Internet DRAFT - draft-ietf-ace-wg-coap-eap
draft-ietf-ace-wg-coap-eap
ACE Working Group R. Marin-Lopez
Internet-Draft University of Murcia
Intended status: Standards Track D. Garcia-Carrillo
Expires: 5 September 2024 University of Oviedo
4 March 2024
EAP-based Authentication Service for CoAP
draft-ietf-ace-wg-coap-eap-10
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), enabling 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 5 September 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 4
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. CoAP-EAP Media type format . . . . . . . . . . . . . . . . . 14
5. CBOR Objects in CoAP-EAP . . . . . . . . . . . . . . . . . . 14
6. Cipher suite negotiation and key derivation . . . . . . . . . 15
6.1. Cipher suite negotiation . . . . . . . . . . . . . . . . 15
6.2. Deriving the OSCORE Security Context . . . . . . . . . . 17
7. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 18
7.1. CoAP as EAP lower layer . . . . . . . . . . . . . . . . . 18
7.2. Size of the EAP lower layer vs EAP method size . . . . . 19
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
8.1. Use of EAP Methods . . . . . . . . . . . . . . . . . . . 20
8.2. Authorization . . . . . . . . . . . . . . . . . . . . . . 20
8.3. Allowing IP traffic connectivity to perform
authentication . . . . . . . . . . . . . . . . . . . . . 20
8.4. Freshness of the key material . . . . . . . . . . . . . . 21
8.5. Channel Binding support . . . . . . . . . . . . . . . . . 21
8.6. Additional Security Considerations . . . . . . . . . . . 21
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
9.1. CoAP-EAP Cipher Suites . . . . . . . . . . . . . . . . . 22
9.2. CoAP-EAP Information elements . . . . . . . . . . . . . . 23
9.3. The Well-Known URI Registry . . . . . . . . . . . . . . . 24
9.4. The EAP lower layer identifier registry . . . . . . . . . 24
9.5. Media Types Registry . . . . . . . . . . . . . . . . . . 25
9.6. CoAP Content-Formats Registry . . . . . . . . . . . . . . 26
9.7. Resource Type (rt=) Link Target Attribute Values
Registry . . . . . . . . . . . . . . . . . . . . . . . . 26
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
10.1. Normative References . . . . . . . . . . . . . . . . . . 26
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10.2. Informative References . . . . . . . . . . . . . . . . . 27
Appendix A. Flow of operation (DTLS establishment) . . . . . . . 30
A.1. Deriving DTLS PSK and identity . . . . . . . . . . . . . 32
Appendix B. Using CoAP-EAP for distributing key material for IoT
networks . . . . . . . . . . . . . . . . . . . . . . . . 33
Appendix C. Examples of Use Case Scenario . . . . . . . . . . . 33
C.1. Example 1: CoAP-EAP in ACE . . . . . . . . . . . . . . . 34
C.2. Example 2: Multi-domain with AAA infrastructures . . . . 36
C.3. Example 3: Single domain with AAA infrastructure . . . . 36
C.4. Example 4: Single domain without AAA infrastructure . . . 36
C.5. Other use cases . . . . . . . . . . . . . . . . . . . . . 36
C.5.1. CoAP-EAP for network access control . . . . . . . . . 36
C.5.2. CoAP-EAP for service authentication . . . . . . . . . 37
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 37
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
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 an
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] in an MSK for this specification. The MSK is used as the
basis for further cryptographic derivations. This way, CoAP messages
are protected after authentication. After CoAP-EAP's operation, an
OSCORE security association is established between the 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. One of the main
applications of CoAP-EAP is Internet of Things (IoT) networks, where
we can find very constrained links (e.g., limited bandwidth) and
devices with limited capabilities. In these IoT scenarios, we
identify the IoT device as the entity that wants to be authenticated
by using EAP to join a domain that is managed by a Controller. The
IoT device is in these cases the EAP peer and the Controller, the
entity steering the authentication, the EAP authenticator. From now
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on, the IoT device is referred to as the EAP peer and the Controller
as the EAP authenticator. In these cases, EAP methods that do not
require many exchanges, have short messages and use cryptographic
algorithms that are manageable by constrained devices are preferable.
The benefits of the EAP framework in IoT are highlighted in
[EAP-framework-IoT].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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. In
this architecture, the EAP peer will act as a CoAP server for this
service, and the domain EAP authenticator as a CoAP client. The
rationale behind this decision is that EAP requests direction is
always from the EAP server to the EAP peer. Accordingly, EAP
responses direction is always from the EAP peer to the EAP server.
