ACE Working Group                                         R. Marin-Lopez
Internet-Draft                                      University of Murcia
Intended status: Standards Track                      D. Garcia-Carrillo
Expires: 10 June 2022                               University of Oviedo
                                                         7 December 2021


               EAP-based Authentication Service for CoAP
                     draft-ietf-ace-wg-coap-eap-06

Abstract

   This document specifies an authentication service that uses the
   Extensible Authentication Protocol (EAP) transported employing
   Constrained Application Protocol (CoAP) messages.  As such, it
   defines an EAP lower layer based on CoAP called CoAP-EAP.  One of the
   main goals is to authenticate a CoAP-enabled IoT device (EAP peer)
   that intends to join a security domain managed by a Controller (EAP
   authenticator).  Secondly, it allows deriving key material to protect
   CoAP messages exchanged between them based on Object Security for
   Constrained RESTful Environments (OSCORE), enabling the establishment
   of a security association between them.

Status of This Memo

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

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

   This Internet-Draft will expire on 10 June 2022.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.



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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  General Architecture  . . . . . . . . . . . . . . . . . . . .   4
   3.  CoAP-EAP Operation  . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Flow of operation (OSCORE establishment)  . . . . . . . .   6
     3.3.  Reauthentication  . . . . . . . . . . . . . . . . . . . .   9
     3.4.  Managing the State of the Service . . . . . . . . . . . .  10
     3.5.  Error handling  . . . . . . . . . . . . . . . . . . . . .  11
       3.5.1.  EAP authentication failure  . . . . . . . . . . . . .  11
       3.5.2.  Non-responding endpoint . . . . . . . . . . . . . . .  12
       3.5.3.  Duplicated message with /.well-known/coap-eap . . . .  12
     3.6.  Proxy operation in CoAP-EAP . . . . . . . . . . . . . . .  13
   4.  CBOR Objects in CoAP-EAP  . . . . . . . . . . . . . . . . . .  13
   5.  Cipher suite negotiation and key derivation . . . . . . . . .  14
     5.1.  Cipher suite negotiation  . . . . . . . . . . . . . . . .  14
     5.2.  Deriving the OSCORE Security Context  . . . . . . . . . .  16
   6.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  CoAP as EAP lower layer . . . . . . . . . . . . . . . . .  17
     6.2.  Size of the EAP lower layer vs EAP method size  . . . . .  18
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
     7.1.  Authorization . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  Freshness of the key material . . . . . . . . . . . . . .  19
     7.3.  Channel Binding support . . . . . . . . . . . . . . . . .  19
     7.4.  Additional Security Consideration . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  Flow of operation (DTLS establishment) . . . . . . .  25
     A.1.  Cryptographic suite negotiation for DTLS  . . . . . . . .  26
     A.2.  Deriving DTLS PSK and identity  . . . . . . . . . . . . .  26
   Appendix B.  Examples of Use Case Scenario  . . . . . . . . . . .  27
     B.1.  Example 1: CoAP-EAP in ACE  . . . . . . . . . . . . . . .  28
     B.2.  Example 2: Multi-domain with AAA infrastructures  . . . .  29
     B.3.  Example 3: Single domain with AAA infrastructure  . . . .  29
     B.4.  Example 4: Single domain without AAA infrastructure . . .  29
     B.5.  Other use cases . . . . . . . . . . . . . . . . . . . . .  29
       B.5.1.  CoAP-EAP for network access control . . . . . . . . .  30



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       B.5.2.  CoAP-EAP for service authentication . . . . . . . . .  30
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  30

1.  Introduction

   This document specifies an authentication service (application) that
   uses the Extensible Authentication Protocol (EAP) [RFC3748] and is
   built on top of the Constrained Application Protocol (CoAP) [RFC7252]
   called CoAP-EAP.  CoAP-EAP is an application that allows
   authenticating two CoAP endpoints by using EAP, and to establish an
   Object Security for Constrained RESTful Environments (OSCORE)
   security association between them.

   More specifically, this document specifies how CoAP can be used as a
   constrained, link-layer independent, reliable EAP lower layer
   [RFC3748] to transport EAP messages between a CoAP server (acting as
   EAP peer) and a CoAP client (acting as EAP authenticator) using CoAP
   messages.  The CoAP client has the option of contacting a backend AAA
   infrastructure to complete the EAP negotiation as described in the
   EAP specification [RFC3748].

   EAP methods transported in CoAP MUST generate cryptographic material
   [RFC5247] for this specification.  This way, CoAP messages are
   protected after the authentication.  After CoAP-EAP's operation, an
   OSCORE security association is established between endpoints of the
   service.  Using the keying material derived from the authentication,
   other security associations could be generated.  Appendix A shows how
   to establish a (D)TLS security association using the keying material
   from the EAP authentication.

1.1.  Requirements Language

   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 [RFC2119] [RFC8174]
   when, and only when, they appear in all capitals, as shown here.

   Readers are expected to be familiar with the terms and concepts of
   described in CoAP [RFC7252], EAP [RFC3748][RFC5247] and OSCORE
   [RFC8613].











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2.  General Architecture

   Figure 1 illustrates the architecture defined in this document.
   Basically, an IoT device, acting as the EAP peer, wants to be
   authenticated by using EAP to join a domain that is managed by a
   Controller acting as EAP authenticator.  The IoT device will act a
   CoAP server for this service, and the EAP authenticator as a CoAP
   client.  The rationale behind this decision, as expanded later, is
   that EAP requests go always from the EAP server to the EAP peer.
   Accordingly, the EAP responses go from the EAP peer to the EAP
   server.

   It is worth noting that the CoAP client (EAP authenticator) MAY
   interact with a backend AAA infrastructure when EAP pass-through mode
   is used, which will place the EAP server in the AAA server that
   contains the information required to authenticate the EAP peer.

   The protocol stack is described in Figure 2.  CoAP-EAP is an
   application built on top of CoAP.  On top of the application, there
   is an EAP state machine that can run any EAP method.  For this
   specification, the EAP method MUST be able to derive keying material.
   CoAP-EAP also relies on CoAP reliability mechanisms in CoAP to
   transport EAP: CoAP over UDP with Confirmable messages ([RFC7252]) or
   CoAP over TCP, TLS and Websocket, which is specified in [RFC8323].

