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ACE Working Group                                               R. Marin
Internet-Draft                                      University of Murcia
Intended status: Standards Track                               D. Garcia
Expires: July 25, 2021                              University of Oviedo
                                                        January 21, 2021

               EAP-based Authentication Service for CoAP


   This document describes an authentication service that uses EAP
   transported by means of CoAP messages with the following purposes:

   o  Authenticate a CoAP-enabled device that enters a new security
      domain against a AAA infrastructure through a domain Controller.

   o  Bootstrap key material to protect CoAP messages exchanged between

   o  Enable the establishment of Security Associations 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
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   This Internet-Draft will expire on July 25, 2021.

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

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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  General Architecture  . . . . . . . . . . . . . . . . . . . .   3
   3.  General Flow Operation  . . . . . . . . . . . . . . . . . . .   4
     3.1.  EAP over CoAP: Running an OSCORE Security Association . .   4
     3.2.  The SeqNum Option . . . . . . . . . . . . . . . . . . . .   7
   4.  Key Derivation for protecting CoAP messages . . . . . . . . .   8
     4.1.  Deriving the OSCORE Security Context  . . . . . . . . . .   8
   5.  Use Case Scenario . . . . . . . . . . . . . . . . . . . . . .  10
   6.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  CoAP as EAP lower-layer . . . . . . . . . . . . . . . . .  11
     6.2.  Size of the EAP lower-layer vs EAP method size  . . . . .  12
     6.3.  Controller as the CoAP Client . . . . . . . . . . . . . .  12
     6.4.  Possible Optimizations  . . . . . . . . . . . . . . . . .  12
       6.4.1.  Empty Token . . . . . . . . . . . . . . . . . . . . .  13
       6.4.2.  Removing SeqNum Option  . . . . . . . . . . . . . . .  13
       6.4.3.  Further re-authentication . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     7.1.  Authorization . . . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Cryptographic suite selection . . . . . . . . . . . . . .  14
     7.3.  Freshness of the key material . . . . . . . . . . . . . .  15
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  15
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  CoAP-EAP with DTLS . . . . . . . . . . . . . . . . .  18
     A.1.  Deriving DTLS_PSK . . . . . . . . . . . . . . . . . . . .  20
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The goal of this document is to describe an authentication service
   that uses the Extensible Authentication Protocol (EAP) [RFC3748].
   The authentication service is built on top of the Constrained
   Application Protocol (CoAP) [RFC7252] and allows authenticating two
   CoAP endpoints by using EAP without the need of additional protocols
   to bootstrap a security association between them.

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   In particular, the document describes how CoAP can be used as a
   constrained link-layer independent EAP lower-layer [RFC3748] to
   transport EAP between a CoAP server (EAP peer) and the CoAP client
   (EAP authenticator) using CoAP messages.  The CoAP client MAY contact
   with a backend AAA infrastructure to complete the EAP negotiation as
   described in the EAP specification [RFC3748].

   The assumption is that the EAP method transported in CoAP MUST
   generate cryptographic material [RFC5247] .  In this way, the CoAP
   messages can be protected.  There are two approaches that we have
   considered in this document:

   o  To define how the OSCORE security association can be established
      based on the cryptographic material generated from the EAP

   o  To establish a DTLS security association using the exported
      cryptographic material after a successful EAP authentication.

   This document also provides some comments about implementation of a
   proof-of-concept of this preliminary idea

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  General Architecture

   The Figure 1 shows the architecture defined in this document.
   Basically a node acting as the EAP peer wants to be authenticated by
   using EAP.  At the time of writing this document, we have considered
   a model where the EAP peer will act as CoAP server for this service
   and the EAP authenticator will act as CoAP client and MAY interact
   with a backend AAA infrastructure, which will place the EAP server
   and contain the information required to authenticate the CoAP client.
   The rationale behind this decision, as we will expand later, is that
   EAP requests go always from the EAP authenticator and the EAP peer
   and the EAP responses from the EAP peer to the EAP authenticator.
   Nevertheless, a model where the EAP peer act as CoAP client and the
   EAP authenticator as CoAP server can be also analyzed in the future.

