| Internet-Draft | CoAP-EAP | February 2021 |
| Marin & Garcia | Expires 1 September 2021 | [Page] |
This document describes an authentication service that uses EAP transported by means of CoAP messages with the following purposes:¶
Generally speaking, this document is specifying an EAP lower layer based on CoAP, to bring the benefits of EAP to IoT.¶
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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 establish a security association between them.¶
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 a 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. The general flow of operation of CoAP-EAP establishes an OSCORE security association specifically for the service. In addition, using the key material derived from the authentication we specify the establishment of other security associations depending on the security requirements of the services:¶
This document also provides comments on how to establish a security association for other types of technologies that rely on CoAP.¶
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].¶
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 to the EAP peer. Accordingly, the EAP responses go from the EAP peer to the EAP authenticator. Nevertheless, a model where the EAP peer acts as CoAP client and the EAP authenticator as CoAP server can be also analyzed in the future.¶
+------------+ +------------+
| EAP peer/ | | EAP auth./ |
| CoAP server|+------+| CoAP client|
+------------+ CoAP +------------+
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, be a not so constrained device. We show the sequence flow in Figure 2 where we depict 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 is be valid. For example EAP-MD5 cannot be used since it does not export key material).¶
The first step to run CoAP-EAP is for the IoT device to discover the Controller, and that it implements the CoAP-EAP service. To do so, we rely on the discovery mechanism of CoAP. The URI of the CoAP-EAP service, is set to "/b" to save bytes over the air. Alternatively, the if the Controller is aware of the presence of the IoT device (e.g., due to a previous authentication) this process can be avoided, and the Controller can directly start the authentication process.¶
The first message that is used to trigger the authentication process is sent by the IoT device, acting as CoAP client. This message uses the No-Response Option [RFC7967] to avoid the response from the Controller to this message. After this, the exchange continues with the Controller acting as CoAP client and the IoT device acting as CoAP server. This is due to the fact that the IoT could be a constrained node, and following the recommendations of [I-D.ietf-lwig-coap] to simplify the implementation of the IoT device, having the Controller the responsibility of handling the retransmissions. In the next section, we refer to the IoT device as the EAP peer and the Controller as the EAP authenticator to elaborate the specifics of the flow of operation.¶
If 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 if it is not needed. So the use of this option will allow signaling the intention the EAP peer to start the authentication process, as a mechanism. Immediately after that, the EAP authenticator will start authentication service. It is worth noting that the EAP authenticator MAY decide start the authentication without waiting for the trigger if it has knowledge about the presence of the peer, for instance, through a previous 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. 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 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) ([RFC5295])¶
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 Section 4.1 The protection of the EAP Success is not a requirement. In our case, we specify this exchange as protected by the lower layer in this scenario with OSCORE. The purpose is double, we can avoid forgery of this message and at the same time we are using the exchange to perform the key confirmation through the establishment of the OSCORE security association. Adding to the previous consideration about the EAP Success, this message does not preclude the operation of the device from continuing as long as there is an alternate success indication that both the EAP peer and authentication can rely on to continue [RFC3748]. This indication can happen in two ways: 1) the reception of the a CoAP message without EAP and with an OSCORE option (following the normal operational communication between the both entities) is an indication that the controller considers the EAP authentication finished. 2) the IoT device is aware that the EAP authentication went well if an MSK is available. In any case, both entities need to prove the possession of the MSK as mentioned in the EAP KMF.¶
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) |---------------------------------------->|
....
| 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
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:¶
The number of the option will be determined by this previous decisions.¶
The number of the SeqNum option will fit this pattern: xxx11111¶
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| | NoCacheKey| U | C |
+---+---+---+---+---+---+---+---+
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
(*) See below.
