ACE Working Group R. Marin-Lopez
Internet-Draft University of Murcia
Intended status: Standards Track D. Garcia-Carrillo
Expires: 25 December 2022 University of Oviedo
23 June 2022
EAP-based Authentication Service for CoAP
draft-ietf-ace-wg-coap-eap-08
Abstract
This document specifies an authentication service that uses the
Extensible Authentication Protocol (EAP) transported employing
Constrained Application Protocol (CoAP) messages. As such, it
defines an EAP lower layer based on CoAP called CoAP-EAP. One of the
main goals is to authenticate a CoAP-enabled IoT device (EAP peer)
that intends to join a security domain managed by a Controller (EAP
authenticator). Secondly, it allows deriving key material to protect
CoAP messages exchanged between them based on Object Security for
Constrained RESTful Environments (OSCORE), enable the establishment
of a security association between them.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 25 December 2022.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. General Architecture . . . . . . . . . . . . . . . . . . . . 4
3. CoAP-EAP Operation . . . . . . . . . . . . . . . . . . . . . 5
3.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Flow of operation (OSCORE establishment) . . . . . . . . 6
3.3. Reauthentication . . . . . . . . . . . . . . . . . . . . 9
3.4. Managing the State of the Service . . . . . . . . . . . . 10
3.5. Error handling . . . . . . . . . . . . . . . . . . . . . 11
3.5.1. EAP authentication failure . . . . . . . . . . . . . 11
3.5.2. Non-responding endpoint . . . . . . . . . . . . . . . 12
3.5.3. Duplicated message with /.well-known/coap-eap . . . . 12
3.6. Proxy operation in CoAP-EAP . . . . . . . . . . . . . . . 13
4. CBOR Objects in CoAP-EAP . . . . . . . . . . . . . . . . . . 13
5. Cipher suite negotiation and key derivation . . . . . . . . . 14
5.1. Cipher suite negotiation . . . . . . . . . . . . . . . . 14
5.2. Deriving the OSCORE Security Context . . . . . . . . . . 16
6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 17
6.1. CoAP as EAP lower layer . . . . . . . . . . . . . . . . . 17
6.2. Size of the EAP lower layer vs EAP method size . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7.1. Authorization . . . . . . . . . . . . . . . . . . . . . . 19
7.2. Freshness of the key material . . . . . . . . . . . . . . 19
7.3. Channel Binding support . . . . . . . . . . . . . . . . . 20
7.4. Additional Security Consideration . . . . . . . . . . . . 20
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
8.1. CoAP-EAP Exporter Label Registry . . . . . . . . . . . . 21
8.2. CoAP-EAP Cipher Suites . . . . . . . . . . . . . . . . . 21
8.3. CoAP-EAP Information elements . . . . . . . . . . . . . . 22
8.4. The Well-Known URI Registry . . . . . . . . . . . . . . . 23
8.5. The EAP lower layer identifier registry . . . . . . . . . 24
8.6. Media Types Registry . . . . . . . . . . . . . . . . . . 24
8.7. CoAP Content-Formats Registry . . . . . . . . . . . . . . 25
8.8. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
10.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Flow of operation (DTLS establishment) . . . . . . . 30
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A.1. Cryptographic suite negotiation for DTLS . . . . . . . . 31
A.2. Deriving DTLS PSK and identity . . . . . . . . . . . . . 31
Appendix B. Examples of Use Case Scenario . . . . . . . . . . . 32
B.1. Example 1: CoAP-EAP in ACE . . . . . . . . . . . . . . . 33
B.2. Example 2: Multi-domain with AAA infrastructures . . . . 34
B.3. Example 3: Single domain with AAA infrastructure . . . . 34
B.4. Example 4: Single domain without AAA infrastructure . . . 34
B.5. Other use cases . . . . . . . . . . . . . . . . . . . . . 34
B.5.1. CoAP-EAP for network access control . . . . . . . . . 35
B.5.2. CoAP-EAP for service authentication . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 35
1. Introduction
This document specifies an authentication service (application) that
uses the Extensible Authentication Protocol (EAP) [RFC3748] and is
built on top of the Constrained Application Protocol (CoAP) [RFC7252]
called CoAP-EAP. CoAP-EAP is an application that allows
authenticating two CoAP endpoints by using EAP, and establishing a
Object Security for Constrained RESTful Environments (OSCORE)
security association between them.
More specifically, this document specifies how CoAP can be used as a
constrained, link-layer independent, reliable EAP lower layer
[RFC3748] to transport EAP messages between a CoAP server (acting as
EAP peer) and a CoAP client (acting as EAP authenticator) using CoAP
messages. The CoAP client has the option of contacting a backend AAA
infrastructure to complete the EAP negotiation as described in the
EAP specification [RFC3748].
EAP methods transported in CoAP MUST generate cryptographic material
[RFC5247] for this specification. This way, CoAP messages are
protected after the authentication. After CoAP-EAP's operation, an
OSCORE security association is established between endpoints of the
service. Using the keying material derived from the authentication,
other security associations could be generated. Appendix A shows how
to establish a (D)TLS security association using the keying material
from the EAP authentication.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119] [RFC8174]
when, and only when, they appear in all capitals, as shown here.
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Readers are expected to be familiar with the terms and concepts of
described in CoAP [RFC7252], EAP [RFC3748][RFC5247] and OSCORE
[RFC8613].
2. General Architecture
Figure 1 illustrates the architecture defined in this document. An
IoT device, acting as the EAP peer, wants to be authenticated by
using EAP to join a domain that is managed by a Controller acting as
an EAP authenticator. The IoT device will act as a CoAP server for
this service, and the EAP authenticator as a CoAP client. The
rationale behind this decision, as expanded later, is that EAP
requests go always from the EAP server to the EAP peer. Accordingly,
the EAP responses go from the EAP peer to the EAP server.
It is worth noting that the CoAP client (EAP authenticator) MAY
interact with a backend AAA infrastructure when EAP pass-through mode
is used, which will place the EAP server in the AAA server that
contains the information required to authenticate the EAP peer.
The protocol stack is described in Figure 2. CoAP-EAP is an
application built on top of CoAP. On top of the application, there
is an EAP state machine that can run any EAP method. For this
specification, the EAP method MUST be able to derive keying material.
CoAP-EAP also relies on CoAP reliability mechanisms in CoAP to
transport EAP: CoAP over UDP with Confirmable messages ([RFC7252]) or
CoAP over TCP, TLS and WebSocket, which is specified in [RFC8323].
+----------+ +--------------+ +----------+
| EAP peer | | EAP | | AAA/ |
| peer |<------>| authenticator|<---------->|EAP Server|
+----------+ CoAP +--------------+ AAA +----------+
(Optional)
<---(SCOPE OF THIS DOCUMENT)---->
Figure 1: CoAP-EAP Architecture
+-------------------------------+
| EAP State Machine |
+-------------------------------+ \
| Application(CoAP-EAP) | | This Document
+-------------------------------+ /
| Request/Responses/Signaling | RFC 7252 / RFC 8323
+-------------------------------+
| Message / Message Framing | RFC 7252 / RFC 8323
+-------------------------------+
|Unreliable / Reliable Transport| RFC 7252 / RFC 8323
+-------------------------------+
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Figure 2: CoAP-EAP Stack
3. CoAP-EAP Operation
Since CoAP-EAP uses reliable delivery in CoAP ([RFC7252], [RFC8323]),
EAP retransmission time is set to infinite as mentioned in [RFC3748].
