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ACE Working Group                                             R. Sanchez
Internet-Draft                                                  R. Marin
Intended status: Experimental                       University of Murcia
Expires: October 11, 2014                                  April 9, 2014


               EAP-based Authentication Service for CoAP
                     draft-marin-ace-wg-coap-eap-00

Abstract

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

   o  Authenticate two CoAP endpoints.

   o  Bootstrap key material to protect CoAP messages exchanged 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 October 11, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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



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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  General Architecture  . . . . . . . . . . . . . . . . . . . .   3
   3.  General Flow Operation  . . . . . . . . . . . . . . . . . . .   3
     3.1.  EAP in CoAP with AUTH option  . . . . . . . . . . . . . .   4
     3.2.  EAP in CoAP with DTLS . . . . . . . . . . . . . . . . . .   7
   4.  Key Derivation for protecting CoAP messages . . . . . . . . .   9
     4.1.  Deriving COAP_AUTH_KEY  . . . . . . . . . . . . . . . . .  10
     4.2.  Deriving DTLS_PSK . . . . . . . . . . . . . . . . . . . .  10
   5.  Generating AUTH option  . . . . . . . . . . . . . . . . . . .  11
   6.  Implementation  . . . . . . . . . . . . . . . . . . . . . . .  12
   7.  Future Work: CoAP Relay . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  13
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     11.2.  Informative References . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

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) [I-D.ietf-core-coap] and allows
   authenticating two CoAP endpoints by using EAP without the need of
   additional protocols to bootstrap a security association between
   them.

   In particular, the document describes how CoAP can be used as 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:





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   o  To define a new AUTH option that includes an authentication tag
      generated with the cryptographic material exported during an EAP
      authentication.  This new option is used to protect the integrity
      of the CoAP message that carries the AUTH option.  (NOTE:
      Encryption will be considered in the future).

   o  To establish a DTLS security association using the exported
      cryptographic material after a successful EAP authentication.
      [I-D.ohba-core-eap-based-bootstrapping]

   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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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.  Nevertheless, a model where the
   EAP peer act as CoAP client and the EAP authenticator as CoAP server
   will be also analyzed in the future.

             +------------+        +------------+       +--------------+
             | 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 the CoAP protocol as transport layer
   for EAP.  CoAP becomes an EAP lower-layer (in EAP terminology).  In
   general, it is assumed that, since the EAP authenticator may need to
   implement an AAA client to interact with the AAA infrastructure, this
   endpoint will have more resources.  We describe two different
   sequence flow.  First, it is shown in Figure 2 where the AUTH option
   is used at the end of EAP authentication.  Second diagram (see
   Figure 3) shows the flow when DTLS is used to protect CoAP messages



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   at the end of the EAP authentication.  As an example, both diagrams
   show the usage of the EAP-PSK method [RFC4764] as authentication
   mechanism.  (NOTE: any EAP method which is able to export
   cryptographic material should be valid).

3.1.  EAP in CoAP with AUTH option

   If the EAP peer discovers the presence of the EAP authenticator and
   wants to start the authentication, it can send a Confirmable "GET /
   auth" request to the node (Step 0).  If the EAP authenticator
   implements the authentication service will return a 2.05 in a
   Acknowledgment with a piggy-backed response (Step 0').  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 a "GET /auth" message.

   In any case, to perform the authentication service, the CoAP client
   (EAP authenticator) sends a Confirmable "POST /auth" request to the
   CoAP Server (Step 1).  POST message indicates to the CoAP server the
   creation of a resource for the EAP-based authentication service.  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.  Moreover if after a period of time (TBD) no further
   message is received from the CoAP client, the CoAP server will free
   the created state.  In this moment, the CoAP server has started a
   resource for the EAP authentication, whose resource identifier value
   will be used together with the Token option value to relate all the
   EAP conversation between both CoAP endpoints.

