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: 2 April 2024 UCLA
R. King
Operant Networks Inc.
30 September 2023
Defined-Trust Transport (DeftT) Protocol for Limited Domains
draft-nichols-iotops-defined-trust-transport-03
Abstract
This document describes a broadcast-oriented, many-to-many Defined-
trust Transport (DeftT) framework 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's communication model is one of
synchronized collections of secured information rather than one-to-
one optionally secured connections. DeftT is part of a Defined-trust
Communications approach with a specific example implementation
available. 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
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Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 2 April 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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 . . . . . . . . . . . . . . . . . . . 7
1.2. Transporting information . . . . . . . . . . . . . . . . 8
1.3. Securing information . . . . . . . . . . . . . . . . . . 10
1.4. Defined-trust Communications Domains . . . . . . . . . . 11
1.5. Current status . . . . . . . . . . . . . . . . . . . . . 13
2. DeftT and Defined-trust Communications . . . . . . . . . . . 13
2.1. Inside DeftT . . . . . . . . . . . . . . . . . . . . . . 15
2.2. syncps: a set reconciliation protocol . . . . . . . . . . 15
2.3. DeftT formats . . . . . . . . . . . . . . . . . . . . . . 18
2.3.1. Top level container TLVs . . . . . . . . . . . . . . 18
2.3.2. Leaf TLVs . . . . . . . . . . . . . . . . . . . . . . 25
2.3.3. TLV header details . . . . . . . . . . . . . . . . . 27
2.3.4. Design rationale . . . . . . . . . . . . . . . . . . 27
2.4. Application and network interface . . . . . . . . . . . . 29
2.5. Synchronizing a collection . . . . . . . . . . . . . . . 30
2.6. Distributors . . . . . . . . . . . . . . . . . . . . . . 32
2.6.1. Certificate distributor . . . . . . . . . . . . . . . 32
2.6.2. Group key distributors . . . . . . . . . . . . . . . 32
2.6.3. Other distributors . . . . . . . . . . . . . . . . . 33
2.7. Schema-based information movement . . . . . . . . . . . . 33
2.8. Congestion control . . . . . . . . . . . . . . . . . . . 35
3. Defined-trust management engine . . . . . . . . . . . . . . . 36
3.1. Communications schemas . . . . . . . . . . . . . . . . . 37
3.2. A schema language . . . . . . . . . . . . . . . . . . . . 38
4. Certificates and identity bundles . . . . . . . . . . . . . . 41
4.1. Obviate CA usage . . . . . . . . . . . . . . . . . . . . 42
4.2. Identity bundles . . . . . . . . . . . . . . . . . . . . 43
5. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.1. Secure Industrial IoT . . . . . . . . . . . . . . . . . . 45
5.2. Secure access to Distributed Energy Resources (DER) . . . 47
6. Using Defined-trust Communications without DeftT . . . . . . 49
7. Terms . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
8. Security Considerations . . . . . . . . . . . . . . . . . . . 51
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 54
10. Normative References . . . . . . . . . . . . . . . . . . . . 54
11. Informative References . . . . . . . . . . . . . . . . . . . 54
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Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 62
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
[DIGN] and carbon capture monitoring [IIOT].
While use of 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.
Yet "good for global communication" isn't the same as "good for
everything". A signficant number of OT uses can be characterized as
Limited Domains [RFC8799]: 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 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
communications model, a flexible pub/sub API, chain-of-trust
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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,
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).
DeftT's intended use is for communications in the constrained
environments that can be characterized as Limited Domains (LDs).
Though the properties enumerated in [RFC8799] are necessary, we do
not claim they are sufficient.
(Artwork only available as svg: ./figs/defttlayer-rfc.svg)
Figure 1: DeftT's place in an IP stack
DeftT is IP-compatible but not Internet-compatible (e.g., not
routable). In contrast with IETF standards track protocols like the
client-server COAP [RFC7252], DeftT is intended to serve the
communication needs of a closed community with common objectives, a
zero-trust Limited Domain (_trust domain_). Foremost among those
needs is the ability to enforce community-specific policy constraints
("who can say what to which"). ABAC (Attribute-Based Access Control)
[NIST] provides a model sufficient to express and enforce these
constraints but a fundamental architectural choice remains to either:
(a) Start with Internet-based communication protocols then "harden
them" by layering an ABAC framework on top, or
(b) Start with an ABAC framework that verifiably enforces the policy
constraints then augment it with the minimum necessary communication
primitives needed to function in a community's deployment
environment.
Existing IETF protocols use approach (a) and, given how few
enforceable security policies are possible on the open Internet, it's
a reasonable choice. For LDs, approach (a) imports all the
(otherwise unneeded) Internet abstraction maintenance machinery
(DHCP, DNS, CAs, PDPs/PIPs, routing, address plans, etc.). When
communication is expressed in terms of Internet abstractions (e.g., a
TLS connection between two IP endpoints), there needs to be a
translation layer to map between these abstractions and the
community's entities, requirements and objectives. All this
machinery is configuration intensive and recent history has
demonstrated that it's all prime attack surface. DeftT has been
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created as a self-contained ABAC framework where the PEP and PDP are
in the transport narrow pub/sub waist and embeds the PIP function in
certificate signing chains so it's self-authenticating and self-
distributing. Further, DeftT's efficient use of its communications
schema obviates the ABAC expectation that "the more granular the
controls, the higher the overhead."
Like COAP/OSCORE nodes, DeftT members start with a pre-existing
identity obtained out of band which means that existing and evolving
bootstrap and enrollment protocols and methodologies can be used.
But DeftT identities are more than a single key pair and only convey
membership in a _specific_ Trust Domain that is using a _particular_
set of rules and a _particular_ trust anchor (TA). Member identities
are in the form of certificate chains containing all relevant
attributes or roles with a private key corresponding to a unique
identity cert at the chain's leaf. As with MUD, members of a trust
domain have specific capabilities and permitted communications that
are explicitly specified. Unlike MUD, each member gets the
communications rules for the domain distributed in binary form in a
cert signed by the same trust anchor that is at the root of the
member identity. This _schema_ specifies the format for membership
identity chains as well as the format of all legal communications and
the attributes required to issue them. Each DeftT has an integrated
trust management engine that makes use of the schema at run-time.
DeftT 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 (used to confer attributes) terminated at the trust
anchor. The private key corresponding to the leaf certificate of the
identity should be securely configured (i.e., not exposed to any
third party) while the security of the identity bundle can be
deployment-specific (i.e., the public certificates it contains may
optionally be protected from third parties). The identity chains of
all communicating members share a common trust anchor as do the rules
that define the permissible signing chains of a Domain, so the bundle
suffices for a member to authenticate and authorize communication
from peers and vice-versa. The identity bundle, DeftT's trust
management engine, and a trust domain's certificate collection
(obtained via DeftT as members initiate connection) allow new members
to join and communicate with no specific knowledge of other members,
thus obviating labor intensive and error-prone device-to-device
association configuration. (More on certificates and identities in
DeftT in Section 2.6 and Section 4.)
In synchronized collections, members communicate about their local
version of the collection state and send additions to the collection
the other members are missing. Along with the out-of-band identity
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bundle, DeftT makes use of both its synchronized collections and its
integrated trust management engine to securely join a particular
Trust Domain. As understanding how a DeftT joins a Trust Domain
should be helpful in in understanding DeftT and how it differs from
other approaches, its certificate distributor module is briefly
described here. A state diagram of the joining process is Figure 2.
