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Versions: (draft-gerdes-core-dcaf-authorize) 00 01 02 03 04

ACE Working Group                                              S. Gerdes
Internet-Draft                                               O. Bergmann
Intended status: Standards Track                              C. Bormann
Expires: September 10, 2015                      Universitaet Bremen TZI
                                                          March 09, 2015


    Delegated CoAP Authentication and Authorization Framework (DCAF)
                   draft-gerdes-ace-dcaf-authorize-02

Abstract

   This specification defines a protocol for delegating client
   authentication and authorization in a constrained environment for
   establishing a Datagram Transport Layer Security (DTLS) channel
   between resource-constrained nodes.  The protocol relies on DTLS to
   transfer authorization information and shared secrets for symmetric
   cryptography between entities in a constrained network.  A resource-
   constrained node can use this protocol to delegate authentication of
   communication peers and management of authorization information to a
   trusted host with less severe limitations regarding processing power
   and memory.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   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 September 10, 2015.

Copyright Notice

   Copyright (c) 2015 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Features  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.1.  Actors  . . . . . . . . . . . . . . . . . . . . . . .   4
       1.2.2.  Other Terms . . . . . . . . . . . . . . . . . . . . .   5
   2.  System Overview . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .   7
     3.2.  Unauthorized Resource Request Message . . . . . . . . . .   8
     3.3.  SAM Information Message . . . . . . . . . . . . . . . . .   9
     3.4.  Access Request  . . . . . . . . . . . . . . . . . . . . .  10
     3.5.  Ticket Request Message  . . . . . . . . . . . . . . . . .  11
     3.6.  Ticket Grant Message  . . . . . . . . . . . . . . . . . .  12
     3.7.  Ticket Transfer Message . . . . . . . . . . . . . . . . .  13
     3.8.  DTLS Channel Setup Between C and S  . . . . . . . . . . .  14
     3.9.  Authorized Resource Request Message . . . . . . . . . . .  15
     3.10. Dynamic Update of Authorization Information . . . . . . .  16
       3.10.1.  Handling of Ticket Transfer Messages . . . . . . . .  17
   4.  Ticket  . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.1.  Face  . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     4.2.  CI  . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     4.3.  Revocation  . . . . . . . . . . . . . . . . . . . . . . .  19
       4.3.1.  Lifetime  . . . . . . . . . . . . . . . . . . . . . .  19
       4.3.2.  Revocation Messages . . . . . . . . . . . . . . . . .  20
   5.  Payload Format and Encoding (application/dcaf+cbor) . . . . .  20
     5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  23
   6.  DTLS PSK Generation Methods . . . . . . . . . . . . . . . . .  25
     6.1.  DTLS PSK Transfer . . . . . . . . . . . . . . . . . . . .  25
     6.2.  Distributed Key Derivation  . . . . . . . . . . . . . . .  25
   7.  Authorization Configuration . . . . . . . . . . . . . . . . .  26
   8.  Trust Relationships . . . . . . . . . . . . . . . . . . . . .  26
   9.  Listing Authorization Manager Information in a Resource
       Directory . . . . . . . . . . . . . . . . . . . . . . . . . .  27
     9.1.  The "auth-request" Link Relation  . . . . . . . . . . . .  27
   10. Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
     10.1.  Access Granted . . . . . . . . . . . . . . . . . . . . .  28
     10.2.  Access Denied  . . . . . . . . . . . . . . . . . . . . .  30
     10.3.  Access Restricted  . . . . . . . . . . . . . . . . . . .  31
     10.4.  Implicit Authorization . . . . . . . . . . . . . . . . .  32



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   11. Specific Usage Scenarios  . . . . . . . . . . . . . . . . . .  33
     11.1.  Combined Authorization Manager and Client  . . . . . . .  33
       11.1.1.  Creating the Ticket Request Message  . . . . . . . .  33
       11.1.2.  Processing the Ticket Grant Message  . . . . . . . .  34
     11.2.  Combined Client Authorization Manager and Server
            Authorization Manager  . . . . . . . . . . . . . . . . .  34
       11.2.1.  Processing the Access Request Message  . . . . . . .  35
       11.2.2.  Creating the Ticket Transfer Message . . . . . . . .  35
     11.3.  Combined Server Authorization Manager and Server . . . .  35
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  36
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  36
     13.1.  DTLS PSK Key Generation Methods  . . . . . . . . . . . .  37
     13.2.  dcaf+cbor Media Type Registration  . . . . . . . . . . .  37
     13.3.  CoAP Content Format Registration . . . . . . . . . . . .  38
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  38
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     14.2.  Informative References . . . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  40

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a transfer
   protocol similar to HTTP which is designed for the special
   requirements of constrained environments.  A serious problem with
   constrained devices is the realization of secure communication.  The
   devices only have limited system resources such as memory, stable
   storage (such as disk space) and transmission capacity and often lack
   input/output devices such as keyboards or displays.  Therefore, they
   are not readily capable of using common protocols.  Especially
   authentication mechanisms are difficult to realize, because the lack
   of stable storage severely limits the number of keys the system can
   store.  Moreover, CoAP has no mechanism for authorization.

   [I-D.gerdes-ace-actors] describes an architecture that is designed to
   help constrained nodes with authorization-related tasks by
   introducing less-constrained nodes.  These Authorization Managers
   perform complex security tasks for their nodes such as managing keys
   for numerous devices, and enable the constrained nodes to enforce the
   authorization policies of their principals.

   DCAF uses access tokens to implement this architecture.  A device
   that wants to access an item of interest on a constrained node first
   has to gain permission in the form of a token from the node's
   Authorization Manager.

   As fine-grained authorization is not always needed on constrained
   devices, DCAF supports an implicit authorization mode where no
   authorization information is exchanged.



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   The main goals of DCAF are the setup of a Datagram Transport Layer
   Security (DTLS) [RFC6347] channel with symmetric pre-shared keys
   (PSK) [RFC4279] between two nodes and to securely transmit
   authorization tickets.

1.1.  Features

   o  Utilize DTLS communication with pre-shared keys.

   o  Authenticated exchange of authorization information.

   o  Simplified authentication on constrained nodes by handing the more
      sophisticated authentication over to less-constrained devices.

   o  Support of secure constrained device to constrained device
      communication.

   o  Authorization policies of the principals of both participating
      parties are ensured.

   o  Simplified authorization mechanism for cases where implicit
      authorization is sufficient.

   o  Using only symmetric encryption on constrained nodes.

1.2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   Readers are expected to be familiar with the terms and concepts
   defined in [I-D.gerdes-ace-actors].

1.2.1.  Actors

   Server (S):   An endpoint that hosts and represents a CoAP resource.

   Client (C):   An endpoint that attempts to access a CoAP resource on
      the Server.

   Server Authorization Manager (SAM):  An entity that prepares and
      endorses authentication and authorization data for a Server.

   Client Authorization Manager (CAM):   An entity that prepares and
      endorses authentication and authorization data for a Client.

   Authorization Manager (AM):  An entity that is either a SAM or a CAM.



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   Client Overseeing Principal (COP):  The principal that is in charge
      of the Client and controls permissions concerning authorized
      representations of a CoAP resource.

   Resource Overseeing Principal (ROP):  The principal that is in charge
      of the CoAP resource and controls its access permissions.

1.2.2.  Other Terms

   Resource:  A CoAP resource.

   Authorization information:   Contains all information needed by S to
      decide if C is privileged to access a resource in a specific way.

   Authentication information:   Contains all information needed by S to
      decide if the entity in possession of a certain key is verified by
      SAM.

   Access information:   Contains authentication information and, if
      necessary, authorization information.

   Access ticket:   Contains the authentication and, if necessary, the
      authorization information needed to access a resource.  A Ticket
      consists of the Ticket Face and the Client Information.  The
      access ticket is a representation of the access information.

   Ticket Face:   The part of the ticket which is generated for the
      Server.  It contains the authorization information and all
      information needed by the Server to verify that it was granted by
      SAM.

