Internet DRAFT - draft-nichols-iotops-defined-trust-transport
draft-nichols-iotops-defined-trust-transport
Network Working Group K. Nichols
Internet-Draft Pollere LLC
Intended status: Informational V. Jacobson
Expires: 4 October 2023 UCLA
R. King
Operant Networks Inc.
2 April 2023
Defined-Trust Transport (DeftT) Protocol for Limited Domains
draft-nichols-iotops-defined-trust-transport-01
Abstract
This document describes a broadcast-friendly, many-to-many Defined-
trust Transport (DeftT) that makes it simple to express and enforce
application and deployment specific integrity, authentication, access
control and behavior constraints directly in the protocol stack.
DeftT is part of a Defined-trust Communications framework with an
example codebase, not a protocol specification. Combined with IPv6
multicast and modern hardware-based methods for securing keys and
code, it provides an easy to use foundation for secure and efficient
communications in Limited Domains (RFC8799), in particular for
Operational Technology (OT) networks.
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
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This Internet-Draft will expire on 4 October 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Environment and use . . . . . . . . . . . . . . . . . . . 4
1.2. Transporting information . . . . . . . . . . . . . . . . 5
1.3. Securing information . . . . . . . . . . . . . . . . . . 7
1.4. Defined-trust Communications Domains . . . . . . . . . . 8
1.5. Current status . . . . . . . . . . . . . . . . . . . . . 9
2. DeftT and Defined-trust Communications . . . . . . . . . . . 10
2.1. Inside DeftT . . . . . . . . . . . . . . . . . . . . . . 11
2.2. syncps: a set reconciliation protocol . . . . . . . . . . 12
2.3. Formats of DeftT Communications . . . . . . . . . . . . . 13
2.3.1. Publications . . . . . . . . . . . . . . . . . . . . 13
2.3.2. Certificates . . . . . . . . . . . . . . . . . . . . 15
2.3.3. cState . . . . . . . . . . . . . . . . . . . . . . . 15
2.3.4. cAdd . . . . . . . . . . . . . . . . . . . . . . . . 16
2.4. Interface between application and network interface . . . 17
2.5. Schema-based information movement . . . . . . . . . . . . 19
2.6. Congestion control . . . . . . . . . . . . . . . . . . . 21
3. Defined-trust management engine . . . . . . . . . . . . . . . 22
3.1. Communications schemas . . . . . . . . . . . . . . . . . 22
3.2. A schema language . . . . . . . . . . . . . . . . . . . . 23
4. Certificates and identity bundles . . . . . . . . . . . . . . 26
4.1. Obviate CA usage . . . . . . . . . . . . . . . . . . . . 27
4.2. Identity bundles . . . . . . . . . . . . . . . . . . . . 28
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.1. Secure Industrial IoT . . . . . . . . . . . . . . . . . . 30
5.2. Secure access to Distributed Energy Resources (DER) . . . 31
6. Using Defined-trust Communications without DeftT . . . . . . 33
7. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
8. Security Considerations . . . . . . . . . . . . . . . . . . . 35
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 38
10. Normative References . . . . . . . . . . . . . . . . . . . . 38
11. Informative References . . . . . . . . . . . . . . . . . . . 38
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 45
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1. Introduction
Decades of success in providing IP connectivity over any physical
media ("IP over everything") has commoditized IP-based
communications. This makes IP an attractive option for Internet of
Things (IoT), Industrial Control Systems (ICS) and Operational
Technologies (OT) applications like building automation, embedded
systems and transportation control, that previously required
proprietary or analog connectivity. For the energy sector in
particular, the growing use of Distributed Energy Resources (DER)
like residential solar has created interest in low cost commodity
networked devices but with added features for security, robustness
and low-power operation [MODOT][OPR][CIDS]. Other emerging uses
include connecting controls and sensors in nuclear power plants and
carbon capture monitoring [DIGN][IIOT].
While moving to an IP network layer is a major advance for OT,
current Internet transport options are a poor match to its needs.
TCP generalized the Arpanet transport notion of a packet "phone call"
between two endpoints into a generic, reliable, bi-directional
bytestream working over IP's stateless unidirectional best-effort
delivery model. Just as the voice phone call model spawned a global
voice communications infrastructure in the 1900s, TCP/IP's two-party
packet sessions are the foundation of today's global data
communication infrastructure. But "good for global communication"
isn't the same as "good for everything". OT applications tend to be
localized and communication-intensive with a primary function of
coordination and control and communication patterns that are many-to-
many. Implementing many-to-many applications over two-party
transport sessions changes the configuration burden and traffic
scaling from the native media's O(_n_) to O(_n_^2) (see Section 1.2).
Further, as OT devices have specific, highly prescribed roles with
strict constraints on "who can say what to which", the opacity of
modern encrypted two-party sessions can make it impossible to enforce
or audit these constraints.
This memo describes a new transport protocol, Defined-trust Transport
(DeftT) for Limited Domains [RFC8799] in which multipoint
communications are enabled through use of a named collection
abstraction and secured by an integrated trust management engine.
DeftT employs multicast (e.g., IPv6 link-local [RFC4291]), a
distributed set reconciliation PDU transport, a flexible pub/sub API,
chain-of-trust membership identities, and secured rules that define
the local context and communication constraints of a deployment in a
declarative language. These rules are used by DeftT's runtime trust
management engine to enforce adherence to the constraints. The
resulting system is efficient, secure and scalable: communication,
signing and validation costs are constant per-publication,
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independent of the richness and complexity of the deployment's
constraints or the number of entites deployed. Like QUIC, DeftT is a
user-space transport protocol that sits between an application and a
system-provided transport like UDP or UDP multicast (see Figure 1).
(Artwork only available as svg: figs/defttlayer-rfc.svg)
Figure 1: DeftT's place in an IP stack
Device enrollment consists of configuring a device with _identity
bundles_ that contains the trust anchor certificate, a compact and
secured copy of the communication rules, and a membership identity
(for domain communications) which comprises all the certs in its
signing chain (which can be used to confer attributes) terminated at
the trust anchor. The secret key corresponding to the leaf
certificate of the identity should be securely configured while the
security of the identity bundle can be deployment-specific. The
identity chains of all communicating members share a common trust
anchor and the rules that define legal signing chains, so the bundle
suffices for a member to authenticate and authorize communication
from peers and vice-versa. New members can join and communicate
without labor intensive and error-prone device-to-device association
configuration.
1.1. Environment and use
Due to physical deployment constraints and the high cost of wiring,
OT networks preferentially use radio as their communication medium.
Use of wires is impossible in many installations (untethered Things,
adding connected devices to home and infrastructure networks,
vehicular uses, etc.). Wiring costs far exceed the cost of current
System-on-Chip Wi-Fi IoT devices and the cost differential is
increasing [WSEN][COST]. For example, the popular ESP32 is a
32bit/320KB SRAM RISC with 60 analog and digital I/O channels plus
complete 802.11b/g/n and bluetooth radios on a 5mm die that consumes
70uW in normal operation. It currently costs $0.13 in small
quantities while the estimated cost of pulling cable to retrofit
nuclear power plants is presently $2000/ft [NPPI].
OT applications are frequently Limited Domain with communications
that are local, have a many-to-many pattern, and use application-
specific identifiers ("topics") for rendezvous. This fits the
generic Publish/Subscribe communications model ("pub/sub") and, as
table 1 in [PRAG] shows, nine of the eleven most widely used IoT
protocols use a topic-based pub/sub transport. For example MQTT, an
open standard developed in 1999 to monitor oil pipelines over
satellite [MQTT][MHST], is now likely the most widely used IoT
protocol (https://mqtt.org/use-cases/ (https://mqtt.org/use-cases/)).
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Microsoft Azure, Amazon AWS, Google Cloud, and Cloudflare all offer
hosted MQTT brokers for collecting and connecting sensor and control
data in addition to providing local pub/sub in buildings, factories
and homes. Pub/sub protocols communicate by using the same topic but
need no knowledge of one another. These protocols are typically
_implemented_ as an application layer protocol over a two-party
Internet transports like TCP or TLS which require in-advance
configuration of peer addresses and credentials at each endpoint and
incur unnecessary communications overhead Section 1.2.
1.2. Transporting information
The smart lighting example of Figure 2 illustrates a topic-based pub/
sub application layer protocol in a wireless broadcast subnet. Each
switch is set up to do triple-duty: one click of its on/off paddle
controls some particular light(s), two clicks control all the lights
in the room, and three clicks control all available lights (five
kitchen plus the four den ceiling). Thus a switch button push may
require a message to as many as nine light devices. On a broadcast
physical network each packet sent by the switch is heard by all nine
devices. IPv6 link-level multicast provides a network layer that can
take advantage of this but current IP transport protocols cannot.
Instead, each switch needs to establish nine bi-lateral transport
associations in order to send the published message for all lights to
turn on. Communicating devices must be configured with each other's
IP address and enrolled identity so, for _n_ devices, both the
configuration burden and traffic scale as O(_n^2_). For example,
when an "_all_" event is triggered, every light's radio will receive
nine messages but discard the eight determined to be "not mine." If
a device sleeps, is out-of-range, or has partial connectivity,
additional application-level mechanisms have to be implemented to
accommodate it.
(Artwork only available as svg: figs/iotDeftt-rfc.svg)
Figure 2: Smart lighting use of Pub/Sub
MQTT and other broker-based pub/sub approaches mitigate this by
adding a _broker_ where all transport connections terminate
(Figure 3). Each entity makes a single TCP transport connection with
the broker and tells the broker the topics to which it subscribes.
Thus the kitchen switch uses its single transport session to publish
commands to topic kitchen/counter, topic kitchen or all. The kitchen
counter light uses its broker session to subscribe to those same
three topics. The kitchen ceiling lights subscribe to topics kitchen
ceiling, kitchen and all while den ceiling lights subscribe to topics
den ceiling, den and all. Use of a broker reduces the configuration
burden from O(_n_^2) to O(_n_): 18 transport sessions to 11 for this
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simple example but for realistic deployments the reduction is often
greater. There are other advantages: besides their own IP addresses
and identities, devices only need to be configured with those of the
broker. Further, the broker can store messages for temporarily
unavailable devices and use the transport session to confirm the
reception of messages. This approach is popular because the pub/sub
application layer protocol provides an easy-to-use API and the broker
reduces configuration burden while maintaining secure, reliable
delivery and providing short-term in-network storage of messages.
Still the broker implementation doubles the per-device configuration
burden by adding an entity that exists only to implement transport
and traffic still scales as O(_n^2_), e.g., any switch publishing to
all lights results in ten (unicast) message transfers over the wifi
network. Further, the broker introduces a single point of failure
into a network that is richly connected physically.