   From now on, the EAP authenticator and the EAP peer will exchange EAP
   packets transported in the CoAP message payload (Steps 3,4,5,6,7).
   The EAP authenticator will use PUT method to send EAP requests to the
   EAP peer.  The EAP peer will use a Piggy-backed response in the
   Acknowledgement 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) ([RFC5295])

   If the new defined AUTH option is used, an authentication tag is
   generated with a new key named COAP_AUTH_KEY, derived from the MSK.
   The Acknowledgment message from the CoAP server will also include an
   AUTH option so that the CoAP client can verify that the CoAP server
   obtained the MSK.  This is shown in Steps 7 and 8.  From that point
   any exchange (for example, Steps 9 and 10) between both CoAP
   endpoints are protected with the AUTH option.  Finally, the CoAP
   client MAY send a Confirmable DELETE request to remove all the state



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   related with the authentication service in the CoAP server (Steps 11
   and 12).  The CoAP server may decide to remove the state after period
   of time in case not receiving a DELETE request.  This may be easier
   if the EAP authenticator sends a session lifetime option (TBD) in the
   Step 7 (where the EAP Success is sent).

   On the contrary, if DTLS is used (see Figure 3), 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)
          -------------                             -------------
               |                                         |
               | CON [0x6af5]                            |
           0)  | Token [0xFA5B45FF4723BB43]              |
               | GET /auth                               |
               |---------------------------------------->|
               |                                         |
               |                                         |
               |                       ACK 2.05 [0x6af5] |
               |              (Token 0xFA5B45FF4723BB43) |
           0') |                           Payload ["OK"]|
               |<----------------------------------------|
               |                                         |
               |                                         |
               |                           CON [0x73DE]  |
               |              (Token 0x78728FD4AC3190FF) |
               |                              POST /auth |
            1) |<----------------------------------------|
               |                                         |
               | ACK [0x73DE]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.01 Created                            |
               | Uri-Path [auth/5]                       |
            2) |---------------------------------------->|
               |                                         |
               |                            CON [0x7654] |
               |              (Token 0x78728FD4AC3190FF) |
               |                   Payload EAP-PSK MSG 1 |
               |                             PUT /auth/5 |
            3) |<----------------------------------------|
               |                                         |
               | ACK [0x7654]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.04 Changed                            |



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               | Payload EAP-PSK MSG 2                   |
            4) |---------------------------------------->|
               |                                         |
               |                            CON [0x9869] |
               |              (Token 0x78728FD4AC3190FF) |
               |                   Payload EAP-PSK MSG 3 |
               |                             PUT /auth/5 |
            5) |<----------------------------------------|
               |                                         |
               | ACK [0x9869]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.04 Changed                            |
               | Payload EAP-PSK MSG 4                   |
            6) |---------------------------------------->|
           MSK |                                         | MSK
            |  |                            CON [0x7811] |  |
       COAP_AUTH_KEY          (Token 0x78728FD4AC3190FF) |COAP_AUTH_KEY
               |                             AUTH option |
               |                     Payload EAP Success | (*)
               |                             PUT /auth/5 |
            7) |<----------------------------------------|
               |                                         |
           (*) | ACK [0x7811]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | AUTH option                             |
               | 2.04 Changed                            |
            8) |---------------------------------------->|

                               .............

               |                                         |
               |                            CON [0x7511] |
               |              (Token 0x55566AF7464646BC) |  (*)
               |                             AUTH option |
               |                               GET /temp |
            9) |<----------------------------------------|
               |                                         |
               | ACK [0x7511]                            |
          (*)  | (Token 0x55566AF7464646BC)              |
               | AUTH option                             |
               | 2.05 Content                            |
               | "22.5C"                                 |
           10) |---------------------------------------->|
                             ................

               |                                         |
               |                            CON [0x7600] |
               |              (Token 0x678443AA678BC690) | (*)



