A Taxonomy of operational security considerations for manufacturer installed keys and Trust Anchors
draft-richardson-t2trg-idevid-considerations-06
| Document | Type | Active Internet-Draft (individual) | |
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| Author | Michael Richardson | ||
| Last updated | 2022-02-03 | ||
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draft-richardson-t2trg-idevid-considerations-06
T2TRG Research Group M. Richardson
Internet-Draft Sandelman Software Works
Intended status: Informational 3 February 2022
Expires: 7 August 2022
A Taxonomy of operational security considerations for manufacturer
installed keys and Trust Anchors
draft-richardson-t2trg-idevid-considerations-06
Abstract
This document provides a taxonomy of methods used by manufacturers of
silicon and devices to secure private keys and public trust anchors.
This deals with two related activities: how trust anchors and private
keys are installed into devices during manufacturing, and how the
related manufacturer held private keys are secured against
disclosure.
This document does not evaluate the different mechanisms, but rather
just serves to name them in a consistent manner in order to aid in
communication.
RFCEDITOR: please remove this paragraph. This work is occurring in
https://github.com/mcr/idevid-security-considerations
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 7 August 2022.
Copyright Notice
Copyright (c) 2022 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
and restrictions with respect to this document. Code Components
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Applicability Model . . . . . . . . . . . . . . . . . . . . . 5
2.1. A reference manufacturing/boot process . . . . . . . . . 6
3. Types of Trust Anchors . . . . . . . . . . . . . . . . . . . 7
3.1. Secured First Boot Trust Anchor . . . . . . . . . . . . . 8
3.2. Software Update Trust Anchor . . . . . . . . . . . . . . 8
3.3. Trusted Application Manager anchor . . . . . . . . . . . 9
3.4. Public WebPKI anchors . . . . . . . . . . . . . . . . . . 9
3.5. DNSSEC root . . . . . . . . . . . . . . . . . . . . . . . 9
3.6. What else? . . . . . . . . . . . . . . . . . . . . . . . 10
4. Types of Identities . . . . . . . . . . . . . . . . . . . . . 10
4.1. Manufacturer installed IDevID certificates . . . . . . . 10
4.1.1. Operational Considerations for Manufacturer IDevID
Public Key Infrastructure . . . . . . . . . . . . . . 11
4.1.2. Key Generation process . . . . . . . . . . . . . . . 11
5. Public Key Infrastructures (PKI) . . . . . . . . . . . . . . 14
5.1. Number of levels of certification authorities . . . . . . 15
5.2. Protection of CA private keys . . . . . . . . . . . . . . 17
5.3. Supporting provisioned anchors in devices . . . . . . . . 17
6. Evaluation Questions . . . . . . . . . . . . . . . . . . . . 18
6.1. Integrity and Privacy of on-device data . . . . . . . . . 18
6.2. Integrity and Privacy of device identify
infrastructure . . . . . . . . . . . . . . . . . . . . . 19
6.3. Integrity and Privacy of included trust anchors . . . . . 19
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 20
8. Security Considerations . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 20
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 20
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 20
12.1. Normative References . . . . . . . . . . . . . . . . . . 20
12.2. Informative References . . . . . . . . . . . . . . . . . 20
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 25
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1. Introduction
An increasing number of protocols derive a significant part of their
security by using trust anchors [RFC4949] that are installed by
manufacturers. Disclosure of the list of trust anchors does not
usually cause a problem, but changing them in any way does. This
includes adding, replacing or deleting anchors. [RFC6024] deals with
how trust anchor stores are managed, while this document deals with
how the associated PKI which is anchor is managed.
Many protocols also leverage manufacturer installed identities.
These identities are usually in the form of [ieee802-1AR] Initial
Device Identity certificates (IDevID). The identity has two
components: a private key that must remain under the strict control
of a trusted part of the device, and a public part (the certificate),
which (ignoring, for the moment, personal privacy concerns) may be
freely disclosed.