(Artwork only available as svg: figs/iotMQTT-rfc.svg)
Figure 3: Brokers enable Pub/Sub over connection/session protocols
Clearly, a transport protocol able to exploit a physical network's
broadcast capabilities would better suit this problem. (Since
unicast is just multicast restricted to peer sets of size 2, a
multicast transport handles all unicast use cases but the converse is
not true.) In the distributed systems literature, communication
associated with coordinating shared objectives has long been modeled
as _distributed set reconciliation_ [WegmanC81][Demers87]. In this
approach, each domain of discourse is a named set, e.g.,
_myhouse.iot_. Each event or action, e.g., a switch button press, is
added as a new element to the instance of _myhouse.iot_ at its point
of origin then the reconciliation process ensures that every instance
of _myhouse.iot_ has this element. In 2000, [MINSKY03] developed a
broadcast-capable set reconciliation algorithm whose communication
cost equaled the set instance _differences_ (which is optimal) but
its polynomial computational cost impeded adoption. In 2011, [DIFF]
used Invertible Bloom Lookup Tables (IBLTs) [IBLT][MPSR] to create a
simple distributed set reconciliation algorithm providing optimal in
both communication and computational cost. DeftT uses this algorithm
(see Section 2.2) and takes advantage of IPv6's self-configuring link
local multicast to avoid all manual configuration and external
dependencies. This restores the system design to Figure 2 where each
device has a single, auto-configured transport that makes use of the
broadcast radio medium without need for a broker or multiple
transport associations. Each button push is broadcast exactly once
to be added to the distributed set.
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1.3. Securing information
Conventional session-based transports combine multiple publications
with independent topics and purposes under a single session key,
providing privacy by encrypting the sessions between endpoints. The
credentials of endpoints (e.g., a website) are usually attested by a
third party certificate authority (CA) and bound to a DNS name; each
secure transport association requires the exchange of these
credentials which allows for secure exchange of a nonce symmetric
key. In Figure 3 each transport session is a separate security
association where each device needs to validate the broker's
credential and the broker has to validate each device's. This
ensures that transport associations are between two enrolled devices
(protecting against outsider and some MITM attacks) but, once the
transport session has been established there are no constraints
whatsoever on what devices can say. Clearly, this does not protect
against the insider attacks that currently plague OT, e.g., [CHPT]
description of a lightbulb taking over a network. For example, the
basic function of a light switch requires that it be allowed to tell
a light to turn on or off but it almost certainly shouldn't be
allowed to tell the light to overwrite its firmware (fwupd), even
though "on/off" and "fwupd" are both standard capabilities of most
smart light APIs. Once a TLS session is established, the transport
handles "fwupd" publications _the same way_ as "on/off" publications.
Such attacks can be prevented using trust management that operates
per-publication, using rules that enable the "fwupd" from the light
switch to be rejected. Combining per-publication trust decisions
with many-to-many communications over broadcast infrastructure
requires per-publication signing rather than session-based signing.
Securing each publication rather than the path it arrives on deals
with a wider spectrum of threats while avoiding the quadratic session
state and traffic burden. In OT, valid messages conform to rigid
standards on syntax and semantics
[IEC61850][ISO9506MMS][ONE][MATR][OSCAL][NMUD][ST][ZCL] that can be
combined with site-specific requirements on identities and
capabilities to create a system's communication rules. These rules
can be employed to secure publications in a trust management system
such as [DLOG] where each publisher is responsible for supplying all
of the "who/what/where/when" information needed for each subscriber
to _prove_ the publication complies with system policies.
Instead of vulnerable third-party CAs [W509], sites employ a local
root of trust and locally created certificates. When the
communication rules are expressed in a _declarative_ language [DLOG],
they can be validated for consistency and completeness then converted
to a compact runtime form which can be authorized and secured via
signing with the system trust anchor. This _communication schema_
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can be distributed as a certificate, then validated using on-device
trusted enclaves [TPM][HSE][ATZ] as part of the device enrollment
process. In DeftT's publication-based transport, the schema is used
to both construct and validate publications, guaranteeing that _all_
parts of the system _always_ conform to and enforce the same rules,
even as those rules evolve to meet new threats (more in Section 3.1).
DeftT embeds the trust management mechanism described above directly
in the publish and subscribe data paths as shown below:
(Artwork only available as svg: figs/trustElements-rfc.svg)
Figure 4: Trust management elements of DeftT.
This approach extends LangSec's [LANG] "be definite in what you
accept" principle by using the authenticated common ruleset for belt-
and-suspenders enforcement at both publication and subscription
functions of the transport. If an application asks the Publication
Builder to publish something and the schema shows it lacks
credentials, an error is thrown and nothing is published.
Independently, the Publication Validator ignores publications that:
* don't have a locally validated, complete signing chain for the
credential that signed it
* the schema shows its signing chain isn't appropriate for this
publication
* have a publication signature that doesn't validate
Note that since an application's subscriptions determine which
publications it wants, only certificates from chains that can sign
publications matching the subscriptions need to be validated or
retained. Thus a device's communication state burden and computation
costs are a function of how many different things are allowed to talk
to it but _not_ how many things it talks to or the total number of
devices in the system. In particular, event driven, publish-only
devices like sensors spend no time or space on validation. Unlike
most 'secure' systems, adding additional constraints to schemas to
reduce attack surface results in devices doing _less_ work.
1.4. Defined-trust Communications Domains
A Defined-trust Communications Limited Domain (or simply, _trust
domain_) is a Limited Domain where all the members communicate via a
DeftT Figure 5 and are configured with the same trust anchor, schema,
a schema-conformant DeftT identity cert chain that terminates at the
trust anchor and the secret key corresponding to the identity chain's
leaf cert. The particular rules for any deployment are application-
specific (e.g., Is it home IoT or a nuclear power plant?) and site-
specific (specific form of credential and idiosyncrasies in rules)
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which DeftT accommodates by being invoked with a ruleset (schema)
particular to a deployment. We anticipate that the efforts to create
common data models (e.g., [ONE]) for specific sectors will lead to
easier and more forms-based configuration of DeftT deployments.
A trust domain is perimeterless and may operate over one or more
subnets, sharing physical media with non-member entities. Member
entities throughout the domain publish and subscribe to its topics
using Publication Builders and Validators as shown in Figure 4.
These Publications become the elements of a set, or named collection,
that is confined to each subnet. DeftT uses a distributed set
reconciliation protocol on _each_ collection and _each_ subnet
independently. Every DeftT maintains at least two collections:
*pubs* for application Publications and *cert* where identity bundle
certs are published. Figure 5
(Artwork only available as svg: figs/trustdomain-rfc.svg)
Figure 5: Trust domain
Trust domains are extended across physically separated subnets,
subnets using different media and/or subdomains on the same subnet
(see Section 2.5) by using *Relays* that have a DeftT in each subnet
and pass Publications between subnets as long as they are valid at
the receiving DeftT Figure 6. Since set reconciliation does not
accept duplicates, Relays are powerful elements in creating efficient
configuration-free meshes. The subnets of the figure could be
different colocated media (e.g. bluetooth, wifi, ethernet) or may be
physically distant. The triangle Relay-only subnet can be carried
over a unicast link. The set reconciliation protocol ensures that
items only transit a subnet once: an item must be specifically
requested in order to be transmitted. More Relay discussion is in
Section 2.5 and Section 5.
(Artwork only available as svg: figs/relayedtrustdomain-rfc.svg)
Figure 6: Relayed trust domain
1.5. Current status
An open-source Defined-trust Communications Toolkit [DCT] with an
example implementation of DeftT is maintained by the corresponding
author's company. [DCT] has examples of using DeftT to implement
secure brokerless message-based pub/sub using UDP/IPv6 multicast and
unicast UDP/TCP and include extending a Trust Domain via a unicast
connection or between two broadcast network segments. Working
implementations and performance improvements are occasionally added
to the repository.
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Massive build out of the renewable energy sector is driving
connectivity needs for both monitoring and control. Author King's
company, Operant, is currently developing extensions of DeftT in a
mix of open-source and proprietary software tailored for commercial
deployment in support of distributed energy resources (DER). Current
small scale use cases have performed well and expanded usage is
planned. Pollere is also working on home IoT uses. Our development
philosophy is to start from solving useful problems with a well-
defined scope and extend from there. As the needs of our use cases
expand, the Defined-trust communications framework will evolve with
increased efficiencies. DeftT's code is open source, as befits any
communications protocol, but even more critical for one attempting to
offer security. DCT itself makes use of the open source
cryptographic library libsodium [SOD] and the project is open to
feedback on potential security issues as well as hearing from
potential collaborators.
The well-known issues with 802.11 multicast [RFC9119] can make DeftT
less efficient than it should be. Target OT deployments primarily
use smaller packet sizes and DeftT's set reconciliation provides
robust delivery that currently mitigates these concerns. DeftT use
may become another force for improved multicast on 802.11, joining
the critical network infrastructure applications of neighbor
discovery, address resolution, DHCP, etc.
Cryptographic signing takes most of the application-to-network time
in DeftT. Though not prohibitively costly, increased use of signing
in transports may incentivize creation of more efficient signing
algorithms.
2. DeftT and Defined-trust Communications
DeftT synchronizes and secures communications between enrolled
members of a Limited Domain [RFC8799]. DeftT's multi-party
synchronized collections of named, schema-conformant Publications
contrast with the bilateral session of TCP or QUIC where a source and
a destination coordinate with one another to transport
undifferentiated streams of information. DeftTs in a trust domain
may hold different subsets of the collection at any time (e.g.,
immediately after entities add elements to the collection) but the
synchronization protocol ensures all converge to holding the complete
set of elements within a few round-trip-times following the changes.
Applications use DeftT to add to and access from a collection of
Publications. DeftT enforces "who can say what to which" as well as
providing required integrity, authenticity and confidentiality.
Transparently to applications, a DeftT both constructs and validates
all Publications against its schema's formal, validated rules. The
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compiled binary communications schema is distributed as a trust-root-
signed certificate and that certificate's thumbprint (see
Section 2.3.2 and Section 7) _uniquely_ identifies each trust domain.
Each DeftT is configured with the trust anchor used in the domain,
the schema cert, and its own credentials for membership in the
domain. To communicate, DeftTs must be in the same domain. Identity
credentials comprise a unique private identity key along with a
public certificate chain rooted at the domain's trust anchor.
Certificates in identity chains are specified in the schema and
contain the attributes granted to the identity. Thus, attributes are
stored in the identity *not* on an external server.
Each member publishes its credentials to the certificate collection
in order to join the domain. DeftT validates credentials as a
certificate chain against the schema and does not accept Publications
without a fully validated signer. This unique approach enables fully
distributed policy enforcement without a secured-perimeter physical
network and/or extensive per-device configuration. DeftT can share
an IP network with non-DeftT traffic as well as DeftT traffic of a
different omain. Privacy via AEAD encryption is automatically
handled within DeftT if selected in the schema.
(Artwork only available as svg: figs/transportBD0v2-rfc.svg)
Figure 7: DeftT's interaction in a network stack
Figure 7 shows the data flow in and out of a DeftT. DeftT uses its
schema to package application information into Publications that are
added to its local view of the collection. Application information
is packaged in Publications which are carried in cAdd PDUs that are
used along with cState PDUs to communicate about and synchronize
Collections. cStates are used to report the state of the local
collection; cAdds carry Publications to other members that need them.
These PDUs are broadcast on their subnet (e.g., UDP multicast).