It is worth noting that the 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, or WebSockets [RFC8323].
+----------+ +--------------+ +----------+
| EAP | | EAP | | AAA/ |
| peer |<------>| authenticator|<---------->|EAP Server|
+----------+ CoAP +--------------+ AAA +----------+
(Optional)
<---(SCOPE OF THIS DOCUMENT)---->
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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
+-------------------------------+
Figure 2: CoAP-EAP Stack
3. CoAP-EAP Operation
Because CoAP-EAP uses reliable delivery defined in CoAP ([RFC7252],
[RFC8323]), EAP retransmission time is set to infinite as mentioned
in [RFC3748]. To keep the ordering guarantee, CoAP-EAP uses
Hypermedia as the Engine of Application State (HATEOAS). Each step
during the EAP authentication accesses a new resource in the CoAP
server (EAP peer). The previous resource is removed once the new
resource is created, indicating the resource that will process the
next 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 from a large
variety of authentication methods.
In CoAP-EAP, the EAP peer will only have one authentication session
with a specific EAP authenticator and it will not process any other
EAP authentication in parallel (with the same EAP authenticator).
That is, a single ongoing EAP authentication is permitted for the
same EAP peer and the same EAP authenticator. It may be worth noting
that the EAP authenticator may have parallel EAP sessions with
multiple EAP peers.
To access the authentication service, this document defines the well-
known URI "coap-eap" (to be assigned by IANA)
3.1. Discovery
Before the CoAP-EAP exchange takes place, the EAP peer needs to
discover the EAP authenticator or the entity that will enable the
CoAP-EAP exchange (e.g., an intermediary proxy). The discovery
process is out of the scope of this document.
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The CoAP-EAP application can be accessed through the 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 also be available in the resource directory
[RFC9176].
Implementation Notes: There are different methods to discover the
IPv6 address of the EAP authenticator or intermediary entity. For
example, on a 6LoWPAN network, the Border Router will typically act
as the EAP authenticator hence, after receiving the Router
Advertisement (RA) messages from the Border Router, the EAP peer may
engage on the CoAP-EAP exchange. Different protocols can be used to
discover the IP of the EAP authenticator. 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 EAP peer knows the EAP authenticator implements the CoAP-EAP
service. A CoAP-EAP message has a media type application/coap-eap,
See Section 9.5.
The steps of the operation are as follows:
* Step 0. The EAP peer MUST start the CoAP-EAP 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 is the only
message where the EAP authenticator acts as a CoAP server and the
EAP peer as a CoAP client. The message also includes a URI in the
payload of the message to indicate the resource where the EAP
authenticator MUST send the next message. The name of the
resource is selected by the CoAP server.
Implementation notes: When generating the URI for a resource of a
step of the authentication, the resource could have the following
format "path/eap/counter", where:
* "path" is some local path on the device to make the path unique.
This could be omitted if desired.
* "eap" is the name that indicates the URI is for the EAP peer.
This has no meaning for the protocol but helps with debugging.
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* "counter' which is an incrementing unique number for every new EAP
request.
So, in Figure 3 for example, the URI for the first resource would be
“a/eap/1"
* Step 1. The EAP authenticator sends a POST message to the
resource indicated in Step 0 (e.g., '/a/eap/1'). The payload in
this message contains the first EAP message (EAP Request/
Identity), the Recipient ID of the EAP authenticator (RID-C) for
OSCORE, and MAY contain a CBOR array with a list of cipher suites
(CS) for OSCORE. If the cipher suite list is not included, the
default cipher suite for OSCORE is used. The details of the
cipher suite negotiation are discussed in Section 6.1.
* Step 2. The EAP peer processes the POST message sending the EAP
request (EAP-Req/Id) to the EAP peer state machine, which returns
an EAP response (EAP Resp/Id). Then, assigns a new resource to
the ongoing authentication process (e.g., '/a/eap/2'), and deletes
the previous one ('/a/eap/1'). 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 EAP peer (RID-I) and the
selected cipher suite for OSCORE are included in the payload. In
this step, the EAP peer MAY create an OSCORE security context (see
Section 6.2). The required Master Session Key (MSK) will be
available once the EAP authentication is successful in step 7.
* Steps 3-6. From now on, the EAP authenticator and the EAP peer
will exchange EAP packets related to the EAP method (EAP-X),
transported in the CoAP message payload. The EAP authenticator
will use the POST method to send EAP requests to the EAP peer.
The EAP peer 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/eap/1') 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/eap/3') for processing the next
EAP request and the ongoing resource (e.g., '/a/eap/2') 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.
