Internet DRAFT - draft-sarikaya-t2trg-sbootstrapping
draft-sarikaya-t2trg-sbootstrapping
Network Working Group M. Sethi
Internet-Draft Ericsson
Intended status: Informational B. Sarikaya
Expires: August 22, 2021 Denpel Informatique
D. Garcia-Carrillo
University of Oviedo
February 18, 2021
Secure IoT Bootstrapping: A Survey
draft-sarikaya-t2trg-sbootstrapping-11
Abstract
This draft provides an overview of the various terms that are used
when discussing bootstrapping of IoT devices. We document terms that
have been used within the IETF as well as other standards bodies. We
investigate if the terms refer to the same phenomena or have subtle
differences. We provide recommendations on the applicability of
terms in different contexts. Finally, this document presents a
survey of secure bootstrapping mechanisms available for smart objects
that are part of an Internet of Things (IoT) network. The survey
does not prescribe any one mechanism and rather presents IoT
developers with different options to choose from, depending on their
use-case, security requirements, and the user interface available on
their IoT devices.
Status of This Memo
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This Internet-Draft will expire on August 22, 2021.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Usage of bootstrapping terminology in standards . . . . . . . 4
3.1. Device Provisioning Protocol (DPP) . . . . . . . . . . . 4
3.2. Open Mobile Alliance (OMA) Lightweight M2M (LwM2M) . . . 5
3.3. Open Connectivity Foundation (OCF) . . . . . . . . . . . 6
3.4. Bluetooth . . . . . . . . . . . . . . . . . . . . . . . . 7
3.5. Fast IDentity Online (FIDO) alliance . . . . . . . . . . 7
3.6. Internet Engineering Task Force (IETF) . . . . . . . . . 8
3.6.1. Enrollment over Secure Transport (EST) . . . . . . . 8
3.6.2. Bootstrapping Remote Secure Key Infrastructures
(BRSKI) . . . . . . . . . . . . . . . . . . . . . . . 8
3.6.3. Secure Zero Touch Provisioning . . . . . . . . . . . 8
3.6.4. Nimble out-of-band authentication for EAP (EAP-NOOB) 9
4. Comparison . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Recommendations . . . . . . . . . . . . . . . . . . . . . . . 9
6. Classification of available mechanisms . . . . . . . . . . . 10
7. IoT Device Bootstrapping Methods . . . . . . . . . . . . . . 11
7.1. Managed Methods . . . . . . . . . . . . . . . . . . . . . 11
7.1.1. Bootstrapping in LPWAN . . . . . . . . . . . . . . . 13
7.2. Peer-to-Peer or Ad-hoc Methods . . . . . . . . . . . . . 14
7.3. Leap-of-faith/Opportunistic Methods . . . . . . . . . . . 15
7.4. Hybrid Methods . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 16
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
11. Informative References . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23
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1. Introduction
We informally define bootstrapping as "any process that takes place
before a device can become operational". While bootstrapping is
necessary for all computing devices, until recently, most of our
devices typically had sufficient computing power and user interface
(UI) for ensuring somewhat smooth operation. For example, a typical
laptop device required the user to setup a username/password as well
as enter network settings for Internet connectivity. Following these
steps ensured that the laptop was fully operational.
The problem of bootstrapping is however exacerbated for Internet of
Things (IoT) networks. The size of an IoT network varies from a
couple of devices to tens of thousands, depending on the application.
Smart objects/things/devices in IoT networks are produced by a
variety of vendors and are typically heterogeneous in terms of the
constraints on their power supply, communication capability,
computation capacity, and user interfaces available. This problem of
bootstrapping in IoT was identified by Sethi et al. [Sethi14] while
developing a bootstrapping solution for smart displays. Although
this document focuses on bootstrapping terminology and methods for
IoT devices, we do not exclude bootstrapping related terminology used
in other contexts.
Bootstrapping devices typically also involves providing them with
some sort of network connectivity. Indeed, the functionality of a
disconnected device is rather limited. Bootstrapping devices often
assumes that some network has been setup a-priori. Setting up and
maintaining a network itself is challenging. For example, users may
need to configure the network name (called as Service Set Identifier
(SSID) in Wi-Fi networks) and passpharse before new devices can be
bootstrapped. Specifications such as the Wi-Fi Alliance Simple
Configuration [simpleconn] help users setup networks. However, this
document is only focused on terminology and processes associated with
bootstrapping devices and excludes any discussion on setting up
networks before devices can be bootstrapped.
In addition to our informal definition, it is helpful to look at
other definitions of bootstrapping. The IoT@Work project defines
bootstrapping in the context of IoT as "the process by which the
state of a device, a subsystem, a network, or an application changes
from not operational to operational" [iotwork]. Vermillard
[vermillard] , on the other hand, describes bootstrapping as the
procedure by which an IoT device gets the URLs and secret keys for
reaching the necessary servers. Vermillard notes that the same
process is useful for re-keying, upgrading the security schemes, and
for redirecting the IoT devices to new servers.