2.1. Inside DeftT
DeftT's reference implementation [DCT] is organized in functional
library modules that interact to prepare application-level
information for transport and to extract application-level
information from packets, see Figure 8. Extensions and alternate
module implementations are possible but the functionality and
interfaces must be preserved. Internals of DeftT are completely
transparent to an application and the reference implementation is
efficient in both lines of code and performance. The schema
determines which modules are used. A DeftT participates in two
required collections and _may_ participate in others if required by
the schema-designated signature managers. One of the required
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collections, descriptive collection name component *pubs*, contains
application Publications (see Table 2). The other required
collection, *cert*, manages the certificates of the trust domain.
Specific signature managers _may_ require group key distribution in
descriptively named collection *keys*.
(Artwork only available as svg: figs/DeftTmodules-rfc.svg)
Figure 8: Run-time library modules
A _shim_ serves as the translator between application semantics and
the named information objects (Publications) whose format is defined
by the schema. The *syncps* module is the set reconciliation
protocol used by DeftT (see Section 2.2). New signature managers,
distributors, and face modules may be added to the library to extend
features. More detail on each module can be found at [DCT] in both
code files and documents.
The signing and validation modules (_signature managers_) are used
for both Publications and cAdds. Following good security practice,
DeftT's Publications are constructed and signed _early_ in their
creation, then are validated (or discarded) early in the reception
process.The _schemaLib_ module provides certificate store access
throughout DeftT along with access to _distributors_ of group keys,
Publication building and structural validation, and other functions
of the trust management engine. This organization of interacting
modules is not possible in a strictly layered implementation.
2.2. syncps: a set reconciliation protocol
DeftT requires a method or protocol that keeps collections
synchronized, where the collection a set and the Publications are the
elements of the set. The *syncps* protocol uses IBLTs
[DIFF][IBLT][MPSR] to solve the multi-party set-difference problem
efficiently without the use of prior context and with communication
proportional to the size of the difference between the sets being
compared. Syncps announces its _collection state_ (set of currently
known Publications) by sending a cState PDU containing an IBLT. The
cState serves as a query for additional data that isn't reflected in
its local state. Receipt of a cState performs three simultaneous
functions: (1) announces new Publications, (2) notifies of
Publications that member(s) are missing and (3) acknowledges
Publication receipt. The first may prompt the recipient to share its
cState to get the new Publication(s). The second results in sending
a cAdd PDU containing all the missing and locally available
Publications that fit. The third may result in a progress
notification sent to other local modules so anything waiting for
delivery confirmation can proceed.
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On broadcast media, syncps uses the cStates it hears to suppress
sending its own and listens for any cAdds that add to its cState.
This means that one-to-many Publications require one cState and one
cAdd to be sent, independently of the number of members desiring the
Publication (the theoretical minimum possible for reliable delivery).
The digest size of a cState can be controlled by Publication
lifetime, dynamically constructing the digest to maximize
communication progress [Graphene][Graphene19] and, if necessary for a
large network, dynamically adapting topic specificity.
A cAdd with new Publication(s) responds to a particular cState so a
receiving syncps removes that cState as a pending query (it will be
replaced with a new cState with the addition of the new items). Any
DeftT that is missing a Publication (due to being out-of-range or
asleep, etc.) can receive it from any other DeftT. A syncps will
continue to send cAdds as long as cStates are received that are
missing any of its active Publications. This results in reliability
that is subscriber-oriented, not publisher-oriented and is efficient
for broadcast media, particularly with protocol features designed to
prevent multiple redundant broadcasts.
2.3. Formats of DeftT Communications
In DeftT, information containers (i.e., Publications, cAdds, Cstate)
hold names, content and signatures in TLVs. Tables 1-3 layout the
formats of Publications, cStates, cAdds and certificates, which are a
special type of Publication (where _keys_ are the information
carried). Publications and cAdds use a compatible format which
allows them to use the same library signing/validation modules
(_sigmgrs_) and the same parser. The cState/cAdd formats and
dynamics were originally prototyped using Named Data Networking.
Although the NDN code and architecture are not used in DeftT or DCT,
a restricted version of the NDNv3 TLV encoding is still used, with
TLV types from NDN's TLV Type Registry [NDNS], as is its IANA
assigned port number [RFC6335].
In Tables 1-3, the Type in level _i_ is contained within the TLV of
the previous level _i-1_ TLV.
2.3.1. Publications
Publications use a Name TLV to encode the name defined in the schema.
A Publication is _valid_ if it starts with the correct TLV, its Name
validates against the schema and it contains the five required Level
1 TLVs in the right order (top-down in Table 1) and nothing else.
MetaInfo contains the ContentType (in DeftT either type Key or Blob).
The Content carries the named information and may be empty.
SignatureInfo indicates the SignatureType used to select the
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appropriate signature manager (Figure 8). The SignatureType for a
collection's Publications is specified in the schema and each
Publication must match it. (A list of current types can be found in
[DCT] file include/dct/sigmgrs/sigmgr.hpp.) The KeyLocator holds the
thumbprint (see Section 2.3.2) of the certificate that signed this
Publication. If the Publication is a certificate, KeyLocator will be
followed by the ValidityPeriod. Finally, SignatureValue is
determined by the SignatureType and its format is verified by the
signature manager.
+=======+================+================+==================+
| Level | Level 1 | Level 2 | Comments |
| 0 | | | |
+=======+================+================+==================+
| Type | | | the value 6 |
+-------+----------------+----------------+------------------+
| | Name | | format specified |
| | | | by schema |
+-------+----------------+----------------+------------------+
| | MetaInfo | | |
+-------+----------------+----------------+------------------+
| | | ContentType | either type Key |
| | | | or Blob |
+-------+----------------+----------------+------------------+
| | Content | | arbitrary byte |
| | | | sequence; may |
| | | | have length 0 |
+-------+----------------+----------------+------------------+
| | SignatureInfo | | |
+-------+----------------+----------------+------------------+
| | | SignatureType | Value specified |
| | | | by schema |
+-------+----------------+----------------+------------------+
| | | KeyLocator | must be a |
| | | | thumbprint |
+-------+----------------+----------------+------------------+
| | | ValidityPeriod | Only for |
| | | | Certificates |
+-------+----------------+----------------+------------------+
| | SignatureValue | | format |
| | | | determined by |
| | | | SignatureType |
+-------+----------------+----------------+------------------+
Table 1: Publication format
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2.3.2. Certificates
Certificates (certs) are Publications with the ContentType set to Key
and both a KeyLocator and a ValidityPeriod. DCT certs are compatible
with the NDN Certificate standard V2 but adhere to a stricter set of
conventions to make them resistant to substitution, work factor and
DoS attacks. The _only_ KeyLocator type allowed in a DCT cert is a
KeyDigest type that must contain the 32 byte SHA256 digest of the
_entire_ signing cert (including SignatureValue). A self-signed cert
(such as a trust anchor) must set this digest to all zero. This
digest, a cert _thumbprint_ [IOTK], is the only locator allowed in
_any_ signed Defined-trust object (e.g., Publications, cAdd, schemas,
certs) and must be present in every signed object. A signed object
using any other type of locator will be considered unverifiable and
silently ignored. Certificate Names use a suffix:
KEY/<keyID>/dct/<version>
where the cert's thumbprint is the keyID and its creation time is the
version.
The original publisher of any signed object must ensure that that
_all_ certs, schemas, etc., needed to validate the object have been
published _before_ the object is published. If a member receives a
signed object that is missing any of its signing dependencies, the
object should be considered unverifiable and silently ignored. Such
objects must not be propagated.
2.3.3. cState
cState and cAdds are are the PDUs exchanged with the system-level
transport in use (e.g., UDP) but are only used by the syncps and face
modules Figure 8: syncps creates cState and cAdd PDUs while the face
manages the protocol interaction within the trust domain. A cState
PDU (see Table 2) is used to report the state of a Collection at its
originator. Collections are denoted by structured names which
include the identifier of a particular trust domain (thumbprint of
its schema cert). in DeftT PDU headers. A cState serves to inform
all subscribing entities of a trust domain about Publications
currently in the Collection, both so an entity can obtain
Publications it is missing and so an entity can add Publications it
has that are not reflected in the received cState.
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+=========+==========+=========+====================================+
| Level 0 | Level 1 | Level 2 | Comments |
+=========+==========+=========+====================================+
| Type | | | the value 5 |
+---------+----------+---------+------------------------------------+
| | Name | | |
+---------+----------+---------+------------------------------------+
| | | Generic | trust domain id |
+---------+----------+---------+------------------------------------+
| | | Generic | descriptive collection name |
+---------+----------+---------+------------------------------------+
| | | Generic | collection state (sender's view) |
+---------+----------+---------+------------------------------------+
| | Nonce | | uniquely distinguishes this |
| | | | cState |
+---------+----------+---------+------------------------------------+
| | Lifetime | | expiry time (ms after arrival) |
+---------+----------+---------+------------------------------------+
Table 2: cState format
A cState is _valid_ if it starts with the correct TLV and it contains
the three required Level 1 TLVs in the right order (top-down in
Table 2) and nothing else. Its Name must start with the trust domain
id of the DeftT, then a descriptive Collection name (of at least one
component) and finally a representation of the the state of the
Collection at the originator. There is no signature for a cState
PDU. (The cState format is a restricted subset of an NDNv3
Interest.)
2.3.4. cAdd
A cAdd PDU is used by _syncps_ to add Publications to a collection
and carries Publications as Content. _syncps_ creates a cAdd PDU
after receiving a cState and only if the recipient has Publications
that are not reflected in the received state. A cAdd is _valid_ if
it starts with the correct TLV, contains the five required Level 1
TLVs in the right order (top-down in Table 3) and nothing else. A
cAdd name is identical to the cState to which it responds.
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+=======+================+================+=======================+
| Level | Level 1 | Level 2 | Comments |
| 0 | | | |
+=======+================+================+=======================+
| Type | | | the value 6 |
+-------+----------------+----------------+-----------------------+
| | Name | | must match Name of |
| | | | cState it's adding to |
+-------+----------------+----------------+-----------------------+
| | MetaInfo | | |
+-------+----------------+----------------+-----------------------+
| | | ContentType | type cAdd |
+-------+----------------+----------------+-----------------------+
| | Content | | |
+-------+----------------+----------------+-----------------------+
| | | Publication(s) | one or more |
| | | | Publications to add |
| | | | to the Collection |
+-------+----------------+----------------+-----------------------+
| | SignatureInfo | | |
+-------+----------------+----------------+-----------------------+
| | | SignatureType | Value indicates which |
| | | | signature manager |
+-------+----------------+----------------+-----------------------+
| | | KeyLocator | Presence depends on |
| | | | SignatureType |
+-------+----------------+----------------+-----------------------+
| | SignatureValue | | Value holds the |
| | | | signature for this |
| | | | PDU |
+-------+----------------+----------------+-----------------------+
Table 3: cAdd format
The structure of the cState and cAdd Names means that nothing about
Publication Names (which are application-oriented) are exposed if
encrypted cAdds are specified in a schema. (The schema itself may be
distributed in an encrypted cAdd if desired).