   Client Information (CI):  The part of the ticket which is generated
      for the Client.  It contains the Verifier and optionally may
      contain authorization information that represent COP's
      authorization policies for C.

   Verifier:   It enables the client to verify that it is communicating
      with an appropriate S.

   Explicit authorization:  SAM informs the S in detail which privileges
      are granted to the Client.

   Implicit authorization:   SAM authenticates the Client for the Server
      without specifying the privileges in detail.  This can be used for
      binary or unrestricted authorization (cf section 4 of
      [I-D.gerdes-ace-actors]).





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2.  System Overview

   Within the DCAF Architecture each Server (S) has a Server
   Authorization Manger (SAM) which conducts the authentication and
   authorization for S.  S and SAM share a symmetric key which has to be
   exchanged initially to provide for a secure channel.  The mechanism
   used for this is not in the scope of this document.

   To gain access to a specific resource on a S, a Client (C) has to
   request an access ticket from the SAM serving S either directly or,
   if it is a constrained device, using its Client Authorization Manager
   (CAM).  In the following, we always discuss the CAM role separately,
   even if that is co-located within a (more powerful) C (see section
   Section 11 for details about co-located actors).

   CAM decides if S is an authorized source for R according to the
   policies set by COP and in this case transmits the request to SAM.
   If SAM decides that C is allowed to access the resource according to
   the policies set by ROP, it generates a DTLS pre-shared key (PSK) for
   the communication between C and S and wraps it into an access ticket.
   For explicit access control, SAM adds the detailed access permissions
   to the ticket in a way that CAM and S can interpret.  CAM checks if
   the permissions in the access ticket comply with COP's authorization
   policies for C, and if this is the case sends it to C.  After C
   presented the ticket to S, C and S can communicate securely.

   To be able to provide for the authentication and authorization
   services, an Authorization Manager has to fulfill several
   requirements:

   o  AM must have enough stable storage (such as disk space) to store
      the necessary number of credentials (matching the number of
      Clients and Servers).

   o  AM must possess means for user interaction, for example directly
      or indirectly connected input/output devices such as keyboard and
      display, to allow for configuration of authorization information
      by the respective Principal.

   o  AM must have enough processing power to handle the authorization
      requests for all constrained devices it is responsible for.

3.  Protocol

   The DCAF protocol comprises three parts:

   1.  transfer of authentication and, if necessary, authorization
       information between C and S;



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   2.  transfer of access requests and the respective ticket grants
       between C and CAM; and

   3.  transfer of access requests and the respective ticket grants
       between SAM and CAM.

3.1.  Overview

   In Figure 1, a DCAF protocol flow is depicted (messages in square
   brackets are optional):

    CAM                   C                    S                   SAM
     | <== DTLS chan. ==> |                    | <== DTLS chan. ==> |
     |                    | [Resource Req.-->] |                    |
     |                    |                    |                    |
     |                    | [<-- SAM Info.]    |                    |
     |                    |                    |                    |
     | <-- Access Req.    |                    |                    |
     |                    |                    |                    |
     | <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> |
     |                    |                    |                    |
     | Ticket Request   ------------------------------------------> |
     |                    |                    |                    |
     | <------------------------------------------    Ticket Grant  |
     |                    |                    |                    |
     | Ticket Transf. --> |                    |                    |
     |                    |                    |                    |
     |                    | <== DTLS chan. ==> |                    |
     |                    | Auth. Res. Req. -> |                    |


                        Figure 1: Protocol Overview

   To determine the SAM in charge of a resource hosted at the S, C MAY
   send an initial Unauthorized Resource Request message to S.  S then
   denies the request and sends the address of its SAM back to C.

   Instead of the initial Unauthorized Resource Request message, C MAY
   look up the desired resource in a resource directory (cf.
   [I-D.ietf-core-resource-directory]) that lists S's resources as
   discussed in Section 9.

   Once C knows SAM's address, it can send a request for authorization
   to SAM using its own CAM.  CAM and SAM authenticate each other and
   each determine if the request is to be authorized.  If it is, SAM
   generates an access ticket for C.  The ticket contains keying
   material for the establishment of a secure channel and, if necessary,
   a representation of the permissions C has for the resource.  C keeps



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   one part of the access ticket and presents the other part to S to
   prove its right to access.  With their respective parts of the
   ticket, C and S are able to establish a secure channel.

   The following sections specify how CoAP is used to interchange
   access-related data between S and SAM so that SAM can provide C and S
   with sufficient information to establish a secure channel, and
   simultaneously convey authorization information specific for this
   communication relationship to S.

   Note:  Special implementation considerations apply when one single
      entity takes the role of more than one actors.  Section 11 gives
      additional advice on some of these usage scenarios.

   This document uses Concise Binary Object Representation (CBOR,
   [RFC7049]) to express authorization information as set of attributes
   passed in CoAP payloads.  Notation and encoding options are discussed
   in Section 5.

3.2.  Unauthorized Resource Request Message

   The optional Unauthorized Resource Request message is a request for a
   resource hosted by S for which no proper authorization is granted.  S
   MUST treat any CoAP request as Unauthorized Resource Request message
   when any of the following holds:

   o  The request has been received on an insecure channel.

   o  S has no valid access ticket for the sender of the request
      regarding the requested action on that resource.

   o  S has a valid access ticket for the sender of the request, but
      this does not allow the requested action on the requested
      resource.

   Note: These conditions ensure that S can handle requests autonomously
   once access was granted and a secure channel has been established
   between C and S.

   Unauthorized Resource Request messages MUST be denied with a client
   error response.  In this response, the Server MUST provide proper SAM
   Information to enable the Client to request an access ticket from S's
   SAM as described in Section 3.3.

   The response code MUST be 4.01 (Unauthorized) in case the sender of
   the Unauthorized Resource Request message is not authenticated, or if
   S has no valid access ticket for C.  If S has an access ticket for C
   but not for the resource that C has requested, S MUST reject the



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   request with a 4.03 (Forbidden).  If S has an access ticket for C but
   it does not cover the action C requested on the resource, S MUST
   reject the request with a 4.05 (Method Not Allowed).

   Note:  The use of the response codes 4.03 and 4.05 is intended to
      prevent infinite loops where a dumb Client optimistically tries to
      access a requested resource with any access token received from
      the SAM.  As malicious clients could pretend to be C to determine
      C's privileges, these detailed response codes must be used only
      when a certain level of security is already available which can be
      achieved only when the Client is authenticated.

3.3.  SAM Information Message

   The SAM Information Message is sent by S as a response to an
   Unauthorized Resource Request message (see Section 3.2) to point the
   sender of the Unauthorized Resource Request message to S's SAM.  The
   SAM information is a set of attributes containing an absolute URI
   (see Section 4.3 of [RFC3986]) that specifies the SAM in charge of S.

   The message MAY also contain a timestamp generated by S.

   Figure 2 shows an example for an SAM Information message payload
   using CBOR diagnostic notation.  (Refer to Section 5 for a detailed
   description of the available attributes and their semantics.)

       4.01 Unauthorized
       Content-Format: application/dcaf+cbor
       {SAM: "coaps://sam.example.com/authorize", TS: 168537}

                 Figure 2: SAM Information Payload Example

   In this example, the attribute SAM points the receiver of this
   message to the URI "coaps://sam.example.com/authorize" to request
   access permissions.  The originator of the SAM Information payload
   (i.e.  S) uses a local clock that is loosely synchronized with a time
   scale common between S and SAM (e.g., wall clock time).  Therefore,
   it has included a time stamp on its own time scale that is used as a
   nonce for replay attack prevention.  Refer to Section 4.1 for more
   details concerning the usage of time stamps to ensure freshness of
   access tickets.

   The examples in this document are written in CBOR diagnostic notation
   to improve readability.  Figure 3 illustrates the binary encoding of
   the message payload shown in Figure 2.