   +----------+        +--------------+            +----------+
   | EAP peer |        |      EAP     |            |   AAA/   |
   |   peer   |<------>| authenticator|<---------->|EAP Server|
   +----------+  CoAP  +--------------+     AAA    +----------+
                                        (Optional)
   <---(SCOPE OF THIS DOCUMENT)---->

                      Figure 1: CoAP-EAP Architecture

                   +-------------------------------+
                   |        EAP State Machine      |
                   +-------------------------------+ \
                   |     Application(CoAP-EAP)     |  | This Document
                   +-------------------------------+ /
                   | Request/Responses/Signaling   | RFC 7252 / RFC 8323
                   +-------------------------------+
                   |    Message / Message Framing  | RFC 7252 / RFC 8323
                   +-------------------------------+
                   |Unreliable / Reliable Transport| RFC 7252 / RFC 8323
                   +-------------------------------+

                          Figure 2: CoAP-EAP Stack




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3.  CoAP-EAP Operation

   Since CoAP-EAP uses reliable delivery in CoAP ([RFC7252], [RFC8323]),
   EAP retransmission time is set to infinite as mentioned in [RFC3748].
   To keep ordering guarantee, CoAP-EAP uses Hypermedia as the Engine of
   Application State (HATEOAS).  Each step during the EAP authentication
   is represented as a new resource in the EAP peer (CoAP server).  The
   previous resource is removed once the new resource is created
   indicating the resource that will process the next expected step of
   the EAP authentication.

   An EAP method that does not export keying material MUST NOT be used.
   One of the benefits of using EAP is that we can choose over a large
   variety of authentication methods.  Although for IoT, where we can
   find very constrained links (e.g., limited bandwidth) and devices
   with limited capabilities, EAP methods that do not require many
   exchanges, with short messages, and that use cryptographic algorithms
   that are manageable by constrained devices are preferable.

   In CoAP-EAP, the IoT device (EAP peer/CoAP server) will only have one
   authentication session with a specific Controller (EAP authenticator/
   CoAP client) and it will not process any other EAP authentication in
   parallel (with the same Controller).  That is, a single ongoing EAP
   authentication is permitted for the same IoT device and same
   Controller.  Moreover, EAP is a lock-step protocol ([RFC3748]).  The
   benefits of the EAP framework in IoT are highlighted in
   [eap-framework].

   To access the authentication service, this document defines the well-
   known URI "/.well-known/coap-eap" (to be assigned by IANA).  This URI
   is referring to the authentication service that is present in the
   Controller so that IoT device can start the service.

3.1.  Discovery

   Prior to the CoAP-EAP exchange takes place, the IoT device needs to
   discovers the Controller or the entity that will enable the exchange
   between the IoT and the Controller (e.g., an intermediary such as a
   proxy).

   The discovery process is out of the scope of this document.  This
   document provides the specification using the mechanisms provided by
   CoAP to discover the Controller for CoAP-EAP.








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   The CoAP-EAP application is designated by the well-known URI "coap-
   eap" for the trigger message (Step 0).  The CoAP-EAP service can be
   discovered by asking directly about the services offered.  This
   information can be also available in the resource directory
   [I-D.ietf-core-resource-directory].

   Implementation Notes: On the methods on how the IPv6 address of the
   Controller or intermediary entity can be discovered, there can be
   different methods depending on the specific deployment.  For example,
   on a 6LoWPAN network, the Border Router will typically act as the
   Controller hence, after receiving the Router Advertisement (RA)
   messages from the Border Router, the IoT device may engage on the
   CoAP-EAP exchange.  Different protocols can be used to discover the
   IP of the Controller.  Examples of such protocols are Multicast DNS
   (mDNS) [RFC6762] or DHCPv6 [RFC8415].

3.2.  Flow of operation (OSCORE establishment)

   Figure 3 shows the general flow of operation for CoAP-EAP to
   authenticate using EAP and establish an OSCORE security context.  The
   flow does not show a specific EAP method.  Instead, we represent the
   chosen EAP method by using a generic name (EAP-X).  The flow assumes
   that the IoT device knows the Controller implements the CoAP-EAP
   service.  The specific mechanism of discovery is out-of-scope of this
   document.  Some comments about Controller discovery is in
   Section 3.1.

   The steps for the operation happens as follows:

   *  Step 0.  The IoT device MUST start the authentication process by
      sending a "POST /.well-known/coap-eap" request (trigger message).
      This message carries the 'No-Response' [RFC7967] CoAP option to
      avoid waiting for a response that is not needed.  This message is
      the only instance where the Controller acts as a CoAP server and
      the IoT device as a CoAP client.  The message also includes a URI
      in the payload of the message to indicate to what resource (e.g.
      '/a/x') the Controller MUST send the first message with the EAP
      authentication.  The name of the resource is selected by the CoAP
      server as it pleases.  After this, the exchange continues with the
      Controller as a CoAP client and the IoT device as a CoAP server.











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   *  Step 1.  The Controller sends a "POST" message to the resource
      indicated by the IoT device in the Step 0 (e.g., '/a/x').  The
      payload in this message contains the first EAP message (EAP
      Request/Identity), the Recipient ID of the Controller (RID-C) for
      OSCORE and it MAY contain a CBOR array containing a list with the
      cipher suites (CS) for OSCORE.  If the cipher suite is not
      included the default cipher suite for OSCORE is used.  The details
      of the cipher suite negotiation are discussed in Section 5.1.

   *  Step 2.  The IoT device processes the POST message by passing the
      EAP request (EAP-Req/Id) to the EAP peer state machine, which
      returns an EAP response (EAP Resp/Id); it assigns a new resource
      to the ongoing authentication process (e.g., '/a/y'), and deletes
      the previous one ('/a/x').  It finally sends a '2.01 Created'
      response with the new resource identifier in the Location-Path
      (and/or Location-Query) options for the next step; the EAP
      response, the Recipient ID of the IoT device (RID-I) and the
      selected cipher suite for OSCORE are in the payload.  In this
      step, the IoT device MAY create a OSCORE security context (see
      Section 5.2).  The required key, the Master Session Key (MSK),
      will be available once the EAP authentication is successful in
      step 7.

   *  Step 3-6.  From now on, the Controller and the IoT device will
      exchange EAP packets related to the EAP method (EAP-X),
      transported in the CoAP message payload.  The Controller will use
      the POST method to send EAP requests to the IoT device.  The IoT
      device will use a response to carry the EAP response in the
      payload.  EAP requests and responses are represented in Figure 3
      using the nomenclature (EAP-X-Req and EAP-X-Resp, respectively.
      When a POST message arrives (e.g, '/a/x') carrying an EAP request
      message, if processed correctly by the EAP peer state machine,
      returns an EAP Response.  Along with each EAP Response, a new
      resource is created (e.g, '/a/z') for processing the next EAP
      request and the ongoing resource (e.g., '/a/y') is erased.  This
      way ordering guarantee is achieved.  Finally, EAP response is sent
      in the payload of a CoAP response that will also indicate the new
      resource in the Location-Path (and/or Location-Query) Options.  In
      case there is an error processing a legitimate message, the server
      will return a (4.00 Bad Request).  There is a discussion about
      error handling in Section 3.5.