2.4. Interface between application and network interface
Figure 7 and Figure 8 show the blocks and modules application
information passes through in DeftT. Its handling of application
information can be illustrated using an example of a new Publication
at a trust domain member and following its progress into a collection
and its reception by other members. For more detail, see the library
at [DCT]. DeftT uses [DCT]'s message-based pub/sub (_mbps_) *shim*
which kicks off all the necessary DeftT startup when an mbps object
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is instantiated by the application. After startup, the *pub* syncps
of each member will maintain a cState containing the IBLT of its view
of the collection. (In the stable, synchronized state, all IBLTs are
the same.)
Applications use an mbps _subscribe_ method to either subscribe to
all messages or to a subset by topic, passing a callback function to
handle matching items. These application-level subscriptions are
turned into syncps subscriptions via mbps. When the application has
new information to communicate, topic items (as parameters) and
message are passed to mbps with a _publish_ call. Only these topic
components and the message, if any, are passed between the
application and mbps. The message may be segmented into multiple
Publications by mbps, if the message size exceeds Publication
content. For each Publication, mbps-specific components are added to
the parameter list and the services of *schemaLib* are invoked in
order to build and publish a valid Publication according to the
schema (no Publication will be built if the correct attributes are
not contained in the member's identity chain). The Publication is
signed using the _sign_ method of the appropriate *sigmgr* and passed
to *syncps*.
syncps adds this Publication to its collection and updates its IBLT
to contain the new Publication. Since its application just created
it, syncps knows this is a new addition to the collection and it is a
response to the current cState. Thus the Publication is packaged
into a cAdd and signed using the _sign_ method of the designated
*sigmgr* and passed to the face. The updated IBLT is packaged into a
new cState that is handed to the face.
Members of the trust domain specifically respond only to IPv6 cAdds
that share their trust domain identifier (Section 2.3.3 and
Section 2.3.4). When a new cAdd is received at a member, the face
ensures it matches an outstanding cState and, if so, passes it on to
matching syncps(es). Syncps validates (both structurally and
cryptographically) the cAdd using the appropriate sigmgr's _validate_
and continues, removing Publications, if valid. Each Publication is
structurally validated via a sigmgr and valid Publications are added
to the local collection and IBLT. syncps passes this updated cState
to the local face. If this Publication matches a subscription it is
passed to mbps, invoking the sigmgr's decrypt if the Publication is
encrypted (Publication decryption is _not_ available at Relays.) mbps
receives the Publication and passes any topic components of interest
to the application along with the content (if any) to the application
via the callback registered when it subscribed. (If the original
content was spread across Publications, mbps will wait until all of
the content is received. The _sCnt_ component of a mbps Publication
Name is used for this.)
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2.5. Schema-based information movement
Although the Internet's transport and routing protocols emphasize
universal reachability with packet forwarding based on destination, a
significant number of applications neither need nor desire to transit
the Internet (e.g., see [RFC8799]). This is true for a wide class of
OT application. Further, liberal acceptance of packets while
depending on the good sending practices of others leaves critical
applications open to misconfiguration and attacks. DeftT *only*
moves its Publications in accordance with fully specified
communication rules. This approach differs from Internet forwarding
but offers new opportunities to address the specific security
requirements of applications in many Limited Domains.
DeftTs on the same subnet may be in different trust domains and
DeftTs in the same trust domain may not be on the same subnet. In
some cases, it is useful to define sub-domains whose DeftTs have a
compatible, but more limited, version of the trust domain's
communications schema. Compatible means there is at least one
publication type and associated signer specification in common or one
schema may be a subset of the other. In the case of sub-domains,
they be deployed on the same subnet or on different subnets. The
rules of a sub-domain compiled to a binary schema distributed as a
schema cert will have a different thumbprint from that of the full
trust domain.
In the case of DeftTs on the same subnet but in different trust
domains or different sub-domains, the cState and cAdd PDUs of
different domains are differentiated by the _domain id_ (thumbprint
of the domain's schema certificate as in Table 2) which can be used
at the face module to determine whether or not to process a PDU. A
particular sync collection is managed on a single subnet: cState and
cAdds are not forwarded off that subnet nor between DeftTs with
different domain ids on the same subnet. Instead, schema-compliant
Relays connect Publications between separate sync collections of the
same trust domain. Collections are differentiated by both subnet
(the physical media) and domain id (a required field of the cState
and cAdd PDUs). Consequently, cStates and cAdds are subnet-sprecific
while Publications belong to a trust domain (or sub-domain).
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A Relay is implemented [DCT] as an entity running on a device with a
DeftT interface on each subnet (two or more) or with multiple DeftT
interfaces to the same subnet Figure 9 where each uses a different
but compatible version of the others' schema. Each DeftT
participates in different sync collections and uses a communication
identity valid for the schema used by the DeftT. Only Publications
(including certs) are relayed between DeftTs and the Publication must
validate against the schema of each DeftT. Consequently cAdd
encryption is unique per collection while Publication encryption
holds across the domain.
As Relays do not originate Publications, their DeftT API module (a
"shim", see Section 2.1) performs _pass-through_ of valid
Publications. The Relay of Figure 9-left is on three separate
wireless subnets. If all three DeftTs are using an identical schema,
a new validated cert added to the cert store of an incoming DeftT is
then passed to the other two, which each validate the cert before
adding to their own cert stores (superfluous in this case, but not a
lot of overhead for additional security). When a valid Publication
is received at one DeftT, it is passed to the other two DeftTs to
validate against their schemas and published if it passes.
(Artwork only available as svg: figs/relayextend-rfc.svg)
Figure 9: Relays connect subnets
A Relay may have different identities and schemas for each DeftT but
_must_ have the same trust anchor and schemas must be identical
copies, proper subsets or overlapping subsets of the domain schema.
Publications that are undefined for a particular DeftT will be
silently discarded when they do not validate upon relay, just as they
are when received from a face. This means the Relay application of
Figure 9-left can remain the same but Publications will only be
published to a different subnet if its DeftT has that specification
in its schema. Relays may filter Publications at the application
level or restrict subscriptions on some of their DeftT interfaces.
Figure 9-right shows extending a trust domain geographically by using
a unicast connection (e.g., over a cell line or tunnel over the
Internet) between two Relays which also interface to local broadcast
subnets. Everything on each local subnet shows up on the other. A
communications schema subset could be used here to limit the types of
Publications sent on the remote link, e.g., logs or alerts. Using
this approach in Figure 9-right, local communications for subnet 1
can be kept local while subnet 2 might send commands and/or collect
log files from subnet 1.
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More generally, Relays can form a mesh of broadcast subnets with no
additional configuration (i.e., Relays on a broadcast network do not
need to be configured with others' identities and can join at any
time). The mesh is efficient: publications are only added to an
individual DeftT's collection once regardless of how it is received.
Relays with overlapping broadcast physical media will only add a
Publication to any of its DeftTs *once*; syncps ensures there are no
duplicates. More on the applicability of DeftT meshes is in
Section 5.
2.6. Congestion control
Each DeftT manages its collection on a single broadcast subnet (since
unicast is a proper subset of multicast, a point-to-point connection
is viewed as a trivial broadcast subnet) thus only has to deal with
that subnet's congestion. As described in the previous section, a
device connected to two or more subnets may create DeftTs having the
same collection name on each subnet with a *Publication* Relay
between them but DeftT never forwards *PDUs* between subnets. It is,
of course, possible to run DeftT over an extended broadcast network
like a PIM multicast group but the result will generally require more
configuration and be less reliable, efficient and secure than DeftT's
self-configuring peer-to-peer Relay mesh.
DeftT sends _at most one_ copy of any Publication over any subnet,
_independent_ of the number of publishers and subscribers on the
subnet. Thus the total DeftT traffic on a subnet is strictly upper
bounded by the application-level publication rate. As described in
Section 2.2, DeftTs publish a cState specifying the set elements they
currently hold. If a DeftT receives a cState specifying the same
elements (Publications) it holds, it doesn't send its cState. Thus
the upper bound on cState publication rate is the number of members
on the subnet divided by the cState lifetime (typically seconds to
minutes) but is typically one per cState lifetime due to the
duplicate suppression. Each member can send at most one cAdd in
response to a cState. This creates a strict request/response flow
balance which upper bounds the cAdd traffic rate to (number of
members - 1) times the cState publication rate. The flow balance
ensures an instance can't send a new cState until it's previous one
is either obsoleted by a cAdd or times out. Similarly a cAdd can
only be sent in response to the cState which it obsoletes. Thus the
number of outstanding PDUs per instance is at most one and DeftT
cannot cause subnet congestion collapse.
If a Relay is used to extend a trust domain over a path whose
bandwidth delay product is many times larger than typical subnet MTUs
(1.5-9KB), the one-outstanding-PDU per member constraint can result
in poor performance (1500 bytes per 100ms transcontinental RTT is
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only 120Kbps). DeftT can run over any lower layer transport and
stream-oriented transports like TCP or QUIC allow for a 'virtual MTU'
that can be set large enough for DeftT to relay at or above the
average publication rate (the default is 64KB which can relay up to
5Mbps of publications into a 100ms RTT). In this case there can be
many lower layer packets in flight for each DeftT cAdd PDU but their
congestion control is handled by TCP or QUIC.
3. Defined-trust management engine
OT applications are distinguished (from general digital
communications) by well-defined roles, behaviors and relationships
that constrain the information to be communicated (e.g., as noted in
[RFC8520]). Structured abstract profiles characterize the
capabilities and attributes of Things and can be machine-readable
(e.g., [ONE][RFC8520][ZCL]). Energy applications in particular have
defined strict role-based access controls [IEC] though proposed
enforcement approaches require interaction of a number of mechanisms
across the communications stack [NERC]. Structured profiles and
rules strictly define permitted behaviors including what types of
messages can be issued or acted on; undefined behaviors are not
permitted. These rules, along with local configuration, are
incorporated directly into the schemas used by DeftT's integrated
trust management engine to both prohibit undefined behaviors and to
construct compliant Publications. This not only provides a fine-
grained security but a highly _usable_ security, an approach that can
make an application writer's job easier since applications do not
need to contain local configuration and security considerations.
DCT [DCT] includes a language for expressing the rules of
communication, its compiler, and other tools to create the
credentials a DeftT needs at run-time. DCT is example code, not
currently optimized for performance.
3.1. Communications schemas
Defined-trust's use of communications schemas has been influenced by
[SNC][SDSI] and the field of _trust management_ defined by Blaze et.
al. [DTM] as the study of security policies, security credentials,
and trust relationships. Li et. al. [DLOG] refined some trust
management concepts arguing that the expressive language for the
rules should be _declarative_ (as opposed to the original work).
Communications schemas also have roots in the _trust schemas_ for
Named-Data Networking, described by Yu et al [STNDN] as "an overall
trust model of an application, i.e., what is (are) legitimate key(s)
for each data packet that the application produces or consumes."