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               |                             AUTH option |
               |                          DELETE /auth/5 |
           11) |<----------------------------------------|
               |                                         |
               | ACK [0x7500]                            |
           (*) | (Token 0x678443AA678BC690)              |
               | AUTH option                             |
               | 2.02 Deleted                            |
           12) |---------------------------------------->|

                     (*) Protected with AUTH option




                 Figure 2: EAP over CoAP with AUTH option

3.2.  EAP in CoAP 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 3.  The Steps 0-6 are
   the same as the case with AUTH option, however, before continuing
   with Steps 7 and 8, the EAP authenticator starts the DTLS handshake
   with the EAP peer (Step 7').  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, new AUTH option is
   not needed.  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)
          -------------                             -------------
               |                                         |
               | CON [0x6af5]                            |
           0)  | Token [0xFA5B45FF4723BB43]              |
               | GET /auth                               |
               |---------------------------------------->|
               |                                         |
               |                                         |
               |                       ACK 2.05 [0x6af5] |
               |              (Token 0xFA5B45FF4723BB43) |



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           0') |                           Payload ["OK"]|
               |<----------------------------------------|
               |                                         |
               |                           CON [0x73DE]  |
               |              (Token 0x78728FD4AC3190FF) |
               |                              POST /auth |
            1) |<----------------------------------------|
               |                                         |
               | ACK [0x73DE]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.01 Created                            |
               | Uri-Path [auth/5]                       |
            2) |---------------------------------------->|
               |                                         |
               |                            CON [0x7654] |
               |              (Token 0x78728FD4AC3190FF) |
               |                   Payload EAP-PSK MSG 1 |
               |                             PUT /auth/5 |
            3) |<----------------------------------------|
               |                                         |
               | ACK [0x7654]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.04 Changed                            |
               | Payload EAP-PSK MSG 2                   |
            4) |---------------------------------------->|
               |                                         |
               |                            CON [0x9869] |
               |              (Token 0x78728FD4AC3190FF) |
               |                   Payload EAP-PSK MSG 3 |
               |                             PUT /auth/5 |
            5) |<----------------------------------------|
               |                                         |
               | ACK [0x9869]                            |
               | (Token 0x78728FD4AC3190FF)              |
               | 2.04 Changed                            |
               | Payload EAP-PSK MSG 4                   |
            6) |---------------------------------------->|
           MSK |                                         | MSK
            |  |                                         |  |
       DTLS_PSK|                                         | DTLS_PSK
               |                                         |
               |              DTLS HANDSHAKE              |
               |          (Initiated by EAP Auth.)       |
           7') |<--------------------------------------->|
               |                                         |
               |                            CON [0x7811] |
               |              (Token 0x78728FD4AC3190FF) |
               |                     Payload EAP Success | (*)



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               |                           PUT /auth/5   |
            7) |<----------------------------------------|
               |                                         |
               | ACK [0x7811]                            |
           (*) | (Token 0x78728FD4AC3190FF)              |
               | 2.04 Changed                            |
            8) |---------------------------------------->|

                                .............

               |                                         |
               |                            CON [0x7511] |
               |              (Token 0xAA763D82F623B7FF) | (*)
               |                               GET /temp |
            9) |<----------------------------------------|
               |                                         |
               | ACK [0x7511]                            |
          (*)  | (Token 0xAA763D82F623B7FF)              |
               | 2.05 Content                            |
               | "22.5C"                                 |
           10) |---------------------------------------->|
                                ................

               |                                         |
               |                            CON [0x7600] |
               |              (Token 0x678443AA678BC690) | (*)
               |                          DELETE /auth/5 |
           11) |<----------------------------------------|
               |                                         |
               | ACK [0x7500]                            |
           (*) | (Token 0x678443AA678BC690)              |
               | 2.02 Deleted                            |
           12) |---------------------------------------->|

                     (*) Protected with DTLS



                     Figure 3: EAP over CoAP with DTLS

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 COAP_AUTH_KEY or
   DTLS_PSK.




