ACE Working Group R. Marin Internet-Draft D. Garcia Intended status: Experimental University of Murcia Expires: October 23, 2017 April 21, 2017 EAP-based Authentication Service for CoAP draft-marin-ace-wg-coap-eap-05 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on October 23, 2017. Copyright Notice Copyright (c) 2017 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 Marin & Garcia Expires October 23, 2017 [Page 1] Internet-Draft CoAP-EAP April 2017 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. CoAP-EAP with DTLS . . . . . . . . . . . . . . . . . . . 7 3.3. CoAP as EAP lower-layer . . . . . . . . . . . . . . . . . 10 3.4. Optimization Considerations . . . . . . . . . . . . . . . 11 4. Key Derivation for protecting CoAP messages . . . . . . . . . 11 4.1. Deriving COAP_PSK . . . . . . . . . . . . . . . . . . . . 11 4.2. Deriving DTLS_PSK . . . . . . . . . . . . . . . . . . . . 12 5. Generating AUTH option . . . . . . . . . . . . . . . . . . . 13 6. Implementation . . . . . . . . . . . . . . . . . . . . . . . 14 7. Future Work: CoAP Intermediaries . . . . . . . . . . . . . . 15 8. Use Case Scenario . . . . . . . . . . . . . . . . . . . . . . 16 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17 10. Security Considerations . . . . . . . . . . . . . . . . . . . 17 10.1. Authorization . . . . . . . . . . . . . . . . . . . . . 18 10.2. Cryptographic suite selection . . . . . . . . . . . . . 18 10.3. Additional Security Consiederation . . . . . . . . . . . 18 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 12. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 12.1. Normative References . . . . . . . . . . . . . . . . . . 18 12.2. Informative References . . . . . . . . . . . . . . . . . 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. 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]. Marin & Garcia Expires October 23, 2017 [Page 2] Internet-Draft CoAP-EAP April 2017 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 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 Marin & Garcia Expires October 23, 2017 [Page 3] Internet-Draft CoAP-EAP April 2017 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 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 Non-Confirmable "POST /auth" request to the node (Step 0). This message, will carry an option developed from this work [RFC7967] called no response. The rationale of these options is avoiding having to listen to a response if is not needed. So the use of this option will allow us to signal the intention of the EAP-Peer to start the authentication process. 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 "POST /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 3,4,5,6,7,8,9). The EAP authenticator will use POST 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]) Marin & Garcia Expires October 23, 2017 [Page 4] Internet-Draft CoAP-EAP April 2017 If the new defined AUTH option is used, an authentication tag is generated with a new key named COAP_PSK, 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 9 and 10. From that point any exchange (for example, Steps 11 and 12) 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 related with the authentication service in the CoAP server (Steps 13 and 14). 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 9 (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) ------------- ------------- | | | NON [0x6af5] | | No-Response (0x1A) | 0) | (Token 0xFA5B45FF4723BB43) | | POST /auth | |---------------------------------------->| | | | | | CON [0x73DE] | | (Token 0x78728FD4AC3190FF) | | POST /auth | | Payload nonce_c | 1) |<----------------------------------------| | | | ACK [0x73DE] | | (Token 0x78728FD4AC3190FF) | | 2.01 Created | | Uri-Path [auth/5] | | Payload nonce_s | 2) |---------------------------------------->| | | | CON [0x7654] | | (Token 0x78728FD4AC3190FF) | | Payload EAP Req/Id | | POST /auth/5 | Marin & Garcia Expires October 23, 2017 [Page 5] Internet-Draft CoAP-EAP April 2017 3) |<----------------------------------------| | | | ACK [0x7654] | | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP Res/Id | 4) |---------------------------------------->| | | | CON [0x7654] | | (Token 0x78728FD4AC3190FF) | | Payload EAP-PSK MSG 1 | | POST /auth/5 | 5) |<----------------------------------------| | | | ACK [0x7654] | | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP-PSK