During configuration, the private identity key should always be
stored so it is both private and secure. The identity key is not
used for signing DeftT packets, but for signing a cert that is
locally created so that signing certs can be updated more often
without the need for update of the identity. In addition, the
identity key can remain within protected hardware like a TPM for
signing while the signing key is intended for use in the
communications path where we can tradeoff the possibility of more
exposure vs the need for speed. Once the signing pair is created and
a cert signed, the DeftT starts the process of joining the Trust
Domain by subscribing to the Domain certificate collection whereupon
it receives the state of the collection. The local signing chain is
added to the local copy of the cert collection then the local
collection state can be compared to the received collection state and
any certs that are not already in that received state will be sent on
the network to be added to the Domain collection. Note this will
always include the identity cert and the signing cert of a new
member, but other certificates of the chain may already have been
added by previously joined members. A DeftT does not consider itself
joined until it receives a collection state from the network that
contains all of its certs, indicating that at least one other member
will be able to receive its signed packets. Whether joined or not,
the cert distributor handles all certs received from the network,
adding them to its local collection when an entire validated signing
chain is received.
(Artwork only available as svg: ./figs/certSD-rfc.svg)
Figure 2: DeftT certificate distributor enables joining Trust Domain
OSCORE [RFC8613] adds object security to COAP specifically to get
around the vulnerability of using only DTLS/TLS with proxies. OSCORE
uses pre-shared keys either acquired out-of-band or via a key
establishment protocol. OSCORE encrypts/signs a COAP message and
carries it as payload in a COAP message with the OSCORE option. A
Security Context is between two endpoints, specific to sender ID and
recipient ID. Sender IDs may be establish o-o-b. As Internet
compatible protocols, COAP/OSCORE/ACE[RFC9200] use 1) cleartext
options in their headers and 2) trusted third parties or resource
servers, both of which can be exploited. A DeftT PDU uses a hash of
its compiled rules cert to identify its trust domain with no options.
In the Internet, PDU headers tell nodes how the packet should be
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handled. In a DeftT trust domain, the hash in the PDU identifies the
packet as part of the domain whose rules will be enforced by any
receiver. These are very different architectures both for
communicating and for securing communications and are expected to
serve different roles although the application spaces may overlap.
Further, DeftT and Defined-trust Communications are early-stage work
compared to COAP/OSCORE and other IETF work, but deployments are
underway by Operant and Pollere.
1.1. Environment and use
Due to physical deployment constraints and the high cost of wiring,
many 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].
Many OT networks are Limited Domains having a defined membership and
communications that are often 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
application communication protocol in IoT (https://mqtt.org/use-
cases/ (https://mqtt.org/use-cases/)). 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.
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1.2. Transporting information
The smart lighting example of Figure 3 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
transmission 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 3: 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 4). 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
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
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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 4: 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.)
More general solutions for this communications paradigm are possible
by moving the view of the problem from message exchange to the
concept of coordinating shared objectives. 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 3 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. (See Section 2.5 to see how members fill in
missing elements.)
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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 4 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 5: Trust management elements of DeftT.
This approach extends LangSec's [LANGSEC] "be definite in what you
accept" principle by using the authenticated common rules of the
schema 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 6) and are configured with the same trust anchor and
schema as well as an individual schema-conformant DeftT identity cert
chain that terminates at the trust anchor and the private 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
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credential and idiosyncrasies in rules) 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. Domain
member entities' DeftTs publish and subscribe using Publication
Builders and Validators as shown in Figure 5. Publications become
the elements of a set, or named collection, that is synchronized
across 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
information Publications and *cert* where identity signing chains are
published.
(Artwork only available as svg: ./figs/trustdomain-rfc.svg)
Figure 6: Trust domain
Trust domains are extended across physically separated subnets,
subnets using different media and/or subdomains on the same subnet
(see Section 2.7) 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 7. 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. Any part of a verifiable
defined-trust identity can be used in the delineation of subdomains,
e.g. specific component(s) of identity names for all DeftTs of a
subdomain can be constrained to be the same so that Publications are
effectively only relayed to a particular "group" as identified by
those components and this is enforced via the secured schema
(non-"group" Publications will not validate). Relay discussion is in
Section 2.7 and Section 5.
(Artwork only available as svg: ./figs/relayedtrustdomain-rfc.svg)
Figure 7: Relayed trust domain
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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] currently has examples of using DeftT to
implement secure brokerless message-based pub/sub using multicast
UDP/IPv6 and unicast UDP/TCP and include extending a trust domain via
a unicast connection or between two broadcast network segments.
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
underway. Pollere is also working on home IoT uses. The development
philosophy for DeftT 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 (e.g., under 20
microseconds on a Mac Studio), 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.,
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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
compiled binary communications schema is distributed as a trust-root-
signed certificate and that certificate's thumbprint (see
Section 2.3.1.4 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.
As illustrated in Figure 2, 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 Domain. Privacy via AEAD
(Authenticated Encryption with Associated Data) is automatically
handled within DeftT if selected in the schema.
(Artwork only available as svg: ./figs/transportBD0v2-rfc.svg)
Figure 8: DeftT's interaction in a network stack
Figure 8 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 collection addition
(cAdd) PDUs that are used along with collection state (cState) PDUs
to communicate about and synchronize Collections. cStates 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).
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2.1. Inside DeftT
DeftT's example 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 9. 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 example 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
collections, *pubs*, contains application Publications. The other
required collection, *cert*, contains 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 9: 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 of
Publications synchronized. Required functionality for such a
protocol can be understood through the example of the *syncps*
protocol included in the example implementation. The *syncps*
protocol uses IBLTs [DIFF][IBLT][MPSR] to solve the multi-party set-
difference problem efficiently without the use of prior context and
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with communication proportional to the size of the difference between
the sets being compared. The state of a local collection is encoded
in an IBLT. A syncps announces its local _collection state_ (set of
currently known Publications) by sending a cState (Section 2.3.1.1)
that also serves as a query for additional data not 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 the
recipient sending a cAdd Section 2.3.1.2 containing all the locally
available missing Publications that fit. The third is used
optionally and may result in a progress notification sent to other
local modules so anything waiting for delivery confirmation can
proceed.
On broadcast media, syncps uses any cStates it hears to reduce
(suppress) sending excess cStates and listens for cAdds that may add
to its collection. This means that one-to-many Publications cause
sending a single cState and a single cAdd 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 as per
(Section 2.3.1.2 item 1). Any DeftT that is missing a Publication
(due to being out-of-range, asleep, channel errors, 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, kept efficient with protocol
features that prevent multiple redundant broadcasts. The example
implementation of syncps prevents redundant broadcasts by having
originating publishers send their responding Publications immediately
while others delay before supplying missing Publications, canceling
if a responding cAdd is overheard. Other approaches are possible.