   *  Step 7.  When the EAP authentication ends with success, the
      Controller obtains the Master Session Key (MSK) exported by the
      EAP method, an EAP Success message and some authorization
      information (i.e. session lifetime) [RFC5247].  The Controller
      creates the OSCORE security context using the MSK and Sender ID
      and Recipient ID exchanged in Step 1 and 2.  The establishment of



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      the OSCORE Security Context is defined in Section 5.2.  Then, the
      Controller sends the POST message protected with OSCORE for key
      confirmation including the EAP Success.  The Controller MAY also
      send a Session Lifetime, in seconds, which is represented with an
      unsigned integer in a CBOR object (see Section 4.  If this Session
      Lifetime is not sent, the IoT device assumes a default value of 8
      hours as RECOMMENDED in [RFC5247].  The verification of the
      received OSCORE protected "POST" message using RID-I (Recipient ID
      of the IoT device) sent in Step 2 is considered by the IoT device
      as an alternate indication of success ([RFC3748]).  The EAP peer
      state machine in the IoT device interprets the alternate
      indication of success similarly the arrival of an EAP Success and
      returns the MSK, which is used for the OSCORE security context in
      the IoT device to process the protected POST message received from
      the Controller.

   *  Step 8.  If the EAP authentication and the verification of the
      OSCORE protected "POST" in Step 7 is successful, then the IoT
      Device answers with an OSCORE protected '2.04 Changed'.  From this
      point on, the communication with the last resource (e.g. '/a/w')
      MUST be protected with OSCORE.  If allowed by application policy,
      same OSCORE security context MAY be used to protect communication
      to other resources between the same endpoints.




























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                  IoT device                              Controller
                -------------                            ------------
                    |  POST /.well-known/coap-eap             |
                 0) |  No-Response                            |
                    |  Payload("/a/x")                        |
                    |---------------------------------------->|
                    |                               POST /a/x |
                    |          Payload(EAP Req/Id||CS||RID-C) |
                 1) |<----------------------------------------|
                    | 2.01 Created Location-Path [/a/y]       |
                    | Payload(EAP Resp/Id||CS||RID-I)         |
                 2) |---------------------------------------->|
                    |                               POST /a/y |
                    |                     Payload(EAP-X Req)  |
                 3) |<----------------------------------------|
                    | 2.01 Created Location-Path [/a/z]       |
                    | Payload(EAP-X Resp)                     |
                 4) |---------------------------------------->|
                                       ....
                    |                             POST /a/q   |
                    |                     Payload(EAP-X Req)  |
                 5) |<----------------------------------------|
                    | 2.01 Created Location-Path [/a/w]       |
                    | Payload (EAP-X Resp)                    |
                 6) |---------------------------------------->|
                    |                                         |  MSK
                    |                               POST /a/w |   |
                    |                                  OSCORE |   V
                    | Payload (EAP Success||*Session-Lifetime)| OSCORE
            MSK  7) |<----------------------------------------| CONTEXT
             |      |                                         |
             V      | 2.04 Changed                            |
           OSCORE   | OSCORE                                  |
         CONTEXT 8 )|---------------------------------------->|

                      (*) Session-Lifetime is optional.

              Figure 3: CoAP-EAP flow of operation with OSCORE

3.3.  Reauthentication

   When the CoAP-EAP state is close to expire, the IoT device MAY want
   to start a new authentication process (re-authentication) to renew
   the state.  The main goal is to derive new and fresh keying material
   (MSK/EMSK) that, in turn, allows deriving a new OSCORE security
   context, increasing the protection against key leakage.  The keying
   material MUST be renewed before the expiration of the Session-
   Lifetime.  By default, the EAP Key Management Framework establishes a



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   default value of 8 hours to refresh the keying material.  Certain EAP
   methods such as EAP-NOOB [I-D.ietf-emu-eap-noob] or EAP-AKA'
   [RFC5448] provides fast reconnect for quicker re-authentication.  The
   EAP re-authentication protocol (ERP) [RFC6696] MAY be also used for
   avoiding the repetition of the entire EAP exchange.

   The message flow for the re-authentication will be the same as the
   one shown in Figure 3.  Nevertheless, two different CoAP-EAP states
   will be active during the re-authentication: the current CoAP-EAP
   state and the new CoAP-EAP state, which will be created once the re-
   authentication has finished with success.  Once the re-authentication
   is completed successfully, the current CoAP-EAP state is deleted and
   the new CoAP-EAP becomes the current one.  If by any reason, the re-
   authentication fails to complete, the current CoAP-EAP state will be
   available until it expires, or it is renewed in another try of re-
   authentication.

   If the re-authentication fails, it is up to the IoT device decide
   when to restart a re-authentication before the current EAP state
   expires.

3.4.  Managing the State of the Service

   The IoT device and the Controller keep a state during the CoAP-EAP
   negotiation.  The CoAP-EAP state includes several important parts:

   *  A reference to an instance of the EAP (peer or authenticator/
      server) state machine.

   *  The resource for the next message in the negotiation (e.g '/a/y')

   *  The MSK exported when the EAP authentication is successful.  In
      particular, CoAP-EAP is able to access to the different variables
      by the EAP state machine (i.e.  [RFC4137]).

   *  A reference to the OSCORE context.

   Once created, the Controller MAY choose to delete it as described in
   Figure 4.  On the other hand, the IoT device may need to renew the
   CoAP-EAP state because the key material is close to expire, as
   mentioned in Section 3.3.

   There are situations where the current CoAP-EAP state might need to
   be removed.  For instance, due to its expiration or a forced removal
   if the IoT device needs to be expelled from the security domain.
   This exchange is illustrated in Figure 4.





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   If the Controller deems necessary, the removal of the CoAP-EAP state
   from the IoT device before it expires, it can send a DELETE command
   in a request to the IoT device, referencing the last CoAP-EAP state
   resource given by the CoAP server, whose identifier will be the last
   one received (e.g., '/a/w' in Figure 3).  This message is protected
   with the OSCORE security association to prevent forgery.  Upon
   reception of this message, the CoAP server sends a response to the
   Controller with the Code '2.02 Deleted', which is also protected with
   the OSCORE security association.  If a response from the IoT device
   does not arrive after EXCHANGE_LIFETIME the Controller will remove
   the state from its side.

                IoT device                             Controller
              -------------                           -------------
                   |                                         |
                   |                             DELETE /a/w |
                   |                                  OSCORE |
                   |<----------------------------------------|
                   |                                         |
                   | 2.02 Deleted                            |
                   | OSCORE                                  |
                   |---------------------------------------->|

                          Figure 4: Deleting state

3.5.  Error handling

   This section elaborates how different errors are handled, from EAP
   authentication failure, a non-responding endpoint, lost messages or
   initial POST message arriving out of place.

3.5.1.  EAP authentication failure

   EAP authentication MAY fail for different situations (e.g.  wrong
   credentials).  The result is that the Controller will send an EAP
   failure because of the EAP authentication (Step 7 in Figure 3).  In
   this case, the IoT device MUST send a response '4.01 Unauthorized' in
   Step 8.  Therefore, Step 7 and Step 8 are not protected in this case
   because no MSK is exported and the OSCORE security context is not
   generated.

   If the EAP authentication fails during the re-authentication and the
   Controller sends an EAP failure, the current CoAP-EAP state will be
   still usable until it expires.