[STNDN] gave a general description of how trust schema rules might be
used by an authenticating interpreter finite state machine to
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validate packets. A new approach to both a trust schema language and
its integration with communications was introduced in [NDNW] and
extended in [DNMP][IOTK][DCT]. In this approach, a schema is
analogous to the plans for constructing a building. Construction
plans serve multiple purposes:
1. Allow permitting authorities to check that the design meets
applicable codes
2. Show construction workers what to build
3. Let building inspectors validate that as-permitted matches as-
built
Construction plans get this flexibility from being declarative: they
describe "what", not "how". As noted in p4 of [DLOG], a declarative
trust management specification based on a formal foundation
guarantees all parties to a communication have the same notion of
what constitutes compliance. This allows a single schema to provide
the same protection as dozens of manually configured, per-node ACL
rules. This approach is a critical part of Defined-trust
Communications which uses the more descriptive term _communication
schema_ as the rules define the communications of a trust domain.
VerSec, an approach to creating schemas, is included with the
Defined-trust Communications Toolkit [DCT]. VerSec includes a
declarative schema specification language with a compiler that checks
the formal soundness of a specification (case 1 above) then converts
it to a signed, compact, binary form. The diagnostic output of the
compiler (including a digraph listing) can be used to inspect that
the intent for the communications schema has indeed been implemented.
The binary form is used by DeftT to build (case 2) or validate (case
3) the Publications (format covered in Section 2.3.1). Certificates
(Section 2.3.2) are a type of Publication, allowing them to be
distributed and validated using DeftT, but they are subject to many
additional constraints that ensure DeftT's security framework is
well-founded.
3.2. A schema language
The VerSec language follows LangSec [LANG] principles to minimize
misconfiguration and attack surface. Its structure is amenable to a
forms-based input or a translator from the structured data profiles
often used by standards [ONE][RFC8520][ZCL]. Declarative languages
are expressive and strongly typed, so they can express the constructs
of these standards in their rules. VerSec continues to evolve and
add new features as its application domain is expanded; the latest
released version is at [DCT]. Other languages and compilers are
possible as long as they supply the features and output needed for
DeftT.
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A communication schema expresses the intent for a domain's
communications in fine-grained rules: "who can say what."
Credentials that define "who" are specified along with complete
definitions of "what". Defined-trust communications has been
targeted at OT networking where administrative control is explicit
and it is not unreasonable to assume that identities and
communication rules can be securely configured at every device. The
schema details the meaning and relationship of individual components
of the filename-like names (URI syntax [RFC3986]) of Publications and
certificates. A simple communications schema (Figure 10) defines a
Publication in this domain as *#pub* with a six component name. The
strings between the slashes are the tags used to reference each
component in the structured format and in the run-time schema
library. An example of this usage is the component constraint
following the "&" where _ts_ is a timestamp (64-bit unix timepoints
in microseconds) which will be set with the current time when a
Publication is created. The first component gets its value from the
variable "domain" and #pubPrefix is designated as having this value
so that the schema contains information on what part of the name is
considered common prefix. For the sake of simplicity, the Figure 10
schema puts no constraints on other name components (not the usual
case for OT applications) but requires that Publications of template
#pub are signed by ("<=") a *mbrCert* whose format and signing rule
(signed by a netCert) is also defined. The Validator lines specify
cryptographic signing and validation algorithms from DCT's run-time
library for both the Publication and the cAdd PDU that carries
Publications. Here, both use EdDSA signing. This schema has no
constraints on the inner four name components (additional constraints
could be imposed by the application but they won't be enforced by
DeftT). Member identity comes from a *mbrCert* which allows it to
create legal communications (using the associated private key in
signing). A signing certificate must adhere to the schema;
Publications or cAdds with unknown signers are discarded. The
timestamp component is used to prevent replay attacks. A DeftT adds
its identity certificate chain to the domain certificate collection
(see Section 4.2) at its startup, thus announcing its identity to all
other members. Using the pre-configured trust anchor and schema, any
member can verify the identity of any other member. This approach
means members are not pre-configured with identities of other members
of a trust domain and new entities can join at any time.
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#pub: /_domain/trgt/topic/loc/arg/_ts & { _ts: timestamp() } <= mbrCert
mbrCert: _domain/_mbrType/_mbrId/_keyinfo <= netCert
netCert: _domain/_keyinfo
#pubPrefix: _domain
#pubValidator: "EdDSA"
#cAddValidator: "EdDSA"
_domain: "example"
_keyinfo: "KEY"/_/"dct"/_
Figure 10: An example communication schema
To keep the communications schema both compact and secure, it is
compiled into a binary format that becomes the content of a schema
certificate. The [DCT] _schemaCompile_ converts the text version
(e.g. Figure 10) of the schema into binary as well as reporting
diagnostics (see Figure 11) used to confirm the intent of the rules
(and to flag problems).
Publication #pub:
parameters: trgt topic loc arg
tags: /_domain/trgt/topic/loc/arg/_ts
Publication #pubPrefix:
parameters:
tags: /_domain
Publication #pubValidator:
parameters:
tags: /"EdDSA"
Publication #cAddValidator:
parameters:
tags: /"EdDSA"
Certificate templates:
cert mbrCert: /"example"/_mbrType/_mbrIdId/"KEY"/_/"dct"/_
cert netCert: /"example"/"KEY"/_/"dct"/_
binary schema is 301 bytes
Figure 11: schemaCompile diagnostic output for example of Figure 10
Even this simple schema provides useful security, using enrolled
identities both to constrain communications actions (via its #*pub*
format) and to convey membership. To increase security, more detail
can be added to Figure 10. For example, different types of members
can be created, e.g., "admin" and "sensor", and communications
privacy can added by specifying AEAD Validator to encrypt cAdds or
AEADSGN (signed AEAD) to encrypt Publications. To make those member
types meaningful, a role-based security policy could be employed by
defining Publications such that only admins can issue _commands_ and
only sensors can issue _status_. Specifying the AEAD validator for
the cAddValidator means that at least one member of a subnet will
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need a key maker attribute in its signing chain. If AEADSGN is
specified for the pubValidator, at least one member of the trust
domain will need key maker capability. In Figure 11 key maker
capability is added to the signing chain of all sensors. WIth AEAD
specified, a key maker is elected during DeftT start up and that key
maker creates, publishes, and periodically updates the shared
encryption key. (Late joining entities are able to discover that a
key maker has already been chosen.) These are the _only_ changes
required in order to increase security and add privacy: neither code
nor binary needs to change and DeftT handles all aspects of
validators. The unique approach to integrating communication rules
into the transport makes it easy to produce secure application code.
adminCert: mbrCert & { _mbrType: "admin" } <= netCert
sensorCert: mbrCert & { _mbrType: "sensor" } <= kmCap
capCert: _network/"CAP"/_capId/_capArg/_keyinfo <= netCert
kmCap: capCert & { _capId: "KM" }
#reportPub: #pub & {topic:"status"} <= sensorCert
#commandPub: #pub & {topic:"command"} <= adminCert
#cAddValidator: "EdDSA"
Figure 12: Enhancing security in the example schema
In DeftT, identities include the member cert and its entire signing
chain. By adding attributes via _capability certificates_ in a
member cert's signing chain, attribute-based security policies can be
implemented without the need for separate servers accessed at run-
time (and the attendant security weaknesses). More on certs will be
covered in Section 4.
Converting desired behavioral structure into a schema is the major
task of applying Defined-trust Communications to an application
domain. Once completed, all the deployment information is contained
in the schema. Although a particular schema cert defines a
particular trust domain, the text version of a schema can be re-used
for related applications. For example, a home IoT schema could be
edited to be specific to a particular home network or a solar rooftop
neighborhood and then signed with a chosen trust anchor.
4. Certificates and identity bundles
Defined-trust's approach is partially based on the seminal SDSI
[SDSI] approach to create user-friendly namespaces that establish
transitive trust through a certificate (cert) chain that validates
locally controlled and managed keys, rather than requiring a global
Public Key Infrastructure (PKI). When certificates are created, they
have a particular context in which they should be utilized and
trusted rather than conferring total authority. This is particularly
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useful in OT where communicating entities share an administrative
control and using a third party to certify identity is both
unnecessary and a potential security vulnerability. Well-formed
certificates and identity deployment are critical elements of this
framework. This section describes certificate requirements and the
identity _bundles_ that are securely distributed to trust domain
members. (DCT includes utilities to create certs and bundles.)
4.1. Obviate CA usage
Use of third party certificate authorities (CAs) is often
antithetical to OT security needs. Any use of a CA (remote or local)
results in a single point of failure that greatly reduces system
reliability. An architecture with a single, local, trust root cert
(trust anchor) and no CAs simplifies trust management and avoids the
well-known CA federation and delegation issues and other weaknesses
of the X.509 architecture (summarized at [W509], original references
include [RSK][NVR]). DCT certificates (see Section 2.3.2) can be
generated and signed locally (using supplied utilities) so there is
no reason to aggregate a plethora of unrelated claims into one cert
(avoiding the Aggregation problem [W509]).
A DCT cert's one and only Subject Name is the Name of the Publication
that contains the public key as its content and neither name nor
content are allowed to contain any optional information or
extensions. Certificates are created with a lifetime; local
production means cert lifetimes can be just as long as necessary (as
recommended in [RFC2693]) so there's no need for the code burden and
increased attack surface associated with certificate revocation lists
(CRLs) or use of on-line certificate status protocol (OSCP). Keys
that require longer lifetimes, like device keys, get new certs before
the current ones expire and may be distributed through DeftT (e.g.,
using a variant of the group key distributors in DCT). If there is a
need to exclude previously authorized identities from a domain, there
are a variety of options. The most expedient is via use of an AEAD
cAdd or Publication validator by ensuring that the group key maker(s)
of a domain exclude that entity from subsequent symmetric key
distributions until its identity cert expires (and it is not issued
an update). Another option is to publish an identity that supplants
that of the excluded member. Though more complex, it is also
possible to distribute a new schema and identities (without changing
the trust anchor), e.g., using remote attestation via the TPM.
From Section 3, a member cert is granted attributes in the schema via
the certs that appear in its member identity chain. Member certs are
_always_ accompanied by their full chain-of-trust, both when
installed and when the member publishes its identity to the cert
collection. Every signing chain in the domain has the same trust
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anchor at its root and its legal form specified in the schema.
Without the entire chain, a signer's right to issue Publications
cannot be validated. Cert validation is according to the schema
which may specify attributes and capabilities for Publication signing
from any certificate in the chain. For this model to be well
founded, each cert's *key locator* must _uniquely_ identify the cert
that actually signed it. This property ensures that each locator
resolves to one and only one signing chain. A cert's key locator is
a *thumbprint*, a SHA256 hash of the _entire_ signer's Publication
(name, content, key locator, and signature), ensuring that each
locator resolves to one and only one cert and signing chain. Use of
the thumbprint locator ensures that certs are not open to the
substitution attacks of name-based locators like X.509's "Authority
Key Identifier" and "Issuer" [ConfusedDep][CAvuln][TLSvuln].
4.2. Identity bundles
Identity bundles comprise the certificates needed to participate in a
trust domain: trust anchor, schema, and the member's identity chain.