There also situations where identities are tied up in the provision
of symmetric shared secrets. A common example is the SIM card
([_3GPP.51.011]), it now comes as a virtual SIM, but which is usually
not provisioned at the factory. The provision of an initial, per-
device default password also falls into the category of symmetric
shared secret.
It is further not unusual for many devices (particularly smartphones)
to also have one or more group identity keys. This is used, for
instance, in [fidotechnote] to make claims about being a particular
model of phone (see [I-D.richardson-rats-usecases]). The key pair
that does this is loaded into large batches of phones for privacy
reasons.
The trust anchors are used for a variety of purposes. Trust anchors
are used to verify:
* the signature on a software update (as per
[I-D.ietf-suit-architecture]),
* a TLS Server Certificate, such as when setting up an HTTPS
connection,
* the [RFC8366] format voucher that provides proof of an ownership
change.
Device identity keys are used when performing enrollment requests (in
[RFC8995], and in some uses of [I-D.ietf-emu-eap-noob]. The device
identity certificate is also used to sign Evidence by an Attesting
Environment (see [I-D.ietf-rats-architecture]).
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These security artifacts are used to anchor other chains of
information: an EAT Claim as to the version of software/firmware
running on a device ([I-D.birkholz-suit-coswid-manifest]), an EAT
claim about legitimate network activity (via [I-D.birkholz-rats-mud],
or embedded in the IDevID in [RFC8520]).
Known software versions lead directly to vendor/distributor signed
Software Bill of Materials (SBOM), such as those described by
[I-D.ietf-sacm-coswid] and the NTIA/SBOM work [ntiasbom] and CISQ/OMG
SBOM work underway [cisqsbom].
In order to manage risks and assess vulnerabilities in a Supply
Chain, it is necessary to determine a degree of trustworthiness in
each device. A device may mislead audit systems as to its
provenance, about its software load or even about what kind of device
it is (see [RFC7168] for a humorous example).
In order to properly assess the security of a Supply Chain it is
necessary to understand the kinds and severity of the threats which a
device has been designed to resist. To do this, it is necessary to
understand the ways in which the different trust anchors and
identities are initially provisioned, are protected, and are updated.
To do this, this document details the different trust anchors (TrAnc)
and identities (IDs) found in typical devices. The privacy and
integrity of the TrAncs and IDs is often provided by a different,
superior artifact. This relationship is examined.
While many might desire to assign numerical values to different
mitigation techniques in order to be able to rank them, this document
does not attempt to do that, as there are too many other (mostly
human) factors that would come into play. Such an effort is more
properly in the purview of a formal ISO9001 process such as ISO14001.
1.1. Terminology
This document is not a standards track document, and it does not make
use of formal requirements language.
This section will be expanded to include needed terminology as
required.
The words Trust Anchor are contracted to TrAnc rather than TA, in
order not to confuse with [I-D.ietf-teep-architecture]'s "Trusted
Application".
This document defines a number of hyphenated terms, and they are
summarized here:
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device-generated: a private or symmetric key which is generated on
the device
infrastructure-generated: a private or symmetric key which is
generated by some system, likely located at the factory that built
the device
mechanically-installed: when a key or certificate is programmed into
non-volatile storage by an out-of-band mechanism such as JTAG
[JTAG]
mechanically-transferred: when a key or certificate is transferred
into a system via private interface, such as serial console, JTAG
managed mailbox, or other physically private interface
network-transferred: when a key or certificate is transferred into a
system using a network interface which would be available after
the device has shipped. This applies even if the network is
physically attached using a bed-of-nails [BedOfNails].
device/infrastructure-co-generated: when a private or symmetric key
is derived from a secret previously synchronized between the
silicon vendor and the factory using a common algorithm.
2. Applicability Model
There is a wide variety of devices to which this analysis can apply.
(See [I-D.bormann-lwig-7228bis].) This document will use a J-group
processor as a sample. This class is sufficiently large to
experience complex issues among multiple CPUs, packages and operating
systems, but at the same time, small enough that this class is often
deployed in single-purpose IoT-like uses. Devices in this class
often have Secure Enclaves (such as the "Grapeboard"), and can
include silicon manufacturer controlled processors in the boot
process (the Raspberry PI boots under control of the GPU).