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3.5.2.  Non-responding endpoint

   If, by any reason, one of the entities becomes non-responding, the
   CoAP-EAP state SHOULD be kept only for a period of time before it is
   removed.  The removal of the CoAP-EAP state in the Controller assumes
   that the IoT device will need to authenticate again.  According to
   CoAP, EXCHANGE_LIFETIME considers the time it takes until a client
   stops expecting a response to a request.  A timer is reset every time
   a message is sent.  If EXCHANGE_LIFETIME has passed waiting for the
   next message, both entities will delete the CoAP-EAP state if the
   authentication process has not finished correctly.

3.5.3.  Duplicated message with /.well-known/coap-eap

   The reception of the trigger message in Step 0 containing /.well-
   known/coap-eap needs some additional considerations, as the resource
   is always available in the EAP authenticator.

   If a trigger message (Step 0) arrives to the Controller during an
   ongoing authentication, the Controller MUST silently discard this
   trigger message.

   If an old "POST /.well-known/coap-eap" (Step 0) arrives to the
   Controller and there is no authentication ongoing, the Controller may
   understand that a new authentication process is requested.
   Consequently, the Controller will start a new EAP authentication.
   However, the IoT device did not start any authentication and
   therefore, it has not selected any resource for the EAP
   authentication.  Thus, IoT device sends a '4.04 Not found' in the
   response (Figure 5).

       IoT device                                 Controller
     -------------                              -------------
           |  *POST /.well-known/coap-eap            |
        0) |  , No-Response                          |
           |  Payload("/a/x")                        |
           |               ------------------------->|
           |                              POST /a/x  |
           |                Payload (EAP Req/Id||CS) |
        1) |<----------------------------------------|
           |                                         |
           | 4.04 Not found                          |
           |---------------------------------------->|
           * Old

       Figure 5: /.well-known/coap-eap with no ongoing authentication
                         from the EAP authenticator




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3.6.  Proxy operation in CoAP-EAP

   The CoAP-EAP operation is intended to be compatible with the use of
   intermediary entities between the IoT device and the Controller, when
   direct communication is not possible.  In this context, CoAP proxies
   can be used as enablers of the CoAP-EAP exchange.

   This specification is limited to use standard CoAP [RFC7252] as well
   as standardized CoAP options [RFC8613].  It does not specify any
   addition in the form of CoAP options.  This is expected to ease the
   integration of CoAP intermediaries in the CoAP-EAP exchange.

   There is a consideration that needs to be considered, when using
   proxies in the CoAP-EAP, as the exchange contains a role-reversal
   process at the beginning of the exchange.  In the first message, the
   IoT device acts as a CoAP client, and the Controller as the CoAP
   server.  After that, remaining exchanges the roles are reversed,
   being the IoT device, the CoAP server and the Controller, the CoAP
   client.

4.  CBOR Objects in CoAP-EAP

   In the CoAP-EAP exchange, there is information that needs to be
   exchanged between the two entities.  Examples of these are the cipher
   suites that need to be negotiated or authorization information
   (Session-lifetime).  There may be also a need of extending the
   information that has to be exchanged in the future.  This section
   specifies the CBOR [RFC8949] data structure to exchange information
   between the IoT device and the Controller in the CoAP payload.

   Next, is the specification of the CBOR Object to exchange information
   in CoAP-EAP

        CoAP-EAP_Info = {
            ?  1 : array,                      ; cipher suite
            ?  2 : bstr,                       ; RID-C
            ?  3 : bstr,                       ; RID-I
            ?  4 : uint                        ; Session-Lifetime
        }

                 Figure 6: CBOR data structure for CoAP-EAP

   The parameters contain the following information:

   1.  cipher suite: It contains a array with the list of the proposed
       or selected CBOR algorithms for OSCORE.  If the field is carried
       over a request, the meaning is the proposed cipher suite, if it
       is carried over a response, corresponds to the response.



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   2.  RID-I: It contains the Recipient ID of the IoT device.  The
       Controller uses this value as Sender ID for its OSCORE Sender
       Context.  The IoT device uses this value as Recipient ID for its
       Recipient Context.

   3.  RID-C: It contains the Recipient ID of the Controller.  The IoT
       device uses this value as Sender ID for its OSCORE Sender
       Context.  The Controller uses this value as Recipient ID for its
       Recipient Context.

   4.  Session-Lifetime: Contains the time the session is valid in
       seconds.

   The indexes from 65000 to 65535 are reserved for experimentation.

5.  Cipher suite negotiation and key derivation

5.1.  Cipher suite negotiation

   OSCORE runs after the EAP authentication, using the cipher suite
   selected in the cipher suite negotiation (Step 1 and 2).  To
   negotiate the cipher suite, CoAP-EAP follows a simple approach: the
   Controller sends a list, in decreasing order or preference, with the
   identifiers of the supported cipher suites (Step 1).  In the response
   to that message (Step 2), the IoT device sends a response with the
   choice.

   This list is included in the payload after the EAP message with a
   CBOR array that contains the cipher suites.  An example of how the
   fields are arranged in the CoAP payload can be seen in Figure 7.  An
   example of the exchange with the cipher suite negotiation is shown in
   Figure 8, where can be appreciated the disposition of both EAP-
   Request/Identity and EAP-Response/Identity, followed by the CBOR
   object defined in Section 4, containing in the cipher suite field the
   CBOR array for the cipher suite negotiation.

   +-----+-----------+-------+------++-------------+
   |Code |Identifier |Length | Data ||cipher suites|
   +-----+-----------+-------+------++-------------+
             EAP Packet                CBOR Object

              Figure 7: cipher suites are in the CoAP payload









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       EAP peer                                  EAP Auth.
    (CoAP server)                              (CoAP client)
    -------------                             -------------
          |                                         |
          |                  ...                    |
          |---------------------------------------->|
          |                              POST /a/x  |
          |  Payload (EAP Req/Id, CBORArray[0,1,2]) |
       1) |<----------------------------------------|
          | 2.01 Created Location-Path [/a/y]       |
          | Payload (EAP Resp/Id, CBORArray[0])     |
       2) |---------------------------------------->|
                             ...

                     Figure 8: cipher suite negotiation

   In case there is no CBOR array stating the cipher suites, the default
   cipher suites are applied.  If the Controller sends a restricted list
   of cipher suites that is willing to accept it MUST include the
   default value 0 since it is mandatory to implement.  The IoT device
   will have at least that option available.

   The cipher suite requirements are inherited from the ones established
   by OSCORE.  By default, the HKDF algorithm is SHA-256 and the AEAD
   algorithm is AES-CCM-16-64-128.  Both are mandatory to implement.
   The other cipher suites supported and negotiated in the cipher suite
   negotiation are the following:

      0.  AES-CCM-16-64-128, SHA-256 (default)

      1.  A128GCM, SHA-256

      2.  A256GCM, SHA-384

      3.  ChaCha20/Poly1305, SHA-256

      4.  ChaCha20/Poly1305, SHAKE256

   This specification uses the (HMAC)-based key derivation function
   (HKDF) defined in [RFC5869] to derive the necessary key material.
   Since the key derivation process uses the MSK, which is considered
   fresh key material, we will use the HKDF-Expand function, which we
   will shorten here as KDF.