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       +------------+        +------------+       +--------------+
       | EAP peer/  |        | EAP auth./ |       |  EAP server/ |
       | CoAP server|+------+| CoAP client|+-----+|  AAA server  |
       +------------+  CoAP  +------------+  AAA  +--------------+

                      Figure 1: CoAP EAP Architecture

3.  General Flow Operation

   The authentication service uses CoAP as transport layer for EAP.  In
   other words, CoAP becomes an EAP lower-layer (in EAP terminology).
   In general, it is assumed that, since the EAP authenticator may
   implement an AAA client to interact with the AAA infrastructure, this
   endpoint will have more resources or, at least, it is not a so
   constrained device.  We describe two different sequence flow.  First,
   it is shown in Figure 2 where the OSCORE is used at the end of EAP
   authentication.  The diagram in Figure 5 shows the flow when DTLS is
   used to protect CoAP messages at the end of the EAP authentication.
   As an example, both diagrams show the usage of a generic EAP method
   that we call EAP-X as authentication mechanism.  (NOTE: any EAP
   method which is able to export cryptographic material should be

3.1.  EAP over CoAP: Running an OSCORE Security Association

   When the EAP peer discovers the presence of the EAP authenticator and
   wants to start the authentication, it can send a Non-Confirmable
   "POST /b" request to the node (Step 0).  This message, will carry an
   option developed from the work on [RFC7967] called no response.  The
   rationale of this option is to avoid waiting for a response if it is
   not needed.  So the use of this option will allow signaling the
   intention of the EAP peer to start the authentication process, as a
   trigger mechanism.  Immediately after that, the EAP authenticator
   will start the authentication service.  It is worth noting that the
   EAP authenticator MAY decide to start the authentication without
   waiting for the trigger message if it has knowledge about the
   presence of the EAP peer, for instance, through a previous
   authentication (re-authentication).

   In any case, to perform the authentication service, the CoAP client
   (EAP authenticator) sends a Confirmable "POST /b" request to the CoAP
   Server (Step 1).  This POST message contains a new option SeqNum that
   holds a sequence number randomly chosen by the CoAP client.  This
   SeqNum is used to provide ordered and reliable delivery of messages
   involved during the whole authentication.  In general, when a CoAP
   request with EAP message is received, the CoAP client considers a
   valid message if only if its sequence number is the expected value.

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   The sequence number is monotonically incremented by 1 so that the
   CoAP server can know what it is the next expected sequence number.

   After receiving the first POST, the CoAP server assigns a resource
   and answers with an Acknowledgment with the piggy-backed resource
   identifier (Uri-Path) (Step 2).  It is assumed that the CoAP server
   will only have an ongoing authentication and will not process
   simultaneous EAP authentications in parallel to save resources.  In
   these two messages, the EAP Req/Id and Rep/ID are exchanged between
   the EAP authenticator and the EAP peer.  The EAP Req/Id message is
   forwarded by the EAP authenticator, when EAP is in pass-through mode,
   to the local AAA server that is in charge of steering the
   conversation, choosing the EAP method to be used (e.g.  EAP-X) if the
   user is local or sending the EAP messages to the home AAA of the EAP
   peer.  At this point, the CoAP server has created a resource for the
   EAP authentication.  The resource identifier value will be used
   together to relate all the EAP conversation between both CoAP
   endpoints.  Since, only an ongoing EAP authentication is permitted
   and EAP is a lock-step protocol a Token of a constant value and 1
   byte can be used throughout the authentication process.  This also
   allows to save bytes through the link.