To keep ordering guarantee, CoAP-EAP uses Hypermedia as the Engine of
Application State (HATEOAS). Each step during the EAP authentication
is represented as a new resource in the EAP peer (CoAP server). The
previous resource is removed once the new resource is created
indicating the resource that will process the next expected step of
the EAP authentication.
An EAP method that does not export keying material MUST NOT be used.
One of the benefits of using EAP is that we can choose over a large
variety of authentication methods. Although for IoT, where we can
find very constrained links (e.g., limited bandwidth) and devices
with limited capabilities, EAP methods that do not require many
exchanges, with short messages, and that use cryptographic algorithms
that are manageable by constrained devices are preferable.
In CoAP-EAP, the IoT device (EAP peer/CoAP server) will only have one
authentication session with a specific Controller (EAP authenticator/
CoAP client) and it will not process any other EAP authentication in
parallel (with the same Controller). That is, a single ongoing EAP
authentication is permitted for the same IoT device and the same
Controller. Moreover, EAP is a lock-step protocol ([RFC3748]). The
benefits of the EAP framework in IoT are highlighted in
[eap-framework].
To access the authentication service, this document defines the well-
known URI "/.well-known/coap-eap" (to be assigned by IANA). This URI
is referring to the authentication service that is present in the
Controller so that IoT devices can start the service.
3.1. Discovery
Before the CoAP-EAP exchange taking place, the IoT device needs to
discovers the Controller or the entity that will enable the exchange
between the IoT and the Controller (e.g., an intermediary such as a
proxy).
The discovery process is out of the scope of this document. This
document provides the specification using the mechanisms provided by
CoAP to discover the Controller for CoAP-EAP.
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The CoAP-EAP application is designated by the well-known URI "coap-
eap" for the trigger message (Step 0). The CoAP-EAP service can be
discovered by asking directly about the services offered. This
information can be also available in the resource directory
[RFC9176].
Implementation Notes: On the methods on how the IPv6 address of the
Controller or intermediary entity can be discovered, there can be
different methods depending on the specific deployment. For example,
on a 6LoWPAN network, the Border Router will typically act as the
Controller hence, after receiving the Router Advertisement (RA)
messages from the Border Router, the IoT device may engage on the
CoAP-EAP exchange. Different protocols can be used to discover the
IP of the Controller. Examples of such protocols are Multicast DNS
(mDNS) [RFC6762] or DHCPv6 [RFC8415].
3.2. Flow of operation (OSCORE establishment)
Figure 3 shows the general flow of operation for CoAP-EAP to
authenticate using EAP and establish an OSCORE security context. The
flow does not show a specific EAP method. Instead, we represent the
chosen EAP method by using a generic name (EAP-X). The flow assumes
that the IoT device knows the Controller implements the CoAP-EAP
service. The specific mechanism of discovery is out-of-scope of this
document. Some comments about Controller discovery is in
Section 3.1.
The steps for the operation are as follows:
* Step 0. The IoT device MUST start the authentication process by
sending a "POST /.well-known/coap-eap" request (trigger message).
This message carries the 'No-Response' [RFC7967] CoAP option to
avoid waiting for a response that is not needed. This message is
the only instance where the Controller acts as a CoAP server and
the IoT device as a CoAP client. The message also includes a URI
in the payload of the message to indicate to what resource (e.g.
'/a/x') the Controller MUST send the first message with the EAP
authentication. The name of the resource is selected by the CoAP
server as it pleases. After this, the exchange continues with the
Controller as a CoAP client and the IoT device as a CoAP server.
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* Step 1. The Controller sends a "POST" message to the resource
indicated by the IoT device in Step 0 (e.g., '/a/x'). The payload
in this message contains the first EAP message (EAP Request/
Identity), the Recipient ID of the Controller (RID-C) for OSCORE
and it MAY contain a CBOR array containing a list with the cipher
suites (CS) for OSCORE. If the cipher suite is not included the
default cipher suite for OSCORE is used. The details of the
cipher suite negotiation are discussed in Section 5.1.
* Step 2. The IoT device processes the POST message passing the EAP
request (EAP-Req/Id) to the EAP peer state machine, which returns
an EAP response (EAP Resp/Id); it assigns a new resource to the
ongoing authentication process (e.g., '/a/y'), and deletes the
previous one ('/a/x'). It finally sends a '2.01 Created' response
with the new resource identifier in the Location-Path (and/or
Location-Query) options for the next step; the EAP response, the
Recipient ID of the IoT device (RID-I) and the selected cipher
suite for OSCORE are in the payload. In this step, the IoT device
MAY create an OSCORE security context (see Section 5.2). The
required key, the Master Session Key (MSK), will be available once
the EAP authentication is successful in step 7.
* Step 3-6. From now on, the Controller and the IoT device will
exchange EAP packets related to the EAP method (EAP-X),
transported in the CoAP message payload. The Controller will use
the POST method to send EAP requests to the IoT device. The IoT
device will use a response to carry the EAP response in the
payload. EAP requests and responses are represented in Figure 3
using the nomenclature (EAP-X-Req and EAP-X-Resp, respectively.
When a POST message arrives (e.g, '/a/x') carrying an EAP request
message, if processed correctly by the EAP peer state machine,
returns an EAP Response. Along with each EAP Response, a new
resource is created (e.g, '/a/z') for processing the next EAP
request and the ongoing resource (e.g., '/a/y') is erased. This
way ordering guarantee is achieved. Finally, an EAP response is
sent in the payload of a CoAP response that will also indicate the
new resource in the Location-Path (and/or Location-Query) Options.
In case there is an error processing a legitimate message, the
server will return a (4.00 Bad Request). There is a discussion
about error handling in Section 3.5.
* Step 7. When the EAP authentication ends with success, the
Controller obtains the Master Session Key (MSK) exported by the
EAP method, an EAP Success message and some authorization
information (i.e. session lifetime) [RFC5247]. The Controller
creates the OSCORE security context using the MSK and Sender ID
and Recipient ID exchanged in Steps 1 and 2. The establishment of
the OSCORE Security Context is defined in Section 5.2. Then, the
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Controller sends the POST message protected with OSCORE for key
confirmation including the EAP Success. The Controller MAY also
send a Session Lifetime, in seconds, which is represented with an
unsigned integer in a CBOR object (see Section 4. If this Session
Lifetime is not sent, the IoT device assumes a default value of 8
hours as RECOMMENDED in [RFC5247]. The verification of the
received OSCORE protected "POST" message using RID-I (Recipient ID
of the IoT device) sent in Step 2 is considered by the IoT device
as an alternate indication of success ([RFC3748]). The EAP peer
state machine in the IoT device interprets the alternate
indication of success in a similar way to the arrival of an EAP
Success and returns the MSK, which is used for the OSCORE security
context in the IoT device to process the protected POST message
received from the Controller.