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There are several terms that have often been used in the context of
bootstrapping:
o Bootstrapping
o Provisioning
o Onboarding
o Enrollment
o Commissioning
o Initialization
o Configuration
o Registration
We attempt to find out whether all these terms refer to the same
phenomena. We begin by looking at how these terms have been used in
various standards and standardization bodies in Section 3. We then
summarize our understanding in Section 4, and provide our
recommendations on their usage in Section 5. Section 6 provides a
taxonomy of bootstrapping methods and Section 7 categorizes methods
according to the taxonomy.
2. Terminology
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 BCP 14
[RFC2119][RFC8174].
3. Usage of bootstrapping terminology in standards
To better understand bootstrapping related terminology, let us first
look at the terms used by some existing specifications:
3.1. Device Provisioning Protocol (DPP)
The Wi-Fi Alliance Device provisioning protocol (DPP) [dpp] describes
itself as a standardized protocol for providing user friendly Wi-Fi
setup while maintaining or increasing the security. DPP relies on a
configurator, e.g. a smartphone application, for setting up all other
devices, called enrollees, in the network. DPP has the following
three phases/sub-protocols:
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o Bootstrapping: The configurator obtains bootstrapping information
from the enrollee using an out-of-band channel such as scanning a
QR code or tapping NFC. The bootstrapping information includes
the public-key of the device and metadata such as the radio
channel on which the device is listening.
o Authentication: In DPP, either the configurator or the enrollee
can initiate the authentication protocol. The side initiating the
authentication protocol is called as the initiator while the other
side is called the responder. The authentication sub-protocol
provides authentication of the responder to an initiator. It can
optionally authenticate the initiator to the responder (only if
the bootstrapping information was exchange out-of-band in both
directions).
o Configuration: Using the key established from the authentication
protocol, the enrollee asks the configurator for network
information such as the SSID and passphrase of the access point.
3.2. Open Mobile Alliance (OMA) Lightweight M2M (LwM2M)
The OMA LwM2M specification [oma] defines an architecture where a new
device (LwM2M client) contacts a Bootstrap-server which is
responsible for "provisioning" essential information such as
credentials. After receiving this essential information, the LwM2M
client device "registers" itself with one or more LwM2M Servers which
will manage the device during its lifecycle. The current standard
defines the following four bootstrapping modes:
o Factory Bootstrap: An IoT device in this case is configured with
all the necessary bootstrap information during manufacturing and
prior to its deployment.
o Bootstrap from Smartcard: An IoT device retrieves and processes
all the necessary bootstrap data from a Smartcard.
o Client Initiated Bootstrap: This mode provides a mechanism for an
IoT client device to retrieve the bootstrap information from a
Bootstrap Server. This requires the client device to have an
account at the Bootstrap Server and credentials to obtain the
necessary information securely.
o Server Initiated Bootstrap: In this bootstrapping mode, the
bootstrapping server configures all the bootstrap information on
the IoT device without receiving a request from the client. This
means that the bootstrap server needs to know if a client IoT
Device is ready for bootstrapping before it can be configured.
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For example, a network may inform the bootstrap server of a new
connecting IoT client device.
3.3. Open Connectivity Foundation (OCF)
The Open Connectivity Foundation (OCF) [ocf] defines the process
before a device is operational as onboarding. The first step of this
onboarding process is "configuring" the ownership, i.e., establishing
a legitimate user that owns the device. For this, the user is
supposed to use an Onboarding tool (OBT) and an Owner Transfer
Methods (OTM).
The OBT is described as a logical entity that may be implemented on a
single or multiple entities such as a home gateway, a device
management tool, etc. OCF lists several optional OTMs. At the end
of the execution of an OTM, the onboarding tool must have
"provisioned" an Owner Credential onto the device. The following
owner transfer methods are specified:
o Just works: Performs an un-authenticated Diffie-Hellman key
exchange over Datagram Transport Layer Security (DTLS). The key
exchange results in a symmetric session key which is later used
for provisioning. Naturally, this mode is vulnerable to Man-in-
The-Middle (MiTM) attackers.
o Random PIN: The device generates a PIN code that is entered into
the onboarding tool by the user. This pin code is used together
with TLS-PSK ciphersuites for establishing a symmetric session
key. OCF recommends PIN codes to have an entropy of 40 bits.
o Manufacturer certificate: An onboarding tool authenticates the
device by verifying a manufacturer installed certificate.
Similarly, the device may authenticate the onboarding tool by
verifying its signature.
o Vendor specific: Vendors implement their own transfer method that
accommodates any specific device constraints.
Once the onboarding tool and the new device have authenticated and
established secure communication, the onboarding tool
"provisions"/"configures" the device with Owner credentials. Owner
credentials may consist of certificates, shared keys, or Kerberos
tickets for example.
The OBT additionally configures/provisions information about the
Access Management Service (AMS), the Credential Management Service
(CMS), and the credentials for interacting with them. The AMS is
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responsible for provisioning access control entries, while the CMS
provisions security credentials necessary for device operation.
3.4. Bluetooth
Bluetooth mesh provisioning. Beacons for discovery. Public-key
exchange followed by authentication. Finally provisioning of the
network key and unicast address. To be expanded.