   From now on, the EAP authenticator and the EAP peer will exchange EAP
   packets related to the EAP method, transported in the CoAP message
   payload (Steps 3,4,5,6).  The EAP authenticator will use the POST
   method to send EAP requests to the EAP peer.  The EAP peer will use a
   Piggy-backed response in the Acknowledgment message to carry the EAP
   response.  At the end of the message exchanges, if everything has
   gone well, the EAP authenticator is able to send an EAP Success
   message and both CoAP endpoints will share a Master Session Key (MSK)

   To establish a security association that will confirm to the EAP peer
   that EAP authenticator received the MSK from the AAA sever, as well
   as to the EAP authenticator that the EAP peer derived the MSK
   correctly, both entities engage in the establishment of a security
   association.  In the context of constrained devices [RFC7228] and
   networks we consider protocols that are designed for these cases.
   Concretely, we show here in the diagram the establishment of the
   OSCORE security association.  This is shown in Steps 7 and 8.  From
   that point any exchange between both CoAP endpoints are protected
   with OSCORE.  Before sending the EAP success to the EAP peer, the EAP
   authenticator is able to derive the OSCORE Security Context, to
   confirm the establishment of the security association.  The details
   of the establishment of the OSCORE Security Context are discussed in
   Section 4.1

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   On the contrary, if DTLS is used (see Figure 5), a DTLS_PSK is
   derived from the MSK.  Moreover, exchanges between both CoAP
   endpoints are protected with DTLS from that point.

            EAP peer                                  EAP Auth.
          (CoAP server)                             (CoAP client)
          -------------                             -------------
               |                                         |
               | NON [0x6af5]                            |
               | POST /b                                 |
               | No-Response                             |
           0)  | Token (0xab)                            |
               |                                         |
               |                            CON [0x7654] |
               |                                 POST /b |
               |                               SeqNum(x) |
               |                            Token (0xac) |
               |                      Payload EAP Req/Id |
            1) |<----------------------------------------|
               |                                         |
               | ACK [0x7654]                            |
               | SeqNum(x)                               |
               | Token (0xac)                            |
               | 2.01 Created                            |
               | Uri-Path [/b/5]                         |
               | Payload EAP Rep/Id                      |
            2) |---------------------------------------->|
               |                                         |
               |                            CON [0x8694] |
               |                               POST /b/5 |
               |                             SeqNum(x+1) |
               |                            Token (0xac) |
               |                     Payload EAP-X MSG 1 |
            3) |<----------------------------------------|
               |                                         |
               | ACK [0x8694]                            |
               | Token (0xac)                           |
               | SeqNum(x+1)                             |
               | 2.04 Changed                            |
               | Payload EAP-X MSG 2                     |
            4) |---------------------------------------->|

               |                                         |
               |                            CON [0x9869] |

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               |                               POST /b/5 |
               |                            SeqNum(x+n/2)|
               |                            Token (0xac) |
               |                 Payload EAP-X MSG (n-1) |
            5) |<----------------------------------------|
               |                                         |
               | ACK [0x9869]                            |
               | SeqNum(x+n/2)                           |
               | Token (0xac)                            |
               | 2.04 Changed                            |
               | Payload EAP-X MSG (n)                   |  MSK
            6) |---------------------------------------->|   |
               |                                         |   V
               |                         CON [0x7811]    | OSCORE
               |                         POST /b/5       | CONTEXT
               |                         SeqNum(x+n/2+1) |
               |                         Token (0xac)    | (*)
               |                         OSCORE Option   |
               |                         EAP success     |
     MSK    7) |<----------------------------------------|
      |        |                                         |
      V    (*) | ACK [0x7811]                            |
    OSCORE     | SeqNum(x+n/2+1)                         |
    CONTEXT    | Token (0xac)                            |
               | OSCORE Option                           |
               | 2.04 Changed                            |
            8) |---------------------------------------->|

               (*) Protected with OSCORE

                   Figure 2: CoAP-EAP with OSCORE option

3.2.  The SeqNum Option

   A new SeqNum option is defined in this document for establishing the
   ordering guarantee of the EAP exchange.  Following guidelines in
   [RFC7252] this option is:

