EAP-based Authentication Service for CoAP
draft-ietf-ace-wg-coap-eap-00
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draft-ietf-ace-wg-coap-eap-00
ACE Working Group R. Marin
Internet-Draft University of Murcia
Intended status: Standards Track D. Garcia
Expires: August 26, 2021 University of Oviedo
February 22, 2021
EAP-based Authentication Service for CoAP
draft-ietf-ace-wg-coap-eap-00
Abstract
This document describes an authentication service that uses EAP
transported by means of CoAP messages with the following purposes:
o Authenticate a CoAP-enabled device that enters a new security
domain managed by a domain Controller.
o Derive key material to protect CoAP messages exchanged between
them, enabling the establishment of a security association between
them.
o Optionally, to generate key material for other types of Security
Associations.
Generally speaking, this document is specifying an EAP lower layer
based on CoAP, to bring the benefits of EAP to IoT.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on August 26, 2021.
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Copyright Notice
Copyright (c) 2021 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. General Architecture . . . . . . . . . . . . . . . . . . . . 4
3. General Flow Operation . . . . . . . . . . . . . . . . . . . 4
3.1. EAP over CoAP flow of operation . . . . . . . . . . . . . 5
3.2. The SeqNum Option . . . . . . . . . . . . . . . . . . . . 8
4. Key Derivation for protecting CoAP messages . . . . . . . . . 9
4.1. Deriving the OSCORE Security Context . . . . . . . . . . 9
4.2. Deriving DTLS_PSK . . . . . . . . . . . . . . . . . . . . 11
5. Examples of Use Case Scenario . . . . . . . . . . . . . . . . 11
5.1. Example 1: CoAP-EAP in ACE . . . . . . . . . . . . . . . 12
5.2. Example 2: Multi-domain with AAA infrastructures . . . . 13
5.3. Example 3: Single domain with AAA infrastructure . . . . 13
5.4. Example 4: Single domain without AAA infrastructure . . . 13
5.5. Other use cases . . . . . . . . . . . . . . . . . . . . . 13
5.5.1. CoAP-EAP for network access control . . . . . . . . . 13
5.5.2. CoAP-EAP for service authentication . . . . . . . . . 14
6. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1. CoAP as EAP lower-layer . . . . . . . . . . . . . . . . . 14
6.2. Need for SeqNum Option . . . . . . . . . . . . . . . . . 15
6.3. Size of the EAP lower-layer vs EAP method size . . . . . 15
6.4. Controller as the CoAP Client . . . . . . . . . . . . . . 16
6.5. Possible Optimizations . . . . . . . . . . . . . . . . . 16
6.5.1. Empty Token . . . . . . . . . . . . . . . . . . . . . 16
6.5.2. Removing SeqNum Option . . . . . . . . . . . . . . . 16
6.5.3. Further re-authentication . . . . . . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7.1. Authorization . . . . . . . . . . . . . . . . . . . . . . 18
7.2. Cryptographic suite selection . . . . . . . . . . . . . . 18
7.3. Freshness of the key material . . . . . . . . . . . . . . 18
7.4. Additional Security Consideration . . . . . . . . . . . . 18
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8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
10.1. Normative References . . . . . . . . . . . . . . . . . . 19
10.2. Informative References . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21
1. Introduction
The goal of this document is to describe an authentication service
that uses the Extensible Authentication Protocol (EAP) [RFC3748].
The authentication service is built on top of the Constrained
Application Protocol (CoAP) [RFC7252] and allows authenticating two
CoAP endpoints by using EAP without the need of additional protocols
to establish a security association between them.
In particular, the document describes how CoAP can be used as a
constrained, link-layer independent, EAP lower-layer [RFC3748] to
transport EAP between a CoAP server (EAP peer) and a CoAP client (EAP
authenticator) using CoAP messages. The CoAP client MAY contact with
a backend AAA infrastructure to complete the EAP negotiation as
described in the EAP specification [RFC3748].
The assumption is that the EAP method transported in CoAP MUST
generate cryptographic material [RFC5247]. In this way, the CoAP
messages can be protected. The general flow of operation of CoAP-EAP
establishes an OSCORE security association specifically for the
service. In addition, using the key material derived from the
authentication we specify the establishment of other security
associations depending on the security requirements of the services:
o OSCORE [RFC8613] security association can be established based on
the cryptographic material generated from the EAP authentication.
o A DTLS security association can be established using the exported
cryptographic material after a successful EAP authentication.