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5.2.  Deriving the OSCORE Security Context

   The derivation of the security context for OSCORE allows securing the
   communication between the IoT device and the Controller once the MSK
   has been exported providing, confidentiality, integrity, key
   confirmation (Step 7 and 8) and detecting a downgrading attack.

   The Master Secret can be derived by using the chosen cipher suite and
   the KDF.  The Master Secret can be derived as follows:

   Master Secret = KDF(MSK, CS | "COAP-EAP OSCORE MASTER SECRET",
   length)

   where:

   *  The algorithms for OSCORE are agreed in the cipher suite
      negotiation.

   *  The MSK exported by the EAP method.  Discussion about the use of
      the MSK for the key derivation is done in Section 7.

   *  CS is the concatenation of the content of the cipher suite
      negotiation, that is, the list of cipher suites sent by the
      Controller (Step 1) the selected option by the IoT device (Step
      2).  If any of the messages did not contain the CBOR array
      (default algorithms), the null string is used.

   *  "COAP-EAP OSCORE MASTER SECRET" is the ASCII code representation
      of the non-NULL terminated string (excluding the double quotes
      around it).

   *  CS and "COAP-EAP OSCORE MASTER SECRET" are concatenated.

   *  length is the size of the output key material.

   The Master Salt, similarly to the Master Secret, can be derived as
   follows:

   Master Salt = KDF(MSK, CS | "OSCORE MASTER SALT", length)

   where:

   *  The algorithms are agreed in the cipher suite negotiation.

   *  The MSK exported by the EAP method.  Discussion about the use of
      the MSK for the key derivation is done in Section 7.





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   *  CS is the concatenation of the content of the cipher suite
      negotiation, in the request and response.  If any of the messages
      did not contain the CBOR array, the null string is used.

   *  "OSCORE MASTER SALT" is the ASCII code representation of the non-
      NULL terminated string (excluding the double quotes around it).

   *  CS and "COAP-EAP OSCORE MASTER SECRET" are concatenated.

   *  length is the size of the output key material.

   Since the MSK is used to derive the Master Key, the correct
   verification of the OSCORE protected request (Step 7) and response
   (Step 8) confirms the Controller and the IoT device have the same
   Master Secret, achieving key confirmation.

   To prevent a downgrading attack, the content of the cipher suites
   negotiation (which we refer to here as CS) is embedded in the Master
   Secret derivation.  If an attacker changes the value of the cipher
   suite negotiation, the result will be different OSCORE security
   contexts, that ends up with a failure in Step 7 and 8.

   The Controller will use the Recipient ID of the IoT device (RID-I) as
   Sender ID for its OSCORE Sender Context.  The IoT device will use
   this value as Recipient ID for its Recipient Context.

   The IoT device will use the Recipient ID of the Controller (RID-C) as
   Sender ID for its OSCORE Sender Context.  The Controller will use
   this value as Recipient ID for its Recipient Context.

6.  Discussion

6.1.  CoAP as EAP lower layer

   This section discusses the suitability of the CoAP protocol as EAP
   lower layer, and reviews the requisites imposed by the EAP protocol
   to any protocol that transports EAP.  What EAP expects from its lower
   layers can be found in section 3.1 of [RFC3748], which is elaborated
   next:

   Unreliable transport.  EAP does not assume that lower layers are
   reliable but it can benefit for a reliable lower layer.  In this
   sense, CoAP provides a reliability mechanism (e.g.  through the use
   of Confirmable messages).

   Lower layer error detection.  EAP relies on lower layer error
   detection (e.g., CRC, Checksum, MIC, etc.).  CoAP goes on top of UDP/
   TCP which provides a checksum mechanism over its payload.



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   Lower layer security.  EAP does not require security services from
   the lower layers.

   Minimum MTU.  Lower layers need to provide an EAP MTU size of 1020
   octets or greater.  CoAP assumes an upper bound of 1024 for its
   payload which covers the requirements of EAP.

   Ordering guarantees.  EAP relies on lower layer ordering guarantees
   for correct operation.  Regarding message ordering, every time a new
   message arrives at the authentication service hosted by the IoT
   device, a new resource is created and this is indicated in a "2.01
   Created" response code along with the name of the new resource via
   Location-Path or Location-Query.  This way the application indicates
   that its state has advanced.  Although the [RFC3748] states: "EAP
   provides its own support for duplicate elimination and
   retransmission", EAP is also reliant on lower layer ordering
   guarantees.  In this regard, [RFC3748] talks about possible
   duplication and says: "Where the lower layer is reliable, it will
   provide the EAP layer with a non-duplicated stream of packets.
   However, while it is desirable that lower layers provide for non-
   duplication, this is not a requirement".  CoAP is providing a non-
   duplicated stream of packets and accomplish the "desirable" non-
   duplication.  In addition, [RFC3748] says that when EAP runs over a
   reliable lower layer "the authenticator retransmission timer SHOULD
   be set to an infinite value, so that retransmissions do not occur at
   the EAP layer."

6.2.  Size of the EAP lower layer vs EAP method size

   Regarding the impact that an EAP lower layer will have to the total
   byte size of the whole exchange, there is a comparison with another
   network layer based EAP lower layer, PANA [RFC5191], in [coap-eap].
   Comparing the EAP lower layer (alone) and taking into account EAP.
   On the one hand, at the EAP lower layer level, the usage of CoAP
   gives important benefits.  On the other hand, when taking into
   account the EAP method overload, this reduction is less but still
   significant if the EAP method generates large EAP messages.  If the
   EAP method is very taxing, the impact of the reduction in size of the
   EAP lower layer is less significant.  This leads to the conclusion
   that possible next steps in this field could be designing new EAP
   methods that can be better adapted to the requirements of IoT devices
   and networks.  For example, authors in [coap-eap] used EAP-PSK as an
   example, since it only involves 4 messages and their length can be
   less than 60 bytes.  Moreover, it only uses symmetric cryptography.

   However, the impact of the EAP lower layer itself cannot be ignored,
   hence the proposal of using CoAP as lightweight protocol for this
   purpose.  Other EAP methods such as EAP-AKA'[RFC5448] or new EAP



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   methods such as EAP-NOOB [I-D.ietf-emu-eap-noob] or EAP-EDHOC
   [I-D.ingles-eap-edhoc] that can benefit, as well as new ones that may
   be proposed in the future with IoT constraints in mind, from a CoAP-
   based EAP lower layer.

7.  Security Considerations

   There are some aspects to be considered such as how authorization is
   managed, the use of MSK as keying material and how the trust in the
   Controller is established.  Additional considerations such as EAP
   channel binding as per [RFC6677] are also discussed here.