   1.  Format opaque (sequence of bytes).

   2.  Critical

   3.  Safe to Forward

   4.  No cacheable and Not part of the Cache-Key

   5.  Not repeatable

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   The number of the option will be determined by this previous

   1.  Critical (C = 1)

   2.  Safe to Forward (1)

   3.  NoCacheKey (111)

   The number of the SeqNum option will fit this pattern: xxx11111

                           0   1   2   3   4   5   6   7
                           |           | NoCacheKey| U | C |

                     Figure 3: Auth Option Number Mask

   The option number is TBD.

   The resultant SeqNum option is:

      | No. | C | U | N | R | Name   | Format | Length | Default |
      | TBD | x |   | x |   | SeqNum | uint   |  0-16  | (none)  |

      C = Critical,   U = Unsafe,   N = NoCacheKey,   R = Repeatable

                          Figure 4: SeqNum option

4.  Key Derivation for protecting CoAP messages

   As a result of a successful EAP authentication, both CoAP server and
   CoAP client share a Master Key Session (MSK).  The assumption is that
   MSK is a fresh key so any derived key from the MSK will be also
   fresh.  We have considered the derivation of either the OSCORE
   Security Context or pre-shared key that can be used for a DTLS
   negotiation (DTLS_PSK) (in the Appendix)

4.1.  Deriving the OSCORE Security Context

   Key material needed to derive the OSCORE Security Context, from the
   MSK can be done as follows.  First, HKDF SHA-256 [RFC5869] is
   mandatory to implement.  We assume the use of the default algorithms

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   HKDF SHA-256 and AES-CCM-16-64-128.  The extract phase of HKDF
   produces a pseudo-random key ( that we refer to here as RK) that is
   used to generate the OSCORE Security Context in the Expand phase.
   The derivation is done as follows:

   RK = HMAC-SHA-256(MSK)


   o  MSK is the Master Session Key derived from the EAP method.

   o  RK is the Random Key that is generated from the MSK in the Extract

   Discussions about the use of the MSK for the key derivation are done
   in Section Section 7.

   Based on the RK generated from the MSK, we can now generate the
   Master Secret and Master Salt.  The key derivation is performed as

   Master_Secret = HKDF(RK, "IETF_OSCORE_MASTER_SECRET", length)


   o  The RK is exported in the Extract Phase, previously commented.

   o  "IETF_OSCORE_MASTER_SECRET" is the ASCII code representation of
      the non-NULL terminated string (excluding the double quotes around

   o  length is the length of the Master_Secret.  We set the length to
      32 bytes

   The Master Salt can be derived as follows:

   Master_Salt = HKDF(PK, "IETF_OSCORE_MASTER_SALT", length)


   o  The RK is exported in the Extract Phase, previously commented.

   o  "IETF_OSCORE_MASTER_SALT" is the ASCII code representation of the
      non-NULL terminated string (excluding the double quotes around

   o  length is the length of the Master Salt.  We set the length to 8

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   The ID Context can be set to the Identity of the EAP peer.

5.  Use Case Scenario

   In the following, we explain a basic example about the usage of CoAP-
   EAP.  There are 5 entities involved in the scenario:

   o  2 nodes (A and B), which are constrained devices.

   o  1 node D, which is considered a no so constrained device, such a
      phone, or a tablet or even a laptop.

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

   o  1 AAA server (AAA).  The AAA is an Authentication, Authorization
      and Accounting Server, which is not constrained.

   Any node wanting to join the domain managed by the controller, MUST
   perform an CoAP-EAP authentication with the controller C.  This
   authentication may involve an external AAA server.  This means that A
   and B, once deployed, will perform this CoAP-EAP once as a
   bootstrapping phase to establish a security association with the
   controller C.  Moreover, any other entity (i.e. node D) , which wants
   to join and establish communications with nodes under the controller
   C's domain must also do the same.