3.5. Fast IDentity Online (FIDO) alliance
The Fast IDentity Online Alliance (FIDO) is currently specifying an
automatic onboarding protocol for IoT devices [fidospec]. The goal
of this protocol is to provide a new IoT device with information for
interacting securely with an online IoT platform. This protocol
allows owners to choose the IoT platform for their devices at a late
stage in the device lifecyle. The draft specification refers to this
feature as "late binding".
The FIDO IoT protocol itself is composed of one Device Initialization
(DI) protocol and 3 Transfer of Ownership (TO) protocols TO0, TO1,
TO2. Protocol messages are encoded in Concise Binary Object
Representation (CBOR) [RFC8949] and can be transported over
application layer protocols such as Constrained Application Protocol
(CoAP) [RFC7252] or directly over TCP, Bluetooth etc. FIDO IoT
however assumes that the device already has IP connectivity to a
rendezvous server. Establishing this initial IP connectivity is
explicitly stated as "out-of-scope" but the draft specification hints
at the usage of Hypertext Transfer Protocol (HTTP) [RFC7230] proxies
for IP networks and other forms of tunneling for non-IP networks.
The specification only provides a non-normative example of the DI
protocol which must be executed in the factory during device
manufacture. This protocol embeds initial ownership and
manufacturing credentials into Restricted Operation Environment (ROE)
on the device. The initial information embedded also includes a
unique device identifier (called as GUID in the specification).
After DI is executed, the manufacturer has an ownership voucher which
is passed along the supply chain to the device owner.
When a device is unboxed and powered on by the new owner, the device
discovers a network-local or an Internet-based rendezvous server.
Protocols (TO0, TO1, and TO2) between the device, the rendezvous
server, and the new owner (as the owner onboarding service) ensure
that the device and the new owner are able to authenticate each
other. Thereafter, the new owner establishes cryptographic control
of the device and provides it with credentials of the IoT platform
which the device should used.
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3.6. Internet Engineering Task Force (IETF)
In this section, we will look at some IETF standards and draft
specifications related to IoT bootstrapping.
3.6.1. Enrollment over Secure Transport (EST)
Enrollment over Secure Transport (EST) [RFC7030] defines a profile of
Certificate Management over CMS (CMC) [RFC5272]. EST relies on
Hypertext Transfer Protocol (HTTP) and Transport Layer Security (TLS)
for exchanging CMC messages and allows client devices to obtain
client certificates and associated Certification Authority (CA)
certificates. A companion specification for using EST over secure
CoAP has also been standardized [I-D.ietf-ace-coap-est]. EST assumes
that some initial information is already distributed so that EST
client and servers can perform mutual authentication before
continuing with protocol. [RFC7030] further defines "Bootstrap
Distribution of CA Certificates" which allows minimally configured
EST clients to obtain initial trust anchors. It relies on human
users to verify information such as the CA certificate "fingerprint"
received over the unauthenticated TLS connection setup. After
successful completion of this bootstrapping step, clients can proceed
to the enrollment step during which they obtain client certificates
and associated CA certificates.
3.6.2. Bootstrapping Remote Secure Key Infrastructures (BRSKI)
The ANIMA working group is working on a bootstrapping solution for
devices that relies on 802.1AR vendor certificates called
Bootstrapping Remote Secure Key Infrastructures (BRSKI)
[I-D.ietf-anima-bootstrapping-keyinfra]. In addition to vendor
installed IEEE 802.1AR certificates, a vendor based service on the
Internet is required. Before being authenticated, a new device only
needs link-local connectivity, and does not require a routable
address. When a vendor provides an Internet based service, devices
can be forced to join only specific domains. The document highlights
that the described solution is aimed in general at non-constrained
(i.e. class 2+ defined in [RFC7228]) devices operating in a non-
challenged network. It claims to scale to thousands of devices
located in hostile environments, such as ISP provided CPE devices
which are drop-shipped to the end user.
3.6.3. Secure Zero Touch Provisioning
[RFC8572] defines a bootstrapping strategy for enabling devices to
securely obtain all the configuration information with no installer
input, beyond the actual physical placement and connection of cables.
Their goal is to enable a secure NETCONF [RFC6241] or RESTCONF
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[RFC8040] connection to the deployment specific network management
system (NMS). This bootstrapping method requires the devices to be
configured with trust anchors in the form of X.509 certificates.
[RFC8572] is similar to BRSKI based on [RFC8366].
3.6.4. Nimble out-of-band authentication for EAP (EAP-NOOB)
EAP-NOOB [I-D.ietf-emu-eap-noob] defines an EAP method where the
authentication is based on a user-assisted out-of-band (OOB) channel
between the server and peer. It is intended as a generic
bootstrapping solution for IoT devices which have no pre-configured
authentication credentials and which are not yet registered on the
authentication server. This method claims to be more generic than
most ad-hoc bootstrapping solutions in that it supports many types of
OOB channels. The exact in-band messages and OOB message contents
are specified and not the OOB channel details. EAP-NOOB also
supports IoT devices with only output (e.g. display) or only input
(e.g. camera). It makes combined use of both secrecy and integrity
of the OOB channel for more robust security than the ad-hoc
solutions.