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   a2                                      # map(2)
          00                                   # unsigned(0) (=SAM)
          78 21                                # text(33)
             636f6170733a2f2f73616d2e6578
             616d706c652e636f6d2f617574686f72
             697a65             # "coaps://sam.example.com/authorize"
          05                                   # unsigned(5) (=TS)
          1a 00029259                          # unsigned(168537)

         Figure 3: SAM Information Payload Example encoded in CBOR

3.4.  Access Request

   To retrieve an access ticket for the resource that C wants to access,
   C sends an Access Request to its CAM.  The Access Request is
   constructed as follows:

   1.  The request method is POST.

   2.  The request URI is set as described below.

   3.  The message payload contains a data structure that describes the
       action and resource for which C requests an access ticket.

   The request URI identifies a resource at CAM for handling
   authorization requests from C.  The URI SHOULD be announced by CAM in
   its resource directory as described in Section 9.

   Note:  Where capacity limitations of C do not allow for resource
      directory lookups, the request URI in Access Requests could be
      hard-coded during provisioning or set in a specific device
      configuration profile.

   The message payload is constructed from the SAM information that S
   has returned in its SAM Information message (see Section 3.3) and
   information that C provides to describe its intended request(s).  The
   Access Request MUST contain the following attributes:

   1.  Contact information for the SAM to use.

   2.  An absolute URI of the resource that C wants to access.

   3.  The actions that C wants to perform on the resource.

   4.  Any time stamp generated by S.






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   An example Access Request from C to CAM is depicted in Figure 4.
   (Refer to Section 5 for a detailed description of the available
   attributes and their semantics.)

      POST client-authorize
      Content-Format: application/dcaf+cbor
      {
        SAM: "coaps://sam.example.com/authorize",
        SAI: ["coaps://temp451.example.com/s/tempC", 5],
        TS: 168537
      }

                 Figure 4: Access Request Message Example

   The example shows an Access Request message payload for the resource
   "/s/tempC" on the Server "temp451.example.com".  Requested operations
   in attribute SAI are GET and PUT.

   The attributes SAM (that denotes the Server Authorization Manager to
   use) and TS (a nonce generated by S) are taken from the SAM
   Information message from S.

   The response to an Authorization Request is delivered by CAM back to
   C in a Ticket Transfer message.

3.5.  Ticket Request Message

   When CAM receives an Access Request message from C and COP specified
   authorization policies for C, CAM MUST check if the requested actions
   are allowed according to these policies.  If this is not the case,
   CAM MUST send a 4.03 response.

   If no authorization policies were specified or the requested action
   is allowed according to the authorization policies, CAM either
   returns a cached response or attempts to create a Ticket Request
   message.

   CAM MAY return a cached response if it is known to be fresh according
   to Max-Age. CAM SHOULD NOT return a cached response if it expires in
   less than a minute.

   If CAM does not send a cached response, it checks whether the request
   payload is of type "application/dcaf+cbor and contains at least the
   fields SAM and SAI.  CAM MUST respond with 4.00 (Bad Request) if the
   type is "application/dcaf+cbor and any of these fields is missing or
   does not conform to the format described in Section 5.  Content
   formats other than application/dcaf+cbor are out of scope of this
   specification.



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   If the payload is correct, CAM creates a Ticket Request message from
   the Access Request received from C as follows:

   1.  The destination of the Ticket Request message is derived from the
       authority information in the URI contained in field "SAM" of the
       Access Request message payload.

   2.  The request method is POST.

   3.  The request URI is constructed from the SAM field received in the
       Access Request message payload.

   4.  The payload is copied from the Access Request sent by C.

   5.  A label that describes the Client is added to the payload

   To send the Ticket Request message to SAM a secure channel between
   CAM and SAM MUST be used.  Depending on the URI scheme used in the
   SAM field of the Access Request message payload (the less-constrained
   devices CAM and SAM do not necessarily use CoAP to communicate with
   each other), this could be, e.g., a DTLS channel (for "coaps") or a
   TLS connection (for "https").  CAM and SAM MUST be able to mutually
   authenticate each other, e.g. based on a public key infrastructure.
   (Refer to Section 8 for a detailed discussion of the trust
   relationship between Client Authorization Managers and Server
   Authorization Managers.)

3.6.  Ticket Grant Message

   When SAM has received a Ticket Request message it has to evaluate the
   access request information contained therein.  First, it checks
   whether the request payload is of type "application/dcaf+cbor" and
   contains at least the fields SAM and SAI.  SAM MUST respond with 4.00
   (Bad Request) for CoAP (or 400 for HTTP) if the type is "application/
   dcaf+cbor" and any of these fields is missing or does not conform to
   the format described in Section 5.

   SAM decides whether or not access is granted to the requested
   resource and then creates a Ticket Grant message that reflects the
   result.  To grant access to the requested resource, SAM creates an
   access ticket comprised of a Face and the Client Information as
   described in Section 4.1.

   The Ticket Grant message then is constructed as a success response
   indicating attached content, i.e. 2.05 for CoAP, or 200 for HTTP,
   respectively.  The payload of the Ticket Grant message is a data
   structure that contains the result of the access request.  When
   access is granted, the data structure contains the Ticket Face, the



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   Client Information, which at this point only consists of the Verifier
   and the Session Key Generation Method.

   The Ticket Grant message MAY provide cache-control options to enable
   intermediaries to cache the response.  The message MAY be cached
   according to the rules defined in [RFC7252] to facilitate ticket
   retrieval when C has crashed and wants to recover the DTLS session
   with S.

   SAM SHOULD set Max-Age according to the ticket lifetime in its
   response (Ticket Grant Message).

   Figure 5 shows an example Ticket Grant message using CoAP.  The Face/
   Verifier information is transferred as a CBOR data structure as
   specified in Section 5.  The Max-Age option tells the receiving CAM
   how long this ticket will be valid.

      2.05 Content
      Content-Format: application/dcaf+cbor
      Max-Age: 86400
      { F: {
              SAI: [ "/s/tempC", 7 ],
              TS: 0("2013-07-10T10:04:12.391"),
              L:  86400,
              G: hmac_sha256
        },
        V: h'f89947160c73601c7a65cb5e08812026
             6d0f0565160e3ff7d3907441cdf44cc9'
      }

                  Figure 5: Example Ticket Grant Message

   A Ticket Grant message that declines any operation on the requested
   resource is illustrated in Figure 6.  As no ticket needs to be
   issued, an empty payload is included with the response.

       2.05 Content
       Content-Format: application/dcaf+cbor

            Figure 6: Example Ticket Grant Message With Reject

3.7.  Ticket Transfer Message

   A Ticket Transfer message delivers the access information sent by SAM
   in a Ticket Grant message to the requesting client C.  The Ticket
   Transfer message is the response to the Access Request message sent
   from C to CAM and includes any access information from SAM contained
   in the Ticket Grant message.



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   The Authorization Information provided by SAM in the Ticket Grant
   Message may grant more permissions than C has requested.  The
   authorization policies of COP and ROP may differ: COP might want
   restrict the resources C is allowed to access, and the actions that C
   is allowed to perform on the resource.

   If CAM must ensure authorization policies COP configured , CAM MUST
   add Authorization Information for C (CAI) to the CI.  Since C and CAM
   use a DTLS channel for communication, the autorization information
   does not need to be encrypted.

   CAM includes the Face and Verifier sent by SAM in the Ticket Transfer
   message.  CAM MUST NOT include any other information SAM provided.
   In particular, CAM MUST NOT include any CAI information provided by
   SAM.

   Figure 7 shows an example Ticket Transfer message that conveys the
   permissions for actions GET, POST, PUT (but not DELETE) on the
   resource "/s/tempC" in field SAI.  As CAM only wants to permit
   outbound GET requests, it restricts C's permissions in the field CAI
   accordingly.