MSG 2 | 6) |---------------------------------------->| | | | CON [0x9869] | | (Token 0x78728FD4AC3190FF) | | Payload EAP-PSK MSG 3 | | POST /auth/5 | 7) |<----------------------------------------| | | | ACK [0x9869] | | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP-PSK MSG 4 | 8) |---------------------------------------->| MSK | | MSK | | CON [0x7811] | | COAP_PSK| (Token 0x78728FD4AC3190FF) |COAP_PSK | AUTH option | | Payload EAP Success | (*) | POST /auth/5 | 9) |<----------------------------------------| | | (*) | ACK [0x7811] | | (Token 0x78728FD4AC3190FF) | | AUTH option | | 2.04 Changed | 10) |---------------------------------------->| ............. | | Marin & Garcia Expires October 23, 2017 [Page 6] Internet-Draft CoAP-EAP April 2017 | CON [0x7511] | | (Token 0x55566AF7464646BC) | (*) | AUTH option | | GET /temp | 11) |<----------------------------------------| | | | ACK [0x7511] | (*) | (Token 0x55566AF7464646BC) | | AUTH option | | 2.05 Content | | "22.5C" | 12) |---------------------------------------->| ................ | | | CON [0x7600] | | (Token 0x678443AA678BC690) | (*) | AUTH option | | DELETE /auth/5 | 13) |<----------------------------------------| | | | ACK [0x7500] | (*) | (Token 0x678443AA678BC690) | | AUTH option | | 2.02 Deleted | 14) |---------------------------------------->| (*) Protected with AUTH option Figure 2: CoAP-EAP with AUTH option 3.2. 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 3. The Steps 0-8 are the same as the case with AUTH option, however, before continuing with Steps 10 and 11, the EAP authenticator starts the DTLS handshake with the EAP peer (Step 9'). 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 Marin & Garcia Expires October 23, 2017 [Page 7] Internet-Draft CoAP-EAP April 2017 DTLS_PSK that, in turn, corroborates that both entities participated in the EAP authentication. EAP peer EAP Auth. (COAP server) (COAP client) ------------- ------------- | | | NON [0x6af5] | | No-Response (0x1A) | 0) | (Token 0xFA5B45FF4723BB43) | | POST /auth | |---------------------------------------->| | | | CON [0x73DE] | | (Token 0x78728FD4AC3190FF) | | POST /auth | | Payload nonce_c | 1) |<----------------------------------------| | | | ACK [0x73DE] | | (Token 0x78728FD4AC3190FF) | | 2.01 Created | | Uri-Path [auth/5] | | Payload nonce_s | 2) |---------------------------------------->| | | | CON [0x7654] | | (Token 0x78728FD4AC3190FF) | | Payload EAP Req/Id | | POST /auth/5 | 3) |<----------------------------------------| | | | ACK [0x7654] | | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP Res/Id | 4) |---------------------------------------->| | | | CON [0x7654] | | (Token 0x78728FD4AC3190FF) | | Payload EAP-PSK MSG 1 | | POST /auth/5 | 5) |<----------------------------------------| | | | ACK [0x7654] | Marin & Garcia Expires October 23, 2017 [Page 8] Internet-Draft CoAP-EAP April 2017 | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP-PSK MSG 2 | 6) |---------------------------------------->| | | | CON [0x9869] | | (Token 0x78728FD4AC3190FF) | | Payload EAP-PSK MSG 3 | | POST /auth/5 | 7) |<----------------------------------------| | | | ACK [0x9869] | | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | | Payload EAP-PSK MSG 4 | 8) |---------------------------------------->| MSK | | MSK | | | | DTLS_PSK| | DTLS_PSK | | | DTLS HANDSHAKE | | (Initiated by EAP Auth.) | 9') |<--------------------------------------->| | | | CON [0x7811] | | (Token 0x78728FD4AC3190FF) | | Payload EAP Success | (*) | POST /auth/5 | 10) |<----------------------------------------| | | | ACK [0x7811] | (*) | (Token 0x78728FD4AC3190FF) | | 2.04 Changed | 11) |---------------------------------------->| ............. | | | CON [0x7511] | | (Token 0xAA763D82F623B7FF) | (*) | GET /temp | 12) |<----------------------------------------| | | | ACK [0x7511] | (*) | (Token 0xAA763D82F623B7FF) | | 2.05 Content | | "22.5C" | 13) |---------------------------------------->| Marin & Garcia Expires October 23, 2017 [Page 9] Internet-Draft CoAP-EAP April 2017 ................ | | | CON [0x7600] | | (Token 0x678443AA678BC690) | (*) | DELETE /auth/5 | 14) |<----------------------------------------| | | | ACK [0x7500] | (*) | (Token 0x678443AA678BC690) | | 2.02 Deleted | 15) |---------------------------------------->| (*) Protected with DTLS Figure 3: EAP over CoAP with DTLS 3.3. 