4. Comparison
There are several stages before a device becomes fully operational.
This typically involves establishing some initial trust after which
credentials and other parameters are configured. For DPP,
bootstrapping is the first step before authentication and
provisioning of credentials can occur. For EST, bootstrapping
happens as the first step when the client devices have no
certificates available for starting enrollment. Provisioning/
configuring are terms used for providing additional information to
devices before they are fully operational. For example, credentials
are provisioned onto the device. But before credential provisioning,
a device is bootstrapped and authenticated. Some protocols may only
deal with parts of the process. For example, TLS maybe used for
authentication after bootstrapping. A separate device management
protocol then may run over this TLS tunnel for provisioning
operational information and credentials.
5. Recommendations
o It is recommended that the IETF use the term "bootstrapping" for
the initial (authentication) step that a device must perform.
Bootstrapping will likely happen before the device has obtained
full network connectivity.
o It is recommended to use the term "provisioning"/"configuring" for
the process of providing necessary information to a device to
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become operational after initial authentication is complete. As
is evident from above, provisioning and configuring may include
bootstrapping and authentication as a sub protocol.
o IETF specifications should aim to avoid mixing terminology or
adding new terminology for better consistency.
6. Classification of available mechanisms
Given the large number of bootstrapping protocols and related
specifications, it can be helpful to classify them. We categorize
the available bootstrapping solutions into the following major
classes:
o Managed methods: These methods rely on pre-established trust
relations and authentication credentials. They typically utilize
centralized servers for authentication, although several such
servers may join to form a distributed federation. Example
methods include Extensible Authentication Protocol (EAP)
[RFC3748], Generic Bootstrapping Architecture (GBA) [TS33220],
Kerberos [RFC4120], Bootstrapping Remote Secure Key
Infrastructures (BRSKI) and vendor certificates [vendorcert]. EAP
Transport Layer Security EAP-TLS [I-D.ietf-emu-eap-tls13] for
instance assumes that both the client and the server have
certificates to authenticate each other. Based on this
authentication, the server authorizes the client for network
access. The Eduroam federation [RFC7593] uses a network of such
servers to support roaming clients.
o Opportunistic and leap-of-faith methods: In these methods, rather
than verifying the initial authentication, the continuity of the
initial identity or connection is verified. Some of these methods
assume that the attacker is not present during the initial setup.
Example methods include Secure Neighbor Discovery (SEND) [RFC3971]
and Cryptographically Generated Addresses (CGA) [RFC3972], Wifi
Protected Setup (WPS) push button [wps], and Secure Shell (SSH)
[RFC4253].
o Peer-to-Peer (P2P) and Ad-hoc methods: These bootstrapping methods
do not rely on any pre-established credentials. Instead, the
bootstrapping protocol results in credentials being established
for subsequent secure communication. Such bootstrapping methods
typically perform an unauthenticated Diffie-Hellman exchange [dh]
and then use an out-of-band (OOB) communication channel to prevent
a man-in-the-middle attack (MitM). Various secure device pairing
protocols fall in this category. Based on how the OOB channel is
used, the P2P methods can be further classified into two sub
categories:
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* Key derivation: Contextual information received over the OOB
channel is used for shared key derivation. For example,
[proximate] relies on the common radio environment of the
devices being paired to derive the shared secret which would
then be used for secure communication.
* Key confirmation: A Diffie-Hellman key exchange occurs over the
insecure network and the established key is used to
authenticate with the help of the OOB channel. For example,
Bluetooth simple pairing [SimplePairing] use the OOB channel to
ask the user to compare pins and approve the completed
exchange.
o Hybrid methods: Most deployed methods are hybrid and use
components from both managed and ad-hoc methods. For instance,
central management may be used for devices after they have been
registered with the server using ad-hoc registration methods.
It is important to note here that categorization of different methods
is not always easy or clear. For example, all the opportunistic and
leap-of-faith methods become managed methods after the initial
vulnerability window. The choice of bootstrapping method used for
devices depends heavily on the business case. Questions that may
govern the choice include: What third parties are available? Who
wants to retain control or avoid work? In each category, there are
many different methods of secure bootstrapping available. The choice
of the method may also be governed by the type of device being
bootstrapped.
7. IoT Device Bootstrapping Methods
In this section we look at additional bootstrapping protocols for IoT
devices which are not covered in Section 3. Protocols already
covered in Section 3 however are mentioned in their respective
classes. This list is non-exhaustive.
7.1. Managed Methods
EAP-TLS is a widely used EAP method for network access authentication
[I-D.ietf-emu-eap-tls13]. It requires certificate-based mutual
authentication and a public key infrastructure. The ZigBee Alliance
has specified an IPv6 stack for IEEE 802.15.4 [IEEE802.15.4] devices
used in smart meters developed primarily for SEP 2.0 (Smart Energy
Profile) application layer traffic [SEP2.0]. The ZigBee IP stack
uses EAP-TLS for secure bootstrapping of devices.