[I-D.ohba-core-eap-based-bootstrapping]
This document also provides comments on how to establish a security
association for other types of technologies that rely on CoAP.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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2. General Architecture
Figure 1 shows the architecture defined in this document. Basically,
a node acting as the EAP peer wants to be authenticated by using EAP.
At the time of writing this document, we have considered a model
where the EAP peer will act as CoAP server for this service and the
EAP authenticator will act as CoAP client and MAY interact with a
backend AAA infrastructure, which will place the EAP server and
contain the information required to authenticate the CoAP client.
The rationale behind this decision, as we will expand later, is that
EAP requests go always from the EAP authenticator to the EAP peer.
Accordingly, the EAP responses go from the EAP peer to the EAP
authenticator. Nevertheless, a model where the EAP peer acts as CoAP
client and the EAP authenticator as CoAP server can be also analyzed
in the future.
+------------+ +------------+
| EAP peer/ | | EAP auth./ |
| CoAP server|+------+| CoAP client|
+------------+ CoAP +------------+
Figure 1: CoAP EAP Architecture
3. General Flow Operation
The authentication service uses CoAP as transport layer for EAP. In
other words, CoAP becomes an EAP lower-layer (in EAP terminology).
In general, it is assumed that, since the EAP authenticator MAY
implement an AAA client to interact with the AAA infrastructure, this
endpoint will have more resources or, at least, be a not so
constrained device. We show the sequence flow in Figure 2 where we
depict the usage of a generic EAP method that we call EAP-X as
authentication mechanism. (NOTE: any EAP method which is able to
export cryptographic material is be valid. For example EAP-MD5
cannot be used since it does not export key material).
The first step to run CoAP-EAP is for the IoT device to discover the
Controller, and that it implements the CoAP-EAP service. To do so,
we rely on the discovery mechanism of CoAP. The URI of the CoAP-EAP
service, is set to "/b" to save bytes over the air. Alternatively,
the if the Controller is aware of the presence of the IoT device
(e.g., due to a previous authentication) this process can be avoided,
and the Controller can directly start the authentication process.
The first message that is used to trigger the authentication process
is sent by the IoT device, acting as CoAP client. This message uses
the No-Response Option [RFC7967] to avoid the response from the
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Controller to this message. After this, the exchange continues with
the Controller acting as CoAP client and the IoT device acting as
CoAP server. This is due to the fact that the IoT could be a
constrained node, and following the recommendations of
[I-D.ietf-lwig-coap] to simplify the implementation of the IoT
device, having the Controller the responsibility of handling the
retransmissions. In the next section, we refer to the IoT device as
the EAP peer and the Controller as the EAP authenticator to elaborate
the specifics of the flow of operation.
3.1. EAP over CoAP flow of operation
If the EAP peer discovers the presence of the EAP authenticator and
wants to start the authentication, it can send a Non-Confirmable
"POST /b" request to the node (Step 0). This message, will carry an
option developed from the work on [RFC7967] called no response. The
rationale of this option is to avoid waiting for a if it is not
needed. So the use of this option will allow signaling the intention
the EAP peer to start the authentication process, as a mechanism.
Immediately after that, the EAP authenticator will start
authentication service. It is worth noting that the EAP
authenticator MAY decide start the authentication without waiting for
the trigger if it has knowledge about the presence of the peer, for
instance, through a previous authentication.
In any case, to perform the authentication service, the CoAP client
(EAP authenticator) sends a Confirmable "POST /b" request to the CoAP
Server (Step 1). This POST message contains a new option SeqNum that
holds a sequence number randomly chosen by the CoAP client. This
SeqNum is used to provide ordered and reliable delivery of messages
involved during the whole authentication. In general, when a CoAP
request with EAP message is received, the CoAP client considers a
valid message if only if its sequence number is the expected value.
The sequence number is monotonically incremented by 1 so that the
CoAP server can know what it is the next expected sequence number.