   Let us assume that the node A wants to communicate with node B (e.g.
   to active a light switch).  The overall process is divided in three
   phases.  Let's start with node A.  In the first phase, the node A
   (EAP peer) does not yet belong to the controller C's domain.  Then,
   it communicates with controller C (EAP authenticator) and
   authenticates with CoAP-EAP, which, in turn, communicates with the
   AAA server to complete the authentication process.  If the
   authentication is successful, key material is distributed to the
   controller C and derived by node A.  This key material allows node A
   to establish a security association with controller C.  Some
   authorization information coming from the AAA infrastructure may be
   also provided in this step.  If authentication and authorization are
   correct, node A is enrolled in the controller C's domain during a
   period of time.  In particular, [RFC5247] recommends 8 hours, though
   the AAA server can establish a different 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 the controller C for authorization to access node B and

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   obtain all the required information to do that in a secure manner
   (e.g. keys, tokens, authorization information, etc.).  It does NOT
   require the usage of CoAP-EAP.  The details of this phase are out of
   scope of this document, but ACE framework can be 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.

   It is worth noting that first phase with CoAP-EAP is ONLY required to
   join the controller C's domain.  Once it is performed with success,
   the communications are local to the controller C's domain so there is
   no need to contact the external AAA server nor performing EAP

6.  Discussion

6.1.  CoAP as EAP lower-layer

   In this section we discuss the suitability of the CoAP protocol as
   EAP lower layer, and review the requisites imposed by the EAP
   protocol to any protocol that transports EAP.  The assumptions EAP
   makes about its lower layers can be found in section 3.1 of the rfc
   [RFC3748], which are enumerated next:

   o  Unreliable transport.  EAP does not assume that lower layers are

   o  Lower layer error detection.  EAP relies on lower layer error
      detection (e.g., CRC, Checksum, MIC, etc.)

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

   o  Minimum MTU.  Lower layers need to provide an EAP MTU size of 1020
      octets or greater.

   o  Possible duplication.  EAP stipulates that, while desirable, it
      does not require for the lower layers to provide non-duplication.

   o  Ordering guarantees.  EAP relies on lower layer ordering
      guarantees for correct operation.

   Regarding the unreliable transport, although EAP assumes a non
   reliable transport, CoAP does provide a reliability mechanism through

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   the use of Confirmable messages.  For the error detection, CoAP goes
   on top of UDP which provides a checksum mechanism over its payload.
   Lower layer security services are not required.  About the minimum
   MTU of 1020 octets, CoAP assumes an upper bound of 1024 for its
   payload which covers the requirements for EAP.  Regarding message
   ordering, we propose the use of a new CoAP option, the SeqNum option
   described in Section (Section 3.2), which will allow keep the order
   in which the different messages are exchanged.

   Regarding the Token, we consider the use of a constant value using a
   small 1 byte Token.  In fact, the EAP server will not send a new EAP
   request until it has processed the expected EAP response.
   Additionally, we are under the assumption that there will a single
   EAP authentication between the constrained device and the same

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

   Using CoAP as EAP lower layer guarantees a constrained transport for
   EAP in constrained environments.  However, it is a fair to ask about
   the level of improvement taking into account the overload represented
   by the EAP method.  In fact, if the EAP method is very taxing in the
   number of messages and the bytes sent over the networks the
   improvement achieved in the EAP lower-layer may be less significant
   ([coap-eap]).  However, if the EAP method is lightweight and suitable
   for constrained networks (e.g.  EAP-PSK, as a representative example
   of a lightweight EAP method) a constrained EAP lower-layer brings
   more benefits.  This leads to the conclusion that possible next steps
   in this field could be also improving or designing new EAP methods
   that can be better adapted to the requirements of constrained devices
   and networks.  Therefore, others EAP methods such as EAP-AKA or new
   lightweight EAP methods such as EAP-EDHOC [I-D.ingles-eap-edhoc] may
   benefit from a CoAP-based EAP lower-layer, as well as any new
   lightweight EAP method.