      2.05 Content
      Content-Format: application/dcaf+cbor
      Max-Age: 86400
      { F: {
              SAI: [ "/s/tempC", 7 ],
              TS: 0("2013-07-10T10:04:12.391"),
              L:  86400,
              G: hmac_sha256
        },
        V: h'f89947160c73601c7a65cb5e08812026
             6d0f0565160e3ff7d3907441cdf44cc9'
        CAI: [ "/s/tempC", 1 ],
        TS: 0("2013-07-10T10:04:12.855"),
        L:  86400
      }

                 Figure 7: Example Ticket Transfer Message

3.8.  DTLS Channel Setup Between C and S

   Using the information contained in a positive response to its Access
   Request (i.e. a Ticket Transfer message that contains a Face and a
   Client Information), C can initiate establishment of a new DTLS
   channel with S.  To use DTLS with pre-shared keys, C follows the PSK
   key exchange algorithm specified in Section 2 of [RFC4279], with the
   following additional requirements:



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   1.  C sets the psk_identity field of the ClientKeyExchange message to
       the ticket Face received in the Ticket Transfer message.

   2.  C uses the ticket Verifier as PSK when constructing the premaster
       secret.

   Note1: As S cannot provide C with a meaningful PSK identity hint in
   response to C's ClientHello message, S SHOULD NOT send a
   ServerKeyExchange message.

   Note2: According to [RFC7252], CoAP implementations MUST support the
   ciphersuite TLS_PSK_WITH_AES_128_CCM_8 [RFC6655].  C is therefore
   expected to offer at least this ciphersuite to S.

   Note3: The ticket is constructed by SAM such that S can derive the
   authorization information as well as the PSK (refer to Section 6 for
   details).

3.9.  Authorized Resource Request Message

   If the Client Information in the Ticket Transfer message contains
   CAI, C MUST ensure that it only sends requests that according to them
   are allowed.  C therefore MUST check CAI, L and T before every
   request.  If CAI is no longer valid according to L, C MUST terminate
   the DTLS connection with S and re-request the CAI from CAM using an
   Access Request Message.

   On the Server side, successful establishment of the DTLS channel
   between C and S ties the SAM authorization information contained in
   the psk_identity field to this channel.  Any request that S receives
   on this channel is checked against these authorization rules.
   Incoming CoAP requests that are not Authorized Resource Requests MUST
   be rejected by S with 4.01 response as described in Section 3.2.

   S SHOULD treat an incoming CoAP request as Authorized Resource
   Request if the following holds:

   1.  The message was received on a secure channel that has been
       established using the procedure defined in Section 3.8.

   2.  The authorization information tied to the secure channel is
       valid.

   3.  The request is destined for S.

   4.  The resource URI specified in the request is covered by the
       authorization information.




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   5.  The request method is an authorized action on the resource with
       respect to the authorization information.

   Note that the authorization information is not restricted to a single
   resource URI.  For example, role-based authorization can be used to
   authorize a collection of semantically connected resources
   simultaneously.  Implicit authorization also provides access rights
   to authenticated clients for all actions on all resources that S
   offers.  As a result, C can use the same DTLS channel not only for
   subsequent requests for the same resource (e.g. for block-wise
   transfer as defined in [I-D.ietf-core-block] or refreshing observe-
   relationships [I-D.ietf-core-observe]) but also for requests to
   distinct resources.

   Incoming CoAP requests received on a secure channel according to the
   procedure defined in Section 3.8 MUST be rejected

   1.  with response code 4.03 (Forbidden) when the resource URI
       specified in the request is not covered by the authorization
       information, and

   2.  with response code 4.05 (Method Not Allowed) when the resource
       URI specified in the request covered by the authorization
       information but not the requested action.

   Since SAM may limit the set of requested actions in its Ticket Grant
   message, C cannot know a priori if an Authorized Resource Request
   will succeed.

3.10.  Dynamic Update of Authorization Information

   Once a security association exists between a Client and a Resource
   Server, the Client can update the Authorization Information stored at
   the Server at any time.  To do so, the Client creates a new Access
   Request for the intended action on the respective resource and sends
   this request to its CAM which checks and relays this request to the
   Server's SAM as described in Section 3.4.

   Note:  Requesting a new Access Ticket also can be a Client's reaction
      on a 4.03 or 4.05 error that it has received in response to an
      Authorized Resource Request.

   Figure 8 depicts the message flow where C requests a new Access
   Tickets after a security association between C and S has been
   established using this protocol.






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    CAM                   C                    S                   SAM
     | <== DTLS chan. ==> | <== DTLS chan. ==> | <== DTLS chan. ==> |
     |                    |                    |                    |
     |                    | [Unauth. R. Req->] |                    |
     |                    |[<- 4.0x+SAM Info.] |                    |
     |                    |                    |                    |
     | <-- Access Req.    |                    |                    |
     |                    |                    |                    |
     | <==== TLS/DTLS channel (CAM/SAM Mutual Authentication) ====> |
     |                    |                    |                    |
     | Ticket Request   ------------------------------------------> |
     |                    |                    |                    |
     | <------------------------------------------    Ticket Grant  |
     |                    |                    |                    |
     | Ticket Transf. --> |                    |                    |
     |                    |                    |                    |
     |                    | <== Update SAI ==> |                    |


              Figure 8: Overview of Dynamic Update Operation

   Processing the Ticket Request is done at the SAM as specified in
   Section 3.6, i.e. the SAM checks whether or not the requested
   operation is permitted by the Resource Principal's policy, and then
   return a Ticket Grant message with the result of this check.  If
   access is granted, the Ticket Grant message contains an Access Ticket
   comprised of a public Ticket Face and a private Ticket Verifier.
   This authorization payload is relayed by CAM to the Client in a
   Ticket Transfer Message as defined in Section 3.7.

   The major difference between dynamic update of Authorization
   Information and the initial handshake is the handling of a Ticket
   Transfer message by the Client that is described in Section 3.10.1.

3.10.1.  Handling of Ticket Transfer Messages

   If the security association with S still exists and S has indicated
   support for session renegotiation according to [RFC5746], the ticket
   Face SHOULD be used to renegotiate the existing DTLS session.  In
   this case, the ticket Face is used as psk_identity as defined in
   Section 3.8.  Otherwise, the Client MUST perform a new DTLS handshake
   according to Section 3.8 that replaces the existing DTLS session.

   After successful completion of the DTLS handshake S updates the
   existing SAM Authorization Information for C according to the
   contents of the ticket Face.





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   Note:  No mutual authentication between C and S is required for
      dynamic updates when a DTLS channel exists that has been
      established as defined in Section 3.8.  S only needs to verify the
      authenticity and integrity of the ticket Face issued by SAM which
      is achieved by having performed a successful DTLS handshake with
      the ticket Face as psk_identity.  This could even be done within
      the existing DTLS session by tunneling a CoDTLS
      [I-D.schmertmann-dice-codtls] handshake.

4.  Ticket

   Access tokens in DCAF are tickets that consist of two parts, namely
   the Face and the Client Information.  The Face goes to S, the CI goes
   to the Client.  The Face and the CI are parts of the same ticket.

   S only needs the information contained in the Ticket Face to
   authorize the client and make sure that SAM generated the Ticket Face
   (S cannot make authorization decisions by itself and hence needs SAM
   to do it).  No additional information about the Client is needed.  S
   keeps the Ticket Face as long as it is valid.

4.1.  Face

   Face is the part of the ticket generated for S.  Face MUST contain
   all information needed for authorized access to a resource:

   o  SAM Authorization Information

   o  A timestamp generated by SAM

   Optionally, Face MAY also contain:

   o  A lifetime (optional)

   o  A DTLS pre-shared key (optional)

   S MUST verify the integrity of Face, i.e. the information contained
   in Face stems from SAM and was not manipulated by anyone else.