After receiving the first POST, the CoAP server assigns a resource
and answers with an Acknowledgment with the piggy-backed resource
identifier (Uri-Path) (Step 2). It is assumed that the CoAP server
will only have an ongoing authentication and will not process
simultaneous EAP authentications in parallel to save resources. In
these two messages, the EAP Req/Id and Rep/ID are exchanged between
the EAP authenticator and the EAP peer. The EAP Req/Id message is
forwarded by the EAP authenticator, when EAP is in pass-through mode,
to the local AAA server that is in charge of steering the
conversation, choosing the EAP method to be used (e.g. EAP-X) if the
user is local or sending the EAP messages to the home AAA of the EAP
peer. At this point, the CoAP server has created a resource for the
EAP authentication. The resource identifier value will be used
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together to relate all the EAP conversation between both CoAP
endpoints. Since, only an ongoing EAP authentication is permitted
and EAP is a lock-step protocol a Token of a constant value and 1
byte can be used throughout the authentication process. This also
allows to save bytes through the link.
From now on, the EAP authenticator and the EAP peer will exchange EAP
packets related to the EAP method, transported in the CoAP message
payload (Steps 3,4,5,6). The EAP authenticator will use POST method
to send EAP requests to the EAP peer. The EAP peer will use a Piggy-
backed response in the Acknowledgment message to carry the EAP
response. At the end of the message exchanges, if everything has
gone well, the EAP authenticator is able to send an EAP Success
message and both CoAP endpoints will share a Master Session Key (MSK)
([RFC5295])
To establish a security association that will confirm to the EAP peer
that EAP authenticator received the MSK from the AAA sever, as well
as to the EAP authenticator that the EAP peer derived the MSK
correctly, both entities engage in the establishment of a security
association. In the context of constrained devices [RFC7228] and
networks we consider protocols that are designed for these cases.
Concretely, we show here in the diagram the establishment of the
OSCORE security association. This is shown in Steps 7 and 8. From
that point any exchange between both CoAP endpoints are protected
with OSCORE. Before sending the EAP success to the EAP peer, the EAP
authenticator is able to derive the OSCORE Security Context, to
confirm the establishment of the security association. The details
of the establishment of the OSCORE Security Context are discussed in
Section Section 4.1 The protection of the EAP Success is not a
requirement. In our case, we specify this exchange as protected by
the lower layer in this scenario with OSCORE. The purpose is double,
we can avoid forgery of this message and at the same time we are
using the exchange to perform the key confirmation through the
establishment of the OSCORE security association. Adding to the
previous consideration about the EAP Success, this message does not
preclude the operation of the device from continuing as long as there
is an alternate success indication that both the EAP peer and
authentication can rely on to continue [RFC3748]. This indication
can happen in two ways: 1) the reception of the a CoAP message
without EAP and with an OSCORE option (following the normal
operational communication between the both entities) is an indication
that the controller considers the EAP authentication finished. 2) the
IoT device is aware that the EAP authentication went well if an MSK
is available. In any case, both entities need to prove the
possession of the MSK as mentioned in the EAP KMF.
EAP peer EAP Auth.
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(CoAP server) (CoAP client)
------------- -------------
| |
| NON [0x6af5] |
| POST /b |
| No-Response |
0) | Token (0xab) |
|---------------------------------------->|
| |
| CON [0x7654] |
| POST /b |
| SeqNum(x) |
| Token (0xac) |
| Payload EAP Req/Id |
1) |<----------------------------------------|
| |
| ACK [0x7654] |
| SeqNum(x) |
| Token (0xac) |
| 2.01 Created |
| Uri-Path [/b/5] |
| Payload EAP Rep/Id |
2) |---------------------------------------->|
| |
| CON [0x8694] |
| POST /b/5 |
| SeqNum(x+1) |
| Token (0xac) |
| Payload EAP-X MSG 1 |
3) |<----------------------------------------|
| |
| ACK [0x8694] |
| Token (0xac) |
| SeqNum(x+1) |
| 2.04 Changed |
| Payload EAP-X MSG 2 |
4) |---------------------------------------->|
....