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4.1.  Deriving COAP_AUTH_KEY

   A key COAP_AUTH_KEY is derived from the MSK to generate the
   authentication tag included in the AUTH option.  COAP_AUTH_KEY is
   derived by using AES-CMAC-PRF-128 [RFC4615], which, in turn, uses
   AES-CMAC-128 [RFC4493].  In this case, rest of CoAP exchanges between
   both entities can be protected with integrity (NOTE: encryption will
   be considered in the future) with AUTH option without the need of
   using DTLS.  Thus, all CoAP messages MUST include AUTH option from
   that point.  (NOTE: We understand that this would not be the standard
   way of protecting CoAP but instead a new way of protecting CoAP
   messages).

   COAP_AUTH_KEY is a 16-byte length key which is computed in the
   following way:

   COAP_AUTH_KEY = AES-CMAC-PRF-128(MSK, "IETF COAP AUTH" || Token
   Option value, 64, length("IETF COAP AUTH" || Token Option value))

   where:

   o  The AES-CMAC-PRF-128 is defined in [RFC4615].  This function uses
      AES-CMAC-128 as building block.

   o  The MSK exported by the EAP method.

   o  "IETF COAP AUTH" is the ASCII code representation of the non-NULL
      terminated string (excluding the double quotes around it).  This
      value is concatenated with the value of the Token Option value.

   o  64 is the length of the MSK.

   o  length("IETF COAP AUTH" || Token Option value) is the length of
      the label "IETF COAP AUTH" (14 bytes) plus the Token Option value.

4.2.  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 as follows:

   DTLS_PSK = AES-CMAC-PRF-128(MSK, "IETF COAP DTLS" || Token Option
   value, 64, length("IETF COAP DTLS" || Token Option value)).  This
   value is concatenated with the value of the Token Option value.

   where:

   o  MSK is exported by the EAP method.



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

   o  64 is the length of the MSK.

   o  length("IETF COAP DTLS" || Token Option value) is the length of
      the label "IETF COAP DTLS" (14 bytes) plus the Token Option Value.

   As mentioned in [RFC4279], a PSK identity is needed.  We are
   considering the usage of the Token Option value chosen during the EAP
   authentication as identity.  In any case, this still needs further
   investigation.

5.  Generating AUTH option

   A new AUTH option is defined in this document for authentication
   purposes.  Following guidelines in [I-D.ietf-core-coap] this option
   is:

   1.  Format opaque (sequence of bytes).

   2.  Elective.

   3.  Unsafe to Forward.

   4.  No cacheable.

   The number of the option will be determined by this previous
   decisions.

   1.  Elective (C = 0)

   2.  Unsafe to Forward (0)

   3.  NoCacheKey (111)

   The number of the AUTH option will fit this pattern: xxx11100

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

                     Figure 4: Auth Option Number Mask

   First available option number is 01011100 (92).

   The resultant AUTH option is:



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        +-----+----+---+---+---+-----------+--------+--------+---------+
        | No. | C  | U | N | R | Name      | Format | Length | Default |
        +-----+----+---+---+---+-----------+--------+--------+---------+
        |  92 |    |   | x |   | AUTH      | opaque | 128    | (none)  |
        +-----+----+---+---+---+-----------+--------+--------+---------+

                           Figure 5: AUTH Option

   C, U, N and R columns indicate the properties, Critical, UnSafe,
   NoCacheKey and Repeatable, respectively.

   To generate the value of the AUTH option, we use AES-CMAC-128 as
   authentication algorithm.  Thus, the AUTH option content will have an
   authentication tag of 16 bytes.

   AUTH Option value = AES-CMAC-128(COAP_AUTH_KEY, MSG, MSG_LENGTH)

   where:

   o  COAP_AUTH_KEY is the key derived in the CoAP Security Association
      process.

   o  MSG is the CoAP message including AUTH option filled with zeros.

   o  MSG_LENGTH.  Length of the CoAP message.