   Face MUST contain a timestamp to verify that the contained
   information is fresh.  As constrained devices may not have a clock,
   timestamps MAY be generated using the clock ticks since the last
   reboot.  To circumvent synchronization problems the timestamp MAY be
   generated by S and included in the first SAM Information message.
   Alternatively, SAM MAY generate the timestamp.  In this case, SAM and
   S MUST use a time synchronization mechanism to make sure that S
   interprets the timestamp correctly.




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   Face MAY be encrypted.  If Face contains a DTLS PSK, the whole
   content of Face MUST be encrypted.

   The integrity of Face can be ensured by various means.  Face may be
   encrypted by SAM with a key it shares with S.  Alternatively, S can
   use a mechanism to generate the DTLS PSK which includes Face.  S
   generates the key from the Face it received.  The correct key can
   only be calculated with the correct Face (refer to Section 6 for
   details).

4.2.  CI

   The CI part of the ticket is generated for C.  It contains the
   Verifier, i.e. the DTLS PSK for C and MAY contain Client
   Authorization Information generated by CAM to provide the COP's
   authorization policies to C.  The Verifier MUST NOT be transmitted
   over insecure channels.

4.3.  Revocation

   The existence of access tickets SHOULD be limited in time to avoid
   stale tickets that waste resources on S and C.  This can be achieved
   either by explicit Revocation Messages to invalidate a ticket or
   implicitly by attaching a lifetime to the ticket.

4.3.1.  Lifetime

   Tickets MAY have a lifetime.  SAM is responsible for defining the
   ticket lifetime.  If SAM sets a lifetime for a ticket, SAM and S MUST
   use a time synchronization method to ensure that S is able to
   interpret the lifetime correctly.  S SHOULD end the DTLS connection
   to C if the lifetime of a ticket has run out and it MUST NOT accept
   new requests.  S MUST NOT accept tickets with an invalid lifetime.

   If CAM provides CAI in the CI part of the ticket, CAM MAY add a
   lifetime for this CAI.  If CI contains a lifetime, CAM and C MUST use
   a time synchronization method to ensure that C is able to interpret
   the lifetime correctly.  C SHOULD end the DTLS connection to S and
   MUST NOT send new requests if the CAI in the ticket is no longer
   valid.  C MUST NOT accept tickets with an invalid lifetime.

   Note: Defining reasonable ticket lifetimes is difficult to
   accomplish.  How long a client needs to access a resource depends
   heavily on the application scenario and may be difficult to decide
   for SAM.






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4.3.2.  Revocation Messages

   SAM MAY revoke tickets by sending a ticket revocation message to S.
   If S receives a ticket revocation message, it MUST end the DTLS
   connection to C and MUST NOT accept any further requests from C.

   If ticket revocation messages are used, S MUST check regularly if SAM
   is still available.  If S cannot contact SAM, it MUST end all DTLS
   connections and reject any further requests from C.

   Note: The loss of the connection between S and SAM prevents all
   access to S.  This might especially be a severe problem if SAM is
   responsible for several Servers or even a whole network.

5.  Payload Format and Encoding (application/dcaf+cbor)

   Various messages types of the DCAF protocol carry payloads to express
   authorization information and parameters for generating the DTLS PSK
   to be used by C and S.  In this section, a representation in Concise
   Binary Object Representation (CBOR, [RFC7049]) is defined.

   DCAF data structures are defined as CBOR maps that contain key value
   pairs.  For efficient encoding, the keys defined in this document are
   represented as unsigned integers in CBOR, i. e. major type 0.  For
   improved reading, we use symbolic identifiers to represent the
   corresponding encoded values as defined in Table 1.

























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                          +---------------+-----+
                          | Encoded Value | Key |
                          +---------------+-----+
                          | 0b000_00000   | SAM |
                          |               |     |
                          | 0b000_00001   | SAI |
                          |               |     |
                          | 0b000_00010   | CAI |
                          |               |     |
                          | 0b000_00011   | E   |
                          |               |     |
                          | 0b000_00100   | K   |
                          |               |     |
                          | 0b000_00101   | TS  |
                          |               |     |
                          | 0b000_00110   | L   |
                          |               |     |
                          | 0b000_00111   | G   |
                          |               |     |
                          | 0b000_01000   | F   |
                          |               |     |
                          | 0b000_01001   | V   |
                          +---------------+-----+

              Table 1: DCAF field identifiers encoded in CBOR

   The following list describes the semantics of the keys defined in
   DCAF.

   SAM:  Server Authorization Manager.  This attribute denotes the
      Server Authorization Manager that is in charge of the resource
      specified in attribute R.  The attribute's value is a string that
      contains an absolute URI according to Section 4.3 of [RFC3986].

   SAI:  SAM Authorization Information.  A data structure used to convey
      authorization information from SAM to S.  It describes C's
      permissions for S according to SAM, e.g., which actions C is
      allowed to perform on an R of S.  The SAI attribute contains an
      AIF object as defined in [I-D.bormann-core-ace-aif].  C uses SAI
      for its Access Request messages.

   CAI:  CAM Authorization Information.  A data structure used to convey
      authorization information from CAM to C.  It describes the C's
      permissions for S according to CAM, e.g., which actions C is
      allowed to perform on an R of S.  The CAI attribute contains an
      AIF object as defined in [I-D.bormann-core-ace-aif].





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   E: Encrypted Ticket Face.  A binary string containing an encrypted
      ticket Face.

   K: Key. A string that identifies the shared key between S and SAM
      that can be used to decrypt the contents of E.  If the attribute E
      is present and no attribute K has been specified, the default is
      to use the current session key for the secured channel between S
      and SAM.

   TS:  Time Stamp.  An optional time stamp that indicates the instant
      when the access ticket request was formed.  This attribute can be
      used by the Server in an SAM Information message to convey a time
      stamp in its local time scale (e.g. when it does not have a real
      time clock with synchronized global time).  When the attribute's
      value is encoded as a string, it MUST contain a valid UTC
      timestamp without time zone information.  When encoded as integer,
      TS contains a system timestamp relative to the local time scale of
      its generator, usually S.

   L: Lifetime.  A lifetime of the ticket.  When encoded as a string, L
      MUST denote the ticket's expiry time as a valid UTC timestamp
      without time zone information.  When encoded as an integer, L MUST
      denote the ticket's validity period in seconds relative to TS.

   G: DTLS PSK Generation Method.  A numeric identifier for the method
      that S MUST use to derive the DTLS PSK from the ticket Face.  This
      attribute MUST NOT be used when attribute V is present within the
      contents of F.  This specification uses symbolic identifiers for
      improved readability.  The corresponding numeric values encoded in
      CBOR are defined in Table 2.  A registry for these codes is
      defined in Section 13.1.

   F: Ticket Face.  An object containing the fields SAI, TS, and
      optionally G, L and V.

   V: Ticket Verifier.  A binary string containing the shared secret
      between C and S.














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                +---------------+-------------+-----------+
                | Encoded Value | Mnemonic    | Support   |
                +---------------+-------------+-----------+
                | 0b000_00000   | hmac_sha256 | mandatory |
                |               |             |           |
                | 0b000_00001   | hmac_sha384 | optional  |
                |               |             |           |
                | 0b000_00010   | hmac_sha512 | optional  |
                +---------------+-------------+-----------+

        Table 2: CBOR encoding for DTLS PSK Key Generation Methods

5.1.  Examples

   The following example specifies a SAM that will be accessed using
   HTTP over TLS.  The request URI is set to
   "/a?ep=%5B2001:DB8::dcaf:1234%5D" (hence denoting the endpoint
   address to authorize).  TS denotes a local timestamp in UTC.

   POST /a?ep=%5B2001:DB8::dcaf:1234%5D HTTP/1.1
   Host: sam.example.com
   Content-Type: application/dcaf+cbor
   {SAM: "https://sam.example.com/a?ep=%5B2001:DB8::dcaf:1234%5D",
    SAI: ["coaps://temp451.example.com/s/tempC", 1],
    TS: 0("2013-07-14T11:58:22.923")}

   The following example shows a ticket for the distributed key
   generation method (cf.  Section 6.2), comprised of a Face (F) and a
   Verifier (V).  The Face data structure contains authorization
   information SAI, a client descriptor, a timestamp using the local
   time scale of S, and a lifetime relative to S's time scale.