| POST /b/5 |
| SeqNum(x+n/2)|
| Token (0xac) |
| Payload EAP-X MSG (n-1) |
5) |<----------------------------------------|
| |
| ACK [0x9869] |
| SeqNum(x+n/2) |
| Token (0xac) |
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| 2.04 Changed |
| Payload EAP-X MSG (n) | MSK
6) |---------------------------------------->| |
| | V
| CON [0x7811] |OSCORE
| POST /b/5 |CONTEXT
| SeqNum(x+n/2+1) |
| Token (0xac) | (*)
| OSCORE Option |
| EAP success |
MSK 7) |<----------------------------------------|
| | |
V (*) | ACK [0x7811] |
OSCORE | SeqNum(x+n/2+1) |
CONTEXT | Token (0xac) |
| OSCORE Option |
| 2.04 Changed |
8) |---------------------------------------->|
(*) Protected with OSCORE
Figure 2: CoAP-EAP flow of operation
3.2. The SeqNum Option
A new SeqNum option is defined in this document for establishing the
ordering guarantee of the EAP exchange. Following guidelines in
[RFC7252] this option is:
1. Format opaque (sequence of bytes).
2. Critical
3. Safe to Forward
4. No cacheable and Not part of the Cache-Key
5. Not repeatable
The number of the option will be determined by this previous
decisions.
1. Critical (C = 1)
2. Safe to Forward (1)
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3. NoCacheKey (111)
The number of the SeqNum option will fit this pattern: xxx11111
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| | NoCacheKey| U | C |
+---+---+---+---+---+---+---+---+
Figure 3: SeqNum Option Number Mask
The option number is TBD.
The resultant SeqNum option is:
+-----+---+---+---+---+--------+--------+--------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+-----+---+---+---+---+--------+--------+--------+---------+
| TBD | x | | x | | SeqNum | uint | 0-16 | (none) |
+-----+---+---+---+---+--------+--------+--------+---------+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable
(*) See below.
Figure 4: SeqNum option
4. Key Derivation for protecting CoAP messages
As a result of a successful EAP authentication, both CoAP server and
CoAP client share a Master Key Session (MSK). The assumption is that
MSK is a fresh key so any derived key from the MSK will be also
fresh. To complete the CoAP-EAP exchange, as part of the design, the
establishment of an OSCORE security association specifically for the
CoAP-EAP service is expected. The security level for the CoAP-EAP
exchanges with OSCORE is with integrity. Additionally, we considered
the derivation of either the OSCORE Security Context or a pre-shared
key that can be used for a DTLS negotiation (DTLS_PSK) for further
communications depending of the security requirements of the services
provided by the AS. The CoAP-EAP OSCORE security context could be
generalized to enable further OSCORE secured communications between
the IoT device and the AS services that require the use of OSCORE.
4.1. Deriving the OSCORE Security Context
Key material needed to derive the OSCORE Security Context, from the
MSK can be done as follows. In this case, rest of CoAP exchanges
between both entities can be protected with OSCORE.
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The Master Secret can be derived by using AES-CMAC-PRF-128 [RFC4615],
which, in turn, uses AES-CMAC-128 [RFC4493]. The Master Secret can
be derived as follows:
Master_Secret = KDF(MSK, "IETF_OSCORE_MASTER_SECRET", 64, length)
where:
o The AES-CMAC-PRF-128 is defined in [RFC4615]. This function uses
AES-CMAC-128 as building block.
o The MSK exported by the EAP method, which by design is a fresh key
material. Discussions about the use of the MSK for the key
derivation are done in Section Section 7.
o "IETF_OSCORE_MASTER_SECRET" is the ASCII code representation of
the non-NULL terminated string (excluding the double quotes around
it).
o 64 is the length of the MSK.
o length is the length of the label "IETF_OSCORE_MASTER_SECRET" (25
bytes).
The Master Salt can be derived similarly to the Master Secret. The
Master Salt can be derived as follows:
Master_Salt = KDF(MSK, "IETF_OSCORE_MASTER_SALT", 64, length)
where:
o The AES-CMAC-PRF-128 is defined in [RFC4615]. This function uses
AES-CMAC-128 as building block.
o The MSK exported by the EAP method, which by design is a fresh key
material. Discussions about the use of the MSK for the key
derivation are done in Section Section 7.
o "IETF_OSCORE_MASTER_SALT" is the ASCII code representation of the
non-NULL terminated string (excluding the double quotes around
it).
o 64 is the length of the MSK.
o length is the length of the label "IETF_OSCORE_MASTER_SALT" (23
bytes).
The ID Context can be set to the Identity of the EAP peer.
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4.2. Deriving DTLS_PSK
In the second alternative, a DTLS_PSK is derived from the MSK between
both CoAP endpoints. So far, DTLS_PSK will have also 16 byte length
and it will derived as follows:
DTLS_PSK = KDF(MSK, "IETF_DTLS_PSK" , 64, length). This value is
concatenated with the value of the Token Option value.
where:
o MSK is exported by the EAP method.
o "IETF_DTLS_PSK" is the ASCII code representation of the non-NULL
terminated string (excluding the double quotes around it).
o 64 is the length of the MSK.
o length is the length of the label "IETF_DTLS_PSK" (13 bytes).