EAP-PSK [RFC4764] is another EAP method that realizes mutual
authentication and session key derivation using a Pre-Shared Key
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(PSK). Given the light-weight nature of EAP-PSK, it can be suitable
for resource-constrained devices. However, secure distribution of a
large number of PSKs can be challenging.
CoAP-EAP [I-D.marin-ace-wg-coap-eap] defines a bootstrapping service
for IoT. The authors propose transporting EAP over CoAP [RFC7252]
for the constrained link, and communication with AAA infrastructures
in the non-constrained link. While the draft discusses the use of
EAP-PSK, the authors claim that they are specifying a new EAP lower
layer and any EAP method which results in generation is suitable.
Protocol for Carrying Authentication for Network Access (PANA)
[RFC5191] is a network layer protocol with which a node can
authenticate itself to gain access to the network. PANA does not
define a new authentication protocol and rather uses EAP over User
Datagram Protocol (UDP) for authentication.
Colin O'Flynn [I-D.oflynn-core-bootstrapping] proposes the use of
PANA for secure bootstrapping of resource constrained devices. He
demonstrates how a 6LowPAN Border Router (PANA Authentication Agent
(PAA)) can authenticate the identity of a joining constrained device
(PANA Client). Once the constrained device has been successfully
authenticated, the border router can also provide network and
security parameters to the joining device.
Hernandez-Ramos et al. [panaiot] also use EAP-TLS over PANA for
secure bootstrapping of smart objects. They extend their
bootstrapping scheme for configuring additional keys that are used
for secure group communication.
Generic Bootstrapping Architecture (GBA) is another bootstrapping
method that falls in centralized category. GBA is part of the 3GPP
standard [TS33220] and is based on 3GPP Authentication and Key
Agreement (3GPP AKA). GBA is an application independent mechanism to
provide a client application (running on the User equipment (UE)) and
any application server with a shared session secret. This shared
session secret can subsequently be used to authenticate and protect
the communication between the client application and the application
server. GBA authentication is based on the permanent secret shared
between the UE's Universal Integrated Circuit Card (UICC), for
example SIM card, and the corresponding profile information stored
within the cellular network operator's Home Subscriber System (HSS)
database. [I-D.sethi-gba-constrained] describes a resource-
constrained adaptation of GBA for IoT.
The four bootstrapping modes specified by the Open Mobile Alliance
(OMA) Light-weight M2M (LwM2M) standard require some sort of pre-
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provisioned credentials on the device. All the four modes are
examples of managed bootstrapping methods.
The Kerberos protocol [RFC4120] is a network authentication protocol
that allows several endpoints to communicate over an insecure
network. Kerberos relies on a symmetric cryptography scheme and
requires a trusted third party, that guarantees the identities of the
various actors. It relies on the use of "tickets" for nodes to prove
identity to one another in a secure manner. There has been research
work on using Kerberos for IoT devices [kerberosiot].
It is also important to mention some of the work done on implicit
certificates and identity-based cryptographic schemes [himmo],
[implicit]. While these are interesting and novel schemes that can
be a part of securely bootstrapping devices, at this point, it is
hard to speculate on whether such schemes would see large-scale
deployment in the future.
7.1.1. Bootstrapping in LPWAN
Low Power Wide Area Network (LPWAN) encompasses a wide variety of
technologies whose link-layer characteristics are severely
constrained in comparison to other typical IoT link-layer
technologies such as Bluetooth or IEEE 802.15.4. While some LPWAN
technologies rely on proprietary bootstrapping solutions which are
not publicly accessible, others simply ignore the challenge of
bootstrapping and key distribution. In this section, we discuss the
bootstrapping methods used by LPWAN technologies covered in
[RFC8376].
o LoRaWAN [LoRaWAN] describes its own protocol to authenticate nodes
before allowing them join a LoRaWAN network. This process is
called as joining and it is based on pre-shared keys (called
AppKeys in the standard). The joining procedure comprises only
one exchange (join-request and join-accept) between the joining
node and the network server. There are several adaptations to
this joining procedure that allow network servers to delegate
authentication and authorization to a backend AAA infrastructure
[RFC2904].
o Wi-SUN Alliance Field Area Network (FAN) uses IEEE 802.1X and EAP-
TLS for network access authentication. It performs a 4-way
handshake to establish a session keys after EAP-TLS
authentication.
o NB-IoT relies on the traditional 3GPP mutual authentication scheme
based on a shared-secret in the Subscriber Identity Module (SIM)
of the device and the mobile operator.
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o Sigfox security is based on unique device identifiers and
cryptographic keys. As stated in [RFC8376], although the
algorithms and keying details are not publicly available, there is
sufficient information to indicate that bootstrapping in Sigfox is
based on pre-established credentials between the device and the
Sigfox network.
From the above, it is clear that all LPWAN technologies rely on pre-
provisioned credentials for authentication between a new device and
the network. Thus, all of them can be categorized as managed
bootstrapping methods.
