single trailing backslash '' to
   indicate line wrapping for long values, as per [RFC8792].  The "\"
   character and leading spaces on wrapped lines are not part of the
   value.

2.  Access Token Formats

   When the AS issues an access token for use at an RS, the RS needs to
   have some means of understanding what the access token is for in
   order to determine how to respond to the request.  The core GNAP
   protocol makes no assumptions or demands on the format or contents of
   the access token, but such token formats can be the topic of
   agreements between the AS and RS.

   Self-contained structured token formats allow for the conveyance of
   information between the AS and RS without requiring the RS to call
   the AS at runtime as described in Section 3.3.

   Some token formats, such as Macaroons and Biscuits, allow for the RS
   to derive sub-tokens without having to call the AS as described in
   Section 4.

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3.  Resource-Server-Facing API

   To facilitate runtime and dynamic connections, the AS can offer an
   RS-Facing API consisting of one or more of the following optional
   pieces.

   *  Discovery

   *  Introspection

   *  Token chaining

   *  Resource reference registration

3.1.  RS-facing AS Discovery

   A GNAP AS offering RS-facing services can publish its features on a
   well-known discovery document using the URL ".well-known/gnap-as-rs".

   This endpoint contains a JSON document [RFC8259] consisting of a
   single JSON object with any combination of the following optional
   fields:

   introspection_endpoint:  The URL of the endpoint offering
      introspection.  Section 3.3

   token_formats_supported:  A list of token formats supported by this
      AS.

   resource_registration_endpoint:  The URL of the endpoint offering
      resource registration.  Section 3.4

   grant_endpoint:  The grant endpoint of the GNAP AS.

3.2.  Protecting RS requests to the AS

   Unless otherwise specified, the RS protects its calls to the AS using
   any of the signature methods defined by GNAP.  This signing method
   MUST cover all of the appropriate portions of the HTTP request
   message, including any body elements, tokens, or headers required for
   functionality.

   The AS MAY require an RS to pre-register its keys or could
   alternatively allow calls from arbitrary keys, in a trust-on-first-
   use model.  The RS MAY present its keys by reference or by value in
   the same fashion as a client instance calling the AS in the core
   protocol of GNAP [I-D.ietf-gnap-core-protocol].

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3.3.  Token Introspection

   The AS issues access tokens representing a set of delegated access
   rights to be used at one or more RSs.  The AS can offer an
   introspection service to allow an RS to validate that a given access
   token:

   *  has been issued by the AS

   *  has not expired

   *  has not been revoked

   *  is appropriate for the RS identified in the call

   When the RS receives an access token, it can call the introspection
   endpoint at the AS to get token information.  [[ See issue #115
   (https://github.com/ietf-wg-gnap/gnap-core-protocol/issues/115) ]]

   +--------+       +------+       +------+
   | Client |--(1)->|  RS  |       |  AS  |
   |Instance|       |      |--(2)->|      |
   |        |       |      |       |      |
   |        |       |      |<-(3)--|      |
   |        |       |      |       +------+
   |        |<-(4)--|      |
   +--------+       +------+

   1.  The client instance calls the RS with its access token.

   2.  The RS introspects the access token value at the AS.  The RS
       signs the request with its own key (not the client instance's key
       or the token's key).

   3.  The AS validates the access token value and the client instance's
       request and returns the introspection response for the token.

   4.  The RS fulfills the request from the client instance.

   The RS signs the request with its own key and sends the access token
   as the body of the request.

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   POST /introspect HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Detached-JWS: ejy0...

   {
       "access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0",
       "proof": "httpsig",
       "resource_server": "7C7C4AZ9KHRS6X63AJAO"
   }

   The AS responds with a data structure describing the token's current
   state and any information the RS would need to validate the token's
   presentation, such as its intended proofing mechanism and key
   material.  The response MAY include any fields defined in an access
   token response.