* Step 8. If the EAP authentication and the verification of the
OSCORE protected "POST" in Step 7 is successful, then the IoT
Device answers with an OSCORE protected '2.04 Changed'. From this
point on, the communication with the last resource (e.g. '/a/w')
MUST be protected with OSCORE. If allowed by application policy,
the same OSCORE security context MAY be used to protect
communication to other resources between the same endpoints.
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IoT device Controller
------------- ------------
| POST /.well-known/coap-eap |
0) | No-Response |
| Payload("/a/x") |
|---------------------------------------->|
| POST /a/x |
| Payload(EAP Req/Id||CS||RID-C) |
1) |<----------------------------------------|
| 2.01 Created Location-Path [/a/y] |
| Payload(EAP Resp/Id||CS||RID-I) |
2) |---------------------------------------->|
| POST /a/y |
| Payload(EAP-X Req) |
3) |<----------------------------------------|
| 2.01 Created Location-Path [/a/z] |
| Payload(EAP-X Resp) |
4) |---------------------------------------->|
....
| POST /a/q |
| Payload(EAP-X Req) |
5) |<----------------------------------------|
| 2.01 Created Location-Path [/a/w] |
| Payload (EAP-X Resp) |
6) |---------------------------------------->|
| | MSK
| POST /a/w | |
| OSCORE | V
| Payload (EAP Success||*Session-Lifetime)| OSCORE
MSK 7) |<----------------------------------------| CONTEXT
| | |
V | 2.04 Changed |
OSCORE | OSCORE |
CONTEXT 8 )|---------------------------------------->|
(*) Session-Lifetime is optional.
Figure 3: CoAP-EAP flow of operation with OSCORE
3.3. Reauthentication
When the CoAP-EAP state is close to expiring, the IoT device MAY want
to start a new authentication process (re-authentication) to renew
the state. The main goal is to derive new and fresh keying material
(MSK/EMSK) that, in turn, allows deriving a new OSCORE security
context, increasing the protection against key leakage. The keying
material MUST be renewed before the expiration of the Session-
Lifetime. By default, the EAP Key Management Framework establishes a
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default value of 8 hours to refresh the keying material. Certain EAP
methods such as EAP-NOOB [RFC9140] or EAP-AKA' [RFC5448] provides
fast reconnect for quicker re-authentication. The EAP re-
authentication protocol (ERP) [RFC6696] MAY be also used for avoiding
the repetition of the entire EAP exchange.
The message flow for the re-authentication will be the same as the
one shown in Figure 3. Nevertheless, two different CoAP-EAP states
will be active during the re-authentication: the current CoAP-EAP
state and the new CoAP-EAP state, which will be created once the re-
authentication has finished with success. Once the re-authentication
is completed successfully, and the current CoAP-EAP state is deleted
and the new CoAP-EAP becomes the current one. If for any reason, the
re-authentication fails to complete, the current CoAP-EAP state will
be available until it expires, or it is renewed in another try of re-
authentication.
If the re-authentication fails, it is up to the IoT device to decide
when to restart a re-authentication before the current EAP state
expires.
3.4. Managing the State of the Service
The IoT device and the Controller keep a state during the CoAP-EAP
negotiation. The CoAP-EAP state includes several important parts:
* A reference to an instance of the EAP (peer or authenticator/
server) state machine.
* The resource for the next message in the negotiation (e.g '/a/y')
* The MSK exported when the EAP authentication is successful. In
particular, CoAP-EAP is able to access to the different variables
by the EAP state machine (i.e. [RFC4137]).
* A reference to the OSCORE context.
Once created, the Controller MAY choose to delete it as described in
Figure 4. On the other hand, the IoT device may need to renew the
CoAP-EAP state because the key material is close to expire, as
mentioned in Section 3.3.
There are situations where the current CoAP-EAP state might need to
be removed. For instance, due to its expiration or a forced removal
if the IoT device needs to be expelled from the security domain.
This exchange is illustrated in Figure 4.
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If the Controller deems it necessary the removal of the CoAP-EAP
state from the IoT device before it expires, it can send a DELETE
command in a request to the IoT device, referencing the last CoAP-EAP
state resource given by the CoAP server, whose identifier will be the
last one received (e.g., '/a/w' in Figure 3). This message is
protected with the OSCORE security association to prevent forgery.
Upon reception of this message, the CoAP server sends a response to
the Controller with the Code '2.02 Deleted', which is also protected
with the OSCORE security association. If a response from the IoT
device does not arrive after EXCHANGE_LIFETIME the Controller will
remove the state from its side.
IoT device Controller
------------- -------------
| |
| DELETE /a/w |
| OSCORE |
|<----------------------------------------|
| |
| 2.02 Deleted |
| OSCORE |
|---------------------------------------->|
Figure 4: Deleting state
3.5. Error handling
This section elaborates on how different errors are handled. From
EAP authentication failure, a non-responding endpoint, lost messages
or initial POST message arriving out of place.
3.5.1. EAP authentication failure
EAP authentication MAY fail for different situations (e.g. wrong
credentials). The result is that the Controller will send an EAP
failure because of the EAP authentication (Step 7 in Figure 3). In
this case, the IoT device MUST send a response '4.01 Unauthorized' in
Step 8. Therefore, Step 7 and Step 8 are not protected in this case
because no MSK is exported and the OSCORE security context is not
generated.
If the EAP authentication fails during the re-authentication and the
Controller sends an EAP failure, the current CoAP-EAP state will be
still usable until it expires.
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3.5.2. Non-responding endpoint
If for any reason, one of the entities becomes non-responding, the
CoAP-EAP state SHOULD be kept only for some time before it is
removed. The removal of the CoAP-EAP state in the Controller assumes
that the IoT device will need to authenticate again. According to
CoAP, EXCHANGE_LIFETIME considers the time it takes until a client
stops expecting a response to a request. A timer is reset every time
a message is sent. If EXCHANGE_LIFETIME has passed while waiting for
the next message, both entities will delete the CoAP-EAP state if the
authentication process has not finished correctly.
3.5.3. Duplicated message with /.well-known/coap-eap
The reception of the trigger message in Step 0 containing /.well-
known/coap-eap needs some additional considerations, as the resource
is always available in the EAP authenticator.
If a trigger message (Step 0) arrives at the Controller during an
ongoing authentication, the Controller MUST silently discard this
trigger message.
If an old "POST /.well-known/coap-eap" (Step 0) arrives to the
Controller and there is no authentication ongoing, the Controller may
understand that a new authentication process is requested.
Consequently, the Controller will start a new EAP authentication.
However, the IoT device did not start any authentication and
therefore, it has not selected any resource for the EAP
authentication. Thus, IoT device sends a '4.04 Not found' in the
response (Figure 5).