5. Examples of Use Case Scenario
For a device to act as a trustworthy entity within a security domain,
certain key material is needed to be shared between the IoT device
and AS. In ACE, the process of Client registration and provisioning
of credentials to the client is not specified. The process of Client
registration and provisioning can be achieved by using CoAP-EAP.
Once the process of authentication with EAP is completed, fresh key
material is shared between the IoT device and the AS.
Next, we elaborate examples of different use case scenarios about the
usage of CoAP-EAP. Generally, we are dealing with 4 entities:
o 2 nodes (A and B), which are constrained devices. They are the
EAP peers.
o 1 controller (C). The controller manages a domain where nodes can
be deployed. It can be considered a more powerful machine than
the nodes. In this scenario, the Controller (and EAP
Authenticator), can be co-located with the AS.
o 1 AAA server (AAA) - Optional. The AAA is an Authentication,
Authorization and Accounting Server, which is not constrained.
Generally, any node wanting to join the domain managed by the
controller, MUST perform a CoAP-EAP authentication with the
controller C. This authentication MAY involve an external AAA
server. This means that A and B, once deployed, will perform this
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CoAP-EAP once as a bootstrapping phase to establish a security
association with the controller C. Moreover, any other entity, which
wants to join and establish communications with nodes under the
controller C's domain must also do the same. By using EAP, we can
have the flexibility of having different types of credentials. For
instance, if we have a device that is not battery dependent, and not
very constrained a we could be using a heavier authentication method.
With very constrained devices we might need to go to other
authentication methods (e.g., EAP-PSK, EAP-EDHOC, etc.) being able to
adapt to different types of devices according to policies or devices
capabilities.
5.1. Example 1: CoAP-EAP in ACE
Next, we exemplify how CoAP-EAP can be used to perform the Client
registration in a general way, to allow two IoT devices (A and B) to
communicate and interact after a successful client registration.
Node A wants to communicate with node B (e.g. to active a light
switch). The overall process is divided in three phases. Let's
start with node A. In the first phase, the node A (EAP peer) does
not yet belong to the controller C's domain. Then, it communicates
with controller C (EAP authenticator) and authenticates with CoAP-
EAP, which, optionally, communicates with the AAA server to complete
the authentication process. If the authentication is successful, key
material is distributed to the controller C and derived by node A.
This key material allows node A to establish a security association
with the controller (C). Some authorization information may be also
provided in this step. If authentication and authorization are
correct, node A is enrolled in the controller C's domain during a
period of time. In particular, [RFC5247] recommends 8 hours, though
the AAA server can establish this lifetime. In the same manner, B
needs to perform the same process with CoAP-EAP to be part of the
controller C's domain.
In the second phase, when node A wants to talk with node B, it
contacts the controller C for authorization to access node B and
obtain all the required information to do that in a secure manner
(e.g. keys, tokens, authorization information, etc.). This phase
does NOT require the usage of CoAP-EAP. The details of this phase
are out of scope of this document, and the ACE framework is used for
this purpose [I-D.ietf-ace-oauth-authz].
In the third phase, the node A can access node B with the credentials
and information obtained from the controller C in the second phase.
This access can be repeated without contacting the controller, while
the credentials given to A are still valid. The details of this
phase are out of scope of this document.
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It is worth noting that first phase with CoAP-EAP is ONLY required to
join the controller C's domain. Once it is performed with success,
the communications are local to the controller C's domain so there is
no need to contact the external AAA server nor performing EAP
authentication.
5.2. 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 domain Controller acting as EAP authenticator, which in turn
communicates with a AAA infrastructure (acting as AAA client).
Through the local AAA server to communicate with the home AAA server
to complete the authentication and integrate the device as a
trustworthy entity into the domain of the controller C. In this
scenario the AS under the role of the Controller, receives the key
material from the AAA infrastructure
5.3. Example 3: Single domain with AAA infrastructure
A University Campus, we have several Faculty buildings and each one
has its own criteria or policies in place to manage IoT devices under
an AS. All buildings belong to the same domain (e.g., um.es). All
these buildings are managed with a AAA infrastructure. A new device
(A) with credentials from the domain (e.g., um.es) will be able to
perform the device registration with a Controller (C) of any building
as long as they are managed by the same general domain.