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

   {
       "active": true,
       "access": [
           "dolphin-metadata", "some other thing"
       ],
       "key": {
           "proof": "httpsig",
           "jwk": {
                   "kty": "RSA",
                   "e": "AQAB",
                   "kid": "xyz-1",
                   "alg": "RS256",
                   "n": "kOB5rR4Jv0GMeL...."
           }
       }
   }

3.4.  Registering a Resource Handle

   If the RS needs to, it can post a set of resources as described in
   the Resource Access Rights section of [I-D.ietf-gnap-core-protocol]
   to the AS's resource registration endpoint.

   The RS MUST identify itself with its own key and sign the request.

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   POST /resource HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Detached-JWS: ejy0...

   {
       "access": [
           {
               "actions": [
                   "read",
                   "write",
                   "dolphin"
               ],
               "locations": [
                   "https://server.example.net/",
                   "https://resource.local/other"
               ],
               "datatypes": [
                   "metadata",
                   "images"
               ]
           },
           "dolphin-metadata"
       ],
       "resource_server": "7C7C4AZ9KHRS6X63AJAO"

   }

   The AS responds with a handle appropriate to represent the resources
   list that the RS presented.

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

   {
       "resource_handle": "FWWIKYBQ6U56NL1"
   }

   The RS MAY make this handle available as part of a response
   (Section 5) or as documentation to developers.

   [[ See issue #117 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/117) ]]

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4.  Deriving a downstream token

   Some architectures require an RS to act as a client instance and use
   a derived access token for a secondary RS.  Since the RS is not the
   same entity that made the initial grant request, the RS is not
   capable of referencing or modifying the existing grant.  As such, the
   RS needs to request or generate a new token access token for its use
   at the secondary RS.  This internal secondary token is issued in the
   context of the incoming access token.

   While it is possible to use a [token format]{#structure} that allows
   for the RS to generate its own secondary token, the AS can allow the
   RS to request this secondary access token using the same process used
   by the original client instance to request the primary access token.
   Since the RS is acting as its own client instance from the
   perspective of GNAP, this process uses the same grant endpoint,
   request structure, and response structure as a client instance's
   request.

   +--------+       +-------+       +------+       +-------+
   | Client |--(1)->|  RS1  |       |  AS  |       |  RS2  |
   |Instance|       |       |--(2)->|      |       |       |
   |        |       |       |<-(3)--|      |       |       |
   |        |       |       |       +------+       |       |
   |        |       |       |                      |       |
   |        |       |       |-----------(4)------->|       |
   |        |       |       |<----------(5)--------|       |
   |        |<-(6)--|       |                      |       |
   +--------+       +-------+                      +-------+

   1.  The client instance calls RS1 with an access token.

   2.  RS1 presents that token to the AS to get a derived token for use
       at RS2.  RS1 indicates that it has no ability to interact with
       the RO.  Note that RS1 signs its request with its own key, not
       the token's key or the client instance's key.

   3.  The AS returns a derived token to RS1 for use at RS2.

   4.  RS1 calls RS2 with the token from (3).

   5.  RS2 fulfills the call from RS1.

   6.  RS1 fulfills the call from the original client instance.

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   If the RS needs to derive a token from one presented to it, it can
   request one from the AS by making a token request as described in
   [I-D.ietf-gnap-core-protocol] and presenting the existing access
   token's value in the "existing_access_token" field.

   Since the RS is acting as a client instance, the RS MUST identify
   itself with its own key in the "client" field and sign the request
   just as any client instance would.

   [[ See issue #116 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/116) ]]

 POST /tx HTTP/1.1
 Host: server.example.com
 Content-Type: application/json
 Detached-JWS: ejy0...

 {
     "access_token": {
         "access": [
             {
                 "actions": [
                     "read",
                     "write",
                     "dolphin"
                 ],
                 "locations": [
                     "https://server.example.net/",
                     "https://resource.local/other"
                 ],
                 "datatypes": [
                     "metadata",
                     "images"
                 ]
             },
             "dolphin-metadata"
         ]
     },
     "client": "7C7C4AZ9KHRS6X63AJAO",
     "existing_access_token": "OS9M2PMHKUR64TB8N6BW7OZB8CDFONP219RP1LT0"
 }

   The AS responds with a token for the downstream RS2 as described in
   [I-D.ietf-gnap-core-protocol].  The downstream RS2 could repeat this
   process as necessary for calling further RS's.