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 lower layers are not assumed reliable. 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. Next we go over the previous points to verify that CoAP does fits the EAP lower layer requirements. Regarding the unreliable transport, although EAP assumes a non reliable transport, CoAP does provide a Marin & Garcia Expires October 23, 2017 [Page 10] Internet-Draft CoAP-EAP April 2017 reliability mechanism through 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 of EAP. About possible duplication, although not required, CoAP provides a message-ID value for deduplication purposes. Finally for the ordering guarantees needed by EAP, CoAP message-ID can be used for this purpose. As we can see, CoAP does fulfill the requirements of EAP to be considered suitable as lower-layer. 3.4. Optimization Considerations Here we consider two possible optimizations for reducing the message length: o A first optimization would be to reduce the URI of the bootstrapping service. For example, the /auth URI could be reduced to /a. o Another optimization would be to use a zero length CoAP token in the exchange. Some use cases as LoRA Networks [I-D.pelov-core-cosol] might take advantage of these reductions to improve the performance of the bootstrapping process. 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_PSK or DTLS_PSK. 4.1. Deriving COAP_PSK A key COAP_PSK is derived from the MSK to generate the authentication tag included in the AUTH option. COAP_PSK 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). Marin & Garcia Expires October 23, 2017 [Page 11] Internet-Draft CoAP-EAP April 2017 COAP_PSK is a 16-byte length key which is computed in the following way: COAP_PSK = KDF(MSK, "IETF_COAP_PSK" || nonce_c || nonce_s, 64, length) 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_PSK" 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 nonce_c is the random value sent by the EAP Authenticator to the EAP Peer in the POST Message. o nonce_s is the random value sent by the EAP Peer to the EAP Authenticator in the Acknowledgment to the POST Message. o 64 is the length of the MSK. o length is the length of the label "IETF_COAP_PSK" (13 bytes). 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 = KDF(MSK, "IETF_DTLS_PSK" || nonce_c || nonce_s, 64, length). This value is concatenated with the value of the Token Option value. where: o MSK is exported by the EAP method. o "IETF_DTLS_PSK" is the ASCII code representation of the non-NULL terminated string (excluding the double quotes around it). o nonce_c is the random value sent by the EAP Authenticator to the EAP Peer in the POST Message. Marin & Garcia Expires October 23, 2017 [Page 12] Internet-Draft CoAP-EAP April 2017 o nonce_s is the random value sent by the EAP Peer to the EAP Authenticator in the Acknowledgment to the POST Message. o 64 is the length of the MSK. o length is the length of the label "IETF_DTLS_PSK" (13 bytes). 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 [RFC7252] 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: Marin & Garcia Expires October 23, 2017 [Page 13] Internet-Draft CoAP-EAP April 2017 +-----+----+---+---+---+-----------+--------+--------+---------+ | 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_PSK, MSG, MSG_LENGTH) where: o COAP_PSK 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_PSK. 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. Marin & Garcia Expires October 23, 2017 [Page 14] Internet-Draft CoAP-EAP April 2017 7. Future Work: CoAP Intermediaries 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 intermediary of CoAP messages between the EAP peer and EAP authenticator. Currently we have a design of three different variants of this entity. o CoAP relay o CoAP proxy o CoAP stateless proxy A proof-of-concept of the CoAP relay and CoAP proxy is evaluated and a CoAP stateless proxy implementation is planned next. 