IoT device Controller
------------- -------------
| *POST /.well-known/coap-eap |
0) | , No-Response |
| Payload("/a/x") |
| ------------------------->|
| POST /a/x |
| Payload (EAP Req/Id||CS) |
1) |<----------------------------------------|
| |
| 4.04 Not found |
|---------------------------------------->|
* Old
Figure 5: /.well-known/coap-eap with no ongoing authentication
from the EAP authenticator
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3.6. Proxy operation in CoAP-EAP
The CoAP-EAP operation is intended to be compatible with the use of
intermediary entities between the IoT device and the Controller when
direct communication is not possible. In this context, CoAP proxies
can be used as enablers of the CoAP-EAP exchange.
This specification is limited to using standard CoAP [RFC7252] as
well as standardized CoAP options [RFC8613]. It does not specify any
addition in the form of CoAP options. This is expected to ease the
integration of CoAP intermediaries in the CoAP-EAP exchange.
There is a consideration that needs to be considered when using
proxies in the CoAP-EAP, as the exchange contains a role-reversal
process at the beginning of the exchange. In the first message, the
IoT device acts as a CoAP client and the Controller as the CoAP
server. After that, in the remaining exchanges the roles are
reversed, being the IoT device, the CoAP server and the Controller,
the CoAP client.
4. CBOR Objects in CoAP-EAP
In the CoAP-EAP exchange, there is information that needs to be
exchanged between the two entities. Examples of these are the cipher
suites that need to be negotiated or authorization information
(Session-lifetime). There may be also a need of extending the
information that has to be exchanged in the future. This section
specifies the CBOR [RFC8949] data structure to exchange information
between the IoT device and the Controller in the CoAP payload.
Next, is the specification of the CBOR Object to exchange information
in CoAP-EAP
CoAP-EAP_Info = {
? 1 : array, ; cipher suite
? 2 : bstr, ; RID-C
? 3 : bstr, ; RID-I
? 4 : uint ; Session-Lifetime
}
Figure 6: CBOR data structure for CoAP-EAP
The parameters contain the following information:
1. cipher suite: It contains an array with the list of the proposed
or selected COSE algorithms for OSCORE. If the field is carried
over a request, the meaning is the proposed cipher suite, if it
is carried over a response, corresponds to the response.
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2. RID-I: It contains the Recipient ID of the IoT device. The
Controller uses this value as Sender ID for its OSCORE Sender
Context. The IoT device uses this value as Recipient ID for its
Recipient Context.
3. RID-C: It contains the Recipient ID of the Controller. The IoT
device uses this value as Sender ID for its OSCORE Sender
Context. The Controller uses this value as Recipient ID for its
Recipient Context.
4. Session-Lifetime: Contains the time the session is valid in
seconds.
The indexes from 65000 to 65535 are reserved for experimentation.
5. Cipher suite negotiation and key derivation
5.1. Cipher suite negotiation
OSCORE runs after the EAP authentication, using the cipher suite
selected in the cipher suite negotiation (Steps 1 and 2). To
negotiate the cipher suite, CoAP-EAP follows a simple approach: the
Controller sends a list, in decreasing order of preference, with the
identifiers of the supported cipher suites (Step 1). In the response
to that message (Step 2), the IoT device sends a response with the
choice.
This list is included in the payload after the EAP message with a
CBOR array that contains the cipher suites. An example of how the
fields are arranged in the CoAP payload can be seen in Figure 7. An
example of the exchange with the cipher suite negotiation is shown in
Figure 8, where can be appreciated the disposition of both EAP-
Request/Identity and EAP-Response/Identity, followed by the CBOR
object defined in Section 4, containing in the cipher suite field the
CBOR array for the cipher suite negotiation.
+-----+-----------+-------+------++-------------+
|Code |Identifier |Length | Data ||cipher suites|
+-----+-----------+-------+------++-------------+
EAP Packet CBOR Object
Figure 7: cipher suites are in the CoAP payload
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EAP peer EAP Auth.
(CoAP server) (CoAP client)
------------- -------------
| |
| ... |
|---------------------------------------->|
| POST /a/x |
| Payload (EAP Req/Id, CBORArray[0,1,2]) |
1) |<----------------------------------------|
| 2.01 Created Location-Path [/a/y] |
| Payload (EAP Resp/Id, CBORArray[0]) |
2) |---------------------------------------->|
...
Figure 8: cipher suite negotiation
In case there is no CBOR array stating the cipher suites, the default
cipher suites are applied. If the Controller sends a restricted list
of cipher suites that are willing to accept it MUST include the
default value 0 since it is mandatory to implement. The IoT device
will have at least that option available.
The cipher suite requirements are inherited from the ones established
by OSCORE. By default, the HKDF algorithm is SHA-256 and the AEAD
algorithm is AES-CCM-16-64-128. Both are mandatory to implement.
The other cipher suites supported and negotiated in the cipher suite
negotiation are the following:
0. AES-CCM-16-64-128, SHA-256 (default)
1. A128GCM, SHA-256
2. A256GCM, SHA-384
3. ChaCha20/Poly1305, SHA-256
4. ChaCha20/Poly1305, SHAKE256
This specification uses the (HMAC)-based key derivation function
(HKDF) defined in [RFC5869] to derive the necessary key material.
Since the key derivation process uses the MSK, which is considered
fresh key material, we will use the HKDF-Expand function, which we
will shorten here as KDF.
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5.2. Deriving the OSCORE Security Context
The derivation of the security context for OSCORE allows securing the
communication between the IoT device and the Controller once the MSK
has been exported providing, confidentiality, integrity, key
confirmation (Steps 7 and 8), and detecting a downgrading attack.
The Master Secret can be derived by using the chosen cipher suite and
the KDF. The Master Secret can be derived as follows:
Master Secret = KDF(MSK, CS | "COAP-EAP OSCORE MASTER SECRET",
length)
where:
* The algorithms for OSCORE are agreed in the cipher suite
negotiation.
* The MSK is exported by the EAP method. Discussion about the use
of the MSK for the key derivation is done in Section 7.
* CS is the concatenation of the content of the cipher suite
negotiation, that is, the list of cipher suites sent by the
Controller (Step 1) the selected option by the IoT device (Step
2). If any of the messages did not contain the CBOR array
(default algorithms), the null string is used.
* "COAP-EAP OSCORE MASTER SECRET" is the ASCII code representation
of the non-NULL terminated string (excluding the double quotes
around it).
* CS and "COAP-EAP OSCORE MASTER SECRET" are concatenated.
* length is the size of the output key material.
The Master Salt, similarly to the Master Secret, can be derived as
follows:
Master Salt = KDF(MSK, CS | "COAP-EAP OSCORE MASTER SALT", length)
where:
* The algorithms are agreed upon in the cipher suite negotiation.
* The MSK is exported by the EAP method. Discussion about the use
of the MSK for the key derivation is done in Section 7.
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* CS is the concatenation of the content of the cipher suite
negotiation, in the request and response. If any of the messages
did not contain the CBOR array, the null string is used.
* "COAP-EAP OSCORE MASTER SALT" is the ASCII code representation of
the non-NULL-terminated string (excluding the double quotes around
it).
* CS and "COAP-EAP OSCORE MASTER SALT" are concatenated.
* length is the size of the output key material.