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* Step 7. When the EAP authentication ends with success, the EAP
authenticator obtains the Master Session Key (MSK) exported by the
EAP method, an EAP Success message, and some authorization
information (e.g., session lifetime) [RFC5247]. The EAP
authenticator creates the OSCORE security context using the MSK
and Recipient ID of both entities exchanged in Steps 1 and 2. The
establishment of the OSCORE Security Context is defined in
Section 6.2. Then, the EAP authenticator sends the POST message
protected with OSCORE for key confirmation including the EAP
Success. The EAP authenticator MAY also send a Session Lifetime,
in seconds, which is represented with an unsigned integer in a
CBOR object (see Section 5). If this Session Lifetime is not
sent, the EAP peer 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 EAP
peer) sent in Step 2 is considered by the EAP peer as an alternate
indication of success ([RFC3748]). The EAP peer state machine in
the EAP peer interprets the alternate indication of success
similarly to the arrival of an EAP Success and returns the MSK,
which is used for the OSCORE security context in the EAP peer to
process the protected POST message received from the EAP
authenticator.
* Step 8. If the EAP authentication and the verification of the
OSCORE-protected POST in Step 7 is successful, then the EAP peer
answers with an OSCORE-protected '2.04 Changed'. From this point
on, communication with the last resource (e.g., '/a/eap/(n)') 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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EAP peer EAP authenticator
------------- ------------
| POST /.well-known/coap-eap |
0)| No-Response |
| Payload("/a/eap/1") |
|---------------------------------------->|
| POST /a/eap/1 |
| Payload(EAP Req/Id||CS||RID-C) |
1)|<----------------------------------------|
| 2.01 Created Location-Path [/a/eap/2] |
| Payload(EAP Resp/Id||CS||RID-I) |
2)|---------------------------------------->|
| POST /a/eap/2 |
| Payload(EAP-X Req) |
3)|<----------------------------------------|
| 2.01 Created Location-Path [/a/eap/3] |
| Payload(EAP-X Resp) |
4)|---------------------------------------->|
....
| POST /a/eap/(n-1) |
| Payload(EAP-X Req) |
5)|<----------------------------------------|
| 2.01 Created Location-Path [/a/eap/(n)] |
| Payload (EAP-X Resp) |
6)|---------------------------------------->|
| | MSK
| POST /a/eap/(n) | |
| OSCORE | V
| Payload (EAP Success||*Session-Lifetime)| OSCORE
MSK 7)|<----------------------------------------| CTX
| | |
V | 2.04 Changed |
OSCORE | OSCORE |
CTX 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 EAP peer 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
default value of 8 hours to refresh the keying material. Certain EAP
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methods such as EAP-NOOB [RFC9140] or EAP-AKA' [RFC5448] provide fast
reconnect for quicker re-authentication. The EAP re-authentication
protocol (ERP) [RFC6696] MAY also be used to avoid the repetition of
the entire EAP exchange.
The re-authentication message flow 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, the current CoAP-EAP state is deleted and
replaced by the new CoAP-EAP state. If, for any reason, the re-
authentication fails, 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 EAP peer to decide
when to start a new re-authentication before the current EAP state
expires.
3.4. Managing the State of the Service
The EAP peer and the EAP authenticator keep 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/
eap/2')
* The MSK is exported when the EAP authentication is successful.
CoAP-EAP can access the different variables by the EAP state
machine (i.e., [RFC4137]).
* A reference to the OSCORE context.
Once created, the EAP authenticator MAY choose to delete the state as
described in Figure 4. Conversely, the EAP peer may need to renew
the CoAP-EAP state because the key material is close to expiring, 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 forced removal,
the EAP peer needs to be expelled from the security domain. This
exchange is illustrated in Figure 4.
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If the EAP authenticator deems it necessary to remove the CoAP-EAP
state from the EAP peer before it expires, it can send a DELETE
command in a request to the EAP peer, referencing the last CoAP-EAP
state resource given by the CoAP server, whose identifier will be the
last one received (e.g., '/a/eap/(n)' in Figure 3). This message is
protected by the OSCORE security association to prevent forgery.
Upon reception of this message, the CoAP server sends a response to
the EAP authenticator with the Code '2.02 Deleted', which is also
protected by the OSCORE security association. If a response from the
EAP peer does not arrive after EXCHANGE_LIFETIME the EAP
authenticator will remove the state.
EAP peer EAP authenticator
------------- -------------
| |
| DELETE /a/eap/(n) |
| 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 an initial POST message arriving out of place.