   The DTLS PSK Generation Method is set to hmac_sha256 denoting that
   the distributed key derivation is used as defined in Section 6.2 with
   SHA-256 as HMAC function.

   The Verifier V contains a shared secret to be used as DTLS PSK
   between C and S.













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   HTTP/1.1 200 OK
   Content-Type: application/dcaf+cbor
   {
     F: {
          SAI: [ "/s/tempC", 1 ],
          TS: 2938749,
          L:  3600,
          G: hmac_sha256
        },
     V: h'48ae5a81b87241d81618f56cab0b65ec
          441202f81faabbe10075b20cb57fa939'
   }

   The Face may be encrypted as illustrated in the following example.
   Here, the field E carries an encrypted Face data structure that
   contains the same information as the previous example, and an
   additional Verifier.  Encryption was done with a secret shared by SAM
   and S.  (This example uses AES128_CCM with the secret { 0x00, 0x01,
   0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c,
   0x0d, 0x0e, 0x0f } and S's timestamp { 0x00, 0x2C, 0xD7, 0x7D } as
   nonce.)  Line breaks have been inserted to improve readability.

   The attribute K describes the identity of the key to be used by S to
   decrypt the contents of attribute E.  Here, The value "key0" in this
   example is used to indicate that the shared session key between S and
   SAM was used for encrypting E.

   {
     E: h'2e75eeae01b831e0b65c2976e06d90f4
          82135bec5efef3be3d31520b2fa8c6fb
          f572f817203bf7a0940bb6183697567c
          e291b03e9fca5e9cbdfa7e560322d4ed
          3a659f44a542e55331a1a9f43d7f',
     K: "key0",
     V: h'48ae5a81b87241d81618f56cab0b65ec
          441202f81faabbe10075b20cb57fa939'
   }

   The decrypted contents of E are depicted below (whitespace has been
   added to improve readability).  The presence of the attribute V
   indicates that the DTLS PSK Transfer is used to convey the session
   key (cf.  Section 6.1).









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   {
     F: {
          SAI: [ "/s/tempC", 1 ],
          TS: 2938749,
          L:  3600,
          G: hmac_sha256
        },
     V: h'48ae5a81b87241d81618f56cab0b65ec
          441202f81faabbe10075b20cb57fa939'
   }

6.  DTLS PSK Generation Methods

   One goal of the DCAF protocol is to provide for a DTLS PSK shared
   between C and S.  SAM and S MUST negotiate the method for the DTLS
   PSK generation.

6.1.  DTLS PSK Transfer

   The DTLS PSK is generated by AS and transmitted to C and S using a
   secure channel.

   The DTLS PSK transfer method is defined as follows:

   o  SAM generates the DTLS PSK using an algorithm of its choice

   o  SAM MUST include a representation of the DTLS PSK in Face and
      encrypt it together with all other information in Face with a key
      K(SAM,S) it shares with S.  How SAM and S exchange K(SAM,S) is not
      in the scope of this document.  SAM and S MAY use their preshared
      key as K(SAM,S).

   o  SAM MUST include a representation of the DTLS PSK in the Verifier.

   o  As SAM and C do not have a shared secret, the Verifier MUST be
      transmitted to C using encrypted channels.

   o  S MUST decrypt Face using K(SAM,S)

6.2.  Distributed Key Derivation

   SAM generates a DTLS PSK for C which is transmitted using a secure
   channel.  S generates its own version of the DTLS PSK using the
   information contained in Face (see also Section 4.1).

   The distributed key derivation method is defined as follows:





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   o  SAM and S both generate the DTLS PSK using the information
      included in Face.  They use an HMAC algorithm on Face with a
      shared key K(SAM,S).  The result serves as the DTLS PSK.  How SAM
      and S exchange K(SAM,S) is not in the scope of this document.
      They MAY use their preshared key as K(SAM,S).  How SAM and S
      negotiate the used HMAC algorithm is also not in the scope of this
      document.  They MAY however use the HMAC algorithm they use for
      their DTLS connection.

   o  SAM MUST include a representation of the DTLS PSK in the Verifier.

   o  As SAM and C do not have a shared secret, the Verifier MUST be
      transmitted to C using encrypted channels.

   o  SAM MUST NOT include a representation of the DTLS PSK in Face.

   o  SAM MUST NOT encrypt Face.

7.  Authorization Configuration

   For the protocol defined in this document, proper configuration of
   CAM and SAM is crucial.  The principals that are in charge of the
   resource, S and SAM, and the principals that are in charge of C and
   CAM need to define the respective permissions.  The data
   representation of these permissions are not in the scope of this
   document.

8.  Trust Relationships

   C has a trust relationship with CAM: C trusts CAM to act in behalf of
   C's principal.  S has a trust relationship with SAM: S trusts SAM to
   act in behalf of S's principal.

   Obviously, CAM trusts C with the specific permissions it hands over
   to it.  How this trust is established, is not in the scope of this
   document.  It may be achieved by using a bootstrapping mechanism
   similar to [bergmann12].

   Additionally, SAM and CAM need to have a trust relationship
   established.  Its establishment is also not in the scope of this
   document.  It fulfills the following conditions:

   1.  SAM has means to authenticate CAM (e.g. it has a certificate of
       CAM or a PKI in which CAM is included) and vice versa

   2.  As far as SAM needs to rely on the different clients of CAM to
       receive different permissions, it can be sure that CAM correctly
       identifies these clients towards SAM and does not leak tickets



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       that have been generated for a specific client C to another
       client.

   SAM trusts C indirectly because it trusts CAM and CAM vouches for C.
   The DCAF Protocol does not provide any means for SAM to validate that
   a resource request stems from C.

   C indirectly trusts SAM with some potentially confidential
   information, and that SAM correctly represents S, because CAM trusts
   SAM.

   CAM trusts S indirectly because it trusts SAM and SAM vouches for S.

   C implicitly trusts S with some potentially confidential information
   because it trusts CAM and because S can prove that it shares a key
   with SAM.

      CAM <------------------> SAM

      /|\                      /|\
       |                        |
      \|/                      \|/

       C .....................  S


9.  Listing Authorization Manager Information in a Resource Directory

   CoAP utilizes the Web Linking format [RFC5988] to facilitate
   discovery of services in an M2M environment.  [RFC6690] defines
   specific link parameters that can be used to describe resources to be
   listed in a resource directory [I-D.ietf-core-resource-directory].

9.1.  The "auth-request" Link Relation

   This section defines a resource type "auth-request" that can be used
   by clients to retrieve the request URI for a server's authorization
   service.  When used with the parameter rt in a web link, "auth-
   request" indicates that the corresponding target URI can be used in a
   POST message to request authorization for the resource and action
   that are described in the request payload.

   The Content-Format "application/dcaf+cbor with numeric identifier
   TBD1 defined in this specification MAY be used to express access
   requests and their responses.






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   The following example shows the web link used by CAM in this document
   to relay incoming Authorization Request messages to SAM.  (Whitespace
   is included only for readability.)

   <client-authorize>;rt="auth-request";ct=TBD1
                     ;title="Contact Remote Authorization Manager"

   The resource directory that hosts the resource descriptions of S
   could list the following description.  In this example, the URI
   "ep/node138/a/switch2941" is relative to the resource context
   "coaps://sam.example.com/", i.e. the Server Authorization Manager
   SAM.

   <ep/node138/a/switch2941>;rt="auth-request";ct=TBD1;ep="node138"
                            ;title="Request Client Authorization"
                            ;anchor="coaps://sam.example.com/"

10.  Examples

   This section gives a number of short examples with message flows for
   the initial Unauthorized Resource Request and the subsequent
   retrieval of a ticket from SAM.  The notation here follows the actors
   conventions defined in Section 1.2.1.  The payload format is encoded
   as proposed in Section 5.  The IP address of SAM is 2001:DB8::1, the
   IP address of S is 2001:DB8::dcaf:1234, and C's IP address is
   2001:DB8::c.