The collection synchronization work of a syncps module is shown as a
state diagram in Figure 10. When a new syncps is started, it always
sends its local cState (starts unsuppressed) on the network and sets
an expiration timer for the cState. If this timer expires, the "new
local cState" actions are repeated and the cState may be suppressed
(thus not sent). For most collections, the initial cState will show
an empty collection (certificate collections will have the local
identity chain). The events that can move the collection forward are
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(1) the arrival of a cState from the network, (2) the arrival of a
cAdd from the network whose csID matches a hash value stored from a
previously received or sent cState, or (3) arrival of a new
Publication from its shim. For an arriving cAdd, each Publication is
extracted, validated, and passed to any registered subscriber
callback(s). Non-validating packets are silently discarded (may
optionally set alerts or count discards). Reception of new Pub(s)
may cause an application process to create and add new Publications
to the local collection. Sending of new Pubs is deferred until the
entire cAdd has been processed. If there are no new Pubs to send,
syncps moves to its "set sendCStateTimer" state where a cancelable
sendCState timer is set to the estimated dispersion delay of this
local subnet. (Dispersion should be << cState lifetime. More on
dispersion delay in Section 2.5.)
(Artwork only available as svg: ./figs/syncpsSD-rfc.svg)
Figure 10: State diagram of a syncps module
Since new Publications are always eligible to send, if any were
created while in "process cAdd", the next state is "process Pubs to
send" with csID set to the csID field of the cAdd on entry to
"process cAdd". In "process Pubs to send" any pending sendCState
will be canceled and eligible Pubs are packaged as content for a new
cAdd. Packaged Publications are subject to a hold time (during which
they are ineligible to send) of twice the dispersion delay to avoid
responding to cStates sent before reception of a cAdd containing the
Pub. If there are Pubs to send, a cAdd with that content and the
passed in csID is sent, then the set sendCStateTimer state is
entered; when there are no Pubs to send, the action moves there
directly.
Another path to exit the "wait for event" state is reception of a
cState which moves to "process cState" where incoming cStates are
recorded, If this cState matches the local cState, the syncps returns
to the wait state. Otherwise, the IBLT is extracted from the cState,
an IBLT is computed on the local Pub collection, and they are peeled
to find the ones the received cState has that are not in the local
collection ("needs") and the ones that are in the local collection
and not in the received cState ("haves"). Syncps enters the "process
Pubs to send" state with csID set to the hash of the cState's name.
Eligible Pubs are "haves" that do not have a hold time set and
locally generated Publications are sent preferentially. Publications
obtained from others are not immediately eligible to send; members
delay to give the originator time to respond, sending these when
further cStates indicate a member continues to need them.
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A syncps also exits the wait state when the attached shim has a
Publication to send. Since a new Pub will not appear in any
previously issued cState, any one can be used, including one issued
locally. In "get stored cState", the best one (the most recent
cState from the network if available) is retrieved and passed to the
"peel IBLT values" state where its cState.csID is used for the csID
value. There will always be at least the one new Publication to send
in this case.
This state diagram is intended to capture the major functionality of
a syncps module while excluding excessive detail. In particular,
Figure 10 does not show "housekeeping" tasks on the collection, e.g.,
removal of expired Publications.
2.3. DeftT formats
All DeftT Information is represented using TLV (Type, Length, Value)
tuples. Types can either be containers (they contain a concatenated
sequence of TLVs) or leaves (they contain a single non-TLV value with
well-defined semantics and serialization). All TLVs have a boolean
'valid()' method that returns 'true' if and only if their content
satisfies all the constraints associated with the TLV's type. For
container types this means, at minimum, that the sum of all the
enclosed TLV Lengths and header sizes exactly equals the Length of
the container and that the valid() method of each of the enclosed
TLVs returns true. Most container types have additional constraints
on the type, ordering and value of the enclosed TLVs that are
described below.
2.3.1. Top level container TLVs
As shown in Figure 9 there are two kinds of top level containers:
PDUs which are exchanged with the system-provided transport and carry
Pubs, the other top level container, which are the elements of the
set synchronization protocol. PDUS and Pubs have similar structure
and share most of their code but are designed to be unambiguously
distinguishable. As indicated in Figure 8 and Figure 9, syncps uses
a pub/sub model for both its shim facing and network facing
interfaces. Thus the first TLV in any top level container is a Name
container comprising the topic name used to mediate the pub/sub
rendezvous. The other TLVs in the top level container depend on its
kind.
There are two kinds of PDU containers: cState and cAdd and two kinds
of of Publications, pubs and certs. All four are described in the
following section.
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2.3.1.1. cState PDUs
A cState PDU (TLV type 5) announces the items a member holds in a
specific collection of a specific trust domain subnet. It must
contain the following three TLVs and they must be in this order:
1. Name TLV containing exactly three type Generic (aka, byte array
or binary blob) components:
C1: Trust domain id consisting of the first 8 bytes of the
SHA-256 thumbprint of the domain's schema cert.
C2: Collection name
C3: Run-length compressed IBLT of the items in the publisher's
instance of the collection (see Section 2.2 for more
information on IBLTs).
2. Nonce (TLV type 10, leaf) whose value must be 4 random bytes
chosen by the publisher at the time the cState is built.
Duplicate cStates can arise from multiple members announcing the
same Name because they hold the same items or because the network
doesn't handle multicast well and lets PDUs loop. The nonce
allows these two cases to be distinguished so looping cStates can
be dropped.
3. Lifetime (TLV type 12, leaf) whose value is the lifetime
(measured in milliseconds since this PDU's arrival) serialized as
an unsigned big-endian integer with all leading zero bytes
suppressed. A member receiving the cState and capable of
publishing into the collection can hold onto the cState for this
lifetime. If the member has an item to publish before the end of
the cState's lifetime, the Publication can be sent immediately in
a responding cAdd.
For example, the initial PDU sent by the home IoT "gate controller"
sample app (in examples/hmIot of [DCT]) looks like:
5 (cState) size 128:
| 7 (Name) size 116:
| | 8 (Generic) size 8: 55d5 7f99 7d8d ba91
| | 8 (Generic) size 4: cert
| | 8 (Generic) size 98: 8201 dd76 eb0f 46ed 89a8 8101 dd76 eb0f 46..
| | beb5 9922 fdd6 7401 cbbe 5bc1 5b57 1c63 84..
| | 79aa ca17 8501 cbbe 5bc1 5b57 1c63 8801 ce..
| | 92f8
| 10 (Nonce) size 4: 8b9f 8134
| 12 (Lifetime) size 2: 4789
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Note that the format inspections of this section are produced by
using the dctwatch tool from [DCT] with the -f option.
2.3.1.2. cAdd PDUs
Note: cAdds, Publications and Certificates all share the same Data
(TLV type 6) container format but are distinguished by its Metainfo
TLV. They all contain the same five TLVs in the same order but each
has different constraints on the value of those TLVs.
A cAdd PDU (TLV type 6) supplies one or more Pubs in response to some
cState. It must contain the following five TLVs and they must be in
this order:
1. Name TLV derived from the cState's Name: the first two
components, domain id and collection name, are the same but the
IBLT component is replaced by a csID (TLV type 35, leaf) whose
value must be the 32 bit big-endian Murmurhash of the cState's
entire Name TLV. (This is done because "Repeat the question and
append the answer" is the common strategy for matching responses
to requests in multicast protocols but an IBLT can be hundreds of
bytes which would drastically reduce the cAdd's payload space so
"the question" is replaced with a compact hash proxy.)
2. Metainfo (TLV type 20) saying the PDU's ContentType is cAdd (42),
i.e., contains one or more Pubs and nothing else so it must be
'structurally validated' on arrival.
3. Content container (TLV type 21) which must contain one or more
complete, valid Pubs. The Pubs must NOT already be in the
cState's IBLT. I.e., the Pubs must be newly created on the cAdd
publisher or in the 'need' set when the difference between the
publisher's IBLT and the cState's IBLT is 'peeled' (see [DIFF]
and the DeftT example implementation's handleCState code
(https://github.com/pollere/DCT/blob/main/include/dct/syncps/
syncps.hpp#L460) for details).