3.5.1. EAP authentication failure
EAP authentication MAY fail in different situations (e.g., wrong
credentials). The result is that the EAP authenticator will send an
EAP Failure message because of the EAP authentication (Step 7 in
Figure 3). In this case, the EAP peer 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 yet generated.
If the EAP authentication fails during the re-authentication and the
EAP authenticator 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 EAP authenticator
assumes that the EAP peer 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. By default, CoAP-EAP adopts the value of
EXCHANGE_LIFETIME as a timer in the EAP peer and Authenticator to
remove the CoAP-EAP state if the authentication process has not
progressed in that time, in consequence, it has not been completed.
The EAP peer will remove the CoAP-EAP state either if the
EXCHANGE_LIFETIME is triggered, or the EAP peer state machine returns
an eapFail.
The EAP authenticator will remove the CoAP-EAP state either if the
EXCHANGE_LIFETIME is triggered, or, when the EAP authenticator is
acting in pass-through mode (i.e., the EAP authentication is
performed against a AAA server), the EAP authenticator state machine
returns an aaaTimemout.
3.5.3. Duplicated message with /.well-known/coap-eap
The reception of the trigger message in Step 0 containing the URI
/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 EAP authenticator during
an ongoing authentication with the same EAP peer, the EAP
authenticator MUST silently discard this trigger message.
If an old "POST /.well-known/coap-eap" (Step 0) arrives at the EAP
authenticator and there is no authentication ongoing, the EAP
authenticator may understand that a new authentication process is
requested. Consequently, the EAP authenticator will start a new EAP
authentication. However, the EAP peer did not start any
authentication and therefore, it did not select any resource for the
EAP authentication. Thus, the EAP peer sends a '4.04 Not found' in
the response (Figure 5).
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EAP peer EAP authenticator
---------- ----------
| *POST /.well-known/coap-eap |
0) | No-Response |
| Payload("/a/eap/1") |
| ------------------------->|
| POST /a/eap/1 |
| 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
3.6. Proxy operation in CoAP-EAP
The CoAP-EAP operation is intended to be compatible with the use of
intermediary entities between the EAP peer and the EAP authenticator
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.
When using proxies in the CoAP-EAP we need to consider that the
exchange contains a role-reversal process at the beginning of the
exchange. In the first message, the EAP peer acts as a CoAP client
and the EAP authenticator as the CoAP server. After that, in the
remaining exchanges the roles are reversed, being the EAP peer, the
CoAP server, and the EAP authenticator, the CoAP client. When using
a proxy in the exchange, for message 0, the proxy will act as
forward, and as reverse for the rest. Additionally, in the first
exchange, the EAP peer as a CoAP client, sends the URI for the next
CoAP message in the payload of a request. This is not the typical
behavior, as URIs referring to new services/resources appear in
Location-Path and/or Location-Query Options in CoAP responses.
Hence, the proxy will have to treat the payload of message 0, as if
it were a Location-Path Option of a CoAP response.
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4. CoAP-EAP Media type format
In the CoAP-EAP exchange, we will have the following format. This is
the format is specified by application/coap-eap media type, see
Section 9.5.
In CoAP-EAP we have two different elements that can be sent over the
payload. The first one is a relative URI. This payload will be
present for the first message (0) in Figure 3.
In all the other cases, we will find and EAP message followed by the
CBOR Object specified in Section 5. A visual example of the second
case can be found in Figure 7.
5. CBOR Objects in CoAP-EAP
In the CoAP-EAP exchange, there is information that needs to be
exchanged between the two entities. Examples of this information are
the cipher suites that need to be negotiated or authorization
information (Session-lifetime). There may also be a need to extend
the information that has to be exchanged in the future. This section
specifies the CBOR [RFC8949] data structure to exchange information
between the EAP peer and the EAP authenticator in the CoAP payload.
Figure 6 shows the specification of the CBOR Object to exchange
information in CoAP-EAP
CoAP-EAP_Info = {
? 1 : [+ int/tstr], ; 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 including 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 agreed-upon
ciphersuites.
2. RID-I: It contains the Recipient ID of the EAP peer. The EAP
authenticator uses this value as a Sender ID for its OSCORE
Sender Context. The EAP peer uses this value as Recipient ID for
its Recipient Context.
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3. RID-C: It contains the Recipient ID of the EAP authenticator.
The EAP peer uses this value as a Sender ID for its OSCORE Sender
Context. The EAP authenticator uses this value as Recipient ID
for its Recipient Context.
4. Session-Lifetime: Contains the time the session is valid, in
seconds.
Other indexes can be used to carry additional values as needed, like
specific authorization parameters.