7.2. Peer-to-Peer or Ad-hoc Methods
While managed methods are viable for many IoT devices, they may not
be suitable or desirable in all scenarios. All the managed methods
assume that some credentials are provisioned into the device. These
credentials may be in the device micro-controller or in a replaceable
smart card such as a SIM card. The methods also sometimes assume
that the manufacturer embeds these credentials during the device
manufacture on the factory floor. However, in many cases the
manufacturer may not have sufficient incentive to do this. In other
scenarios, it may be hard to completely trust and rely on the device
manufacturer to securely perform this task. Therefore, many times,
P2P or Ad-hoc methods of bootstrapping are used. We discuss a few
example next.
P2P or ad-hoc bootstrapping methods are used for establishing keys
and credential information for secure communication without any pre-
provisioned information. These bootstrapping mechanisms typically
rely on an out-of-band (OOB) channel in order to prevent man-in-the-
middle (MitM) attacks. P2P and ad-hoc methods have typically been
used for securely pairing personal computing devices such as smart
phones. [devicepairing] provides a survey of such secure device
pairing methods. Many original pairing schemes required the user to
enter the same key string or authentication code to both devices or
to compare and approve codes displayed by the devices. While these
methods can provide reasonable security, they require user
interaction that is relatively unnatural and often considered a
nuisance. Thus, there is ongoing research for more natural ways of
pairing devices. To reduce the amount of user-interaction required
in the pairing process, several proposals use contextual or location-
dependent information, or natural user input such as sound or
movement, for device pairing [proximate].
The local association created between two devices may later be used
for connecting/introducing one of the devices to a centralized
server. Such methods would however be classified as hybrids.
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EAP-NOOB [I-D.ietf-emu-eap-noob] is an example of P2P and ad-hoc
bootstrapping method that establishes a security association between
an IoT device (node) and an online server (unlike pairing two devices
for local connections over WiFi or Bluetooth).
Thread Group commissioning [threadcommissioning] introduces a two
phased process i.e. Petitioning and Joining. Entities involved are
leader, joiner, commissioner, joiner router and border router.
Leader is the first device in Thread network that must be
commissioned using out-of-band process and is used to inject correct
user generated Commissioning Credentials (can be changed later) into
Thread Network. Joiner is the node that intends to get authenticated
and authorized on Thread Network. Commissioner is either within the
Thread Network (Native) or connected with Thread Network via a WLAN
(External).
Under some topologies, Joiner Router and Border Router facilitate the
Joiner node to reach Native and External Commissioner, respectively.
Petitioning begins before Joining process and is used to grant sole
commissioning authority to a Commissioner. After an authorized
Commissioner is designated, eligible thread devices can join network.
Pair-wise key is shared between Commissioner and Joiner, network
parameters (such as network name, security policy, etc.,) are sent
out securely (using pair-wise key) by Joiner Router to Joiner for
letting Joiner to join the Thread Network. Entities involved in
Joining process depends on system topology i.e. location of
Commissioner and Joiner. Thread networks only operate using IPv6.
Thread devices can devise GUAs (Global Unicast Addresses) [RFC4291].
Provision also exist via Border Router, for Thread device to acquire
individual global address by means of DHCPv6 or using SLAAC
(Stateless Address Autoconfiguration) address derived with advertised
network prefix.
7.3. Leap-of-faith/Opportunistic Methods
Bergmann et al. [simplekey] develop a secure bootstrapping mechanism
that does not rely on pre-provisioned credentials using resurrecting-
duckling imprinting scheme. Their bootstrapping protocol involves
three distinct phases: discover (the duckling node searches for
network nodes that can act as mother node), imprint (the mother node
imprints a shared secret establishing a secure channel once a
positive response is received for the imprinting request) and
configure (additional configuration information such as network
prefix and default gateway are configured). In this model for
bootstrapping, a small initial vulnerability window is acceptable and
can be mitigated using techniques such as a Faraday Cage (securing
the communication physically) to protect the environment of the
mother and duck nodes, though this may be inconvenient for the user.
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7.4. Hybrid Methods
[RFC7250] defines how raw public keys can be used for mutual
authentication of devices and servers. The extension specified in
[RFC7250] simplifies client_certificate_type and
server_certificate_type to carry only SubjectPublicKeyInfo structure
with the raw public key instead of many other parameters found in
typical X.509 version 3 certificates. Each side validates the keys
received with pre-configured values stored. Using raw public keys
for bootstrapping can be seen as a hybrid method. This is because it
generally requires an out-of-band (OOB) step (P2P/Ad-hoc) where the
raw public keys [RFC7250] are provided to the authenticating
entities, after which the actual authentication occurs online
(managed). CoAP already provides support for using raw public keys
(see Section 9.1.3.2. of [RFC7252])
8. Security Considerations
This draft does not take any posture on the security properties of
the different bootstrapping protocols discussed. Specific security
considerations of bootstrapping protocols are present in the
respective specifications.
Nonetheless, we briefly discuss some important security aspects which
are not fully explored in various specifications.