4. SigInfo container (TLV type 22) which must contain a SigType (TLV
27, leaf) containing a valid keyed or unkeyed signature type from
the types listed in Section 2.3.2. If and only if the signature
type is keyed (i.e., validation requires the public key cert of a
public/private keypair), the SigInfo must contain KeyLocator (TLV
28) containing a KeyDigest (TLV 29, leaf) of length 32 bytes
containing the thumbprint of the cert needed for validation. The
SigType must match the type of the PDU signature validator
associated with the collection.
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5. SigValue (TLV type 23, leaf) containing the result of signing the
cAdd PDU with using the algorithm and key, if any, specified by
the SigInfo. The Length of the TLV must match the length used by
the signature type as per Section 2.3.2. The PDU signature
validator must successfully validate the signature.
For example, what follows is the frontdoor's cAdd responding to the
cState shown above. The collection being synced here is the
certificate distributor which can't use any of the signature types
that depend on keys since it's responsible for obtaining the keys
that would be needed to validate a PDU's signature. Thus it is the
only collection allowed to use an unkeyed [RFC7693] BLAKE2 MAC to
integrity check the PDU. Since the content of the cAdd is self
authenticating public key certs, this doesn't cause security issues.
6 (Data) size 561:
| 7 (Name) size 22:
| | 8 (Generic) size 8: 55d5 7f99 7d8d ba91
| | 8 (Generic) size 4: cert
| | 35 (csID) size 4: f6d7 3d84
| 20 (MetaInfo) size 3:
| | 24 (ContentType) size 1: 42 (CAdd)
| 21 (Content) size 489:
... (489 bytes of Content elided)
| 22 (SigInfo) size 3:
| | 27 (SigType) size 1: 9 (RFC7693)
| 23 (SigValue) size 32: af8e 1412 e659 103f 5237 f1e1 0e7b 0af8 9c..
Except for this collection, PDU and Pub signature types are specified
in the schema. PDUs typically use AEAD with a locally elected cover
key distributor to protect the content privacy. Pubs typically use
EdDSA to provide provenance and ABAC attributes via the signing chain
or a combined AEAD and EdDSA signature type (AEADSGN) to constrain
content disclosure to some limited group. All encrypted content must
remain encrypted, in motion or at rest, from point of origin to
point(s) of use. The syncps subscribe upcall may decrypt a piece of
content for _ephemeral_ use but the callee must NOT retain the
plaintext form.
2.3.1.3. Publications
As noted above, a Publication must be in a Data TLV containing the
same five TLVs in the same order as cAdds and Certificates.
Publications are distinguished by having a Metainfo ContentType of
Blob (0).
A Publication (TLV type 6) must contain the following five TLVs and
they must be in this order:
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1. Name TLV which must contain at least three components and the
first component's length must be non-zero. The schema specifies
the format of the Name including number and type of components,
allowed values, allowed signers, etc. Implementations must
construct and sign Pubs so that they are consistent with the
schema. (The example implementation's applications show that
this can be done automatically with minimal application
involvement, e.g., see the phone app
(https://github.com/pollere/DCT/blob/main/examples/office/
phone.cpp) in the office control example.) Implementations must
fully validate Publications both cryptographically and against
the schema before adding them to the collection. Implementations
must NOT add a Publication to a collection that already contains
it.
2. Metainfo (TLV type 20) saying the Publication's ContentType is
Blob (0), i.e., contains arbitrary bytes that can't be
'structurally' validated (but are always cryptographically
validated for integrity and authorization by the signature
check)..
3. Content container (TLV type 21) containing _Length_ bytes.
_Length_ may be zero.
4. SigInfo container (TLV type 22) which must contain exactly two
TLVs: a SigType (TLV type 27, leaf) containing a valid keyed
signature type from the types listed in Section 2.3.2 followed by
a KeyLocator (TLV type 28) containing a KeyDigest (TLV type 29,
leaf) of length 32 bytes containing the thumbprint of the cert
needed to validate the signature. The SigType in the Publication
must match the collection's Publication validator which must
match the #pubValidator specified in the schema.
5. SigValue (TLV type 23, leaf) containing the result of signing the
Publication using the algorithm and key specified by the SigInfo.
The Length of the TLV must match the length used by the signature
type as per Section 2.3.2. The collection's publication
signature validator must successfully validate the signature.
For example, what follows are two consecutive Publications made to
the pubs collection. First, operator alice publishes a command for
all lock devices to lock themselves (similar to the multiple
subscriptions per-light shown in Figure 3, the schema requires that
all lockable devices subscribe to the iot1/lock/command/all prefix in
pubs):
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6 (Data) size 216:
| 7 (Name) size 68:
| | 8 (Generic) size 4: iot1
| | 8 (Generic) size 4: lock
| | 8 (Generic) size 7: command
| | 8 (Generic) size 3: all
| | 8 (Generic) size 4: lock
| | 8 (Generic) size 17: p38863@aphone.local
| | 37 (SequenceNum) size 4: b4a1 ea2a
| | 37 (SequenceNum) size 0:
| | 36 (Timestamp) size 7: 23-09-18@19:40:45.591793
| 20 (MetaInfo) size 3:
| | 24 (ContentType) size 1: 0 (Blob)
| 21 (Content) size 32: Msg #3 from operator:alice-38863
| 22 (SigInfo) size 39:
| | 27 (SigType) size 1: 8 (EdDSA)
| | 28 (KeyLocator) size 34:
| | | 29 (KeyDigest) size 32: 7096 5de9 6848 7543 d2c8 e459 24fb 7b0..
| 23 (SigValue) size 64: 61b3 fc3c 03df 2c89 7a0c ddae 27a2 f883 dd..
| 2699 899f 1c91 46c1 3127 9da8 8948 e783 68..
Three milliseconds later, the gate publishes that it has locked
itself:
6 (Data) size 214:
| 7 (Name) size 69:
| | 8 (Generic) size 4: iot1
| | 8 (Generic) size 4: lock
| | 8 (Generic) size 5: event
| | 8 (Generic) size 4: gate
| | 8 (Generic) size 6: locked
| | 8 (Generic) size 17: p59280@rpi2.local
| | 37 (SequenceNum) size 4: e131 5a4b
| | 37 (SequenceNum) size 0:
| | 36 (Timestamp) size 7: 23-09-18@19:40:45.594867
| 20 (MetaInfo) size 3:
| | 24 (ContentType) size 1: 0 (Blob)
| 21 (Content) size 29: Msg #3 from device:gate-59280
| 22 (SigInfo) size 39:
| | 27 (SigType) size 1: 8 (EdDSA)
| | 28 (KeyLocator) size 34:
| | | 29 (KeyDigest) size 32: 3dde 0f21 beae 2c20 3ea3 5c2e 77ca 9d4..
| 23 (SigValue) size 64: 3913 011d 7e74 807c 94b5 e725 a8e7 5b2f 09..
| bc99 9c8b fa9f f929 4722 f23a 1fbe cd84 b6..