5.4. Example 4: Single domain without AAA infrastructure
Another case, without a AAA infrastructure, we have a Controller that
has co-located the AAA server and using EAP standalone mode we are
able to manage all the devices within the same domain locally.
Client registration of a node (A) with Controller (C) can also be
performed in the same manner, transparent to the IoT device. In this
scenario the AAA server is co-located within the Controller (C)-
5.5. Other use cases
5.5.1. CoAP-EAP for network access control
One of the first steps for an IoT device life-cycle is to perform the
authentication to gain access to the network. To do so, the device
first has to be authenticated and granted authorization to gain
access to the network. Additionally, security parameters such as
credentials can be derived from the authentication process allowing
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the trustworthy operation of the IoT device in a particular network
by joining the security domain. By using EAP, we are able to achieve
this with flexibility and scalability, because of the different EAP
methods available and the ability relying in AAA infrastructures if
needed to support multi-domain scenarios, which is a key feature when
the IoT devices deployed under the same security domain, belong to
different organizations. Given that EAP is also used for network
access control, it is possible that this service can be used to
provide network access control service (e.g., LoRa network). In this
specific case, we could leverage the compression by SCHC for CoAP.
5.5.2. CoAP-EAP for service authentication
It is not uncommon that the infrastructure where the device is
deployed and the services the IoT device are managed by different
organizations. Therefore, in addition to the authentication for
network access control, we have to consider the possibility of a
secondary authentication to access different services. This process
of authentication, for example, will provide with the necessary key
material to establish a secure channel and interact with the entity
in charge of granting access to different services.
6. Discussion
6.1. CoAP as EAP lower-layer
In this section we discuss the suitability of the CoAP protocol as
EAP lower layer, and review the requisites imposed by the EAP
protocol to any protocol that transports EAP. The assumptions EAP
makes about its lower layers can be found in section 3.1 of
[RFC3748], which are enumerated next:
o Unreliable transport. EAP does not assume that lower layers are
reliable.
o Lower layer error detection. EAP relies on lower layer error
detection (e.g., CRC, Checksum, MIC, etc.)
o Lower layer security. EAP does not require security services from
the lower layers.
o Minimum MTU. Lower layers need to provide an EAP MTU size of 1020
octets or greater.
o Possible duplication. EAP stipulates that, while desirable, it
does not require for the lower layers to provide non-duplication.
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o Ordering guarantees. EAP relies on lower layer ordering
guarantees for correct operation.
Regarding the unreliable transport, although EAP assumes a non
reliable transport, CoAP does provide a reliability mechanism through
the use of Confirmable messages. For the error detection, CoAP goes
on top of UDP which provides a checksum mechanism over its payload.
Lower layer security services are not required. About the minimum
MTU of 1020 octets, CoAP assumes an upper bound of 1024 for its
payload which covers the requirements of EAP. Regarding message
ordering, we propose the use of a new CoAP option, the SeqNum option
described in Section (Section 3.2), which will allow us to determine
the order in which the different messages are exchanged. Regarding
the Token, we consider the use of a constant value using a small 1
byte Token. In fact, the EAP server will not send a new EAP request
until it has processed the expected EAP response. Additionally, we
are under the assumption that there will a single EAP authentication
between the constrained device and the same Controller.
As we can see, CoAP can fulfil the requirements of EAP to be
considered suitable as lower-layer.
6.2. Need for SeqNum Option
We consider the use of the SeqNum Option due to the independence of
how the CoAP engine is implemented. Since we do not know before hand
if the implementation will allow us to pre-establish the MSG-ID or
the Token from the application perspective, we need to be sure we are
able to provide order delivery. If the implementation of CoAP allows
us to pre-establish the MSD-ID and Token, we could avoid using this
option, due to the characteristics of the CoAP-EAP exchange, i.e.,
the EAP exchange is done in lock-step and only one session is
considered at a time. Another consideration regarding the workings
of the SeqOption, is that since the initial number from which is
monotonically increased by 1, could cause the overflow of the number.
To manage this scenario, the SeqNum Option performs rounding, going
to zero and continue from there.
6.3. Size of the EAP lower-layer vs EAP method size
Regarding the impact an EAP lower layer will have to the total byte
size of the whole exchange, there is a comparison with another
network layer based EAP lower-layer, PANA [RFC5191] in [coap-eap].