The private key corresponding to the leaf certificate of the member's
identity chain should be installed securely when a device is first
commissioned (e.g., out-of-band) for a network. The public certs of
the bundle may be placed in a file in a well-known location or may,
in addition, have their integrity attested or even be encrypted.
Secure device configuration and on-boarding should be carried out
using the best practices most applicable to a particular deployment.
The process of enrolling a device by provisioning an initial secret
and identity in the form of public-private key pair and using this
information to securely onboard a device to a network has a long
history. Current and emergent industry best practices provide a
range of approaches for both secure installation and update of
private keys. For example, the private key of the bundle can be
secured using the Trusted Platform Module, the best current practice
in IoT [TATT][DMR][IAWS][TPM][OTPM][SIOT][QTPM][SKH][RFC8995], or
secure enclave or trusted execution environment (TEE) [ATZ]. In that
case, an authorized configurer adding a new device can use TPM tools
to secure the private signing key and install the rest of the bundle
file in a known location before deploying the device in the network.
Where entities have public-private key pair identities of _any_
(e.g., non-DCT) type, these can be leveraged for DeftT identity
installation. Figure 13 shows the steps involved in configuring
entities and their correspondence of the steps to the "building
plans" model. (The corresponding tools available in DCT are shown
across the bottom and the relationship to the "building plans" model
is shown across the top.)
(Artwork only available as svg: figs/tools.config-rfc.svg)
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Figure 13: Creating and configuring identity bundles
In the examples at [DCT], an identity bundle is given directly to an
application via the command line, useful for development, and the
application passes callbacks to utility functions that supply the
certs and a signing pair separately. For deployment, good key
hygiene using best current practices must be followed e.g., [COMIS].
In deployment, a small application manager may be programmed for two
specific purposes. First, it is registered with a supervisor [SPRV]
(or similar process control) for its own (re)start to serve as a
bootstrap for the application. Second, it can have access to the TPM
functions and the ability to create "short-lived" (~hours to several
days) public/private key pair(s) that are signed by the installed
(commissioned) private identity key using the TPM. This publication
signing key pair is created at (re)start and at the periodicity of
the signing cert lifetime. Since the signing happens via requests to
the TPM, the identity key cannot be exfiltrated.
The DCT examples and library use member identities to create signing
certs (with associated secret keys) and the example schemas give the
format for these signing cert names. A DeftT will request a new
signing cert shortly before expiration of the one in use.
Upon each signing cert update, only the new cert needs to be
published via DeftT's cert distributor. Figure 14 outlines a
representative procedure.
(Artwork only available as svg: figs/InstallIdbundle-rfc.svg)
Figure 14: Representative commissioning and signing key maintenance
All DCT certs have a validity period. Certs that sign publications
are generated locally so they can easily be refreshed as needed.
Trust anchors, schemas, and the member identity chain are higher
value and often require generation under hermetic conditions by some
authority central to the organization. Their lifetime should be
application- and deployment-specific, but the higher difficulty of
cert production and distribution often necessitates liftetimes of
weeks to years.
Updating schemas and other certificates over the deployed network
(OTA) is application-domain specific and can either make use of
domain best practices or develop custom DeftT-based distribution.
Changing the trust anchor is considered a re-commissioning. The
example here is merely illustrative; with pre-established secure
identities and well-founded approaches to secure on-line
communications, a trust domain could be created OTA using secure
identities established through some other system of identity.
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5. Use Cases
5.1. Secure Industrial IoT
IIoT sensors offer significant advantages in industrial process
control including improved accuracy, process optimization, predictive
maintenance and analysis, higher efficiency, low-cost remote
accessibility and monitoring, reduced downtime, power savings, and
reduced costs [IIOT]. The large physical scale of many industrial
processes necessitates that expensive cabling costs be avoided
through wireless transport and battery power. This is a particular
issue in nuclear power plant applications where radioactive shielding
walls are very thick concrete and security regulations make any plant
modifications to add cabling subject to expensive and time-consuming
reviews and permitting. Wireless sensor deployments in an industrial
environment can suffer from signal outages due to shielding walls and
interference caused by rotating machinery and electrical generators.
Multiple _gateway_ devices can receive sensor information and
transmit it to monitor/controllers and servers. These gateway
devices can run DeftT Relay applications and be deployed in a robust
wireless mesh that is resilient against transmission outages,
facilitating reliability. DeftT forms meshes with no additional
configuration (beyond DeftT's usual identity bundle and private key)
as Publications are sent once and heard by all in-range members while
Publications missing from one DeftT's set can be supplied by another
within range. Several gateways may typically be within a single
sensor's wireless range, reducing the number of lost sensor packets.
Other meshed gateways can relay the sensor's Publications either
wirelessly or via a wired ethernet backhaul.
IIoT sensors require tight security. Critical Digital Assets (CDA)
are a class of industrial assets such as power plants or chemical
factories which must be carefully controlled to avoid loss-of-life
accidents. Even when IIoT sensors are not used for direct control of
CDA, spoofed sensor readings can lead to destructive behavior. There
are real-life examples (such as uranium centrifuges) of nation-state
actors changing sensor readings through cyberattacks leading to
equipment damage. These risks result in a requirement for stringent
security reviews and regulation of CDA sensor networks. Despite the
advantages of deploying CDA sensors, adequate security is
prerequisite to deploying the CDA sensors. Information conveyed via
DeftT has an ensured provenance and may be encrypted in an efficient
implementation making it ideal for this use.
IIoT sensors may be mobile (including drone-based) and different
gateways may receive packets from a particular sensor over time. A
DeftT mesh captures Publications _anywhere_ within its combined
network coverage area and ensures it efficiently reaches all members
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as long as they are in range of at least one member that has received
the information. An out-of-service or out-of-range member can
receive all active subscribed publications once it is in range and/or
able to communicate. This use of DeftT is illustrated in Figure 15
where bluetooth-using devices (BT Dev) are deployed as sensors,
switches, cameras, lock openers, etc. A WiFi network includes tablet
devices and a monitor/controller computer. Gateway devices each have
a Relay using both a Bluetooth interface and a WiFi interface.
Gateways are placed so that there is always at least one in range of
a BT device and at least one other Gateway (or the Controller) in its
WiFi range. WiFi tablets can move around within range of one or more
Gateways. All the DeftTs may use the same schema, giving devices on
the WiFi network access to all of the BT devices. Applications on
any particular device may subscribe to a subset of the information
available. If privacy of longer-range data is required, the WiFi
DeftTs can use a schema that requires encrypting its cAdds. These
configuration choices are made by changes in the schemas alone, the
application code is exactly the same. No configuration is needed to
make devices recognize one another and syncps will keep
communications efficient, ensuring that all DeftTs in the trust
domain know what information is available. The face ensures that
identical cStates are only sent once (within broadcast range). These
features mean that DeftT forms efficient broadcast meshes with no
additional configuration beyond identity bundles, an important
advantage.
(Artwork only available as svg: figs/meshEx-rfc.svg)
Figure 15: IIOT meshed gateways connect a single trust domain
In addition to specifying encryption and signing types, schema rules
control which users can access specific sensors. For example, an
outside predictive maintenance analysis vendor can be allowed access
to the vibration sensor data from critical motors, relayed through
the Internet, while only plant Security can see images from on-site
cameras.
5.2. Secure access to Distributed Energy Resources (DER)
The electrical power grid is evolving to encompass many smaller
generators with complex interconnections. Renewable energy systems
such as smaller-scale wind and solar generator sites must be
economically accessed by multiple users such as building owners,
renewable asset aggregators, utilities, and maintenance personnel
with varying levels of access rights. North American Electric
Reliability Corporation Critical Infrastructure Protection (NERC CIP)
regulations specify requirements for communications security and
reliability to guard against grid outages [DER]. Legacy NERC CIP
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compliant utility communications approaches, using dedicated
physically secured links to a few large generators, are no longer
practical. DeftT offers multiple advantages over bilateral TLS
sessions for this use case:
* Security. Encryption, authentication, and authorization of all
information objects. Secure brokerless pub/sub avoids single-
point broker vulnerabilities. Large generation assets of hundreds
of megawatts to more than 1 gigawatt, particularly nuclear power
plants must be controlled securely or risk large-scale loss of
life accidents. Hence, they are attractive targets for
sophisticated nation-state cyber attackers seeking damage with
national security implications. Even small-scale DER generators
are susceptible to a coordinated attack which could still bring
down the electric grid.
* Scalability. Provisioning, maintaining, and distributing multiple
keys with descriptive, institutionalized, hierarchical names.
DeftT allows keys to be published and securely updated on-line.
Where historically a few hundred large-scale generators could
supply all of the energy needs for a wide geographic area, now
small-scale DER such as residential solar photovoltaic (PV)
systems are located at hundreds of thousands of geographically
dispersed sites. Many new systems are added daily and must be
accommodated economically to spur wider adoption.
* Resiliency. A mesh network of multiple client users, redundant
servers, and end devices adds reliability without sacrificing
security. Generation assets must be kept on-line continuously or
failures risk causing a grid-wide blackout. Climate change is
driving frequent natural disasters including wildfires,
hurricanes, and temperature extremes which can impact the
communications infrastructure. If the network is not resilient
communications breakdowns can disable generators on the grid
leading to blackouts.
* Efficiency. Data can be published once from edge gateways over
expensive cellular links and be accessed through servers by
multiple authorized users, without sacrificing security. For
small residential DER systems, economical but reliable
connectivity is required to spur adoption of PV compared to
purchasing from the grid. However, for analytics, maintenance and
grid control purposes, regular updates from the site by multiple
users are required. Pub/sub via DeftT allows both goals to be met
efficiently.
* Flexible Trust rules: Varying levels of permissions are possible
on a user-by-user and site-by-site basis to tightly control user
security and privacy at the information object level. In an
energy ecosystem with many DER, access requirements are quite
complex. For example, a PV and battery storage system can be
monitored on a regular basis by a homeowner. Separate equipment
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vendors for batteries and solar generation assets, including
inverters, need to perform firmware updates or to monitor that the
equipment is operating correctly for maintenance and warranty
purposes. DER aggregators may contract with a utility to supply
and control multiple DER systems, while the utility may want to
access production data and perform some controls themselves such
as during a fire event where the system must be shut down.
Different permissions are required for each user. For example,
hourly usage data which gives detailed insight into customer
behaviors can be seen by the homeowner, but for privacy reasons
might only be shared with the aggregator if permission is given.
These roles and permissions can be expressed in the communication
rules and then secured by DeftT's use of compiled schemas.
The specificity of the requirements of NERC CIP can be used to create
communication schemas that contain site-specifics, allowing
applications to be streamlined and generic for their functionality,
rather than containing security and site-specifics.
6. Using Defined-trust Communications without DeftT
Parts of the defined-trust communications framework could be used
without the DeftT protocol. There are two main elements used in
DeftT: the integrated trust management engine and the multi-party
communications networking layer that makes use of the properties of a
broadcast medium. It's possible to make use of either of these
without DeftT. For example, a message broker could implement the
trust management engine on messages as they arrive at the broker
(e.g., via TLS) to ensure the sender has the proper identity to
publish such a message. If a credential is required in order to
subscribe to certain messages, that could also be checked. Set
reconciliation could be used at the heart of a transport protocol
without using defined-trust security, though signing, encryption, or
integrity hashing could still be employed.