10.1.  Access Granted

   This example shows an Unauthorized PUT request from C to S that is
   answered with a SAM Information message.  C then sends a POST request
   to CAM with a description of its intended request.  CAM forwards this
   request to SAM using CoAP over a DTLS-secured channel.  The response
   from SAM contains an access ticket that is relayed back to CAM.

   C --> S
   PUT a/switch2941 [Mid=1234]
   Content-Format: application/senml+json
   {"e": [{"bv": "1"}]}

   C <-- S
   4.01 Unauthorized  [Mid=1234]
   Content-Format: application/dcaf+cbor
   {SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}

   C --> CAM
   POST client-authorize [Mid=1235,Token="tok"]
   Content-Format: application/dcaf+cbor



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   {
     SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
     SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
   }

   CAM --> SAM [Mid=23146]
   POST ep/node138/a/switch2941
   Content-Format: application/dcaf+cbor
   {
     SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
     SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 4]
   }

   CAM <-- SAM
   2.05 Content  [Mid=23146]
   Content-Format: application/dcaf+cbor
   { F: {
          SAI: ["a/switch2941", 5],
          TS: 0("2013-07-04T20:17:38.002"),
          G: hmac_sha256
        },
     V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
          9503611917b014ee6ec2a570d857987a'
   }

   C <-- CAM
   2.05 Content  [Mid=1235,Token="tok"]
   Content-Format: application/dcaf+cbor
   { F: {
          SAI: ["a/switch2941", 5],
          TS: 0("2013-07-04T20:17:38.002"),
          G: hmac_sha256
        },
     V: h'7ba4d9e287c8b69dd52fd3498fb8d26d
          9503611917b014ee6ec2a570d857987a'
   }

   C --> S
   ClientHello (TLS_PSK_WITH_AES_128_CCM_8)

   C <-- S
   ServerHello (TLS_PSK_WITH_AES_128_CCM_8)
   ServerHelloDone

   C --> S
   ClientKeyExchange
     psk_identity=0xa301826c612f73776974636832393431
                  0x0505c077323031332d30372d30345432



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                  0x303a31373a33382e3030320700

   (C decodes the contents of V and uses the result as PSK)
   ChangeCipherSpec
   Finished

   (S calculates PSK from SAI, TS and its session key
      HMAC_sha256(0xa301826c612f73776974636832393431
                  0x0505c077323031332d30372d30345432
                  0x303a31373a33382e3030320700,
                  0x736563726574)
   = 0x7ba4d9e287c8...
   )

   C <-- S
   ChangeCipherSpec
   Finished


10.2.  Access Denied

   This example shows a denied Authorization request for the DELETE
   operation.




























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   C --> S
   DELETE a/switch2941

   C <-- S
   4.01 Unauthorized
   Content-Format: application/dcaf+cbor
   {SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941"}

   C --> CAM
   POST client-authorize
   Content-Format: application/dcaf+cbor
   {
     SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
     SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
   }

   CAM --> SAM
   POST ep/node138/a/switch2941
   Content-Format: application/dcaf+cbor
   {
     SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
     SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 8]
   }

   CAM <-- SAM
   2.05 Content
   Content-Format: application/dcaf+cbor

   C <-- CAM
   2.05 Content
   Content-Format: application/dcaf+cbor

10.3.  Access Restricted

   This example shows a denied Authorization request for the operations
   GET, PUT, and DELETE.  SAM grants access for PUT only.















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   CAM --> SAM
   POST ep/node138/a/switch2941
   Content-Format: application/dcaf+cbor
   {
     SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
     SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 13]
   }

   CAM <-- SAM
   2.05 Content
   Content-Format: application/dcaf+cbor
   { F: {
          SAI: ["a/switch2941", 5],
          TS: 0("2013-07-04T21:33:11.930"),
          G: hmac_sha256
        },
     V: h'c7b5774f2ddcbd548f4ad74b30a1b2e5
          b6b04e66a9995edd2545e5a06216c53d'
   }

10.4.  Implicit Authorization

   This example shows an Authorization request using implicit
   authorization.  CAM initially requests the actions GET and POST on
   the resource "coaps://[2001:DB8::dcaf:1234]/a/switch2941".  SAM
   returns a ticket that has no SAI field in its ticket Face, hence
   implicitly authorizing C.

   CAM --> SAM
   POST ep/node138/a/switch2941
   Content-Format: application/dcaf+cbor
   {
      SAM: "coaps://[2001:DB8::1]/ep/node138/a/switch2941",
      SAI: ["coaps://[2001:DB8::dcaf:1234]/a/switch2941", 3]
   }

   CAM <-- SAM
   2.05 Content
   Content-Format: application/dcaf+cbor
   { F: {
          TS: 0("2013-07-16T10:15:43.663"),
          G: hmac_sha256
         },
     V: h'4f7b0e7fdcc498fb2ece648bf6bdf736
          61a6067e51278a0078e5b8217147ea06'
   }





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11.  Specific Usage Scenarios

   The general DCAF architure outlined in Section 3.1 illustrates the
   various actors who participate in the message exchange for
   authenticated authorization.  The message types defined in this
   document cover the most general case where all four actors are
   separate entities that may or may not reside on the same device.

   Special implementation considerations apply when one single entity
   takes the role of more than one actor.  This section gives advice on
   the most common usage scenarios where the Client Authorization
   Manager and Client, the Server Authorization Manager and Server or
   both Authorization Managers reside on the same (less-constrained)
   device and have a means of secure communication outside the scope of
   this document.

11.1.  Combined Authorization Manager and Client

   When CAM and C reside on the same (less-constrained) device, the
   Access Request and Ticket Transfer messages can be substituted by
   other means of secure communication.  Figure 9 shows a simplified
   message exchange for a combined CAM+C device.

    CAM+C                 S                 SAM
     |                    | <== DTLS chan. ==> |
     | [Resource Req.-->] |                    |
     |                    |                    |
     |  [<-- SAM Info.]   |                    |
     |                    |                    |
     | <==== TLS/DTLS chan. (Mutual Auth) ===> |
     |                    |                    |
     | Ticket Request   ---------------------> |
     |                    |                    |
     | <---------------------    Ticket Grant  |
     |                    |                    |
     | <== DTLS chan. ==> |                    |
     | Auth. Res. Req. -> |                    |

        Figure 9: Combined Client Authorization Manager and Client

11.1.1.  Creating the Ticket Request Message

   When CAM+C receives an SAM Information message as a reaction to an
   Unauthorized Request message, it creates a Ticket Request message as
   follows:






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   1.  The destination of the Ticket Request message is derived from the
       authority information in the URI contained in field "SAM" of the
       SAM Information message payload.

   2.  The request method is POST.

   3.  The request URI is constructed from the SAM field received in the
       SAM Information message payload.

   4.  The payload contains the SAM field from the SAM Information
       message, an absolute URI of the resource that CAM+C wants to
       access, the actions that CAM+C wants to perform on the resource,
       and any time stamp generated by S that was transferred with the
       SAM Information message.

   5.  A label that describes CAM+C is added to the payload.

11.1.2.  Processing the Ticket Grant Message

   Based on the Ticket Grant message, CAM+C is able to establish a DTLS
   channel with S.  To do so, CAM+C sets the psk_identity field of the
   DTLS ClientKeyExchange message to the ticket Face received in the
   Ticket Grant message and uses the ticket Verifier as PSK when
   constructing the premaster secret.

11.2.  Combined Client Authorization Manager and Server Authorization
       Manager

   In certain scenarios, CAM and SAM may be combined to a single entity
   that knows both, C and S, and decides if their actions are
   authorized.  Therefore, no explicit communication between CAM and SAM
   is necessary, resulting in omission of the Ticket Request and Ticket
   Grant messages.  Figure 10 depicts the resulting message sequence in
   this simplified architecture.

