Since the MSK is used to derive the Master Key, the correct
verification of the OSCORE protected request (Step 7) and response
(Step 8) confirms the Controller and the IoT device have the same
Master Secret, achieving key confirmation.
To prevent a downgrading attack, the content of the cipher suites
negotiation (which we refer to here as CS) is embedded in the Master
Secret derivation. If an attacker changes the value of the cipher
suite negotiation, the result will be different OSCORE security
contexts, that ends up with a failure in Step 7 and 8.
The Controller will use the Recipient ID of the IoT device (RID-I) as
Sender ID for its OSCORE Sender Context. The IoT device will use
this value as Recipient ID for its Recipient Context.
The IoT device will use the Recipient ID of the Controller (RID-C) as
Sender ID for its OSCORE Sender Context. The Controller will use
this value as Recipient ID for its Recipient Context.
6. Discussion
6.1. CoAP as EAP lower layer
This section discusses the suitability of the CoAP protocol as EAP
lower layer, and reviews the requisites imposed by the EAP protocol
on any protocol that transports EAP. What EAP expects from its lower
layers can be found in the section 3.1 of [RFC3748], which is
elaborated next:
Unreliable transport. EAP does not assume that lower layers are
reliable but it can benefit from a reliable lower layer. In this
sense, CoAP provides a reliability mechanism (e.g. through the use
of Confirmable messages).
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Lower layer error detection. EAP relies on lower layer error
detection (e.g., CRC, Checksum, MIC, etc.). CoAP goes on top of UDP/
TCP which provides a checksum mechanism over its payload.
Lower layer security. EAP does not require security services from
the lower layers.
Minimum MTU. Lower layers need to provide an EAP MTU size of 1020
octets or greater. CoAP assumes an upper bound of 1024 for its
payload which covers the requirements of EAP.
Ordering guarantees. EAP relies on lower layer ordering guarantees
for correct operation. Regarding message ordering, every time a new
message arrives at the authentication service hosted by the IoT
device, a new resource is created and this is indicated in a "2.01
Created" response code along with the name of the new resource via
Location-Path or Location-Query. This way the application shows that
its state has advanced. Although the [RFC3748] states: "EAP provides
its own support for duplicate elimination and retransmission", EAP is
also reliant on lower layer ordering guarantees. In this regard,
[RFC3748] talks about possible duplication and says: "Where the lower
layer is reliable, it will provide the EAP layer with a non-
duplicated stream of packets. However, while it is desirable that
lower layers provide for non-duplication, this is not a requirement".
CoAP is providing a non-duplicated stream of packets and accomplish
the "desirable" non-duplication. In addition, [RFC3748] says that
when EAP runs over a reliable lower layer "the authenticator
retransmission timer SHOULD be set to an infinite value, so that
retransmissions do not occur at the EAP layer."
6.2. Size of the EAP lower layer vs EAP method size
Regarding the impact that an EAP lower layer will have on the total
byte size of the whole exchange, there is a comparison with another
network layer-based EAP lower layer, PANA [RFC5191], in [coap-eap].
Comparing the EAP lower layer (alone) and taking into account EAP.
On the one hand, at the EAP lower layer level, the usage of CoAP
gives important benefits. On the other hand, when taking into
account the EAP method overload, this reduction is less but still
significant if the EAP method generates large EAP messages. If the
EAP method is very taxing, the impact of the reduction in the size of
the EAP lower layer is less significant. This leads to the
conclusion that possible next steps in this field could be designing
new EAP methods that can be better adapted to the requirements of IoT
devices and networks. For example, authors in [coap-eap] used EAP-
PSK is an example, since it only involves 4 messages and their length
can be less than 60 bytes. Moreover, it only uses symmetric
cryptography.
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However, the impact of the EAP lower layer itself cannot be ignored,
hence the proposal of using CoAP as a lightweight protocol for this
purpose. Other EAP methods such as EAP-AKA'[RFC5448] or new EAP
methods such as EAP-NOOB [RFC9140] or EAP-EDHOC
[I-D.ingles-eap-edhoc] that can benefit, as well as new ones that may
be proposed in the future with IoT constraints in mind, from a CoAP-
based EAP lower layer.
7. Security Considerations
There are some aspects to be considered such as how authorization is
managed, the use of MSK as keying material and how the trust in the
Controller is established. Additional considerations such as EAP
channel binding as per [RFC6677] are also discussed here.
7.1. Authorization
Authorization is part of bootstrapping. It serves to establish
whether the node can join and the set of conditions it has to adhere
to. The authorization data will be gathered from the organization
that is responsible for the IoT device and sent to the EAP
authenticator in case of AAA infrastructure is deployed.
In standalone mode, the authorization information will be in the
Controller. If the pass-through mode is used, authorization data
received from the AAA server can be delivered by the AAA protocol
(e.g. RADIUS or Diameter). Providing more fine-grained
authorization data can be with the transport of SAML in RADIUS
[RFC7833].
After bootstrapping, additional authorization information to operate
in the security domain, e.g., access services offered by other nodes
can be taken care of by the solutions proposed in the ACE WG.
7.2. Freshness of the key material
In CoAP-EAP there is no nonce exchange to provide freshness to the
keys derived from the MSK. The MSK and Extended Master Session Key
(EMSK) keys according to the EAP Key Management Framework [RFC5247]
are fresh key material. Since only one authentication is established
per EAP authenticator, there is no need for generating additional key
material. In case a new MSK is required, a re-authentication can be
done, by running the process again, or using a more lightweight EAP
method to derive additional key material as elaborated in
Section 3.3.
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7.3. Channel Binding support
According to the [RFC6677], channel binding related to EAP, is sent
through the EAP method that supports it.
To satisfy the requirements of the document, we need to send the EAP
lower layer identifier (To be assigned by IANA), in the EAP Lower-
Layer Attribute if RADIUS is used.
7.4. Additional Security Consideration
In the process of authentication, there is a possibility of an entity
forging messages to generate a denial of service (DoS) attacks on any
of the entities involved. For instance, an attacker can forge
multiple initial messages to start an authentication (Step 0) with
the Controller as if they were sent by different IoT devices.
Consequently, the Controller will start an authentication per each
message received in Step 0, sending the EAP Request/Id (Step 1).
To minimize the effects of this DoS attack, it is RECOMMENDED that
the Controller limits the rate at which it processes incoming
messages in Step 0 to provide robustness against denial of service
(DoS) attacks. The details of rate limiting are outside the scope of
this specification. Nevertheless, the rate of these messages is also
limited by the bandwidth available between the IoT device and the
Controller. This bandwidth will be especially limited in constrained
links (e.g., LPWAN). Lastly, it is also RECOMMENDED to reduce at a
minimum the state in the Controller at least until the EAP Response/
Ids received by the Controller.
Other security-related concerns can be how to ensure that the IoT
device joining the security domain can trust the Controller. This
issue is elaborated in the EAP Key Management Framework [RFC5247].
In particular, the IoT device knows it can trust the Controller
because the key that is used to establish the security association is
derived from the MSK. If the Controller has the MSK, it is clear the
AAA Server of the node trusted the Controller, which can be
considered a trusted party.
8. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding the registration of values related to
CoAP-EAP.
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8.1. CoAP-EAP Exporter Label Registry
IANA has created a new registry titled "CoAP-EAP Exporter Label"
under the new group name "CoAP-EAP protocol". The registration
procedure is "Expert Review". The columns of the registry are Label
and Description. Label is an ASCII code representation of the non-
NULL terminated string. Description is a text string. The initial
contents of the registry are:
* Label: "CoAP-EAP OSCORE MASTER SALT"
* Description: Label used to derive the CoAP-EAP OSCORE master salt
* Label: "COAP-EAP OSCORE MASTER SECRET"
* Description: Label used to derive the COAP-EAP OSCORE master
secret
8.2. CoAP-EAP Cipher Suites
IANA has created a new registry titled "CoAP-EAP Cipher Suites" under
the new group name "CoAP-EAP protocol". The registration procedure
is "Expert Review". The columns of the registry are Value, Array and
Description, where Value is an integer and the other columns are text
strings. The initial contents of the registry are:
* Value: -24
* Array: N/A
* Description: Reserved for Private Use
* Value: -23
* Array: N/A
* Description: Reserved for Private Use
* Value: -22
* Array: N/A
* Description: Reserved for Private Use
* Value: -21
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* Array: N/A
* Description: Reserved for Private Use
* Value: 0
* Array: 10, -16
* Description: AES-CCM-16-64-128, SHA-256
* Value: 1
* Array: 1, -16
* Description: A128GCM, SHA-256
* Value: 2
* Array: 3, -43
* Description: A256GCM, SHA-384
* Value: 3
* Array: 24, -16
* Description: ChaCha20/Poly1305, SHA-256
* Value: 4
* Array: 24, -45
* Description: ChaCha20/Poly1305, SHAKE256
8.3. CoAP-EAP Information elements
IANA has created a new registry titled "CoAP-EAP Information element"
under the new group name "CoAP-EAP protocol". The registration
procedure is "Expert Review". The columns of the registry are Value,
Name and Description, where Value is an integer and the other columns
are text strings. The initial contents of the registry are:
* Value: 1
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* Name: cipher suite
* Description: List of the proposed or selected COSE algorithms for
OSCORE
* Value: 2
* Name: RID-C
* Description: It contains the Recipient ID of the IoT device
* Value: 3
* Name: RID-I
* Description: It contains the Recipient ID of the Controller
* Value: 4
* Name: Session-Lifetime
* Description: Contains the time the session is valid in seconds
* Values: 65000 to 65535
* Name: Reserved
* Description: Reserved for experimentation
8.4. The Well-Known URI Registry
IANA has added the well-known URI "coap-eap" to the "Well-Known URIs"
registry under the group name "Well-Known URIs" defined by [RFC8615].
The registration procedure is "Expert Review".
* URI suffix: coap-eap
* Change controller: IETF
* Specification document(s): [[this document]]
* Related information: None
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8.5. The EAP lower layer identifier registry
IANA has added the identifier of CoAP-EAP to the registry "EAP Lower
Layer" under the Extensible Authentication Protocol (EAP) Registry
defined by [RFC6677]. The registration procedure is "Expert Review".
* Value: TBD.
* Lower Layer: CoAP-EAP
* Specification document(s): [[this document]]
8.6. Media Types Registry
IANA has added the media types "application/coap-eap" to the "Media
Types" registry defined by [RFC6838] and [RFC4855]. The registration
procedure is "Expert Review".
* Type name: application
* Subtype name: coap-eap
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary
* Security considerations: See Section Section 7 of this document.
* Interoperability considerations: N/A
* Published specification: [[this document]]
* Applications that use this media type: To be identified
* Fragment identifier considerations: N/A
* Additional information:
- Magic number(s): N/A
- File extension(s): N/A
- Macintosh file type code(s): N/A
* Person and email address to contact for further information: See
"Authors' Addresses" section.
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* Intended usage: COMMON
* Restrictions on usage: N/A
* Author: See "Authors' Addresses" section.
* Change Controller: IESG
8.7. CoAP Content-Formats Registry
IANA has added the media types "application/coap-eap"to the "CoAP
Content-Formats" registry under the group name "Constrained RESTful
Environments (CoRE) Parameters" defined by [RFC6690]. The
registration procedure is "Expert Review".
+-----------------------+----------+------+-------------------+
| Media Type | Encoding | ID | Reference |
+-----------------------+----------+------+-------------------+
| application/coap-eap | - | TBD | [[this document]] |
+-----------------------+----------+------+-------------------+
Figure 9: CoAP Content-Formats Registry
8.8. Resource Type (rt=) Link Target Attribute Values Registry
IANA has added the resource type "core.coap-eap" to the "Resource
Type (rt=) Link Target Attribute Values" registry under the group
name "Constrained RESTful Environments (CoRE) Parameters" defined by
[RFC6690]. The registration procedure is "Expert Review".
* Value: "core.coap-eap"
* Description: CoAP-EAP resource.
* Reference: [[this document]]
9. Acknowledgments
We would like to thank as the reviewers of this work: Carsten
Bormann, Mohit Sethi, Benjamin Kaduk, Christian Amsuss, John
Mattsson, Goran Selander, Alexandre Petrescu, Pedro Moreno-Sanchez
and Eduardo Ingles-Sanchez.
We would also like to thank Gabriel Lopez-Millan for the first review
of this document and we would like to thank Ivan Jimenez-Sanchez for
the first proof-of-concept implementation of this idea, and Julian
Niklas Schimmelpfennig for the implementation of the Erbium-based IoT
device implementation.
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And thank for their valuable comments to Alexander Pelov and Laurent
Toutain, especially for the potential optimizations of CoAP-EAP.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<https://www.rfc-editor.org/info/rfc3748>.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<https://www.rfc-editor.org/info/rfc5247>.
[RFC6677] Hartman, S., Ed., Clancy, T., and K. Hoeper, "Channel-
Binding Support for Extensible Authentication Protocol
(EAP) Methods", RFC 6677, DOI 10.17487/RFC6677, July 2012,
<https://www.rfc-editor.org/info/rfc6677>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
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[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[RFC8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC8949, December 2020,
<https://www.rfc-editor.org/info/rfc8949>.
10.2. Informative References
[coap-eap] Garcia-Carrillo, D. and R. Marin-Lopez, "Lightweight CoAP-
Based Bootstrapping Service for the Internet of Things -
https://www.mdpi.com/1424-8220/16/3/358", March 2016.
[eap-framework]
Sethi, M. and T. Aura, "Secure Network Access
Authentication for IoT Devices: EAP Framework vs.
Individual Protocols -
https://ieeexplore.ieee.org/document/9579387", October
2021.
[I-D.ietf-ace-oauth-authz]
Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments (ACE) using the OAuth 2.0
Framework (ACE-OAuth)", Work in Progress, Internet-Draft,
draft-ietf-ace-oauth-authz-46, 8 November 2021,
<https://www.ietf.org/archive/id/draft-ietf-ace-oauth-
authz-46.txt>.