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 behavior) and the EAP peer is not assumed to be aware of the presence of the CoAP relay. The CoAP stateless proxy provides an approach between the previous two. It behaves as a proxy, but avoid generating any state related to the ongoing exchange. +------------+ +------------+ +--------------+ | EAP peer/ | | CoAP | | EAP auth | | CoAP server|+------+|intermediary|+------+| CoaP client | +------------+ CoAP +------------+ CoAP +--------------+ | AAA | | +--------------+ | EAP server/ | | AAA server | +--------------+ Figure 6: CoAP EAP Intermediary Architecture Once the EAP peer has been authenticated, CoAP intermediary should not be needed anymore for this EAP peer. Marin & Garcia Expires October 23, 2017 [Page 15] Internet-Draft CoAP-EAP April 2017 Development of this new service may modify the "Unsafe to Forward" flag of the AUTH option. 8. 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 as 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, however it may have some constrained resources. 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, as depicted in Figure 6, 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. One use case is the following. 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 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 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 the controller C for authorization to access node B and Marin & Garcia Expires October 23, 2017 [Page 16] Internet-Draft CoAP-EAP April 2017 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. 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. Another use case is the following. Node D wants to communicate with node A (e.g. to obtain a temperature measurement). To do that, first of all, node D must join the controller C's domain. To do that it performs a CoAP-EAP authentication and authorization with the controller C (first phase). If everything ends with success, the node D can request access to node A to C (second phase). Then if node D is authorized can access to node A (third phase). So, in the end, node D also implements CoAP-EAP as any other constrained node. 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- EAP. This work has been partly funded by European Commission through the FP7-SMARTIE-609062 EU Project. 10. 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. Marin & Garcia Expires October 23, 2017 [Page 17] Internet-Draft CoAP-EAP April 2017 10.1. 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 (RADIUS in this case) can be delivered in RADIUS attributes such as NAS-Filter-Rules, Framed-MTU, Session-Timeout, etc. 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. 10.2. 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. To support the protocol all implementations MUST at least support AES-CMAC-PRF-128 as the KDF and AES-CMAC-128 as default. 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. 10.3. Additional Security Consiederation Other security related concerns can be how to ensure that the node joining the security domain can in fact trust the Controller. This issue is elaborated in the EAP KMF[RFC5247] . Summarizing, the node 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 trusts the Controller, which confirms it is a trusted party. 11. IANA Considerations This document has no actions for IANA. 12. References 12.1. Normative References [I-D.kumar-dice-dtls-relay] Kumar, S., Keoh, S., and O. Garcia-Morchon, "DTLS Relay for Constrained Environments", draft-kumar-dice-dtls- relay-02 (work in progress), October 2014. Marin & Garcia Expires October 23, 2017 [Page 18] Internet-Draft CoAP-EAP April 2017 [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. [I-D.pelov-core-cosol] 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, . [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, . [RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible Authentication Protocol (EAP) Key Management Framework", RFC 5247, DOI 10.17487/RFC5247, August 2008, . Marin & Garcia Expires October 23, 2017 [Page 19] Internet-Draft CoAP-EAP April 2017 [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, . [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, . [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, . [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, . 12.2. 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 Marin & Garcia Expires October 23, 2017 [Page 20] Internet-Draft CoAP-EAP April 2017 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 Dan Garcia Carrillo University of Murcia Campus de Espinardo S/N, Faculty of Computer Science Murcia 30100 Spain Phone: +34 868 88 78 82 Email: dan.garcia@um.es Marin & Garcia Expires October 23, 2017 [Page 21]