As described in Section 8, these Publications' schema is designed for
spoofing and replay protection. Section 2.2 notes that the per-
publication EdDSA signature prevents spoofing or modification. Since
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all collections ignore duplicates of an existing publication, replays
of anything in the collection will be ignored. To keep collections
from growing without bound, publications are removed after a
collection-dependent lifetime but arriving are pubs are ignored as
"expired" if their timestamp (name component 9) plus a collection-
dependent "expiry time" is after the node's local time. "lifetime" is
substantially larger then "expiry time" to account for clock skew so
the combination of these two mechanisms prevents all replay.
2.3.1.4. Certificates
As noted above, a Certificate must be in a Data TLV containing the
same five TLVs in the same order as cAdds and Publications.
Certificates are distinguished by having a Metainfo ContentType of
Key (2) and by having a Validity Period specified according to a more
rigorous subset of the rules in [RFC1422] section 3.3.6 as described
in item 5 below.
A Certificate (TLV type 6) must contain the following five TLVs and
they must be in this order:
1. Name TLV which must contain at least five components and the
first component's length must be non-zero. The schema specifies
the format of the Name including number and type of components,
allowed values, allowed signers, etc. Implementations must
construct and sign certs so that they are consistent with the
schema. (Tools to do this are supplied with the example
implementation.) Implementations must fully validate certs both
cryptographically and against the schema before adding accepting
them. "Fully validating" requires that the cert's signer has
been accepted thus a cert cannot be accepted until its entire
signing chain has been accepted.
2. Metainfo (TLV type 20) saying the Cert's ContentType is Key (2),
This means the container has no TLV structure to validate.
3. Content container (TLV type 21) containing _Length_ bytes.
_Length_ must equal the size of the public key associated with
the cert's SigInfo SigType
4. SigInfo container (TLV type 22) which must contain exactly two
TLVs: a SigType (TLV type 27, leaf) containing a valid keyed
signature type from the types listed in Section 2.3.2 followed by
a KeyLocator (TLV type 28) containing a KeyDigest (TLV type 29,
leaf) of length 32 bytes containing the thumbprint of the cert
needed to validate the signature. The KeyDigest must be followed
by a Validity Period (TLV 253) containing a NotBefore (TLV 254,
leaf) containing a valid 15 character ISO 8601-1:2019 format GMT
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timepoint followed by a NotAfter (TLV 255, leaf) containing a
valid 15 character ISO 8601-1:2019 format GMT timepoint. The
cert must be ignored if the NotBefore value is >= the NotAfter
value, if the NotAfter value is < the current time or if the
validity period is not completely contained within its signing
cert's validity period. The SigType in the Cert must match the
#certValidator type specified in the schema.
5. SigValue (TLV type 23, leaf) containing the result of signing the
Cert using the algorithm and key specified by the SigInfo. The
Length of the TLV must match the length used by the signature
type as per Section 2.3.2.
For example, what follows is the frontdoor's identity cert used in
the home IoT example:
6 (Data) size 240:
| 7 (Name) size 50:
| | 8 (Generic) size 4: iot2
| | 8 (Generic) size 6: device
| | 8 (Generic) size 9: frontdoor
| | 8 (Generic) size 3: KEY
| | 8 (Generic) size 4: 0eaf f793
| | 8 (Generic) size 3: dct
| | 36 (Timestamp) size 7: 23-02-18@18:17:46.088971
| 20 (MetaInfo) size 3:
| | 24 (ContentType) size 1: 2 (Key)
| 21 (Content) size 32: de19 4605 7f77 a7bd 1317 de41 002c fe15 1bc..
| 22 (SigInfo) size 81:
| | 27 (SigType) size 1: 8 (EdDSA)
| | 28 (KeyLocator) size 34:
| | | 29 (KeyDigest) size 32: 8c7f 1de9 ebc9 17b6 a8e9 dce9 056a 74c..
| | 253 (Validity) size 38:
| | | 254 (NotBefore) size 15: 20230219T021746
| | | 255 (NotAfter) size 15: 20240219T021746
| 23 (SigValue) size 64: c8b9 5883 4b9a 8aac 9ad0 e5e4 5eef 0a18 4b..
| 1b3a 1574 58d4 0528 1740 883e d90c 836f ed..
2.3.2. Leaf TLVs
Most of DeftT's leaf TLVs were described above but there are two
important enumeration types, name components and signature types,
with particular constraints and implications.
There are four types of components allowed in a Name (TLV 7):
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+=============+=====+===========================================+
| Type | TLV | Description |
+=============+=====+===========================================+
| Generic | 8 | Arbitrary blob of bytes |
+-------------+-----+-------------------------------------------+
| csID | 35 | 32-bit murmurhash of cState name (number) |
+-------------+-----+-------------------------------------------+
| Timestamp | 36 | GMT time point in microseconds (number) |
+-------------+-----+-------------------------------------------+
| SequenceNum | 37 | unsigned 64-bit integer (number) |
+-------------+-----+-------------------------------------------+
Table 1: Name Component Types
"Number" types are encoded in big-endian order (MSB first) with all
leading zero bytes suppressed. Thus their length can be zero to
eight bytes. For example, a SequenceNum of 0 would be [37, 0], 100
would be [37, 1, 100] and 1,000,000 would be [37, 3, 15, 66, 64].
There are five types of signature allowed in a SigType (TLV 27) and
each requires the SigValue (TLV 23) in a Data with that SigType have
a particular size:
+===========+=======+==========+===========================+
| Type | Value | SigValue | Description |
| | | length | |
+===========+=======+==========+===========================+
| stSHA256 | 0 | 32 | SHA256 data integrity |
+-----------+-------+----------+---------------------------+
| stAEAD | 7 | 40 | [RFC8103] content privacy |
| | | | plus full data integrity |
+-----------+-------+----------+---------------------------+
| stEdDSA | 8 | 64 | Ed25519 provenance and |
| | | | full data integrity |
+-----------+-------+----------+---------------------------+
| stRFC7693 | 9 | 64 | [RFC7693] full data |
| | | | integrity |
+-----------+-------+----------+---------------------------+
| stAEADSGN | 13 | 104 | [RFC8103] content privacy |
| | | | with Ed25519 provenance |
| | | | and data integrity |
+-----------+-------+----------+---------------------------+
Table 2: Signature Types
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2.3.3. TLV header details
All TLV headers use the same format. They occupy either 2 or 4
bytes, depending on the value of _L_. _L_ specifies the length in
bytes of _V_. Lengths in the range 0 to 252 occupy one byte. A
length of zero is allowed and indicates there are no _V_ bytes.
Lengths in the range 253 to 65535 occupy three bytes: a 'flag byte'
of 253 followed by the two bytes of the 16 bit length in big endian
order. Lengths greater than 65535 (deliberately) can not be
represented so a DeftT object can be no larger than 65535+4 = 65539
bytes. (Objects of arbitrary size can be handled by a segmentation/
reassembly layer above DeftT such as dct/shims/mbps.hpp in the
example implementation.)
_L_ must use the minimum description length coding. For example, a
length of 0 must be encoded as the single byte [0], not as the 3
bytes [253, 0, 0], 252 is encoded as [252], 253 as [253, 0, 253], 256
as [253, 1, 0] and 65535 as [253, 255, 255].
_T_ specifies the type of data in the container. It occupies one
byte, must be an element of the valid types set defined below, and
must conform to that element's rules.
2.3.4. Design rationale
DeftT's Publication, PDU and serialization formats were strongly
influenced by the [LANGSEC] observation that most security issues are
due to improper input handling. For example, Part II of [LangSecErr]
found that this class of errors accounted for 75% of the 47 OpenSSL
security vulnerabilities reported in the 18 months following
2015-1-1. Also, as of 2023-7-5, _all_ 25 of the protobuf CVEs listed
in the NIST National Vulnerability Database (https://nvd.nist.gov/)
are of this class.