Authors compared focusing EAP lower-layer (alone) and taking into
account EAP. On the one hand, at EAP lower-layer level, the usage of
CoAP gives us an important benefits. On the other hand, when taking
into account the EAP method overload, this reduction is less but
still significant if the EAP method is lightweight (we used EAP-PSK
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as a representative example of a lightweight EAP method). If the EAP
method is very taxing the improvement achieved in the EAP lower-layer
is less significant. This leads to the conclusion that possible next
steps in this field could be also improving or designing new EAP
methods that can be better adapted to the requirements of constrained
devices and networks. However, we cannot ignore the impact of the
EAP lower-layer itself and try to propose something light as we do
proposing CoAP. We consider that may be others EAP methods such as
EAP-AKA or new lightweight EAP methods such as EAP-EDHOC
[I-D.ingles-eap-edhoc] that can benefit from a CoAP-based EAP lower-
layer, as well as new ones that may be proposed in the future with
IoT constraints in mind.
6.4. Controller as the CoAP Client
Due to the constrained capacities of the devices, to relieve them of
the retransmission tasks, we set the Controller as the CoAP client,
for the main exchange following the recommendations of the
[I-D.ietf-lwig-coap] document to simplify the constrained device
implementation.
6.5. Possible Optimizations
6.5.1. Empty Token
Assuming that the bootstrapping service runs before any other
service, and that no other service will run concurrently until it has
finished, we could use an Empty Token value to save resources, since
there will be no other endpoint or CoAP exchange.
6.5.2. Removing SeqNum Option
An alternative to consider would be to try to rely on the Message ID
values as a way of achieving the order delivery throughout the
authentication exchange. Here we have two approximations: 1)
Removing the option from the ACKs and 2) removing the option
completely.
1. Since the ACKs are piggybacked by design, there is only 1 ongoing
authentication process and the EAP exchange is done in a lockstep
fashion, when we get a response we will get the same Message ID
of the request and we can confirm the SeqNum of the Request.
2. An alternative to consider would be to try to solely rely on the
Message ID values as a way of achieving the order delivery
throughout the authentication exchange. Here we also have two
approaches: A) To expect randomly generated Message IDs and B)
set the Message ID to increase monotonically by 1.
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A. Regarding the use of the Message ID, their values in the
requests sent by the Controller are generated randomly, as
suggested by CoAP. The Controller selects a new Message ID
value each time a new request is sent to the CoAP server,
until the bootstrapping service finishes. Moreover, the
Controller stores the last Message ID sent until correctly
receiving the corresponding ACK. The CoAP server keeps track
of the last received Message ID to identify retransmissions,
and the previous Message IDs during the current bootstrapping
to identify old messages. In general, a request is
considered valid in terms of the Message ID if either this
value matches the last value received, which means a
retransmission of the last response is required, or the
arrival of a new Message ID, which therefore represents a new
message. If these rules do not apply (i.e., an old Message
ID has been received), the CoAP server silently discards the
request. This is possible because the bootstrapping service
is designed as lockstep, i.e. the Controller will not send a
new request until it has received the expected response.
When the bootstrapping exchange finishes successfully, the
CoAP server can free the tracked Message IDs, except for the
last received Message ID at the end of the bootstrapping,
just in case a retransmission is required.
B. This case would avoid having to keep track of the already
used Message IDs, monotonically increasing by 1 the message
ID value once the first is randomly picked by the Controller.
6.5.3. Further re-authentication
Since the initial bootstrapping is usually taxing, it is assumed to
be done only once over a long period of time. If further re-
authentications for refreshing the key material are necessary, there
are other methods that can be used to perform these re-
authentications. For example, the EAP re-authentication (EAP-ERP)
[RFC6696] can be used to avoid repeating the entire EAP exchange in
few exchanges.
7. Security Considerations
There are some aspects to be considered such as how authorization is
managed, how the cryptographic suite is selected and how the trust in
the Controller is established.
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7.1. Authorization
Authorization is part of the bootstrapping. It serves to establish
whether the node can join and the set of conditions it has to adhere.
The authorization data received from the AAA server can be delivered
by the AAA protocol (e.g. Diameter). Providing more fine grained
authorization data can be with the transport of SAML in RADIUS
[RFC7833]. After bootstrapping, additional authorization to operate
in the security domain, e.g., access services offered by other nodes,
can be taken care of by the solutions proposed in the ACE WG.