7. Terms
* certificate thumbprint: the 32 byte SHA256 digest of an _entire_
certificate including its signature ensuring that each thumbprint
resolves to one and only one cert and signing chain
* collection: a set of elements denoted by a structured name that
includes the identifier of a particular communications schema
* communications schema: defined set of rules that cover the
communications for a particular application domain. Where it is
necessary to distinguish between the human readable version and
the compiled binary version, the modifiers "text" or "binary" will
be used. The binary version is placed in a certificate signed by
the domain trust anchor.
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* DCT: Defined-trust Communications Toolkit. Running code, examples
and documentation for defined-trust communications tools, a schema
language and compiler, a DeftT implementation, and illustrative
examples
* face: maintains tables of DeftT's cState PDUs to manage efficient
communications with the system transport in use (UDP multicast,
TCP, etc.)
* identity: a certificate signing chain with a particular domain's
trust anchor at its root and a unique member certificate as the
leaf. The public certificates in the chain contain attributes and
capabilities for the leaf member cert. The secret key associated
with the public key of the member cert should be securely
configured for member use.
* identity bundle - entities in a trust domain are commissioned with
an identity bundle of trust anchor, signed communication schema
cert, and the signing chain associated with a particular identity
* Publication: a named information object exchanged used by DeftT
where name structure and the required identity roles and
capabilities for Publications are specified by the schema.
Publications are the elements of the sets that are reconciled by
DeftT's sync protocol. (Capitalization is used to distinguish
this specific use from both the action and more generic use of the
term.)
* protocol data unit (PDU): a single unit of information transmitted
among entities of a network composed of protocol-specific control
information and user data. DeftT uses two types: cState: (from
"collection state") and cAdd: (from "collection additions")
* sync protocol: a synchronization protocol that implements set
reconciliation of Publications on a subnet making use of cState
and cAdd PDUs
* Things: as per [RFC8520], networked digital devices specifically
not intended to be used for general purpose computing
* trust anchor: NIST SP 800-57 Part 1 Rev. 5 definition "An
authoritative entity for which trust is assumed. In a PKI, a
trust anchor is a certification authority, which is represented by
a certificate that is used to verify the signature on a
certificate issued by that trust-anchor. The security of the
validation process depends upon the authenticity and integrity of
the trust anchor's certificate. Trust anchor certificates are
often distributed as self-signed certificates." In defined-trust
communications, a trust anchor is a self-signed certificate which
is the ultimate signer of all certificates in use in a trust
domain, including the communications schema. From RFC4949: trust
anchor definition: An established point of trust (usually based on
the authority of some person, office, or organization) which
allows the validation of a certification chain.
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* trust domain: a shorthand form for _Defined-trust Communications
Limited Domain_, a zero trust domain governed by a single trust
anchor and communications schema which is enforced at run-time by
a library (e.g., DCT) using a signed binary copy of the schema at
each member. Nothing is accepted without validation; non-
conforming communications are silently discarded. As the schema
cert is signed by the trust anchor, a trust comain is uniquely
identified by the schema cert's thumbprint. Where context is
clear, just _domain_ may be used.
* trust-based Relay: (or just Relay) a special-purpose entity that
connects a trust domain across different subnets
8. Security Considerations
This document presents a transport protocol that secures the
information it conveys (COMSEC in the language of [RFC3552]).
Security of data in the application space is out-of-scope for this
document, but use of a trusted execution environment (TEE), e.g.,
ARM's TrustZone, is recommended where this is of concern.
Unauthorized changes to DeftT code could bypass validation of
received PDUs or modify the content of outgoing PDUs prior to signing
(but only valid PDUs are accepted at receiver; invalid PDUs are
dropped by uncompromised member). Although securing DeftT's code is
out-of-scope for this document, DeftT has been designed to be easily
deployed with a TEE. Revisiting Figure 4, Figure 16 highlights how
all of the DeftT code and data can be placed in the secure zone
(long-dashed line), reachable _only_ via callgates for the Publish
and Subscribe API calls.
(Artwork only available as svg: figs/hwtrust-rfc.svg)
Figure 16: DeftT secured with a Trusted Execution Environment
Providing crypto functions is out-of-scope of this document. The
example implementation uses *libsodium*, an open source library
maintained by experts in the field [SOD]. Crypto functions used in
any alternative implementation should be of similar high quality.
Enrollment of devices is out of scope. A range of solutions are
available and selection of one is dependent on specifics of a
deployment. Example approaches include the Open Connectivity
Foundation (OCF) onboarding and BRSKI [RFC8995]. NIST NCCOE network
layer onboarding might be adapted, treating a communications schema
like a MUD URL.
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Protecting private identity and signing keys is out-of-scope for this
document. Good key hygiene should be practiced, securing private
credentials using best practices for a particular application class,
e.g. [COMIS][OWASP].
DeftT's unit of information transfer is a Publication. It is an
atomic unit sized to fit in a lower layer transport PDU (if needed,
fragmentation and reassembly are done in shim or application). All
Publications must be signed and the signature must be validated. All
Publications start with a Name (Section 2.3.1). Publications are
used both for ephemeral communication, like commands and status
reports, and long-lived information like certs. The set
reconciliation-based syncps protocol identifies Publications using a
hash of the _entire_ Publication, including its signature. A sync
collection can contain at most one instance of any Publication so
replays of Publications in the collection are discarded as duplicates
on arrival. The current DeftT implementation requires weakly
synchronized clocks with a known maximum skew. Publications have a
lifetime enforced by their sync collection; their names include a
timestamp used both to enforce that lifetime and prevent replay
attacks by keeping a Publication in the local collection (but not
advertising its existence) until its lifetime plus the skew has
passed. (Lifetimes in current applications range from days or years
for certs to milliseconds for status and command communications).
Publications arriving a skew time before their timestamp or a skew
time plus lifetime after their timestamp are discarded.
An attacker can modify, drop, spoof, or replay any DeftT PDU or
Publication but DeftT is designed for this to have minimal effect.
1. modification - all DeftT cAdd PDUs must be either signed or AEAD
encrypted with a securely distributed nonce group key. This
choice is specified in the schema and each DeftT checks at
startup that one of these two properties holds for the schema and
throws an error if not.
* for signed PDUs each receiving DeftT must already have the
complete, fully validated signing chain of the signer or the
PDU is dropped. The signing cert must validate the PDU's
signature or the PDU is dropped.
* for encrypted PDUs (and Publications) the symmetric group key
is automatically and securely distributed using signing
identities. Each receiver uses its copy of the current
symmetric key to validate the AEAD MAC and decrypt the PDU
content. Invalid or malformed PDUs and Publications are
dropped.
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cState modification to continually send an older, less complete
state in order to generate the sending of cAdds could create a
DoS attack but counter measures could be implemented using
available DeftT information in order to isolate that entity or
remove it from the trust domain.
2. dropped PDUs - DeftT's sync protocol periodically republishes
cState messages which results in (re)sending dropped cAdds.
Unlike unicast transports, DeftT can and will obtain any
Publications missing from its collection from any member that has
a valid copy.
3. spoofing - DeftT uses a trust management engine that validates
the signing. Malformed Publications and PDUs are dropped as
early as possible.
4. replay - A cAdd is sent in response to a specific cState, so a
replayed cAdd that matches a current cState simply serves a
retransmit of the cAdd's Publication which will be filtered for
duplicates and obsolescence as described above. A cAdd that
doesn't match a current cState will be dropped on arrival.
Peer member authentication in DeftT comes through the integrated
trust management engine. Every DeftT instance is started with an
identity bundle that includes the domain trust anchor, the schema in
certificate format signed by this trust anchor, and its own member
identity chain with a private identity key and the chain signed at
the root by trust anchor. Members publish their identity chains
before any Publications are sent. The trust management engine
unconditionally drops any Publication or PDU that does not have a
valid signer or whose signer lacks the role or capabilities required
for that particular Publication or PDU.
DeftT takes a modular approach to signing/validation of its PDUs and
Publications, so a number of approaches to integrity, authenticity,
and confidentiality are possible (and several are available at
[DCT]). Security features that are found to have vulnerabilities
will be removed or updated and new features are easily added.
A compromised member of a trust domain can only build messages that
match the role and attributes in its signing chain. Thus, a
compromised lightbulb can lie about its state or refuse to turn on,
but it can't tell the front door to unlock or send camera footage to
a remote location. Multiple PDUs could be generated, resulting in
flooding the subnet. There are possible counter-measures that could
be taken if some detection code is added to the current DeftT, but
this is deferred for specific applications with specific types of
threats and desired responses.
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The example encryption modules provide for encryption on both cAdd
PDUs and Publications. The latter _must_ be signed by the originator
in addition to being encrypted. This is not required for cAdd PDUs,
so the specific entity that sent the cAdd cannot be determined but
the Publications it carries _must_ be signed, even if not encrypted.
In DeftT, any member can resend a Publication from any other member
(without modification) so group encryption (in effect, group signing)
is no different. Some other encryption approaches are provided whose
potential vulnerabilities are described with their implementations
and a signed, encrypted approach is also available [DCT].
DeftT relies on libsodium and linux random implementations with
respect to entropy issues. In general, these are quite application-
dependent and should be further addressed for particular deployments.
9. IANA Considerations
This document has no IANA actions.
10. Normative References
[RFC8799] Carpenter, B. and B. Liu, "Limited Domains and Internet
Protocols", RFC 8799, DOI 10.17487/RFC8799, July 2020,
<https://www.rfc-editor.org/info/rfc8799>.
[RFC9119] Perkins, C., McBride, M., Stanley, D., Kumari, W., and JC.
Zuñiga, "Multicast Considerations over IEEE 802 Wireless
Media", RFC 9119, DOI 10.17487/RFC9119, October 2021,
<https://www.rfc-editor.org/info/rfc9119>.
11. Informative References
[ATZ] Ngabonziza, B., Martin, D., Bailey, A., Cho, H., and S.
Martin, "TrustZone Explained: Architectural Features and
Use Cases", 2016, <https://doi.org/10.1109/CIC.2016.065>.
[CAvuln] Marlinspike, M., "More Tricks for Defeating SSL in
Practice", 2009, <http://2015.hack.lu/archive/2009/moxie-
marlinspike-
some_tricks_for_defeating_ssl_in_practice.pdf>.
[CHPT] CheckPoint, "The Dark Side of Smart Lighting: Check Point
Research Shows How Business and Home Networks Can Be
Hacked from a Lightbulb", February 2020,
<https://www.globenewswire.com/news-
release/2020/02/05/1980090/0/en/The-Dark-Side-of-Smart-
Lighting-Check-Point-Research-Shows-How-Business-and-Home-
Networks-Can-Be-Hacked-from-a-Lightbulb.html>.
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[CIDS] OperantNetworks, "Cybersecurity Intrusion Detection System
for Large-Scale Solar Field Networks", 2021,
<https://www.sbir.gov/sbirsearch/detail/2104327>.