[LangSecErr] suggests these vulnerabilities could have been avoided
by designing the protocol following three rules:
| The acceptable input to a program should be:
|
| 1. _well-defined_ (i.e., via a grammar)
| 2. _as simple as possible_ (on the Chomsky scale of syntactic
| complexity)
| 3. _fully validated before use_ (no "shotgun parsing")
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2.3.4.1. Making DeftT 'well-defined'
A DeftT domain's "acceptable inputs" are specified using its
_communication rules_ declarative language (see Section 3.1) then
compiled by an LALR parser into a compact binary "schema" that avoids
any need for runtime parsing -- given the schema, the DeftT runtime
can construct or validate any legal domain input in constant time.
The compiler will fail to construct a schema if the domain
communication rules are incomplete or inconsistent.
After successful compilation, the schema is authorized, authenticated
and integrity protected by cryptographic signing using the domain's
trust anchor. This signed schema is supplied to every member as part
of their identity bundle (Section 4.2) and the SHA-256 thumbprint
(see Section 2.3.1.4) of the schema is the first component of every
PDU's topic name. This ensures not only that the rules are well
defined but also that all publishers and subscribers are playing by
the _same_ rules.
2.3.4.2. Making DeftT 'as simple as possible'
All DeftT Information is represented using TLV (Type, Length, Value)
tuples for the reasons noted by Dan Berstein
[netstrings][tnetstrings]:
* Unlike delimitter-based approaches like XML or JSON, TLVs are
resistant to buffer overflow and false pairing attacks.
* TLVs are self-describing and trivial to parse or validate.
* They can be used recursively -- containers can contain other
containers.
* TLVs are fast, cache friendly and not resource intensive.
* TLVs make no assumptions about contents and can store binary data
without escaping or encoding.
* TLVs are transport agnostic.
Attackers regard the 'seams' between protocol layers as prime attack
surface since a lower layer can pass up partial information that it
later finds to be inconsistent or invalid (an anti-pattern known as
_shotgun parsing_ [LangSecErr]). DeftT deliberately reuses a small
set of formatting conventions to construct its TLV containers in
contrast to the Internet convention of constructing its PDUs in
separate layers with rules chosen by different committees. For
example, DeftT PDUs, Publications and Certs have essentially the same
format so they can _all_ be structurally validated (e.g., the
contents of a container are the type expected in the order expected
and exactly fill their container) by _one_ simple, generic, recursive
descent validation pass over each arriving PDU performed at the point
where it arrives.
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As described in Section 1.3, DeftT validates every Publication and
PDU both cryptographically and syntactically using the domain's
_communications rules_ to enforce who-can-say-what-to-which-where-
when. DeftT does both serialization and validation using rules bound
at runtime (Figure 5) not compile time. It can do this at rates
competitive with protobufs by taking advantage of the "definiteness"
of local-domain communication:
* Since the same rules are used both to produce and validate
Publications/PDUs, encoding order is fixed and known in advance.
Thus every top-level object can be validated by a single
sequential pass through it.
* Every party to the communication is guaranteed to be using the
same rules so there are no options and no negotiation thus no
combinatorial explosion of variants to check.
* Communication rules can be extended and amended at any time and
the resultant binary schema published to members with no changes
to their code. Thus the current ruleset should always be the
_minimum_ necessary to support existing applications and policies,
not the open-ended monster needed to support any possible future.
2.4. Application and network interface
Figure 8 and Figure 9 show the blocks and modules application
information passes through in DeftT. Refer to those figures for this
discussion of how application information originates at a trust
domain member and progressing to a Publication in a collection that
is sent in a PDU via the system network layer to be received by other
members of the domain. (For more detail, see the library at [DCT].)
DeftT uses a *shim* to interface with the application's model of
information exchange. The only currently available shims in the
example implementation [DCT] provides a message-based publish/
subscribe (_mbps_) model to the application, although it should be
possible to construct a shim that provides a different model (e.g.,
streaming). All the necessary DeftT startup is kicked off when an
mbps object 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 members of a collection will have the same IBLTs.)
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
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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.
Trust domain members only process cAdds that share their trust domain
identifier (Section 2.3.1.1 and Section 2.3.1.2). 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.)
2.5. Synchronizing a collection
This section has thus far covered implementation of DeftT at a member
and the format of its communications. Although DeftT works on
unicast (as a special case of multicast) links, it is designed to
take full advantage of a multicast subnet (e.g., link-level IPv6
multicast on broadcast media). This subsection is an introduction to
how syncps orchestrates its collection-based communications on a
shared channel. A sequence diagram of the interaction of multiple
members' syncps modules interacting on a multicast subnet to keep
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their collections synchronized is shown in Figure 11. Starting with
all members _connected_ to the collection (having confirmed
publication of their identity credentials) and with an empty *pubs*
collection (i.e., no members have active Publications), member2's
application passes content to its DeftT (via an mbps.publish()),
which creates and sends a cAdd PDU. The cAdd uses a hash of the
shared (empty) cState as its cState identifier (third component of
the Name Section 2.3.1.2 item 1) to indicate the Publication(s) it
carries are additions to the collection in that state. Member2's new
local cState (with the new Publication) is scheduled to be sent at a
computed delay of twice the subnet's dispersion time (_d)_ plus a
small random value (_r_). (Dispersion time is an estimate of the
expected time for a cAdd to reach every member's collection. It may
be a fixed or adaptive estimate and syncps is robust to inaccuracies:
an overestimate may lead to longer delays and an underestimate may
mean more cState traffic.) Members receive and validate the cAdd,
then extract and validate Publication(s), passing it to
subscriptions. To avoid excessive cState traffic, each member
schedules the acknowledging cState for _d+r_. (Scheduling a
sendCState cancels any pending value.) When the sendCState timer
expires, a new local cState is created with an IBLT that contains the
new Publication. This cState's expiration time is scheduled (value
significantly longer than _d_) and the member sends the cState unless
it is suppressed. *syncps* suppresses cStates that are identical to
one that has been heard twice. If member2 is waiting to confirm the
Publication, it can do so with the first of these cStates it
receives. In Figure 11, member6 did not receive the cAdd but
reception of one of the new cStates shows it there is a new
Publication in the collection so it immediately sends its own local
cState (which has an empty collection, lacking member2's
Publication). Here, all members receive that cState, but member2, as
the originator, responds preferentially, sending a new cAdd
immediately. All other members set a timer (to 2*_d+r_) to send a
cAdd with the Publication. That timer is cancelled when an overheard
cAdd responding to that cState contains the Publication. Meanwhile,
member6 receives this new cAdd, adds the Publication to its
collection and schedules a new cState for _d+r_. That cState should
be suppressed as it will match those already sent by the other
members. Now the distributed collection is synchronized with a state
of one Publication (p1). If no other application content is created,
cStates will be sent at ~cStateLifetime. On the wire, we will see
one cState per ~cStateLifetime since they overlap enough to suppress
others. When p1 expires, it will be removed at each local collection
and the subsequent cState will show an empty collection.