7.2. Cryptographic suite selection
How the cryptographic suit is selected is also important. To reduce
the overhead of the protocol we use a default cryptographic suite.
As OSCORE is assumed to run after the EAP authentication, the same
default crypto-suite is used in this case as explained in the Key
Derivation Section Section 4 The cryptographic suite is not
negotiated. If the cryptographic suite to be used by the node is
different from default, the AAA server will send the specific
parameters to the Authenticator. If the cryptographic suite is not
supported, the key derivation process would result in a security
association failure.
7.3. Freshness of the key material
In this design, we do not exchange nonces to provide freshness to the
keys derived from the MSK. This is done under the assumption that
the MSK and EMSK keys derived following the EAP KMF [RFC5247] are
fresh key material by the specifications of the EAP KMF. Since only
one session key is derived from the MSK we do not have to concern
ourselves with the generation of additional key material. In case
another session has to be established, a re-authentication can be
done, by running process again, or using a more lightweight EAP
method to derive additional key material such as EAP-ERP.
7.4. Additional Security Consideration
Other security related concerns can be how to ensure that the node
joining the security domain can in fact trust the Controller. This
issue is elaborated in the EAP KMF [RFC5247]. Summarizing, the node
knows it can trust the Controller, because the key that is used to
establish the security association is derived from the MSK. If the
Controller has the MSK, it is clear the AAA Server of the node trusts
the Controller, which confirms it is a trusted party.
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8. IANA Considerations
This document has no actions for IANA.
9. Acknowledgments
We would like to thank Pedro Moreno-Sanchez and Gabriel Lopez-Millan
for the first review of this document. Also, we would like to thank
Ivan Jimenez-Sanchez for the first proof-of-concept implementation of
this idea.
We also thank for their valuables comments to Alexander Pelov and
Laurent Toutain, specially for the potential optimizations of CoAP-
EAP.
As well as the reviewers of this work, Alexandre Petrescu, Pedro
Moreno-Sanchez, Eduardo Ingles-Sanchez and Benjamin Kaduk.
10. References
10.1. Normative References
[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)", draft-ietf-ace-oauth-authz-36
(work in progress), November 2020.
[I-D.ietf-lwig-coap]
Kovatsch, M., Bergmann, O., and C. Bormann, "CoAP
Implementation Guidance", draft-ietf-lwig-coap-06 (work in
progress), July 2018.
[I-D.ingles-eap-edhoc]
Sanchez, E., Garcia-Carrillo, D., and R. Marin-Lopez, "EAP
method based on EDHOC Authentication", draft-ingles-eap-
edhoc-01 (work in progress), November 2020.
[I-D.ohba-core-eap-based-bootstrapping]
Das, S. and Y. Ohba, "Provisioning Credentials for CoAP
Applications using EAP", draft-ohba-core-eap-based-
bootstrapping-01 (work in progress), March 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
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[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>.
[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
2006, <https://www.rfc-editor.org/info/rfc4493>.
[RFC4615] Song, J., Poovendran, R., Lee, J., and T. Iwata, "The
Advanced Encryption Standard-Cipher-based Message
Authentication Code-Pseudo-Random Function-128 (AES-CMAC-
PRF-128) Algorithm for the Internet Key Exchange Protocol
(IKE)", RFC 4615, DOI 10.17487/RFC4615, August 2006,
<https://www.rfc-editor.org/info/rfc4615>.
[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>.
[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>.
[RFC5295] Salowey, J., Dondeti, L., Narayanan, V., and M. Nakhjiri,
"Specification for the Derivation of Root Keys from an
Extended Master Session Key (EMSK)", RFC 5295,
DOI 10.17487/RFC5295, August 2008,
<https://www.rfc-editor.org/info/rfc5295>.
[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>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[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>.
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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>.
[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>.
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.
Authors' Addresses
Rafa Marin-Lopez
University of Murcia
Campus de Espinardo S/N, Faculty of Computer Science
Murcia 30100
Spain
Phone: +34 868 88 85 01
Email: rafa@um.es
Dan Garcia-Carrillo
University of Oviedo
Calle Luis Ortiz Berrocal S/N, Edificio Polivalente
Gijon, Asturias 33203
Spain
Email: garciadan@uniovi.es
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