[COMIS] Lydersen, L., "Commissioning Methods for IoT", February
2019,
<https://www.silabs.com/documents/public/presentations/ew-
2019-iot-security-commissioning-methods-for-iot.pdf>.
[COST] Guy, W., "Wireless Industrial Networking Alliance, Wired
vs. Wireless: Cost and Reliability", October 2005,
<https://www.fierceelectronics.com/embedded/wired-vs-
wireless-cost-and-reliability>.
[ConfusedDep]
Support, G. C., "Additional authenticated data guide",
July 2021, <https://cloud.google.com/kms/docs/additional-
authenticated-data#confused_deputy_attack_example>.
[DCT] Pollere, "Defined-trust Communications Toolkit", 2022,
<https://github.com/pollere/DCT>.
[DER] NERC, "North American Electric Reliability Corporation:
Distributed Energy Resources: Connection, Modeling, and
Reliability Considerations", February 2017,
<https://www.nerc.com/pa/RAPA/ra/
Reliability%20Assessments%20DL/
Distributed_Energy_Resources_Report.pdf>.
[DIFF] Eppstein, D., Goodrich, M. T., Uyeda, F., and G. Varghese,
"What's the difference?: efficient set reconciliation
without prior context", 2011.
[DIGN] Bandyk, M., "As Dominion, others target 80-year nuclear
plants, cybersecurity concerns complicate digital
upgrades", November 2019,
<https://www.utilitydive.com/news/as-nuclear-plants-look-
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[DLOG] Li, N., Grosof, B., and J. Feigenbaum, "Delegation logic",
February 2003, <https://doi.org/10.1145/605434.605438>.
[DMR] al., M. C. E., "Device Management Requirements to Secure
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requirements-to-secure-enterprise-iot-edge-
infrastructure/>.
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[DNMP] Nichols, K., "Lessons Learned Building a Secure Network
Measurement Framework Using Basic NDN", September 2019.
[DTM] Blaze, M., Feigenbaum, J., and J. Lacy, "Decentralized
Trust Management", June 1996,
<https://doi.org/10.1109/SECPRI.1996.502679>.
[Demers87] Demers, A. J., Greene, D. H., Hauser, C., Irish, W.,
Larson, J., Shenker, S., Sturgis, H. E., Swinehart, D. C.,
and D. B. Terry, "Epidemic Algorithms for Replicated
Database Maintenance", 1987,
<https://doi.org/10.1145/41840.41841>.
[Graphene] Ozisik, A. P., Andresen, G., Bissias, G., Houmansadr, A.,
and B. N. Levine, "Graphene: A New Protocol for Block
Propagation Using Set Reconciliation", 2017,
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[Graphene19]
Ozisik, A. P., Andresen, G., Levine, B. N., Tapp, D.,
Bissias, G., and S. Katkuri, "Graphene: efficient
interactive set reconciliation applied to blockchain
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<https://doi.org/10.1145/3341302.3342082>.
[HSE] Kapersky, "Secure Element", 2022,
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element/>.
[IAWS] Ganapathy, K., "Using a Trusted Platform Module for
endpoint device security in AWS IoT Greengrass", November
2019, <Using a Trusted Platform Module for endpoint device
security in AWS IoT Greengrass>.
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lookup tables", 2011,
<https://doi.org/10.1109/Allerton.2011.6120248>.
[IEC] IEC, "Power systems management and associated information
exchange - Data and communications security - Part 8:
Role-based access control for power system management",
2022, <https://webstore.iec.ch/publication/61822>.
[IEC61850] Wikipedia, "IEC 61850", 2021,
<https://en.wikipedia.org/wiki/IEC_61850>.
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[IIOT] Rajiv, "Applications of Industrial Internet of Things
(IIoT)", June 2018, <https://www.rfpage.com/applications-
of-industrial-internet-of-things/>.
[IOTK] Nichols, K., "Trust schemas and {ICN:} key to secure home
IoT", 2021, <https://doi.org/10.1145/3460417.3482972>.
[ISO9506MMS]
ISO, "Industrial automation systems --- Manufacturing
Message Specification --- Part 1: Service definition",
2003, <https://www.iso.org/obp/ui/#iso:std:iso:9506:-1:ed-
2:v1:en>.
[LANG] LANGSEC, "LANGSEC: Language-theoretic Security "The View
from the Tower of Babel"", 2021, <http://langsec.org>.
[MATR] Alliance, C. S., "Matter is the foundation for connected
things", 2021, <https://buildwithmatter.com/>.
[MHST] Wikipedia, "MQTT", 2022,
<https://en.wikipedia.org/wiki/MQTT>.
[MINSKY03] Minsky, Y., Trachtenberg, A., and R. Zippel, "Set
reconciliation with nearly optimal communication
complexity", 2003,
<https://doi.org/10.1109/TIT.2003.815784>.
[MODOT] Saleem, D., Granda, S., Touhiduzzaman, M., Hasandka, A.,
Hupp, W., Martin, M., Hossain-McKenzie, S., Cordeiro, P.,
Onunkwo, I., and D. Jose, "Modular Security Apparatus for
Managing Distributed Cryptography for Command and Control
Messages on Operational Technology Networks (Module-OT)",
January 2022,
<https://www.nrel.gov/docs/fy22osti/79974.pdf>.
[MPSR] Mitzenmacher, M. and R. Pagh, "Simple multi-party set
reconciliation", 2018.
[MQTT] OASIS, "MQTT: The Standard for IoT Messaging", 2022,
<mqtt.org>.
[NDNS] NDN, "Named Data Networking Packet Format Specification
0.3", 2022,
<https://named-data.net/doc/NDN-packet-spec/current/>.
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[NDNW] Jacobson, V., "Watching NDN's Waist: How Simplicity
Creates Innovation and Opportunity", July 2019,
<http://ice-ar.named-data.net/meetings/2019-ICE-WEN-
Annual/0-ICNWEN-Van-Keynote.pdf>.
[NERC] NERC, "Emerging Technology Roundtable - Substation
Automation/IEC 61850", November 2016,
<https://www.nerc.com/pa/CI/Documents/roundtable%20-%20IEC
%2061850%20slides%20%20(20161115).pdf>.
[NMUD] al, D. D. E., "Securing Small-Business and Home Internet
of Things (IoT) Devices: Mitigating Network-Based Attacks
Using Manufacturer Usage Description (MUD)", May 2021,
<https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
NIST.SP.1800-15.pdf>.
[NPPI] Hashemian, H. M., "Nuclear Power Plant Instrumentation and
Control", 2011, <https://cdn.intechopen.com/pdfs/21051/InT
echNuclear_power_plant_instrumentation_and_control.pdf>.
[NVR] Gutmann, P., "Everything you Never Wanted to Know about
PKI but were Forced to Find Out", 2002,
<https://www.cs.auckland.ac.nz/~pgut001/pubs/
pkitutorial.pdf>.
[ONE] OneDM, "One Data Model", 2022, <https://onedm.org/>.
[OPR] King, R., "Commercialization of NDN in Cybersecure Energy
System Communications video", 2019, <https://www.nist.gov/
news-events/events/2019/09/ndn-community-meeting>.
[OSCAL] NIST, "OSCAL: the Open Security Controls Assessment
Language", 2022, <https://pages.nist.gov/OSCAL/>.
[OTPM] Hinds, L., "Keylime - An Open Source TPM Project for
Remote Trust", November 2019,
<https://www.youtube.com/watch?v=YtPsruEqGeY>.
[OWASP] owasp.org/www-project-sidekek/, "SideKEK README", June
2020, <https://github.com/OWASP/SideKEK>.
[PRAG] e}bowicz, J. W., Cabaj, K., and J. Krawiec, "Messaging
Protocols for IoT Systems---A Pragmatic Comparison", 2021,
<https://www.mdpi.com/1424-8220/21/20/6904>.
[QTPM] Arthur, D. C. W., "Quick Tutorial on TPM 2.0", January
2015, <https://link.springer.com/
chapter/10.1007/978-1-4302-6584-9_3>.
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[RFC2693] Ellison, C., Frantz, B., Lampson, B., Rivest, R., Thomas,
B., and T. Ylonen, "SPKI Certificate Theory", RFC 2693,
DOI 10.17487/RFC2693, September 1999,
<https://www.rfc-editor.org/info/rfc2693>.
[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC
Text on Security Considerations", BCP 72, RFC 3552,
DOI 10.17487/RFC3552, July 2003,
<https://www.rfc-editor.org/info/rfc3552>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[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>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
[RSK] Ellison, C. and B. Schneier, "Ten Risks of PKI: What
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Data", January 2017, <https://tonytruong.net/how-to-use-
the-tpm-to-secure-your-iot-device-data/>.
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[SKH] Yates, T., "Secure key handling using the TPM", October
2018, <https://lwn.net/Articles/768419/>.
[SNC] Smetters, D. K. and V. Jacobson, "Securing Network
Content", October 2009, <https://named-data.net/wp-
content/uploads/securing-network-content-tr.pdf>.
[SOD] Bernstein, D., Lange, T., and P. Schwabe, "libsodium",
2022, <https://doc.libsodium.org/>.
[SPRV] AgendalessConsulting, "Supervisor: A Process Control
System", 2022, <http://supervisord.org/>.
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st-api.html##operation/listCapabilities>.
[STNDN] Yu, Y., Afanasyev, A., Clark, D. D., claffy, K., Jacobson,
V., and L. Zhang, "Schematizing Trust in Named Data
Networking", 2015.
[TATT] Microsoft, "TPM attestation", June 2021,
<https://docs.microsoft.com/en-us/azure/iot-dps/concepts-
tpm-attestation>.
[TLSvuln] al., C. B. E., "Using Frankencerts for Automated
Adversarial Testing of Certificate Validation in SSL/TLS
Implementations", November 2014,
<https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232952/>.
[TPM] Griffiths, P., "TPM 2.0 and Certificate-Based IoT Device
Authentication", September 2020,
<https://www.globalsign.com/en/resources/white-papers-
ebooks/white-paper-tpm-20-and-certificate-based-iot-
device-authentication>.
[W509] Wikipedia, "X.509: Security", October 2021,
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Bauman, "Wireless Sensors: Technology and Cost-Savings for
Commercial Buildings", 2002,
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SS02_Panel7_Paper10.pdf>.
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[WegmanC81]
Wegman, M. N. and L. Carter, "New Hash Functions and Their
Use in Authentication and Set Equality", 1981,
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[ZCL] zigbeealliance, "Zigbee Cluster Library Specification
Revision 6", 2019, <https://zigbeealliance.org/wp-content/
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specification.pdf>.
Contributors
Lixia Zhang
UCLA
Email: lixia@cs.ucla.edu
Roger Jungerman
Operant Networks Inc.
Roger contributed much to Section 5.
Authors' Addresses
Kathleen Nichols
Pollere LLC
Email: nichols@pollere.net
Van Jacobson
UCLA
Email: vanj@cs.ucla.edu
Randy King
Operant Networks Inc.
Email: randy.king@operantnetworks.com
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