6.3.  Controller as the CoAP Client

   Due to the constrained capacities of the devices, to relieve them of
   the retransmission tasks, we set the Controller as the CoAP client,
   for the main exchange following the recommendations of the
   [I-D.ietf-lwig-coap] document to simplify the constrained device

6.4.  Possible Optimizations

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6.4.1.  Empty Token

   Assuming that the bootstrapping service runs before any other
   service, and that no other service will run concurrently until it has
   finished, we could use an Emtpy Token value to save resources, since
   there will be no other endpoint or CoAP exchange.

6.4.2.  Removing SeqNum Option

   An alternative to consider would be to try to rely on the Message ID
   values as a way of achieving the order delivery throughout the
   authentication exchange.  Here we have two approximations: 1)
   Removing the option from the ACKs and 2) removing the option

   1.  Since the ACKs are piggybacked by design, there is only 1 ongoing
       authentication process and the EAP exchange is done in a lockstep
       fashion, when we get a response we will get the same Message ID
       of the request and we can confirm the SeqNum of the Request.

   2.  An alternative to consider would be to try to solely rely on the
       Message ID values as a way of achieving the order delivery
       throughout the authentication exchange.  Here we also have two
       approaches: A) To expect randomly generated Message IDs and B)
       set the Message ID to increase monotonically by 1.

       A.  Regarding the use of the Message ID, their values in the
           requests sent by the Controller are generated randomly, as
           suggested by CoAP.  The Controller selects a new Message ID
           value each time a new request is sent to the CoAP server,
           until the bootstrapping service finishes.  Moreover, the
           Controller stores the last Message ID sent until correctly
           receiving the corresponding ACK.  The CoAP server keeps track
           of the last received Message ID to identify retransmissions,
           and the previous Message IDs during the current bootstrapping
           to identify old messages.  In general, a request is
           considered valid in terms of the Message ID if either this
           value matches the last value received, which means a
           retransmission of the last response is required, or the
           arrival of a new Message ID, which therefore represents a new
           message.  If these rules do not apply (i.e., an old Message
           ID has been received), the CoAP server silently discards the
           request.  This is possible because the bootstrapping service
           is designed as lockstep, i.e. the Controller will not send a
           new request until it has received the expected response.
           When the bootstrapping exchange finishes successfully, the
           CoAP server can free the tracked Message IDs, except for the

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           last received Message ID at the end of the bootstrapping,
           just in case a retransmission is required.

       B.  This case would avoid having to keep track of the already
           used Message IDs, monotonically increasing by 1 the message
           ID value once the first is randomly picked by the Controller.

6.4.3.  Further re-authentication

   Since the initial bootstrapping is usually taxing, it is assumed to
   be done only once over a long period of time.  If further re-
   authentications for refreshing the key material are necessary, there
   are other methods that can be used to perform these re-
   authentications.  For example, the EAP re-authentication (EAP-ERP)
   [RFC6696] can be used to avoid repeating the entire EAP exchange in
   few exchanges.

7.  Security Considerations

   There are some aspects to be considered such as how authorization is
   managed, how the cryptographic suite is selected and how the trust in
   the Controller is established.

7.1.  Authorization

   Authorization is part of the bootstrapping.  It serves to establish
   whether the node can join and the set of conditions it has to adhere.
   The authorization data received from the AAA server can be delivered
   by the AAA protocol (e.g.  Diameter).  Providing more fine grained
   authorization data can be with the transport of SAML in RADIUS
   [RFC7833] After bootstrapping, additional authorization 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.  Cryptographic suite selection

   How the cryptographic suit is selected is also important.  To reduce
   the overhead of the protocol we use a default cryptographic suite.
   As OSCORE is assumed to run after the EAP authentication, the same
   default crypto-suite is used in this case as explained in the Key
   Derivation Section Section 4 The cryptographic suite is not
   negotiated.  If the cryptographic suite to be used by the node is
   different from default, the AAA server will send the specific
   parameters to the Authenticator.  If the cryptographic suite is not
   supported, the key derivation process would result in a security
   association failure.