(Artwork only available as svg: ./figs/syncseq-rfc.svg)
Figure 11: Seven members using DeftT on a multicast subnet
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Although Figure 11 shows one Publication at a time for clarity, the
logic works if multiple members are publishing simultaneously or at
close intervals (less than _d_ or the cStateLifeTime). The
distributed collection is always moving toward synchronization but
during periods of intense interaction, times when all members are
synchronized may be infrequent; this is not considered problematic.
2.6. Distributors
Distributors implement services a Deft requires for its operation.
Distributors optional to general operation are specified in the
communications schema.
2.6.1. Certificate distributor
DeftT's certificate distributor is a required module. It implements
a collection of all the signing chain certificates in the Domain.
When a new DeftT is instantiated, it must publish all the
certificates from its identity bundle as well as its locally created
signing certificate. This joining process was shown in Figure 2.
Since many certificates in a member's chain are shared, that will be
reflected in each cState and those certs will not be sent on the
subnet. A member DeftT must receive a cState showing its signing
chain in another member's local collection before a DeftT can be
considered "connected" to the trust domain. This ensures there is at
least one other member that can receive the PDUs it sends.
2.6.2. Group key distributors
Group key distributors are optional in DeftT but required, and
automatically supplied, if encryption is specified in the schema.
When present, they are instantiated after their local certificate
distributor has "connected." The example implementation contains two
types of group key distributors. A group key distributor handles
creation and distribution of a single symmetric key to all members of
the Domain to use to encrypt either Publications or PDUs (if both are
encrypted, there is a group key distribuor for each). A subscriber
group key distributor distinguishes subscribers that can decrypt PDUs
and/or Publication and publishers that encrypt PDUs and/or
Publications (a member can be both subscriber and publisher). The
group key distributor is briefly described here.
A trust domain using group key encryption must have at least one
member with the attribute or capability of "keymaker" in its identity
chain. Keymaker-capable members of a Domain elect a keymaker that
makes a new symmetric encryption key upon winning the election. The
non-keymakers publish key requests that the keymaker uses to create a
list of current members. Requests and the symmetric key both have
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limited lifetimes. The keymaker uses each member's signing cert to
encrypt a copy of the current key and creates and publishes as many
Publications as needed to carry all the encrypted keys. In these
Publication, entries are indexed by the thumbprint of associated
signing cert and the range of thumbprints is used in the Publication
name. Members only accept such Publications from keymaker-capable
signers and, in case of conflict, use the key sent by a member whose
signing cert thumbprint is the smallest.
If the keymaker receives a new key request in between making new
keys, a copy of the key will be encrypted for it and published.
There is no explicit revocation but a blacklist can be implemented
and either published or passed from an application and a new group
key can be made and distributed to non-blacklisted members ahead of
the normal schedule.
2.6.3. Other distributors
Distributors may be used for other types of key distribution and for
distributing other types of information, e.g. blacklisted members,
domain statistics.
2.7. 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. Internet
protocols use header information to tell them how to forward packets.
DeftT's header only contains a trust domain id and a collection name.
Each DeftT has a trust management engine with a copy of rules for its
domain. DeftT *only* creates and moves its Publications in
accordance with that fully specified communications schema and never
moves a PDU between subnets. This approach differs in both intent
and execution from Internet forwarding and may not be appropriate for
all use cases but offers new opportunities to address the specific
security requirements of many Limited Domain use cases.
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 (introduced in Section 1.3 and further
discussed in Section 3). "Compatible" means there is at least one
Publication type and associated signer specification in common or one
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schema may be a subset of the other. In the case of sub-domains,
they may 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.) A possible use for multi-subnet trust domains with
different sub-schemas is where a unicast link is used to connect two
remotely located subnets of the same parent trust domain but only
certain types of Publications should go through the unicast link and
there may be slightly different rules used at each subnet. Different
sub-schemas on the same subnet might be used where certain members
have more limited access, either due to the technology of their
devices or to limit their access (e.g., guests of a network). Relays
could limit Publications by filtering Publications or subscribing to
limited Publication subsets but delineating these limits in a schema
provides _enforcement_ of Publication movement.
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 Section 2.3.1.1 item C1)
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-specific while Publications belong to a trust
domain (or sub-domain).
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 12 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 12-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
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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 12: 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 12-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 12-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 12-right, local communications for subnet 1
can be kept local while subnet 2 might send commands and/or collect
log files from subnet 1.
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.
2.8. 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
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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 fully
connected 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
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]). The capabilities and attributes of Things can be
characterized in structured abstract profiles that can be machine-
readable (e.g., [ONE][RFC8520][ZCL]). Energy applications in
particular have defined strict attribute- and role-based access
controls [IEC] though proposed enforcement approaches require
interaction of a number of mechanisms across the communications stack
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[NERC]. In Defined-trust Communications, 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 both to 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 that is
provided as open source. It is frequently updated to improve
features and performance but may have bugs or unoptimized features in
any particular release..
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 in [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 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 on paragraph 4 of [DLOG]:
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| 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 is a critical part of Defined-trust Communications which uses
the more descriptive term _communication schema_ (or _schema_ where
its use is clearly with respect to defined-trust communications) for
the rules that define the communications of a trust domain. A single
schema, securely provided to all members, provides the same
protection as dozens of manually configured, per-node ACL rules.
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 2.3.1.3).
Certificates (Section 2.3.1.4) 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 [LANGSEC] 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.
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 13) defines a
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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 13
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.
#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 13: An example communication schema in VerSec [DCT]
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 13) of the schema into binary as well as reporting
diagnostics (see Figure 14) used to confirm the intent of the rules
(and to flag problems).
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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 14: schemaCompile diagnostic output for example of Figure 13
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 13. 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 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 need a
key maker attribute, which is conferred via a _capability
certificate_ in a member's signing chain. Since DeftT identities
include the member cert and its entire signing chain, adding
attributes via _capability certificates_ to a signing chain lets
attribute-based security policies 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.
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If AEADSGN is specified for the pubValidator, at least one member of
the trust domain will need key maker capability. In Figure 14 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 15: Enhancing security in the example schema
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
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.)
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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 2.3.1.4) 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
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
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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] where
available. 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 16 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)
Figure 16: 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
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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 recreated at the
periodicity of the signing cert lifetime. Since the signing of the
public cert happens via requests to the TPM, the _identity_ key (used
to sign the cert) cannot be exfiltrated. The locally created
_signing_ key is used in the communications path where TPM signing
overhead is prohibitive.
The DCT examples and library use configured member identities to sign
locally created signing certs (with associated private keys) so 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 17
outlines a representative procedure.
(Artwork only available as svg: ./figs/InstallIdbundle-rfc.svg)
Figure 17: Representative commissioning and signing key maintenance
All DCT certs have a validity period. Publication signing key pairs
(with public signing certs) 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.
5. Use Cases
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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]. 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 and
where IIoT sensors require tight security. 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
efficiently encrypted making it ideal for this use.
IIoT sensors may be fixed or mobile (including drone-based); mobility
and envirnomental factors may cause different sensor gateways to
receive measurements 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 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. DeftT forms meshes with no additional configuration
(beyond DeftT's usual identity bundle and private key) needed to make
devices recognize one another in the trust domain. To see how DeftT
propagates information throughout a partially connected mesh,
consider Figure 18 where sensor S1's signal can reach devices D1-D4
but not D5 and D6. (Refer to Section 2.2 and Section 2.5.)