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. To complete the CoAP-EAP exchange, as part of the design, the establishment of an OSCORE security association specifically for the CoAP-EAP service is expected. The security level for the CoAP-EAP exchanges with OSCORE is with integrity. Additionally, we considered the derivation of either the OSCORE Security Context or a pre-shared key that can be used for a DTLS negotiation (DTLS_PSK) for further communications depending of the security requirements of the services provided by the AS. The CoAP-EAP OSCORE security context could be generalized to enable further OSCORE secured communications between the IoT device and the AS services that require the use of OSCORE.¶
Key material needed to derive the OSCORE Security Context, from the MSK can be done as follows. In this case, rest of CoAP exchanges between both entities can be protected with OSCORE.¶
The Master Secret can be derived by using AES-CMAC-PRF-128 [RFC4615], which, in turn, uses AES-CMAC-128 [RFC4493]. The Master Secret can be derived as follows:¶
Master_Secret = KDF(MSK, "IETF_OSCORE_MASTER_SECRET", 64, length)¶
where:¶
The Master Salt can be derived similarly to the Master Secret. The Master Salt can be derived as follows:¶
Master_Salt = KDF(MSK, "IETF_OSCORE_MASTER_SALT", 64, length)¶
where:¶
The ID Context can be set to the Identity of the EAP peer.¶
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" , 64, length). This value is concatenated with the value of the Token Option value.¶
where:¶
For a device to act as a trustworthy entity within a security domain, certain key material is needed to be shared between the IoT device and AS. In ACE, the process of Client registration and provisioning of credentials to the client is not specified. The process of Client registration and provisioning can be achieved by using CoAP-EAP. Once the process of authentication with EAP is completed, fresh key material is shared between the IoT device and the AS.¶
Next, we elaborate examples of different use case scenarios about the usage of CoAP-EAP. Generally, we are dealing with 4 entities:¶
Generally, any node wanting to join the domain managed by the controller, MUST perform a CoAP-EAP authentication with the controller C. This authentication MAY involve an external AAA server. This means that A and B, once deployed, will perform this CoAP-EAP once as a bootstrapping phase to establish a security association with the controller C. Moreover, any other entity, which wants to join and establish communications with nodes under the controller C's domain must also do the same. By using EAP, we can have the flexibility of having different types of credentials. For instance, if we have a device that is not battery dependent, and not very constrained a we could be using a heavier authentication method. With very constrained devices we might need to go to other authentication methods (e.g., EAP-PSK, EAP-EDHOC, etc.) being able to adapt to different types of devices according to policies or devices capabilities.¶
Next, we exemplify how CoAP-EAP can be used to perform the Client registration in a general way, to allow two IoT devices (A and B) to communicate and interact after a successful client registration.¶
Node A wants to communicate with node B (e.g. to 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, optionally, 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 the 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 obtain all the required information to do that in a secure manner (e.g. keys, tokens, authorization information, etc.). This phase does NOT require the usage of CoAP-EAP. The details of this phase are out of scope of this document, and the ACE framework is used for this purpose [I-D.ietf-ace-oauth-authz].¶
In the third phase, the node A can access node B with the credentials and information obtained from the controller C in the second phase. This access can be repeated without contacting the controller, while the credentials given to A are still valid. The details of this phase are out of scope of this document.¶
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 authentication.¶
We assume we have a device (A) of the domain acme.org, which uses a specific kind of credential (e.g., AKA) and intends to join the um.es domain. This user does not belong to this domain, for which first it performs a client registration using CoAP-EAP. For this it interacts with the domain Controller acting as EAP authenticator, which in turn communicates with a AAA infrastructure (acting as AAA client). Through the local AAA server to communicate with the home AAA server to complete the authentication and integrate the device as a trustworthy entity into the domain of the controller C. In this scenario the AS under the role of the Controller, receives the key material from the AAA infrastructure¶
A University Campus, we have several Faculty buildings and each one has its own criteria or policies in place to manage IoT devices under an AS. All buildings belong to the same domain (e.g., um.es). All these buildings are managed with a AAA infrastructure. A new device (A) with credentials from the domain (e.g., um.es) will be able to perform the device registration with a Controller (C) of any building as long as they are managed by the same general domain.¶