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7.3.  Freshness of the key material

   In this design, we do not exchange nonces to provide freshness to the
   keys derived from the MSK.  This is done under the assumption that
   the MSK and EMSK keys derived are fresh key material by the
   specifications of the EAP KMF.  Since only one session key is derived
   from the MSK we do not have to concern ourselves with the generation
   of additional key material.  In case we need to refresh the MSK, a
   re-authentication can be done, by running process again, using a more
   lightweight mechanism to derive additional key material such as ERP

8.  IANA Considerations

   This document has no actions for IANA.

9.  Acknowledgments

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

   We also thank for their valuables comments to Alexander Pelov and
   Laurent Toutain, specially for the potential optimizations of CoAP-

10.  References

10.1.  Normative References

              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)", draft-ietf-ace-oauth-authz-36
              (work in progress), November 2020.

              Kovatsch, M., Bergmann, O., and C. Bormann, "CoAP
              Implementation Guidance", draft-ietf-lwig-coap-06 (work in
              progress), July 2018.

              Sanchez, E., Garcia-Carrillo, D., and R. Marin-Lopez, "EAP
              method based on EDHOC Authentication", draft-ingles-eap-
              edhoc-01 (work in progress), November 2020.

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              Kumar, S., Keoh, S., and O. Garcia-Morchon, "DTLS Relay
              for Constrained Environments", draft-kumar-dice-dtls-
              relay-02 (work in progress), October 2014.

              Das, S. and Y. Ohba, "Provisioning Credentials for CoAP
              Applications using EAP", draft-ohba-core-eap-based-
              bootstrapping-01 (work in progress), March 2012.

              Pelov, A., Toutain, L., and Y. Delibie, "Constrained
              Signaling Over LP-WAN", draft-pelov-core-cosol-01 (work in
              progress), February 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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,

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
              2006, <https://www.rfc-editor.org/info/rfc4493>.

   [RFC4615]  Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
              Advanced Encryption Standard-Cipher-based Message
              Authentication Code-Pseudo-Random Function-128 (AES-CMAC-
              PRF-128) Algorithm for the Internet Key Exchange Protocol
              (IKE)", RFC 4615, DOI 10.17487/RFC4615, August 2006,

   [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,

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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>.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, DOI 10.17487/RFC5247, August 2008,

   [RFC5295]  Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
              "Specification for the Derivation of Root Keys from an
              Extended Master Session Key (EMSK)", RFC 5295,
              DOI 10.17487/RFC5295, August 2008,

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,

   [RFC6345]  Duffy, P., Chakrabarti, S., Cragie, R., Ohba, Y., Ed., and
              A. Yegin, "Protocol for Carrying Authentication for
              Network Access (PANA) Relay Element", RFC 6345,
              DOI 10.17487/RFC6345, August 2011,

   [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,

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,

   [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,

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   [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>.

10.2.  Informative References

              Mills, A., "Cantcoap implementation of CoAP", January

              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.

   [Contiki]  Contiki, "Contiki: The Open Source OS for the Internet of
              Things", March 2014.

   [hostapd]  hostapd, "hostapd: IEEE 802.11 AP, IEEE
              802.1X/WPA/WPA2/EAP/RADIUS Authenticator", March 2014.