Firstly, an IoT system may deal with authorization for resources and
services separately from bootstrapping and authentication in terms of
timing as well as protocols. As an example, two resource-constrained
devices A and B may perform mutual authentication using credentials
provided by an offline third-party X before device A obtains
authorization for running a particular application on device B from
an online third-party Y. In some cases, authentication and
authorization maybe tightly coupled, e.g., successful authentication
also means successful authorization.
Secondly, re-bootstrapping of IoT devices may be required since keys
have limited lifetimes and devices may be lost or resold. Protocols
and systems must have adequate provisions for revocation and re-
bootstrapping. Re-bootstrapping must be as secure as the initial
bootstrapping regardless of whether this re-bootstrapping is done
manually or automatically over the network.
Lastly, some IoT networks use a common group key for multicast and
broadcast traffic. As the number of devices in a network increase
over time, a common group key may not be scalable and the same
network may need to be split into separate groups with different
keys. Bootstrapping and provisioning protocols may need appropriate
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mechanisms for identifying and distributing keys to the current
member devices of each group.
9. IANA Considerations
There are no IANA considerations for this document.
10. Acknowledgements
We would like to thank Tuomas Aura, Hannes Tschofenig, and Michael
Richardson for providing extensive feedback as well as Rafa Marin-
Lopez for his support.
11. Informative References
[devicepairing]
Mirzadeh, S., Cruickshank, H., and R. Tafazolli, "Secure
Device Pairing: A Survey", IEEE Communications Surveys and
Tutorials , pp. 17-40, 2014.
[dh] Diffie, W. and M. Hellman, "New directions in
cryptography", IEEE Transactions on Information Theory ,
vol. 22, no. 6, pp. 644-654, 1976.
[dpp] Wi-Fi Alliance, "Wi-Fi Device Provisioning Protocol
(DPP)", Wi-Fi Alliance , 2018, <https://www.wi-
fi.org/download.php?file=/sites/default/files/private/
Device_Provisioning_Protocol_Specification_v1.1_1.pdf>.
[fidospec]
Fast Identity Online Alliance, "FIDO IoT Spec", Fido
Alliance , August 2020, <https://fidoalliance.org/specs/
internet-of-things/FIDO-IoT-spec.html>.
[himmo] Garcia-Morchon, O., Rietman, R., Sharma, S., Tolhuizen,
L., and J. Torre-Arce, "DTLS-HIMMO: Efficiently Securing a
Post-Quantum World with a Fully-Collusion Resistant KPS",
Submitted to NIST Workshop on Cybersecurity in a Post-
Quantum World , version 20141225:065757, December 2014,
<https://eprint.iacr.org/2014/1008>.
[I-D.ietf-ace-coap-est]
Stok, P., Kampanakis, P., Richardson, M., and S. Raza,
"EST over secure CoAP (EST-coaps)", draft-ietf-ace-coap-
est-18 (work in progress), January 2020.
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[I-D.ietf-anima-bootstrapping-keyinfra]
Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
keyinfra-45 (work in progress), November 2020.
[I-D.ietf-emu-eap-noob]
Aura, T., Sethi, M., and A. Peltonen, "Nimble out-of-band
authentication for EAP (EAP-NOOB)", draft-ietf-emu-eap-
noob-03 (work in progress), December 2020.
[I-D.ietf-emu-eap-tls13]
Mattsson, J. and M. Sethi, "Using EAP-TLS with TLS 1.3",
draft-ietf-emu-eap-tls13-13 (work in progress), November
2020.
[I-D.marin-ace-wg-coap-eap]
Marin-Lopez, R. and D. Garcia-Carrillo, "EAP-based
Authentication Service for CoAP", draft-marin-ace-wg-coap-
eap-07 (work in progress), January 2021.
[I-D.oflynn-core-bootstrapping]
Sarikaya, B., Ohba, Y., Cao, Z., and R. Cragie, "Security
Bootstrapping of Resource-Constrained Devices", draft-
oflynn-core-bootstrapping-03 (work in progress), November
2010.
[I-D.sethi-gba-constrained]
Sethi, M., Lehtovirta, V., and P. Salmela, "Using Generic
Bootstrapping Architecture with Constrained Devices",
draft-sethi-gba-constrained-01 (work in progress),
February 2014.
[IEEE802.15.4]
"IEEE Std. 802.15.4-2015", April 2016,
<http://standards.ieee.org/findstds/
standard/802.15.4-2015.html>.
[implicit]
Porambage, P., Schmitt, C., Kumar, P., Gurtov, A., and M.
Ylianttila, "Pauthkey: A pervasive authentication protocol
and key establishment scheme for wireless sensor networks
in distributed iot applications", International Journal of
Distributed Sensor Networks , Hindawi Publishing
Corporation , 2014.
[iotwork] European Commission FP7, "IoT@Work bootstrapping
architecture Deliverable D2.2", June 2011.
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[kerberosiot]
Hardjono, T., "Kerberos for Internet-of-Things", February
2014, <https://kit.mit.edu/sites/default/files/documents/
Kerberos_Internet_of%20Things.pdf>.