The indexes from 65000 to 65535 are reserved for experimentation.
6. Cipher suite negotiation and key derivation
6.1. 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
EAP authenticator 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 EAP peer sends a response with
the choice.
This list is included in the payload after the EAP message through a
CBOR as explained previously. 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 5, containing the cipher suite field 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/eap/1 |
| Payload (EAP Req/Id, CBORArray[0,1,2]) |
1) |<----------------------------------------|
| 2.01 Created Location-Path [/a/eap/2] |
| 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 EAP authenticator 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
EAP peer will have at least that option available.
The cipher suite requirements are inherited from the ones established
by OSCORE [RFC8613], which are COSE algorithms [RFC9053]. By
default, the HKDF algorithm is SHA-256 and the AEAD algorithm is AES-
CCM-16-64-128 [RFC9053]. Both are mandatory to implement. The other
supported and negotiated cipher suites 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. Why the use of this funtion is enough, and
it is not necessary to use KDF-Extract is explained in Section 8.1.
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6.2. Deriving the OSCORE Security Context
The derivation of the security context for OSCORE allows securing the
communication between the EAP peer and the EAP authenticator once the
MSK has been exported, providing confidentiality, integrity, key
confirmation (Steps 7 and 8), and downgrading attack detection.
The Master Secret can be derived by using the chosen cipher suite and
the KDF as follows:
Master Secret = KDF(MSK, CS | "COAP-EAP OSCORE MASTER SECRET", length)
where:
* The algorithms for OSCORE agreed in the cipher suite negotiation.
* The MSK exported by the EAP method. Discussion about the use of
the MSK for key derivation is done in Section 8.
* CS is the concatenation of the content of the cipher suite
negotiation, that is, the list of cipher suites sent by the EAP
authenticator (Step 1) to the selected option by the EAP peer
(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 8.
* 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.
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* "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 EAP authenticator and the EAP peer 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, which ends up with a failure in Steps 7 and 8.
The EAP authenticator will use the Recipient ID of the EAP peer (RID-
I) as the Sender ID for its OSCORE Sender Context. The EAP peer will
use this value as Recipient ID for its Recipient Context.
The EAP peer will use the Recipient ID of the EAP authenticator (RID-
C) as the Sender ID for its OSCORE Sender Context. The EAP
authenticator will use this value as Recipient ID for its Recipient
Context.
7. Discussion
7.1. CoAP as EAP lower layer
This section discusses the suitability of the CoAP protocol as EAP
lower layer and reviews the requisites imposed by EAP on any protocol
transporting EAP. What EAP expects from its lower layers can be
found in 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., using Confirmable
messages).
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.
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Minimum MTU. Lower layers need to provide an EAP MTU size of 1020
octets or greater. CoAP assumes an upper bound of 1024 octets for
its payload, which covers the EAP requirements when in the CoAP
payload only the EAP message is sent. In the case of Messages 1 and
2 in Figure 3, those messages have extra information apart from EAP.
Nevertheless, the EAP Req/Id has a fixed length of 5 bytes. Message
2 with the EAP Resp/Id, would limit the length of the identity being
used to the CoAP payload maximum size (1024) - len( CS || RID-I ) -
EAP Response header (5 bytes), which leaves enough space for sending
even lengthy identities. Nevertheless, this limitation can be
overcome by using CoAP block-wise transfer[RFC7959]. Note: When EAP
is proxied over an AAA framework, the Access-Request packets in
RADIUS MUST contain a Framed-MTU attribute with the value 1024, and
the Framed-MTU AVP to 1024 in DIAMETER This attribute signals the AAA
server that it should limit its responses to 1024 octets.
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 EAP peer,
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 options. 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 provides a
non-duplicated stream of packets and accomplishes 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."
7.2. Size of the EAP lower layer vs EAP method size
Regarding the impact that an EAP lower layer will have on the number
of bytes of the whole authentication exchange, there is a comparison
with another network layer-based EAP lower layer, PANA [RFC5191], in
[coap-eap].
The EAP method being transported will take most of the exchange,
however, the impact of the EAP lower layer cannot be ignored,
especially in very constrained communication technologies, such as
the ones we can find in IoT, with limited capabilities.
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Note: For constrained devices and network scenarios, the use of the
latest versions of EAP methods (EAP-TLS 1.3 [RFC9190]) is recommended
in favor of older versions with the goal of economization.
8. Security Considerations
There are some security aspects to be considered such as how
authorization is managed, the use of MSK as key material, and how
trust in the EAP authenticator is established. Additional
considerations such as EAP channel binding as per [RFC6677] are also
discussed here.