Appendix A.  CoAP-EAP with DTLS

   Other possibility at our disposal is to do a DTLS handshake after the
   MSKs generation and continue the communication between endpoints
   using CoAP through DTLS as we can see at Figure 5.  The Steps 0-6 are
   the same as the case with OSCORE, however, before continuing with
   Steps 7 and 8, the EAP authenticator starts the DTLS handshake with
   the EAP peer (Step 6').  To establish a DTLS Security Association, a
   key named DTLS-PSK is derived from MSK (see Section 4 ).  In this
   case the CoAP client can start DTLS before sending the last message
   containing the EAP Success.  Once DTLS is established, any posterior
   CoAP exchange is protected.  Thus, OSCORE in this instance is not
   needed for key confirmation, since a successful DTLS negotiation
   confirms the possession of DTLS_PSK that, in turn, corroborates that
   both entities participated in the EAP authentication.

                EAP peer                                   EAP Auth.
              (COAP server)                             (COAP client)
              -------------                             -------------
                   |                                         |
                   | NON [0x6af5]                            |
                   | POST /b                                 |
                   | No-Response                             |
               0)  | Token (0xab)                            |

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                   |                                         |
                   |                            CON [0x7654] |
                   |                                 POST /b |
                   |                               SeqNum(x) |
                   |                            Token (0xac) |
                   |                      Payload EAP Req/Id |
                1) |<----------------------------------------|
                   |                                         |
                   | ACK [0x7654]                            |
                   | SeqNum(x)                               |
                   | Token (0xac)                            |
                   | 2.01 Created                            |
                   | Uri-Path [/b/5]                         |
                   | Payload EAP Rep/Id                      |
                2) |---------------------------------------->|
                   |                                         |
                   |                            CON [0x8694] |
                   |                               POST /b/5 |
                   |                             SeqNum(x+1) |
                   |                           Token (0xac)  |
                   |                     Payload EAP-X MSG 1 |
                3) |<----------------------------------------|
                   |                                         |
                   | ACK [0x8694]                            |
                   | Token (0xac)                            |
                   | SeqNum(x+1)                             |
                   | 2.04 Changed                            |
                   | Payload EAP-X MSG 2                     |
                4) |---------------------------------------->|

                   |                                         |
                   |                            CON [0x9869] |
                   |                               POST /b/5 |
                   |                            SeqNum(x+n/2)|
                   |                            Token (0xac) |
                   |                 Payload EAP-X MSG (n-1) |
                5) |<----------------------------------------|
                   |                                         |
                   | ACK [0x9869]                            |
                   | SeqNum(x+n/2)                           |
                   | Token (0xac)                            |
                   | 2.04 Changed                            |
                   | Payload EAP-X MSG (n)                   |
             MSK 6)|---------------------------------------->| MSK
               |   |                                         |  |
           DTLS_PSK|                                         | DTLS_PSK
                   |                                         |

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                   |              DTLS HANDSHAKE             |
                   |          (Initiated by EAP Auth.)       |
               6') |<--------------------------------------->|
                   |                                         |
                   |                            CON [0x7811] |
                   |                            POST /b/5    |
                   |                            Token (0xac) |
                   |                     Payload EAP Success | (*)
               7)  |<----------------------------------------|
                   |                                         |
                   | ACK [0x7811]                            |
               (*) | Token (0xac)                            |
                   | 2.04 Changed                            |
                8) |---------------------------------------->|

                   (*) Protected with DTLS

                     Figure 5: EAP over CoAP with DTLS

A.1.  Deriving DTLS_PSK

   In the second alternative, a DTLS_PSK is derived from the MSK between
   both CoAP endpoints.  So far, DTLS_PSK will have also 16 byte length
   and it will derived from the RK (generated as done in
   Section Section 4) as follows:

   DTLS_PSK = HKDF(RK, "IETF_DTLS_PSK" , length).  This value is
   concatenated with the value of the Token Option value.


   o  RK is the Random Key generated in the Extract phase, from the MSK.

   o  "IETF_DTLS_PSK" is the ASCII code representation of the non-NULL
      terminated string (excluding the double quotes around it).

   o  length is the length of the DTLS_PSK (16 bytes).

Authors' Addresses

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

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

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

   Email: garciadan@uniovi.es

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