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     C                 CAM+SAM                 S
     | <== DTLS chan. ==> | <== DTLS chan. ==> |
     |                    |                    |
     | [Resource Req.----------------------->] |
     |                    |                    |
     | [<-------------------- SAM Information] |
     |                    |                    |
     | Access Request --> |                    |
     |                    |                    |
     | <-- Ticket Transf. |                    |
     |                    |                    |
     | <===========  DTLS channel ===========> |
     |                    |                    |
     | Authorized Resource Request ----------> |

        Figure 10: Combined Client Authorization Manager and Server
                           Authorization Manager

11.2.1.  Processing the Access Request Message

   When receiving an Access Request message, CAM+SAM performs the checks
   specified in Section 3.5 and returns a 4.00 (Bad Request) response in
   case of failure.  Otherwise, if the checks have succeeded, CAM+SAM
   evaluates the contents of Access Request message as described in
   Section 3.6.

   The decision on the access request is performed by CAM+SAM with
   respect to the stored policies.  When the requested action is
   permitted on the respective resource, CAM+SAM generates an access
   ticket as outlined in Section 4.1 and creates a Ticket Transfer
   message to convey the access ticket to the Client.

11.2.2.  Creating the Ticket Transfer Message

   A Ticket Transfer message is constructed as a 2.05 response with the
   access ticket contained in its payload.  The response MAY contain a
   Max-Age option to indicate the ticket's lifetime to the receiving
   Client.

   This specification defines a CBOR data representation for the access
   ticket as illustrated in Section 3.6.

11.3.  Combined Server Authorization Manager and Server

   If SAM and S are colocated in one entity (SAM+S), the main objective
   is to allow CAM to delegate access to C.  Accordingly, the
   authorization information could be replaced by a nonce internal to
   SAM+S.  (TBD.)



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   CAM                    C                  SAM+S
     | <== DTLS chan. ==> |                    |
     |                    | [Resource Req.-->] |
     |                    |                    |
     |                    |  [<-- SAM Info.]   |
     |                    |                    |
     | <-- Access Req.    |                    |
     |                    |                    |
     | <========= TLS/DTLS channel  =========> |
     |                    |                    |
     | Ticket Request   ---------------------> |
     |                    |                    |
     | <---------------------    Ticket Grant  |
     |                    |                    |
     | Ticket Transf. --> |                    |
     |                    |                    |
     |                    | <== DTLS chan. ==> |
     |                    | Auth. Res. Req. -> |


        Figure 11: Combined Server Authorization Manager and Server

12.  Security Considerations

   As this protocol builds on transitive trust between Authorization
   Managers as mentioned in Section 8, SAM has no direct means to
   validate that a resource request originates from C.  It has to trust
   CAM that it correctly vouches for C and that it does not give
   authorization tickets meant for C to another client nor disclose the
   contained session key.

   The Authorization Managers also could constitute a single point of
   failure.  If the Server Authorization Manager fails, the resources on
   all Servers it is responsible for cannot be accessed any more.  If a
   Client Authorization Manager fails, all clients it is responsible are
   not able to access resources on a Server.  Thus, it is crucial for
   large networks to use Authorization Managers in a redundant setup.

13.  IANA Considerations

   The following registrations are done following the procedure
   specified in [RFC6838].

   Note to RFC Editor: Please replace all occurrences of "[RFC-XXXX]"
   with the RFC number of this specification.






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13.1.  DTLS PSK Key Generation Methods

   A sub-registry for the values indicating the PSK key generation
   method as contents of the field G in a payload of type application/
   dcaf+cbor is defined.  Values in this sub-registry are numeric
   integers encoded in Concise Binary Object Notation (CBOR, [RFC7049]).
   This document follows the notation of [RFC7049] for binary values,
   i.e. a number starts with the prefix "0b".  The major type is
   separated from the actual numeric value by an underscore to emphasize
   the value's internal structure.

   Initial entries in this sub-registry are as follows:

               +---------------+-------------+------------+
               | Encoded Value | Name        | Reference  |
               +---------------+-------------+------------+
               | 0b000_00000   | hmac_sha256 | [RFC-XXXX] |
               |               |             |            |
               | 0b000_00001   | hmac_sha384 | [RFC-XXXX] |
               |               |             |            |
               | 0b000_00010   | hmac_sha512 | [RFC-XXXX] |
               +---------------+-------------+------------+

                 Table 3: DTLS PSK Key Generation Methods

   New methods can be added to this registry based on designated expert
   review according to [RFC5226].

   (TBD: criteria for expert review.)

13.2.  dcaf+cbor Media Type Registration

   Type name: application

   Subtype name: dcaf+cbor

   Required parameters: none

   Optional parameters: none

   Encoding considerations: Must be encoded as using a subset of the
   encoding allowed in [RFC7049].  Specifically, only the primitive data
   types String and Number are allowed.  The type Number is restricted
   to unsigned integers (i.e., no negative numbers, fractions or
   exponents are allowed).  Encoding MUST be UTF-8.  These restrictions
   simplify implementations on devices that have very limited memory
   capacity.




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   Security considerations: TBD

   Interoperability considerations: TBD

   Published specification: [RFC-XXXX]

   Applications that use this media type: TBD

   Additional information:

   Magic number(s): none

   File extension(s): dcaf

   Macintosh file type code(s): none

   Person & email address to contact for further information: TBD

   Intended usage: COMMON

   Restrictions on usage: None

   Author: TBD

   Change controller: IESG

13.3.  CoAP Content Format Registration

   This document specifies a new media type application/dcaf+cbor (cf.
   Section 13.2).  For use with CoAP, a numeric Content-Format
   identifier is to be registered in the "CoAP Content-Formats" sub-
   registry within the "CoRE Parameters" registry.

   Note to RFC Editor: Please replace all occurrences of "RFC-XXXX" with
   the RFC number of this specification.

         +-----------------------+----------+------+------------+
         |            Media type | Encoding | Id.  | Reference  |
         +-----------------------+----------+------+------------+
         | application/dcaf+cbor | -        | TBD1 | [RFC-XXXX] |
         +-----------------------+----------+------+------------+

14.  References








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14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279, December
              2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

   [RFC6838]  Freed, N., Klensin, J., and T. Hansen, "Media Type
              Specifications and Registration Procedures", BCP 13, RFC
              6838, January 2013.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, October 2013.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014.

14.2.  Informative References

   [I-D.bormann-core-ace-aif]
              Bormann, C., "An Authorization Information Format (AIF)
              for ACE", draft-bormann-core-ace-aif-01 (work in
              progress), July 2014.

   [I-D.gerdes-ace-actors]
              Gerdes, S., "Actors in the ACE Architecture", draft-
              gerdes-ace-actors-02 (work in progress), October 2014.

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-16 (work in progress), October 2014.



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   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-16 (work in progress), December 2014.

   [I-D.ietf-core-resource-directory]
              Shelby, Z. and C. Bormann, "CoRE Resource Directory",
              draft-ietf-core-resource-directory-02 (work in progress),
              November 2014.

   [I-D.schmertmann-dice-codtls]
              Schmertmann, L., Hartke, K., and C. Bormann, "CoDTLS: DTLS
              handshakes over CoAP", draft-schmertmann-dice-codtls-01
              (work in progress), August 2014.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

   [bergmann12]
              Bergmann, O., Gerdes, S., Schaefer, S., Junge, F., and C.
              Bormann, "Secure Bootstrapping of Nodes in a CoAP
              Network", IEEE Wireless Communications and Networking
              Conference Workshops (WCNCW), April 2012.

Authors' Addresses

   Stefanie Gerdes
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63906
   Email: gerdes@tzi.org


   Olaf Bergmann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63904
   Email: bergmann@tzi.org



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   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org











































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