8.1. Use of EAP Methods
This documents limits the use of EAP methods to the ones compliant
with [RFC4017] specification, yielding strong and fresh session keys
such as the MSK. By this assumption, the HKDF-Expand function is
used directly, as clarified in [RFC5869].
8.2. Authorization
Authorization is part of bootstrapping. It serves to establish
whether the EAP peer can join and the set of conditions it must
adhere to. The authorization data will be gathered from the
organization that is responsible for the EAP peer and sent to the EAP
authenticator in case AAA infrastructure is deployed.
In standalone mode, the authorization information will be in the EAP
authenticator. 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
may be needed to operate in the security domain. This can be taken
care of by the solutions proposed in the ACE WG such as the use of
OAuth [RFC9200], among other solutions, to provide Authentication and
Authorization for Constrained Environments.
8.3. Allowing IP traffic connectivity to perform authentication
Since CoAP is an application protocol, CoAP-EAP assumes certain IP
connectivity in the link between the EAP peer and the EAP
authenticator to run. This link MUST authorize exclusively
unprotected IP traffic to be sent between the EAP peer and the EAP
authenticator during the authentication with CoAP-EAP. Once the
authentication is successful, the key material generated by the EAP
authentication (MSK) and any other traffic can be authorized if it is
protected. It is worth noting that if the EAP authenticator is not
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in the same link as the EAP peer and intermediate entity help with
this process (i.e., CoAP proxy) and the same comment applies to the
communication between the EAP peer and the intermediary.
8.4. 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 to generate 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.
8.5. Channel Binding support
According to the [RFC6677], channel binding related to EAP, is sent
through the EAP method supporting 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.
8.6. Additional Security Considerations
In the authentication process, there is a possibility of an entity
forging messages to generate 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 EAP authenticator as if they were sent by different EAP peers.
Consequently, the EAP authenticator will start an authentication
process for 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 EAP authenticator 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 EAP peer and the EAP
authenticator. 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 EAP authenticator at least until
the EAP Response/Id is received by the EAP authenticator.
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Another security-related concern is how to ensure that the EAP peer
joining the security domain can trust the EAP authenticator. This
issue is elaborated in the EAP Key Management Framework [RFC5247].
In particular, the EAP peer knows it can trust the EAP authenticator
because the key that is used to establish the security association is
derived from the MSK. If the EAP authenticator has the MSK, it is
because the AAA Server of the EAP peer trusted the EAP authenticator.
9. IANA Considerations
This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding the registration of values related to
CoAP-EAP.
9.1. 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
* Array: N/A
* Description: Reserved for Private Use
* Value: 0
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* 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
9.2. 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
* 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 EAP peer
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* Value: 3
* Name: RID-I
* Description: It contains the Recipient ID of the EAP authenticator
* 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
9.3. 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 "Specification Required".
* URI suffix: coap-eap
* Change controller: IETF
* Specification document(s): [[this document]]
* Related information: None
* Status: permanent
9.4. 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]]
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9.5. Media Types Registry
IANA has added the media types "application/coap-eap" to the "Media
Types" registry. The registration procedure is "Expert Review".
Section 4 defines the format.
* Type name: application
* Subtype name: coap-eap
* Required parameters: N/A
* Optional parameters: N/A
* Encoding considerations: binary
* Security considerations: See Section 8 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.
* Intended usage: COMMON
* Restrictions on usage: N/A
* Author: See "Authors' Addresses" section.
* Change Controller: IESG
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9.6. 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" following the specification in
Section 12.3 of [RFC7252]. The registration procedure is "IETF
Review".
+-----------------------+----------+------+-------------------+
| Media Type | Encoding | ID | Reference |
+-----------------------+----------+------+-------------------+
| application/coap-eap | - | TBD | [[this document]] |
+-----------------------+----------+------+-------------------+
Figure 9: CoAP Content-Formats Registry
9.7. 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".
* Value: "core.coap-eap"
* Description: CoAP-EAP resource.
* Reference: [[this document]]
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>.
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[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>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[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>.
[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",
2016, <https://www.mdpi.com/1424-8220/16/3/358>.
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[EAP-framework-IoT]
Sethi, M., "Secure Network Access Authentication for IoT
Devices: EAP Framework vs. Individual Protocols", 2021,
<https://ieeexplore.ieee.org/document/9579387>.