7.1.  Authorization

   Authorization is part of bootstrapping.  It serves to establish
   whether the node can join and the set of conditions it has to adhere.
   The authorization data will be gathered from the organization that is
   responsible for the IoT device and sent to the EAP authenticator in
   case of AAA infrastructure is deployed.

   In standalone mode, the authorization information will be in the
   Controller.  If the pass-through mode is used, authorization data
   received from the AAA server can be delivered by the AAA protocol
   (e.g.  RADIUS or Diameter).  Providing more fine-grained
   authorization data can be with the transport of SAML in RADIUS
   [RFC7833].

   After bootstrapping, additional authorization information to operate
   in the security domain, e.g., access services offered by other nodes,
   can be taken care of by the solutions proposed in the ACE WG.

7.2.  Freshness of the key material

   In CoAP-EAP there is no nonce exchange to provide freshness to the
   keys derived from the MSK.  The MSK and Extended Master Session Key
   (EMSK) keys according to the EAP Key Management Framework [RFC5247]
   are fresh key material.  Since only one authentication is established
   per EAP authenticator, there is no need for generating additional key
   material.  In case a new MSK is required, a re-authentication can be
   done, by running the process again, or using a more lightweight EAP
   method to derive additional key material as elaborated in
   Section 3.3.

7.3.  Channel Binding support

   According to the [RFC6677], channel binding related with EAP, is sent
   through the EAP method that supports it.




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   To satisfy the requirements of the document, we need to send the EAP
   lower layer identifier (To be assigned by IANA), in the EAP Lower-
   Layer Attribute if RADIUS is used.

7.4.  Additional Security Consideration

   In the process of authentication, there is a possibility of an entity
   forging messages to generate denial of service (DoS) attacks on any
   of the entities involved.  For instance, an attacker can forge
   multiple initial message to start an authentication (Step 0) with the
   Controller as if they were sent by different IoT devices.
   Consequently, the Controller will start an authentication per each
   message received in Step 0, sending the EAP Request/Id (Step 1).

   To minimize the effects of this DoS attack, it is RECOMMENDED that
   the Controller limits the rate at which it processes incoming
   messages in Step 0 to provide robustness against denial of service
   (DoS) attacks.  The details of rate limiting are outside the scope of
   this specification.  Nevertheless, the rate of these messages are
   also limited by the bandwidth available between the IoT device and
   the Controller.  This bandwidth will be specially limited in
   constrained links (e.g., LPWAN).  Lastly, it is also RECOMMENDED to
   reduce at a minimum the state in the Controller at least until the
   EAP Response/Ids received by the Controller.

   Other security-related concerns can be how to ensure that the IoT
   device joining the security domain can in fact trust the Controller.
   This issue is elaborated in the EAP Key Management Framework
   [RFC5247].  In particular, the IoT device knows it can trust the
   Controller because the key that is used to establish the security
   association is derived from the MSK.  If the Controller has the MSK,
   it is clear the AAA Server of the node trusted the Controller, which
   can be considered as a trusted party.

8.  IANA Considerations

   Considerations for IANA regarding this document:

   *  Assignment of EAP lower layer identifier.

   *  Assignment of the URI /.well-known/coap-eap

   *  Assignment of the media type "application/coap-eap"

   *  Assignment of the content format "application/coap-eap"

   *  Assignment of the resource type (rt=) "core.coap-eap"




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   *  Assignment of the numbers assigned for the cipher suite
      negotiation

   *  Assignment of the numbers assigned for the numbers of the CBOR
      object in CoAP-EAP

9.  Acknowledgments

   We would like to thank as the reviewers of this work: Carsten
   Bormann, Mohit Sethi, Benjamin Kaduk, Christian Amsuss, John
   Mattsson, Goran Selander, Alexandre Petrescu, Pedro Moreno-Sanchez
   and Eduardo Ingles-Sanchez.

   We would also like to thank Gabriel Lopez-Millan for the first review
   of this document and we would like to thank Ivan Jimenez-Sanchez for
   the first proof-of-concept implementation of this idea.

   And thank for their valuable comments to Alexander Pelov and Laurent
   Toutain, especially for the potential optimizations of CoAP-EAP.

10.  References

10.1.  Normative References

   [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>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <https://www.rfc-editor.org/info/rfc3748>.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, DOI 10.17487/RFC5247, August 2008,
              <https://www.rfc-editor.org/info/rfc5247>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC6677]  Hartman, S., Ed., Clancy, T., and K. Hoeper, "Channel-
              Binding Support for Extensible Authentication Protocol
              (EAP) Methods", RFC 6677, DOI 10.17487/RFC6677, July 2012,
              <https://www.rfc-editor.org/info/rfc6677>.



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   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

   [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>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

10.2.  Informative References

   [coap-eap] Garcia-Carrillo, D. and R. Marin-Lopez, "Lightweight CoAP-
              Based Bootstrapping Service for the Internet of Things -
              https://www.mdpi.com/1424-8220/16/3/358", March 2016.

   [eap-framework]
              Sethi, M. and T. Aura, "Secure Network Access
              Authentication for IoT Devices: EAP Framework vs.
              Individual Protocols -
              https://ieeexplore.ieee.org/document/9579387", October
              2021.





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   [I-D.ietf-ace-oauth-authz]
              Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments (ACE) using the OAuth 2.0
              Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
              draft-ietf-ace-oauth-authz-36, 16 November 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-ace-oauth-
              authz-36.txt>.

   [I-D.ietf-core-resource-directory]
              Amsüss, C., Shelby, Z., Koster, M., Bormann, C., and P.
              Van der Stok, "CoRE Resource Directory", Work in Progress,
              Internet-Draft, draft-ietf-core-resource-directory-28, 7
              March 2021, <https://www.ietf.org/archive/id/draft-ietf-
              core-resource-directory-28.txt>.

   [I-D.ietf-emu-eap-noob]
              Aura, T., Sethi, M., and A. Peltonen, "Nimble out-of-band
              authentication for EAP (EAP-NOOB)", Work in Progress,
              Internet-Draft, draft-ietf-emu-eap-noob-05, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-emu-eap-noob-
              05.txt>.

   [I-D.ingles-eap-edhoc]
              Sanchez, E., Garcia-Carrillo, D., and R. Marin-Lopez, "EAP
              method based on EDHOC Authentication", Work in Progress,
              Internet-Draft, draft-ingles-eap-edhoc-01, 2 November
              2020, <https://www.ietf.org/internet-drafts/draft-ingles-
              eap-edhoc-01.txt>.

   [lo-coap-eap]
              Garcia-Carrillo, D., Marin-Lopez, R., Kandasamy, A., and
              A. Pelov, "A CoAP-Based Network Access Authentication
              Service for Low-Power Wide Area Networks: LO-CoAP-EAP -
              https://www.mdpi.com/1424-8220/17/11/2646", November 2017.

   [RFC4137]  Vollbrecht, J., Eronen, P., Petroni, N., and Y. Ohba,
              "State Machines for Extensible Authentication Protocol
              (EAP) Peer and Authenticator", RFC 4137,
              DOI 10.17487/RFC4137, August 2005,
              <https://www.rfc-editor.org/info/rfc4137>.