[I-D.ingles-eap-edhoc]
Sanchez, E., Garcia-Carrillo, D., and R. Marin-Lopez, "EAP
method based on EDHOC Authentication", Work in Progress,
Internet-Draft, draft-ingles-eap-edhoc-01, 2 November
2020, <https://www.ietf.org/internet-drafts/draft-ingles-
eap-edhoc-01.txt>.
[lo-coap-eap]
Garcia-Carrillo, D., Marin-Lopez, R., Kandasamy, A., and
A. Pelov, "A CoAP-Based Network Access Authentication
Service for Low-Power Wide Area Networks: LO-CoAP-EAP -
https://www.mdpi.com/1424-8220/17/11/2646", November 2017.
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[RFC4137] Vollbrecht, J., Eronen, P., Petroni, N., and Y. Ohba,
"State Machines for Extensible Authentication Protocol
(EAP) Peer and Authenticator", RFC 4137,
DOI 10.17487/RFC4137, August 2005,
<https://www.rfc-editor.org/info/rfc4137>.
[RFC4764] Bersani, F. and H. Tschofenig, "The EAP-PSK Protocol: A
Pre-Shared Key Extensible Authentication Protocol (EAP)
Method", RFC 4764, DOI 10.17487/RFC4764, January 2007,
<https://www.rfc-editor.org/info/rfc4764>.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, DOI 10.17487/RFC4855, February 2007,
<https://www.rfc-editor.org/info/rfc4855>.
[RFC5191] Forsberg, D., Ohba, Y., Ed., Patil, B., Tschofenig, H.,
and A. Yegin, "Protocol for Carrying Authentication for
Network Access (PANA)", RFC 5191, DOI 10.17487/RFC5191,
May 2008, <https://www.rfc-editor.org/info/rfc5191>.
[RFC5448] Arkko, J., Lehtovirta, V., and P. Eronen, "Improved
Extensible Authentication Protocol Method for 3rd
Generation Authentication and Key Agreement (EAP-AKA')",
RFC 5448, DOI 10.17487/RFC5448, May 2009,
<https://www.rfc-editor.org/info/rfc5448>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6696] Cao, Z., He, B., Shi, Y., Wu, Q., Ed., and G. Zorn, Ed.,
"EAP Extensions for the EAP Re-authentication Protocol
(ERP)", RFC 6696, DOI 10.17487/RFC6696, July 2012,
<https://www.rfc-editor.org/info/rfc6696>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
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[RFC7833] Howlett, J., Hartman, S., and A. Perez-Mendez, Ed., "A
RADIUS Attribute, Binding, Profiles, Name Identifier
Format, and Confirmation Methods for the Security
Assertion Markup Language (SAML)", RFC 7833,
DOI 10.17487/RFC7833, May 2016,
<https://www.rfc-editor.org/info/rfc7833>.
[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
No Server Response", RFC 7967, DOI 10.17487/RFC7967,
August 2016, <https://www.rfc-editor.org/info/rfc7967>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC8824] Minaburo, A., Toutain, L., and R. Andreasen, "Static
Context Header Compression (SCHC) for the Constrained
Application Protocol (CoAP)", RFC 8824,
DOI 10.17487/RFC8824, June 2021,
<https://www.rfc-editor.org/info/rfc8824>.
[RFC9140] Aura, T., Sethi, M., and A. Peltonen, "Nimble Out-of-Band
Authentication for EAP (EAP-NOOB)", RFC 9140,
DOI 10.17487/RFC9140, December 2021,
<https://www.rfc-editor.org/info/rfc9140>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9176] Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
P. van der Stok, "Constrained RESTful Environments (CoRE)
Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
2022, <https://www.rfc-editor.org/info/rfc9176>.
[TS133.501]
ETSI, "5G; Security architecture and procedures for 5G
System - TS 133 501 V15.2.0 (2018-10)", 2018.
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Appendix A. Flow of operation (DTLS establishment)
CoAP-EAP makes it possible to derive a PSK for (D)TLS to allow PSK-
based authentication between the IoT device and the Controller. In
the instance of using (D)TLS to establish a security association,
there is a limitation to the use of intermediaries between the IoT
device and the Controller, as (D)TLS breaks the end-to-end
communications when using intermediaries such as proxies.
IoT device Controller
------------- -------------
...
| 2.01 Created Location-Path [/a/w] |
| Payload (EAP-X Resp) |
6)|---------------------------------------->|
| | MSK
| (D)TLS 1.3 Client Hello | |
MSK 7) |<----------------------------------------| V
| | | DTLS_PSK
V |===============DTLS hanshake=============|
DTLS_PSK | |
*...
(*) Protected with (D)TLS
Figure 10: CoAP-EAP flow of operation with DTLS
Figure 10 shows the last steps of the operation for CoAP-EAP when
(D)TLS is used to protect the communication between the IoT device
and the Controller using the keying material exported by the EAP
authentication. The general flow is essentially the same as in the
case of OSCORE, except that DTLS negotiation is established in Step
7). Once DTLS negotiation has finished successfully the IoT device
is granted access to the domain. Step 7 MUST be interpreted by the
IoT device as an alternate success indication, which will end up with
the MSK and the DTLS_PSK derivation for the (D)TLS authentication
based on PSK.
According to [RFC8446] the provision of the PSK out-of-band also
requires the provision of the KDF hash algorithm and the PSK
identity. To simplify the design in CoAP-EAP, the KDF hash algorithm
can be included in the list of cipher suites exchange in Step 1 and
Step 2 if DTLS wants to be used instead of OSCORE. For the same
reason, the PSK identity is derived from (RID-C) (RID-I) as defined
in Appendix A.2.
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A.1. Cryptographic suite negotiation for DTLS
It is also possible to derive a pre-shared key for DTLS to establish
a DLTS security association after a successful EAP authentication.
Analogously to how the cipher suite is negotiated for OSCORE
Section 5.1, the Controller sends a list, in decreasing order of
preference, with the identifiers of the cipher suites supported (Step
1). In the response, the IoT device sends the choice.
This list is included in the payload after the EAP message with a
CBOR array that contains the cipher suites. This CBOR array is
enclosed as one of the elements of the CBOR Object used for
transporting information in CoAP-EAP (See Section 4. An example of
how the fields are arranged in the CoAP payload can be seen in
Figure 7.
In case there is no CBOR array stating the cipher suites, the default
cipher suites are applied. If the Controller sends a restricted list
of cipher suites that is willing to accept it MUST include the
default value 0 since it is mandatory to implement. The IoT device
will have at least that option available.
The cipher suites are the following:
3. TLS_SHA256
4. TLS_SHA384
5. TLS_SHA512
A.2. Deriving DTLS PSK and identity
To enable DTLS after an EAP authentication using the key material
generated, we define the Identity and the PSK for DTLS. The Identity
in this case is generated by concatenating the exchanged Sender ID
and the Recipient ID.
CoAP-EAP PSK Identity = RID-C || RID-I
It is also possible to derive a pre-shared key for DTLS [RFC9147],
refereed to here as "DTLS PSK", from the MSK between both IoT device
and Controller if required. The length of the DTLS PSK will depend
on the cipher suite. To have keying material with sufficient length
a key of 32 bytes is derived that can be later truncated if needed:
DTLS PSK = KDF(MSK, "CoAP-EAP DTLS PSK", length).
where:
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* MSK is exported by the EAP method.