   After applying AES-CMAC-128 function, the AUTH option value will be
   set in the AUTH option replacing the zeros.

6.  Implementation

   At the time of writing this document, we have developed a proof-of-
   concept based on libcoap ([libCoAP]) in PC platform and started the
   development of a simulation with COOJA network simulator for Contiki
   ([Contiki]).

   So far, we have implemented an authentication tag by using AES-
   CMAC-128.  However this authentication tag has been included in the
   payload of two final messages after sending the EAP Success.  The
   implementation of the AUTH option will come soon.  Moreover, we have
   used AES-CMAC-128 for COAP_AUTH_KEY.  Since this function does not
   allow a key longer than 16 bytes, we have used the most significative
   16 bytes of the MSK as input key.  Since AES-CMAC-PRF-128 eliminates
   this limitation, we will implement this version instead.

   We are using (for the PC version) libeap in wpa-supplicant and
   hostapd open source software ([hostapd]) to implement the EAP stack
   and, in particular, the EAP-PSK method.



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7.  Future Work: CoAP Relay

   Architecture explained in Figure 1 assumes that EAP peer can
   communicate with the EAP authenticator.  In certain scenarios, EAP
   authenticator may not be reachable from the EAP peer if the EAP
   authenticator is placed several hops-away.  In this scenario,
   described in Figure 6, we are considering the usage a new service
   that assists the authentication.  This service acts as a relay of
   CoAP messages between the EAP peer and EAP authenticator.  We have
   called the entity in charge of performing this service CoAP relay.
   The strategy is similar to the one described in PANA Relay
   ([RFC6345]) or DTLS Relay ([I-D.kumar-dice-dtls-relay]).  Unlike CoAP
   proxy, the CoAP relay is not intended to keep any state (stateless
   behaviour) and the EAP peer is not assumed to be aware of the
   presence of the CoAP relay.  In any case, this part needs further
   investigation since CoAP already provides the concept of CoAP proxy
   and, particular, CoAP-to-CoAP proxy that might be used instead.



            +------------+        +------------+        +--------------+
            | EAP peer/  |        |            |        |  EAP auth    |
            | CoAP server|+------+| CoAP relay |+------+|  CoaP client |
            +------------+  CoAP  +------------+  CoAP  +--------------+
                                                                |
                                                            AAA |
                                                                |
                                                         +--------------+
                                                         |  EAP server/ |
                                                         |  AAA server  |
                                                         +--------------+



                   Figure 6: CoAP EAP Relay Architecture

   Once the EAP peer has been authenticated, CoAP relay service should
   not be needed anymore for this EAP peer.

   Development of this new service may modify the "Unsafe to Forward"
   flag of the AUTH option.

8.  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.



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9.  Security Considerations

   TBD.

10.  IANA Considerations

   This document has no actions for IANA.

11.  References

11.1.  Normative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

   [I-D.kumar-dice-dtls-relay]
              Kumar, S., Keoh, S., and O. Garcia-Morchon, "DTLS Relay
              for Constrained Environments", draft-kumar-dice-dtls-
              relay-00 (work in progress), October 2013.

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

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

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
              3748, June 2004.

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

   [RFC4493]  Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
              AES-CMAC Algorithm", RFC 4493, June 2006.

   [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, August 2006.





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   [RFC4764]  Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
              Pre-Shared Key Extensible Authentication Protocol (EAP)
              Method", RFC 4764, January 2007.

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

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

11.2.  Informative References

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

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

   [libCoAP]  "C-Implementation of CoAP", January 2013.

Authors' Addresses

   Raul Sanchez-Sanchez
   University of Murcia
   Campus de Espinardo S/N, Faculty of Computer Science
   Murcia  30100
   Spain

   Phone: +34 868 88 92 81
   Email: raul@um.es


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

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




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