[I-D.ingles-eap-edhoc]
Sanchez, E. I., Garcia-Carrillo, D., Marin-Lopez, R.,
Selander, G., and J. P. Mattsson, "Using the Extensible
Authentication Protocol with Ephemeral Diffie-Hellman over
COSE (EDHOC)", Work in Progress, Internet-Draft, draft-
ingles-eap-edhoc-05, 12 September 2023,
<https://datatracker.ietf.org/doc/html/draft-ingles-eap-
edhoc-05>.
[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",
2017, <https://www.mdpi.com/1424-8220/17/11/2646>.
[RFC4017] Stanley, D., Walker, J., and B. Aboba, "Extensible
Authentication Protocol (EAP) Method Requirements for
Wireless LANs", RFC 4017, DOI 10.17487/RFC4017, March
2005, <https://www.rfc-editor.org/info/rfc4017>.
[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>.
[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>.
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[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>.
[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>.
[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>.
[RFC9031] Vučinić, M., Ed., Simon, J., Pister, K., and M.
Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH",
RFC 9031, DOI 10.17487/RFC9031, May 2021,
<https://www.rfc-editor.org/info/rfc9031>.
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[RFC9053] Schaad, J., "CBOR Object Signing and Encryption (COSE):
Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
August 2022, <https://www.rfc-editor.org/info/rfc9053>.
[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>.
[RFC9190] Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
Extensible Authentication Protocol with TLS 1.3",
RFC 9190, DOI 10.17487/RFC9190, February 2022,
<https://www.rfc-editor.org/info/rfc9190>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/info/rfc9200>.
[THREAD] Thread Group, "Thread specification 1.3", 2023.
[TS133.501]
ETSI, "5G; Security architecture and procedures for 5G
System - TS 133 501 V15.2.0 (2018-10)", 2018.
[ZigbeeIP] Zigbee Alliance, "ZigBee IP Specification (Zigbee Document
095023r34)", 2014.
Appendix A. Flow of operation (DTLS establishment)
CoAP-EAP makes it possible to derive a PSK from the MSK to allow
(D)TLS PSK-based authentication between the EAP peer and the EAP
authenticator instead of using OSCORE. In the case of using (D)TLS
to establish a security association, there is a limitation on the use
of intermediaries between the EAP peer and the EAP authenticator, as
(D)TLS breaks the end-to-end communications when using intermediaries
such as proxies.
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EAP peer EAP authenticator
------------- -------------
...
| 2.01 Created Location-Path [/a/eap/(n)] |
| 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 EAP peer and
the EAP authenticator 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 EAP peer is
granted access to the domain. Step 7 MUST be interpreted by the EAP
peer 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 exchanged 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.1.
Analogously to how the cipher suite is negotiated for OSCORE
Section 6.1, the EAP authenticator sends a list, in decreasing order
of preference, with the identifiers of the hash algorithms supported
(Step 1). In the response, the EAP peer 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 5). An example of
how the fields are arranged in the CoAP payload can be seen in
Figure 7.
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In case there is no CBOR array stating the cipher suites, the default
cipher suites are applied. If the EAP authenticator 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
EAP peer will have at least that option available.
For DTLS, the negotiated cipher suite is used to determine the hash
function to be used to derive the “DTLS PSK” from the MSK:
The hash algorithms considered are the following:
* 5) TLS_SHA256
* 6) TLS_SHA384
* 7) TLS_SHA512
The inclusion of any of these values, apart from indicating the hash
algorithm, indicates if the EAP authenticator intends to establish an
OSCORE security association or a DTLS security association after the
authentication is completed. If both options appear in the cipher
suite negotiation, the OSCORE security association will be preferred
over DTLS.
A.1. Deriving DTLS PSK and identity
To enable DTLS after an EAP authentication, 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],
referred to here as "DTLS PSK", from the MSK between both the EAP
peer and EAP authenticator 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:
* 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).
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* length is the size of the output key material.
Appendix B. Using CoAP-EAP for distributing key material for IoT
networks
Similarly, to the example of Appendix A.1, where a shared key PSK for
DTLS is derived, it is possible to provide key material to different
link-layers after the CoAP-EAP authentication is complete.
One example is that we could use CoAP-EAP to derive the required PSK
required to run the 6TiSCH Constrained Join Protocol (CoJP)
[RFC9031].
Another example is when a shared Network Key is required by the
devices that join a network. An example of this Network Key can be
found in ZigBee IP [ZigbeeIP] or THREAD protocol [THREAD]. After
CoAP-EAP execution a security association based on OSCORE protects
any exchange between the EAP peer and the EAP authenticator. This
security association can be used for distributing the Network Key
securely and other required parameters. How the Network Key is
distributed after a successful CoAP-EAP authentication is out of the
scope of this document.