* "CoAP-EAP DTLS PSK" is the ASCII code representation of the non-
NULL terminated string (excluding the double quotes around it).
* length is the size of the output key material.
Appendix B. Examples of Use Case Scenario
For an IoT device to act as a trustworthy entity within a security
domain, certain key material is needed to be shared between the IoT
device and the Controller.
Next, we elaborate on examples of different use case scenarios about
the usage of CoAP-EAP. Generally, we are dealing with 4 entities:
* 2 nodes (A and B), which are IoT devices. They are the EAP peers.
* 1 controller (C). The controller manages a domain where nodes can
be deployed. It can be considered a more powerful machine than
the IoT devices.
* 1 AAA server (AAA) - Optional. The AAA is an Authentication,
Authorization and Accounting Server, which is not constrained.
Here, the Controller acts as EAP authenticator in pass-through
mode.
Generally, any IoT device wanting to join the domain managed by the
Controller MUST perform a CoAP-EAP authentication with the Controller
(C). This authentication MAY involve an external AAA server. This
means that A and B, once deployed, will run CoAP-EAP once, as a
bootstrapping phase, to establish a security association with C.
Moreover, any other entity, which wants to join and establish
communications with nodes under C's domain must also do the same. By
using EAP, we can have the flexibility of having different types of
credentials. For instance, if we have a device that is not battery-
dependent, and not very constrained, we could use a heavier
authentication method. With varied IoT devices and networks we might
need to resort to more lightweight authentication methods (e.g., EAP-
NOOB[RFC9140], EAP-AKA'[RFC5448], EAP-PSK[RFC4764], EAP-
EDHOC[I-D.ingles-eap-edhoc], etc.) being able to adapt to different
types of devices according to organization policies or devices
capabilities.
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B.1. Example 1: CoAP-EAP in ACE
In ACE, the process of Client registration and provisioning of
credentials to the client is not specified. The process of Client
registration and provisioning can be achieved using CoAP-EAP. Once
the process of authentication with EAP is completed, the fresh key
material is shared between the IoT device and the Controller. In
this instance, the Controller and the Authorization Server (AS) of
ACE can be co-located.
Next, we exemplify how CoAP-EAP can be used to perform the Client
registration in a general way, to allow two IoT devices (A and B) to
communicate and interact after a successful client registration.
Node A wants to communicate with node B (e.g. to activate a light
switch). The overall process is divided into three phases. Let's
start with node A. In the first phase, node A (EAP peer) does not
yet belong to Controller C's domain. Then, it communicates with C
(EAP authenticator) and authenticates with CoAP-EAP, which,
optionally, communicates with the AAA server to complete the
authentication process. If the authentication is successful, a fresh
MSK is shared between C and node A. This key material allows node A
to establish a security association with the C. Some authorization
information may be also provided in this step. In case EAP is used
in standalone mode, the AS itself having information about the
devices can be the entity providing said authorization information.
If authentication and authorization are correct, node A is enrolled
in controller C's domain for some time. In particular, [RFC5247]
recommends 8 hours, though the the entity providing the authorization
information can establish this lifetime. In the same manner, B needs
to perform the same process with CoAP-EAP to be part of controller
C's domain.
In the second phase, when node A wants to talk with node B, it
contacts controller C for authorization to access node B and obtain
all the required information to do that securely (e.g. keys, tokens,
authorization information, etc.). This phase does NOT require the
usage of CoAP-EAP. The details of this phase are out of scope of
this document, and the ACE framework is used for this purpose
[I-D.ietf-ace-oauth-authz].
In the third phase, node A can access node B with the credentials and
information obtained from controller C in the second phase. This
access can be repeated without contacting the controller, while the
credentials given to A are still valid. The details of this phase
are out of scope of this document.
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It is worth noting that the first phase with CoAP-EAP is required to
join the controller C's domain. Once it is performed with success,
the communications are local to the controller C's domain and there
is no need to perform a new EAP authentication as long as the key
material is still valid. When the keys are about to expire, the IoT
device can engage in a re-authentication as explained in Section 3.3,
to renew the key material.
B.2. Example 2: Multi-domain with AAA infrastructures
We assume we have a device (A) of the domain acme.org, which uses a
specific kind of credential (e.g., AKA) and intends to join the um.es
domain. This user does not belong to this domain, for which first it
performs a client registration using CoAP-EAP. For this, it
interacts with the controller's domain acting as an EAP
authenticator, which in turn communicates with a AAA infrastructure
(acting as AAA client). Through the local AAA server to communicate
with the home AAA server to complete the authentication and integrate
the device as a trustworthy entity into the domain of controller C.
In this scenario, the AS under the role of the Controller receives
the key material from the AAA infrastructure
B.3. Example 3: Single domain with AAA infrastructure
As a University Campus, we have several Faculty buildings and each
one has its criteria or policies in place to manage IoT devices under
an AS. All buildings belong to the same domain (e.g., um.es). All
these buildings are managed with a AAA infrastructure. A new device
(A) with credentials from the domain (e.g., um.es) will be able to
perform the device registration with a Controller (C) of any building
as long as they are managed by the same general domain.
B.4. Example 4: Single domain without AAA infrastructure
In another case, without a AAA infrastructure, we have a Controller
that has co-located the EAP server and using EAP standalone mode we
can manage all the devices within the same domain locally. Client
registration of a node (A) with Controller (C) can also be performed
in the same manner.
B.5. Other use cases
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B.5.1. CoAP-EAP for network access control
One of the first steps for an IoT device's 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 can achieve
this with flexibility and scalability, because of the different EAP
methods available and the ability to rely on AAA infrastructures if
needed to support multi-domain scenarios, which is a key feature when
the IoT devices deployed under the same security domain, belong to
different organizations. Given that EAP is also used for network
access control, we can adapt this service for other technologies.
For instance, to provide network access control to very constrained
technologies (e.g., LoRa network). Authors in [lo-coap-eap] provide
an study of a minimal version of CoAP-EAP for LPWAN networks with
interesting results. In this specific case, we could leverage the
compression by SCHC for CoAP [RFC8824].
B.5.2. CoAP-EAP for service authentication
It is not uncommon that the infrastructure where the device is
deployed and the services of the IoT device are managed by different
organizations. Therefore, in addition to the authentication for
network access control, we have to consider the possibility of a
secondary authentication to access different services. This process
of authentication, for example, will provide the necessary key
material to establish a secure channel and interact with the entity
in charge of granting access to different services. In 5G, for
example, consider a primary and secondary authentication using EAP
[TS133.501].
Authors' Addresses
Rafa Marin-Lopez
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
30100 Murcia
Spain
Phone: +34 868 88 85 01
Email: rafa@um.es
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Dan Garcia-Carrillo
University of Oviedo
Calle Luis Ortiz Berrocal S/N, Edificio Polivalente
33203 Gijon Asturias
Spain
Email: garciadan@uniovi.es
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