[LoRaWAN] Sornin, N., Luis, M., Eirich, T., and T. Kramp, "LoRa
Specification V1.0", January 2015, <https://www.lora-
alliance.org/portals/0/specs/
LoRaWAN%20Specification%201R0.pdf>.
[ocf] Open Connectivity Foundation, "OCF Security
Specification", Open Connectivitiy Foundation , June 2017,
<https://openconnectivity.org/specs/
OCF_Security_Specification_v1.0.0.pdf>.
[oma] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification: Core", Open Mobile Alliance ,
November 2020,
<www.openmobilealliance.org/release/LightweightM2M/
V1_2-20201110-A/OMA-TS-LightweightM2M_Core-
V1_2-20201110-A.pdf>.
[panaiot] Hernandez-Ramos, J., Carrillo, D., Marin-Lopez, R., and A.
Skarmeta, "Dynamic Security Credentials PANA-based
Provisioning for IoT Smart Objects", 2nd World Forum on
Internet of Things (WF-IoT) , IEEE , 2015.
[proximate]
Mathur, S., Miller, R., Varshavsky, A., Trappe, W., and N.
Mandayam, "Proximate: proximity-based secure pairing using
ambient wireless signals.", Proceedings of MobiSys
International Conference , pp. 211-224, June 2011.
[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>.
[RFC2904] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
D. Spence, "AAA Authorization Framework", RFC 2904,
DOI 10.17487/RFC2904, August 2000,
<https://www.rfc-editor.org/info/rfc2904>.
[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>.
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[RFC3971] Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
"SEcure Neighbor Discovery (SEND)", RFC 3971,
DOI 10.17487/RFC3971, March 2005,
<https://www.rfc-editor.org/info/rfc3971>.
[RFC3972] Aura, T., "Cryptographically Generated Addresses (CGA)",
RFC 3972, DOI 10.17487/RFC3972, March 2005,
<https://www.rfc-editor.org/info/rfc3972>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<https://www.rfc-editor.org/info/rfc4120>.
[RFC4253] Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH)
Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253,
January 2006, <https://www.rfc-editor.org/info/rfc4253>.
[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 4291, DOI 10.17487/RFC4291, February
2006, <https://www.rfc-editor.org/info/rfc4291>.
[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>.
[RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS
(CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
<https://www.rfc-editor.org/info/rfc5272>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
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[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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <https://www.rfc-editor.org/info/rfc7250>.
[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>.
[RFC7593] Wierenga, K., Winter, S., and T. Wolniewicz, "The eduroam
Architecture for Network Roaming", RFC 7593,
DOI 10.17487/RFC7593, September 2015,
<https://www.rfc-editor.org/info/rfc7593>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[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>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
<https://www.rfc-editor.org/info/rfc8376>.
[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/info/rfc8572>.
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[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>.
[SEP2.0] ZigBee Alliance, "ZigBee IP Specification", March 2014,
<hhttp://www.zigbee.org/non-menu-pages/zigbee-ip-
download/>.
[Sethi14] Sethi, M., Oat, E., Di Francesco, M., and T. Aura, "Secure
Bootstrapping of Cloud-Managed Ubiquitous Displays",
Proceedings of ACM International Joint Conference on
Pervasive and Ubiquitous Computing (UbiComp 2014), pp.
739-750, Seattle, USA , September 2014,
<http://dx.doi.org/10.1145/2632048.2632049>.
[simpleconn]
Wi-Fi Alliance, "Wi-Fi Simple Configuration", Wi-Fi
Alliance , 2019, <https://www.wi-
fi.org/download.php?file=/sites/default/files/private/Wi-F
i_Simple_Configuration_Technical_Specification_v2.0.7.pdf>
.
[simplekey]
Bergmann, O., Gerdes, S., and C. Bormann, "Simple Keys for
Simple Smart Objects", Smart Object Security Workshop,
IETF 83 , March 2012.
[SimplePairing]
Bluetooth, SIG, "Simple pairing whitepaper", Technical
report , 2007.
[threadcommissioning]
Thread Group, "Thread Commissioning", Thread Group, Inc. ,
2015.
[TS33220] 3GPP, "3rd Generation Partnership Project; Technical
Specification Group Services and System Aspects; Generic
Authentication Architecture (GAA); Generic Bootstrapping
Architecture (GBA) (Release 14)", December 2016,
<http://www.3gpp.org/DynaReport/33220.htm>.
[vendorcert]
IEEE std. 802.1ar-2009, "Standard for local and
metropolitan area networks - secure device identity",
December 2009.
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[vermillard]
Vermillard, J., "Bootstrapping device security with
lightweight M2M", Appeared on blog at medium.com ,
February 2015.
[wps] Wi-Fi Alliance, "Wi-fi protected setup", Wi-Fi Alliance ,
2007.
Authors' Addresses
Mohit Sethi
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: mohit@piuha.net
Behcet Sarikaya
Denpel Informatique
Email: sarikaya@ieee.org
Dan Garcia-Carrillo
University of Oviedo
Oviedo 33207
Spain
Email: garciadan@uniovi.es
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