Another case, without a AAA infrastructure, we have a Controller that has co-located the AAA server and using EAP standalone mode we are able to manage all the devices within the same domain locally. Client registration of a node (A) with Controller (C) can also be performed in the same manner, transparent to the IoT device. In this scenario the AAA server is co-located within the Controller (C)-¶
One of the first steps for an IoT device life-cycle is to perform the authentication to gain access to the network. To do so, the device first has to be authenticated and granted authorization to gain access to the network. Additionally, security parameters such as credentials can be derived from the authentication process allowing the trustworthy operation of the IoT device in a particular network by joining the security domain. By using EAP, we are able to achieve this with flexibility and scalability, because of the different EAP methods available and the ability relying in AAA infrastructures if needed to support multi-domain scenarios, which is a key feature when the IoT devices deployed under the same security domain, belong to different organizations. Given that EAP is also used for network access control, it is possible that this service can be used to provide network access control service (e.g., LoRa network). In this specific case, we could leverage the compression by SCHC for CoAP.¶
It is not uncommon that the infrastructure where the device is deployed and the services the IoT device are managed by different organizations. Therefore, in addition to the authentication for network access control, we have to consider the possibility of a secondary authentication to access different services. This process of authentication, for example, will provide with the necessary key material to establish a secure channel and interact with the entity in charge of granting access to different services.¶
In 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 [RFC3748], which are enumerated next:¶
Regarding the unreliable transport, although EAP assumes a non reliable transport, CoAP does provide a 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. Regarding message ordering, we propose the use of a new CoAP option, the SeqNum option described in Section Section 3.2, which will allow us to determine 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 Controller.¶
As we can see, CoAP can fulfil the requirements of EAP to be considered suitable as lower-layer.¶
We consider the use of the SeqNum Option due to the independence of how the CoAP engine is implemented. Since we do not know before hand if the implementation will allow us to pre-establish the MSG-ID or the Token from the application perspective, we need to be sure we are able to provide order delivery. If the implementation of CoAP allows us to pre-establish the MSD-ID and Token, we could avoid using this option, due to the characteristics of the CoAP-EAP exchange, i.e., the EAP exchange is done in lock-step and only one session is considered at a time. Another consideration regarding the workings of the SeqOption, is that since the initial number from which is monotonically increased by 1, could cause the overflow of the number. To manage this scenario, the SeqNum Option performs rounding, going to zero and continue from there.¶
Regarding the impact an EAP lower layer will have to the total byte size of the whole exchange, there is a comparison with another network layer based EAP lower-layer, PANA [RFC5191] in [coap-eap]. Authors compared focusing EAP lower-layer (alone) and taking into account EAP. On the one hand, at EAP lower-layer level, the usage of CoAP gives us an important benefits. On the other hand, when taking into account the EAP method overload, this reduction is less but still significant if the EAP method is lightweight (we used EAP-PSK as a representative example of a lightweight EAP method). If the EAP method is very taxing the improvement achieved in the EAP lower-layer is less significant. 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. However, we cannot ignore the impact of the EAP lower-layer itself and try to propose something light as we do proposing CoAP. We consider that may be others EAP methods such as EAP-AKA or new lightweight EAP methods such as EAP-EDHOC [I-D.ingles-eap-edhoc] that can benefit from a CoAP-based EAP lower-layer, as well as new ones that may be proposed in the future with IoT constraints in mind.¶
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 implementation.¶
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 Empty Token value to save resources, since there will be no other endpoint or CoAP exchange.¶
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 completely.¶
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.¶
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.¶
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.¶
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.¶
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.¶
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 following the EAP KMF [RFC5247] 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 another session has to be established, a re-authentication can be done, by running process again, or using a more lightweight EAP method to derive additional key material such as EAP-ERP.¶
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.¶
This document has no actions for IANA.¶
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.¶
As well as the reviewers of this work, Alexandre Petrescu, Pedro Moreno-Sanchez, Eduardo Ingles-Sanchez and Benjamin Kaduk.¶