   [RFC4764]  Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
              Pre-Shared Key Extensible Authentication Protocol (EAP)
              Method", RFC 4764, DOI 10.17487/RFC4764, January 2007,
              <https://www.rfc-editor.org/info/rfc4764>.





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   [RFC5191]  Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
              and A. Yegin, "Protocol for Carrying Authentication for
              Network Access (PANA)", RFC 5191, DOI 10.17487/RFC5191,
              May 2008, <https://www.rfc-editor.org/info/rfc5191>.

   [RFC5448]  Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
              Extensible Authentication Protocol Method for 3rd
              Generation Authentication and Key Agreement (EAP-AKA')",
              RFC 5448, DOI 10.17487/RFC5448, May 2009,
              <https://www.rfc-editor.org/info/rfc5448>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC6696]  Cao, Z., He, B., Shi, Y., Wu, Q., Ed., and G. Zorn, Ed.,
              "EAP Extensions for the EAP Re-authentication Protocol
              (ERP)", RFC 6696, DOI 10.17487/RFC6696, July 2012,
              <https://www.rfc-editor.org/info/rfc6696>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC7833]  Howlett, J., Hartman, S., and A. Perez-Mendez, Ed., "A
              RADIUS Attribute, Binding, Profiles, Name Identifier
              Format, and Confirmation Methods for the Security
              Assertion Markup Language (SAML)", RFC 7833,
              DOI 10.17487/RFC7833, May 2016,
              <https://www.rfc-editor.org/info/rfc7833>.

   [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
              Richardson, M., Jiang, S., Lemon, T., and T. Winters,
              "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
              RFC 8415, DOI 10.17487/RFC8415, November 2018,
              <https://www.rfc-editor.org/info/rfc8415>.

   [RFC8824]  Minaburo, A., Toutain, L., and R. Andreasen, "Static
              Context Header Compression (SCHC) for the Constrained
              Application Protocol (CoAP)", RFC 8824,
              DOI 10.17487/RFC8824, June 2021,
              <https://www.rfc-editor.org/info/rfc8824>.

   [TS133.501]
              ETSI, "5G; Security architecture and procedures for 5G
              System - TS 133 501 V15.2.0 (2018-10)", 2018.





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Appendix A.  Flow of operation (DTLS establishment)

   CoAP-EAP makes possible to derive a PSK for (D)TLS to allow PSK-based
   authentication between the IoT device and the Controller.  In the
   instance of using (D)TLS to establish a security association, there
   is a limitation to the use of intermediaries between the IoT device
   and the Controller, as (D)TLS breaks the end-to-end communications
   when using intermediaries such as proxies.

          IoT device                                Controller
        -------------                             -------------
                                ...
              | 2.01 Created Location-Path [/a/w]       |
              | Payload (EAP-X Resp)                    |
            6)|---------------------------------------->|
              |                                         | MSK
              |          (D)TLS 1.3 Client Hello        |  |
      MSK  7) |<----------------------------------------|  V
       |      |                                         | DTLS_PSK
       V      |===============DTLS hanshake=============|
    DTLS_PSK  |                                         |
                                  *...
                    (*) Protected with (D)TLS

               Figure 9: CoAP-EAP flow of operation with DTLS

   Figure 9 shows the last steps of the operation for CoAP-EAP when
   (D)TLS is used to protect the communication between the IoT device
   and the Controller using the keying material exported by the EAP
   authentication.  The general flow is essentially the same as in the
   case of OSCORE, except that DTLS negotiation is established in Step
   7).  Once DTLS negotiation has finished successfully the IoT device
   is granted access to the domain.  Step 7 MUST be interpreted by the
   IoT device as an alternate success indication, which will end up with
   the MSK and the DTLS_PSK derivation for the (D)TLS authentication
   based on PSK.

   According to [RFC8446] the provision of the PSK out-of-band also
   requires the provision of the KDF hash algorithm and the PSK
   identity.  To simplify the design in CoAP-EAP, the KDF hash algorithm
   can be included in the list of cipher suites exchange in Step 1 and
   Step 2 if DTLS wants to be used instead of OSCORE.  For the same
   reason, the PSK identity is derived from (RID-C) (RID-I) as defined
   in Appendix A.2.







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A.1.  Cryptographic suite negotiation for DTLS

   It is also possible to derive a pre-shared key for DTLS to establish
   a DLTS security association after a successful EAP authentication.
   Analogously to how the cipher suite is negotiated for OSCORE
   Section 5.1, the Controller sends a list, in decreasing order of
   preference, with the identifiers of the cipher suites supported (Step
   1).  In the response, the IoT device sends the choice.

   This list is included in the payload after the EAP message with a
   CBOR array that contains the cipher suites.  This CBOR array is
   enclosed as one of the elements of the CBOR Object used for
   transporting information in CoAP-EAP (See Section 4.  An example of
   how the fields are arranged in the CoAP payload can be seen in
   Figure 7.

   In case there is no CBOR array stating the cipher suites, the default
   cipher suites are applied.  If the Controller sends a restricted list
   of cipher suites that is willing to accept it MUST include the
   default value 0 since it is mandatory to implement.  The IoT device
   will have at least that option available.

   The cipher suites are the following:

      3.  TLS_SHA256

      4.  TLS_SHA384

      5.  TLS_SHA512

A.2.  Deriving DTLS PSK and identity

   To enable DTLS after an EAP authentication using the key material
   generated, we define the Identity and the PSK for DTLS.  The Identity
   in this case is generated by concatenating the exchanged Sender ID
   and the Recipient ID.

   CoAP-EAP PSK Identity = RID-C || RID-I

   It is also possible to derive a pre-shared key for DTLS [RFC6347],
   refereed to here as "DTLS PSK", from the MSK between both IoT device
   and Controller if required.  The length of the DTLS PSK will depend
   on the cipher suite.  To have keying material with sufficient length
   a key of 32 bytes is derived that can be later truncated if needed:

   DTLS PSK = KDF(MSK, "CoAP-EAP DTLS PSK", length).

   where:



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   *  MSK is exported by the EAP method.

   *  "CoAP-EAP DTLS PSK" is the ASCII code representation of the non-
      NULL terminated string (excluding the double quotes around it).

   *  length is the size of the output key material.

Appendix B.  Examples of Use Case Scenario

   For a IoT device to act as a trustworthy entity within a security
   domain, certain key material is needed to be shared between the IoT
   device and the Controller.

   Next, we elaborate on examples of different use case scenarios about
   the usage of CoAP-EAP.  Generally, we are dealing with 4 entities:

   *  2 nodes (A and B), which are IoT devices.  They are the EAP peers.

   *  1 controller (C).  The controller manages a domain where nodes can
      be deployed.  It can be considered a more powerful machine than
      the IoT devices.