Almost all larger systems (servers, laptops, desktops) include a
Baseboard Management Controller (BMC), which ranges from a M-Group
Class 3 MCU, to a J-Group Class 10 CPU (see, for instance [openbmc]
which uses a Linux kernel and system inside the BMC). As the BMC
usually has complete access to the main CPU's memory, I/O hardware
and disk, the boot path security of such a system needs to be
understood first as being about the security of the BMC.
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2.1. A reference manufacturing/boot process
In order to provide for immutability and privacy of the critical
TrAnc and IDs, many CPU manufacturers will provide for some kind of
private memory area which is only accessible when the CPU is in
certain privileged states. See the Terminology section of
[I-D.ietf-teep-architecture], notably TEE, REE, and TAM, and also
section 4, Architecture.
The private memory that is important is usually non-volatile and
rather small. It may be located inside the CPU silicon die, or it
may be located externally. If the memory is external, then it is
usually encrypted by a hardware mechanism on the CPU, with only the
key kept inside the CPU.
The entire mechanism may be external to the CPU in the form of a
hardware-TPM module, or it may be entirely internal to the CPU in the
form of a firmware-TPM. It may use a custom interface to the rest of
the system, or it may implement the TPM 1.2 or TPM 2.0
specifications. Those details are important to performing a full
evaluation, but do not matter much to this model (see initial-
enclave-location below).
During the manufacturing process, once the components have been
soldered to the board, the system is usually put through a system-
level test. This is often done as a "bed-of-nails" test
[BedOfNails], where the board has key points attached mechanically to
a test system. A [JTAG] process tests the System Under Test, and
then initializes some firmware into the still empty flash storage.
It is now common for a factory test image to be loaded first: this
image will include code to initialize the private memory key
described above, and will include a first-stage bootloader and some
kind of (primitive) Trusted Application Manager (TAM). (The TAM is a
piece of software that lives within the trusted execution
environment.)
Embedded in the stage one bootloader will be a Trust Anchor that is
able to verify the second-stage bootloader image.
After the system has undergone testing, the factory test image is
erased, leaving the first-stage bootloader. One or more second-stage
bootloader images are installed. The production image may be
installed at that time, or if the second-stage bootloader is able to
install it over the network, it may be done that way instead.
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There are many variations of the above process, and this section is
not attempting to be prescriptive, but to be provide enough
illustration to motivate subsequent terminology.
The process may be entirely automated, or it may be entirely driven
by humans working in the factory, or a combination of the above.
These steps may all occur on an access-controlled assembly line, or
the system boards may be shipped from one place to another (maybe
another country) before undergoing testing.
Some systems are intended to be shipped in a tamper-proof state, but
it is usually not desirable that bed-of-nails testing be possible
without tampering, so the initialization process is usually done
prior to rendering the system tamper-proof. An example of a one-way
tamper-proof, weather resistant treatment might to mount the system
board in a case and fill the case with resin.
Quality control testing may be done prior to as well as after the
application of tamper-proofing, as systems which do not pass
inspection may be reworked to fix flaws, and this should ideally be
impossible once the system has been made tamper-proof.
3. Types of Trust Anchors
Trust Anchors (TrAnc) are fundamentally public keys with
authorizations implicitly attached through the code that references
them.
They are used to validate other digitally signed artifacts.
Typically, these are chains of PKIX certificates leading to an End-
Entity certificate (EE).
The chains are usually presented as part of an externally provided
object, with the term "externally" to be understood as being as close
as untrusted flash, to as far as objects retrieved over a network.
There is no requirement that there be any chain at all: the trust
anchor can be used to validate a signature over a target object
directly.