How a particular link-layer technology uses the MSK to derive further
key material for protecting the link-layer or use the OSCORE
protection to distribute key material is out of the scope of this
document.
Appendix C. Examples of Use Case Scenario
In IoT, for an EAP peer to act as a trustworthy entity within a
security domain, certain key material needs to be shared between the
EAP peer and the EAP authenticator.
Next, we elaborate on examples of different use case scenarios about
the usage of CoAP-EAP.
Generally, we are dealing with 4 entities:
* 2 EAP peers (A and B), which are EAP peers. They are the EAP
peers.
* 1 EAP authenticator (C). The EAP authenticator manages a domain
where EAP peers can be deployed. In IoT, it can be considered a
more powerful machine than the EAP peers.
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* 1 AAA server (AAA) - Optional. The AAA is an Authentication,
Authorization, and Accounting Server, which is not constrained.
Here, the EAP authenticator acts as an EAP authenticator in pass-
through mode.
Generally, any EAP peer wanting to join the domain managed by the EAP
authenticator MUST perform a CoAP-EAP authentication with the EAP
authenticator (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 EAP peers 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 EAP peers 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.
C.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 EAP peer and the EAP authenticator.
In this instance, the EAP authenticator and the Authorization Server
(AS) of ACE can be co-located.
Next, we exemplify how CoAP-EAP can be used to perform Client
registration in a general way, to allow two EAP peers (A and B) to
communicate and interact after a successful client registration.
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EAP peer A wants to communicate with EAP peer B (e.g., to activate a
light switch). The overall process is divided into three phases.
Let's start with EAP peer A. In the first phase, EAP peer A does not
yet belong to EAP authenticator C's domain. Then, it communicates
with C 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 EAP peer A. This key material allows EAP peer 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, EAP peer A has been
enrolled in the EAP authenticator C's domain for some time. In
particular, [RFC5247] recommends 8 hours, though 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 EAP authenticator C's domain.
In the second phase, when EAP peer A wants to talk with EAP peer B,
it contacts EAP authenticator C for authorization to access EAP peer
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
the scope of this document, and the ACE framework is used for this
purpose [RFC9200].
In the third phase, EAP peer A can access EAP peer B with the
credentials and information obtained from EAP authenticator C in the
second phase. This access can be repeated without contacting the EAP
authenticator, while the credentials given to A are still valid. The
details of this phase are out of the scope of this document.
It is worth noting that the first phase with CoAP-EAP is required to
join the EAP authenticator C's domain. Once it is performed with
success, the communications are local to the EAP authenticator 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 EAP peer can engage in a re-authentication as explained
in Section 3.3, to renew the key material.
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C.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 EAP authenticator's domain, which in turn
communicates with an AAA infrastructure (acting as AAA client).
Through the local AAA server communicate with the home AAA server to
complete the authentication and integrate the device as a trustworthy
entity into the domain of EAP authenticator C. In this scenario, the
AS under the role of the EAP authenticator receives the key material
from the AAA infrastructure
C.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 EAP peers under
an AS. All buildings belong to the same domain (e.g., um.es). All
these buildings are managed with AAA infrastructure. A new device
(A) with credentials from the domain (e.g., um.es) will be able to
perform the device registration with an EAP authenticator (C) of any
building if they are managed by the same general domain.
C.4. Example 4: Single domain without AAA infrastructure
In another case, without a AAA infrastructure, we have an EAP
authenticator 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 an EAP peer (A) with Controller (C)
can also be performed in the same manner.
C.5. Other use cases
C.5.1. CoAP-EAP for network access control
One of the first steps for an EAP peer's life cycle is to perform the
authentication to gain access to the network. To do so, the device
first must 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 EAP peer 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 EAP
peers deployed under the same security domain, belong to different
organizations.
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Given that EAP is also used for network access control, we can adapt
this service to other technologies. For instance, to provide network
access control to very constrained technologies (e.g., LoRa network).
Authors in [lo-coap-eap] provide a 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].
C.5.2. CoAP-EAP for service authentication
It is not uncommon that the infrastructure where the device is
deployed and the services of the EAP peer 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 primary and secondary authentication
using EAP [TS133.501].
Acknowledgments
We would like to thank the reviewers of this work: Paul Wouters,
Heikki Vatiainen, Josh Howlett, Deb Cooley, Eliot Lear, Alan DeKok,
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.
And thank for their valuable comments to Alexander Pelov and Laurent
Toutain, especially for the potential optimizations of CoAP-EAP.
Authors' Addresses
Rafa Marin-Lopez
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
30100 Murcia
Spain
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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