   *  1 AAA server (AAA) - Optional.  The AAA is an Authentication,
      Authorization and Accounting Server, which is not constrained.
      Here, the Controller acts as EAP authenticator in pass-through
      mode.

   Generally, any IoT device wanting to join the domain managed by the
   Controller MUST perform a CoAP-EAP authentication with the Controller
   (C).  This authentication MAY involve an external AAA server.  This
   means that A and B, once deployed, will run CoAP-EAP once, as a
   bootstrapping phase, to establish a security association with C.
   Moreover, any other entity, which wants to join and establish
   communications with nodes under C's domain must also do the same.  By
   using EAP, we can have the flexibility of having different types of
   credentials.  For instance, if we have a device that is not battery
   dependent, and not very constrained, we could use a heavier
   authentication method.  With varied IoT devices and networks we might
   need to resort to more lightweight authentication methods (e.g., EAP-
   NOOB[I-D.ietf-emu-eap-noob], EAP-AKA'[RFC5448], EAP-PSK[RFC4764],
   EAP-EDHOC[I-D.ingles-eap-edhoc], etc.) being able to adapt to
   different types of devices according to organization policies or
   devices capabilities.








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B.1.  Example 1: CoAP-EAP in ACE

   In ACE, the process of Client registration and provisioning of
   credentials to the client is not specified.  The process of Client
   registration and provisioning can be achieved using CoAP-EAP.  Once
   the process of authentication with EAP is completed, fresh key
   material is shared between the IoT device and the Controller.  In
   this instance, the Controller and the Authorization Server (AS) of
   ACE can be co-located.

   Next, we exemplify how CoAP-EAP can be used to perform the Client
   registration in a general way, to allow two IoT devices (A and B) to
   communicate and interact after a successful client registration.

   Node A wants to communicate with node B (e.g. to activate a light
   switch).  The overall process is divided into three phases.  Let's
   start with node A.  In the first phase, the node A (EAP peer) does
   not yet belong to Controller C's domain.  Then, it communicates with
   C (EAP authenticator) and authenticates with CoAP-EAP, which,
   optionally, communicates with the AAA server to complete the
   authentication process.  If the authentication is successful, a fresh
   MSK is shared between C and node A.  This key material allows node A
   to establish a security association with the C.  Some authorization
   information may be also provided in this step.  In case EAP is used
   in standalone mode, the AS itself having information about the
   devices can be the entity providing said authorization information.
   If authentication and authorization are correct, node A is enrolled
   in controller C's domain for a period of time.  In particular,
   [RFC5247] recommends 8 hours, though the the entity providing the
   authorization information can establish this lifetime.  In the same
   manner, B needs to perform the same process with CoAP-EAP to be part
   of the controller C's domain.

   In the second phase, when node A wants to talk with node B, it
   contacts controller C for authorization to access node B and obtain
   all the required information to do that securely (e.g. keys, tokens,
   authorization information, etc.).  This phase does NOT require the
   usage of CoAP-EAP.  The details of this phase are out-of-scope of
   this document, and the ACE framework is used for this purpose
   [I-D.ietf-ace-oauth-authz].

   In the third phase, the node A can access node B with the credentials
   and information obtained from the controller C in the second phase.
   This access can be repeated without contacting the controller, while
   the credentials given to A are still valid.  The details of this
   phase are out-of-scope of this document.





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   It is worth noting that first phase with CoAP-EAP is required to join
   the controller C's domain.  Once it is performed with success, the
   communications are local to the controller C's domain and there is no
   need to perform a new EAP authentication as long as the key material
   is still valid.  When the keys are about to expire, the IoT device
   can engage in a re-authentication as explained in Section 3.3, to
   renew the key material.

B.2.  Example 2: Multi-domain with AAA infrastructures

   We assume we have a device (A) of the domain acme.org, which uses a
   specific kind of credential (e.g., AKA) and intends to join the um.es
   domain.  This user does not belong to this domain, for which first it
   performs a client registration using CoAP-EAP.  For this, it
   interacts with the controller's domain acting as EAP authenticator,
   which in turn communicates with a AAA infrastructure (acting as AAA
   client).  Through the local AAA server to communicate with the home
   AAA server to complete the authentication and integrate the device as
   a trustworthy entity into the domain of controller C.  In this
   scenario, the AS under the role of the Controller receives the key
   material from the AAA infrastructure

B.3.  Example 3: Single domain with AAA infrastructure

   A University Campus, we have several Faculty buildings and each one
   has its own criteria or policies in place to manage IoT devices under
   an AS.  All buildings belong to the same domain (e.g., um.es).  All
   these buildings are managed with a AAA infrastructure.  A new device
   (A) with credentials from the domain (e.g., um.es) will be able to
   perform the device registration with a Controller (C) of any building
   as long as they are managed by the same general domain.

B.4.  Example 4: Single domain without AAA infrastructure

   In another case, without a AAA infrastructure, we have a Controller
   that has co-located the EAP server and using EAP standalone mode we
   can manage all the devices within the same domain locally.  Client
   registration of a node (A) with Controller (C) can also be performed
   in the same manner.

B.5.  Other use cases










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B.5.1.  CoAP-EAP for network access control

   One of the first steps for an IoT device life-cycle is to perform the
   authentication to gain access to the network.  To do so, the device
   first has to be authenticated and granted authorization to gain
   access to the network.  Additionally, security parameters such as
   credentials can be derived from the authentication process allowing
   the trustworthy operation of the IoT device in a particular network
   by joining the security domain.  By using EAP, we are able to achieve
   this with flexibility and scalability, because of the different EAP
   methods available and the ability to rely on AAA infrastructures if
   needed to support multi-domain scenarios, which is a key feature when
   the IoT devices deployed under the same security domain, belong to
   different organizations.  Given that EAP is also used for network
   access control, we can adapt this service for other technologies.
   For instance, to provide network access control to very constrained
   technologies (e.g., LoRa network).  Authors in [lo-coap-eap] provide
   an study of a minimal version of CoAP-EAP for LPWAN networks with
   interesting results.  In this specific case, we could leverage the
   compression by SCHC for CoAP [RFC8824].

B.5.2.  CoAP-EAP for service authentication

   It is not uncommon that the infrastructure where the device is
   deployed and the services of the IoT device are managed by different
   organizations.  Therefore, in addition to the authentication for
   network access control, we have to consider the possibility of a
   secondary authentication to access different services.  This process
   of authentication, for example, will provide with the necessary key
   material to establish a secure channel and interact with the entity
   in charge of granting access to different services.  In 5G, for
   example, consider a primary and secondary authentication using EAP
   [TS133.501].

Authors' Addresses

   Rafa Marin-Lopez
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   30100 Murcia
   Spain

   Phone: +34 868 88 85 01
   Email: rafa@um.es







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   Dan Garcia-Carrillo
   University of Oviedo
   Calle Luis Ortiz Berrocal S/N, Edificio Polivalente
   33203 Gijon Asturias
   Spain

   Email: garciadan@uniovi.es












































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