(Artwork only available as svg: ./figs/robust-rfc.svg)
Figure 18: Members out-of-range of a Publication's originator can
receive from non-originating members
1. S1 sends a cAdd with its latest measurement Publication that is
received by D1-D4 and added to their collections after which they
synchronize their cStates
2. Either a device in range of D5 and/or D6 sends a cState that
shows the new Publication which results in a cState from D5 and/
or D6 that serves as a request for that Publication *or* D5 and/
or D6 send a periodic cState update that lacks the new
Publication and is received by at least one of D1-D4.
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3. When devices that have received the Publication hear those
lacking cStates, they wait a _dispersion delay_ (plus a small
random value) so that the originator or some other device might
respond, after which they send the Publication in a cAdd unless a
cAdd responding to the specific lacking cState is overheard.
The large physical scale of many industrial processes necessitates
that expensive cabling costs be avoided through wireless transport
and battery power. 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.In particular, nuclear power plant applications have
radioactive shielding walls of very thick concrete and security
regulations make any plant modifications to add cabling subject to
expensive and time-consuming reviews and permitting. Consider such
an industrial setting where LoRa sensors collect a wide range of
information (e.g., temperature, movement/vibration, light levels,
etc.) they broadcast in layer 2 LoRaWAN messages (see Figure 19). A
WiFi network includes fixed displays, mobile tablets, and devices
with both a LoRaWAN gateway interface and a WiFi interface
(Gateways). Any particular WiFi-enabled device may subscribe to a
subset of the information available through DeftT. Privacy of data
is ensured by encrypting cAdd PDUs. The presence of several Gateways
within a single sensor's broadcast range reduces the number of lost
sensor packets and the DeftT WiFi mesh is resilient against
transmission outages, further facilitating reliability. 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.
Figure 19's example deploys WiFi devices near the doorways in
shielded walls to connect mesh members. A Controller is sited in a
control room and connected via an Ethernet cable to an enhanced
Gateway with an Ethernet interface and running a DeftT relay. Mobile
WiFi devices can move throughout the site and maintain connection to
both the sensors (through the gateways) and the Controller with its
longer-term storage of sensor readings.
Figure 19 assumes LoRaWAN server components are integrated with the
Gateway devices (as in many existing MQTT-based deployments) but
these devices communicate via DeftT over adhoc WiFi. Only one
Gateway has the join server _capability_ (via the communications
schema). Multiple _Gateways_ can receive sensor messages which they
package as Publications with the device identifier (DevAddr) and
unique count (uplink FCnt) as part of the name and publish in a
collection of sensor measurements. If Publications are encrypted
with a group key, the full name will be unique and only those members
of the collection who did not receive the broadcast directly from a
sensor will obtain it via the DeftT WiFi interface. (Collection
synchronization performs the deduplication process.) Otherwise, if
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packets are signed with the identity of the Gateway, the shim can
discard duplicates before passing to the subscribing application.
All Gateways participate in a collection for join messages but only
the join server originates LoRaWAN join-accept Publications. The
join server may distribute the (encrypted) application key to
Gateways that display sensor information or to WiFi devices that may
perform more sophisticated tasks e.g., a tablet that analyzes and
displays historical sensor input. The Controller receives
Publications of sensor data via a TCP connection to the TCP face of
the DeftT relay.
(Artwork only available as svg: ./figs/meshEx-rfc.svg)
Figure 19: IIOT meshed gateways connect a single trust domain
In addition to specifying encryption and signing types, the 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
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.
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* 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
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.
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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, e.g., to secure structured messages..
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.
* 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.
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* 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.
* 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
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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 5, Figure 20 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 20: 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.
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 2.3.1.3). 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
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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.
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 publishes
cStates regardless of whether the collection has changed,
resulting in (re)sending dropped cAdds (if any). Unlike
connection-oriented transports, DeftT can and will obtain any
Publications missing from its collection from any member that has
a valid copy.
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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 must match a current cState and, if so, the cAdd's
Publication(s) 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.
DeftT's modular structure allows for any cryptographic methods to be
used as sigmgrs. New methods can easily be added to the transport as
long as they present the same API.
The example implementation's 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
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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]. [DCT] relies on the crypto library
libsodium and on 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
[RFC1422] Kent, S., "Privacy Enhancement for Internet Electronic
Mail: Part II: Certificate-Based Key Management",
RFC 1422, DOI 10.17487/RFC1422, February 1993,
<https://www.rfc-editor.org/info/rfc1422>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
[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.
Zúñiga, "Multicast Considerations over IEEE 802 Wireless
Media", RFC 9119, DOI 10.17487/RFC9119, October 2021,
<https://www.rfc-editor.org/info/rfc9119>.
[RFC9200] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
H. Tschofenig, "Authentication and Authorization for
Constrained Environments Using the OAuth 2.0 Framework
(ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
<https://www.rfc-editor.org/info/rfc9200>.
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>.
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[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>.
[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.
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[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-
to-digitize-controls-and-enhance-performance-
cyber/566478/>.
[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
Enterprise IoT Edge Infrastructure", April 2021,
<https://www.wwt.com/white-paper/device-management-
requirements-to-secure-enterprise-iot-edge-
infrastructure/>.
[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,
<https://doi.org/10.1007/978-3-319-67816-0\_24>.
[Graphene19]
Ozisik, A. P., Andresen, G., Levine, B. N., Tapp, D.,
Bissias, G., and S. Katkuri, "Graphene: efficient
interactive set reconciliation applied to blockchain
propagation", 2019,
<https://doi.org/10.1145/3341302.3342082>.
[HSE] Kapersky, "Secure Element", 2022,
<https://encyclopedia.kaspersky.com/glossary/secure-
element/>.
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[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>.
[IBLT] Goodrich, M. T. and M. Mitzenmacher, "Invertible bloom
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>.
[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>.
[LANGSEC] LANGSEC, "LANGSEC: Language-theoretic Security "The View
from the Tower of Babel"", 2021, <http://langsec.org>.
[LangSecErr]
Momot, F., Bratus, S., Hallberg, S. M., and M. L.
Patterson, "The Seven Turrets of Babel: {A} Taxonomy of
LangSec Errors and How to Expunge Them", 2016,
<https://langsec.org/papers/langsec-cwes-secdev2016.pdf>.
[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>.
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[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>.
[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>.
[NIST] Hu, C., Ferraiolo, D., Kuhn, D., Schnitzer, A., Sandlin,
K., Miller, R., and K. Scarfone, "Guide to Attribute Based
Access Control (ABAC) Definition and Considerations",
August 2019, <https://www.nist.gov/publications/guide-
attribute-based-access-control-abac-definition-and-
considerations-0>.
[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>.
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[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>.
[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>.
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[RFC4949] Shirey, R., "Internet Security Glossary, Version 2",
FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<https://www.rfc-editor.org/info/rfc4949>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7693] Saarinen, M., Ed. and J. Aumasson, "The BLAKE2
Cryptographic Hash and Message Authentication Code (MAC)",
RFC 7693, DOI 10.17487/RFC7693, November 2015,
<https://www.rfc-editor.org/info/rfc7693>.
[RFC8103] Housley, R., "Using ChaCha20-Poly1305 Authenticated
Encryption in the Cryptographic Message Syntax (CMS)",
RFC 8103, DOI 10.17487/RFC8103, February 2017,
<https://www.rfc-editor.org/info/rfc8103>.
[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>.
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and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
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Contributors
Lixia Zhang
UCLA
Email: lixia@cs.ucla.edu
Roger Jungerman
Operant Networks Inc.
Roger contributed significantly 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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