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5.  Requesting Resources With Insufficient Access

   If the client instance calls an RS without an access token, or with
   an invalid access token, the RS MAY respond to the client instance
   with an authentication header indicating that GNAP needs to be used
   to access the resource.  The address of the GNAP endpoint MUST be
   sent in the "as_uri" parameter.  The RS MAY additionally return a
   resource reference that the client instance MAY use in its access
   token request.  This resource reference handle SHOULD be sufficient
   for at least the action the client instance was attempting to take at
   the RS.  The RS MAY use the dynamic resource handle request
   (Section 3.4) to register a new resource handle, or use a handle that
   has been pre-configured to represent what the RS is protecting.  The
   content of this handle is opaque to the RS and the client instance in
   both cases.

   WWW-Authenticate: \
     GNAP as_uri=https://server.example/tx,access=FWWIKYBQ6U56NL1

   The client instance then makes a call to the "as_uri" as described in
   [I-D.ietf-gnap-core-protocol], with the value of "access" as one of
   the members of the "access" array in the "access_token" portion of
   the request.  The client instance MAY request additional resources
   and other information, and MAY request multiple access tokens.

   POST /tx HTTP/1.1
   Host: server.example.com
   Content-Type: application/json
   Detached-JWS: ejy0...

   {
       "access_token": {
           "access": [
               "FWWIKYBQ6U56NL1",
               "dolphin-metadata"
           ]
       },
       "client": "KHRS6X63AJ7C7C4AZ9AO"
   }

   [[ See issue #118 (https://github.com/ietf-wg-gnap/gnap-core-
   protocol/issues/118) ]]

6.  Acknowledgements

   (TODO: the ACK section should probably be split between the
   documents)

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7.  IANA Considerations

   [[ TBD: There are a lot of items in the document that are expandable
   through the use of value registries. ]]

8.  Security Considerations

   [[ TBD: There are a lot of security considerations to add. ]]

   All requests have to be over TLS or equivalent as per [BCP195].  Many
   handles act as shared secrets, though they can be combined with a
   requirement to provide proof of a key as well.

9.  Privacy Considerations

   [[ TBD: There are a lot of privacy considerations to add. ]]

   When introspection is used, the AS is made aware of a particular
   token being used at a particular AS, and the AS would not otherwise
   have insight into this.

   When the client instance receives information about the protecting AS
   from an RS, this can be used to derive information about the
   resources being protected without releasing the resources themselves.

10.  Normative References

   [BCP195]   Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", May 2015,
              <https://www.rfc-editor.org/info/bcp195>.

   [I-D.ietf-gnap-core-protocol]
              Richer, J., Parecki, A., and F. Imbault, "Grant
              Negotiation and Authorization Protocol", Work in Progress,
              Internet-Draft, draft-ietf-gnap-core-protocol-04, 22
              February 2021, <https://www.ietf.org/archive/id/draft-
              ietf-gnap-core-protocol-04.txt>.

   [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>.

   [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>.

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   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/info/rfc8259>.

   [RFC8792]  Watsen, K., Auerswald, E., Farrel, A., and Q. Wu,
              "Handling Long Lines in Content of Internet-Drafts and
              RFCs", RFC 8792, DOI 10.17487/RFC8792, June 2020,
              <https://www.rfc-editor.org/info/rfc8792>.

Appendix A.  Document History

   *  -00

      -  Extracted resource server section.

Authors' Addresses

   Justin Richer (editor)
   Bespoke Engineering

   Email: ietf@justin.richer.org
   URI:   https://bspk.io/

   Aaron Parecki
   Okta

   Email: aaron@parecki.com
   URI:   https://aaronparecki.com

   Fabien Imbault
   acert.io

   Email: fabien.imbault@acert.io
   URI:   https://acert.io/

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