The trust anchors are often stored in the form of self-signed
certificates. The self-signature does not offer any cryptographic
assurance, but it does provide a form of error detection, providing
verification against non-malicious forms of data corruption. If
storage is at a premium (such as inside-CPU non-volatile storage)
then only the public key itself need to be stored. For a 256-bit
ECDSA key, this is 32 bytes of space.
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When evaluating the degree of trust for each trust anchor there are
four aspects that need to be determined:
* can the trust anchor be replaced or modified?
* can additional trust anchors be added?
* can trust anchors be removed?
* how is the private key associated with the trust anchor,
maintained by the manufacturer, maintained?
The first three things are device specific properties of how the
integrity of the trust anchor is maintained.
The fourth property has nothing to do with the device, but has to do
with the reputation and care of the entity that maintains the private
key.
Different anchors have different authorizations associated with them.
These are:
3.1. Secured First Boot Trust Anchor
This anchor is part of the first-stage boot loader, and it is used to
validate a second-stage bootloader which may be stored in external
flash. This is called the initial software trust anchor.
3.2. Software Update Trust Anchor
This anchor is used to validate the main application (or operating
system) load for the device.
It can be stored in a number of places. First, it may be identical
to the Secure Boot Trust Anchor.
Second, it may be stored in the second-stage bootloader, and
therefore its integrity is protected by the Secured First Boot Trust
Anchor.
Third, it may be stored in the application code itself, where the
application validates updates to the application directly (update in
place), or via a double-buffer arrangement. The initial (factory)
load of the application code initializes the trust arrangement.
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In this situation the application code is not in a secured boot
situation, as the second-stage bootloader does not validate the
application/operating system before starting it, but it may still
provide measured boot mechanism.
3.3. Trusted Application Manager anchor
This anchor is the secure key for the [I-D.ietf-teep-architecture]
Trusted Application Manager (TAM). Code which is signed by this
anchor will be given execution privileges as described by the
manifest which accompanies the code. This privilege may include
updating anchors.
3.4. Public WebPKI anchors
These anchors are used to verify HTTPS certificates from web sites.
These anchors are typically distributed as part of desktop browsers,
and via desktop operating systems.
The exact set of these anchors is not precisely defined: it is
usually determined by the browser vendor (e.g., Mozilla, Google,
Apple, Safari, Microsoft), or the operating system vendor (e.g.,
Apple, Google, Microsoft, Ubuntu). In most cases these vendors look
to the CA/Browser Forum [CABFORUM] for inclusion criteria.
3.5. DNSSEC root
This anchor is part of the DNS Security extensions. It provides an
anchor for securing DNS lookups. Secure DNS lookups may be important
in order to get access to software updates. This anchor is now
scheduled to change approximately every 3 years, with the new key
announced several years before it is used, making it possible to
embed keys that will be valid for up to five years.
This trust anchor is typically part of the application/operating
system code and is usually updated by the manufacturer when they do
updates. However, a system that is connected to the Internet may
update the DNSSEC anchor itself through the mechanism described in
[RFC5011].
There are concerns that there may be a chicken and egg situation for
devices that have remained in a powered off state (or disconnected
from the Internet) for some period of years. That upon being
reconnected, that the device would be unable to do DNSSEC validation.
This failure would result in them being unable to obtain operating
system updates that would then include the updates to the DNSSEC key.
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3.6. What else?
TBD?
4. Types of Identities
Identities are installed during manufacturing time for a variety of
purposes.
Identities require some private component. Asymmetric identities
(e.g., RSA, ECDSA, EdDSA systems) require a corresponding public
component, usually in the form of a certificate signed by a trusted
third party.
This certificate associates the identity with attributes.
The process of making this coordinated key pair and then installing
it into the device is called identity provisioning.
4.1. Manufacturer installed IDevID certificates
[ieee802-1AR] defines a category of certificates that are installed
by the manufacturer, which contain at the least, a device unique
serial number.
A number of protocols depend upon this certificate.
* [RFC8572] and [RFC8995] introduce mechanisms for new devices
(called pledges) to be onboarded into a network without
intervention from an expert operator. A number of derived
protocols such as [I-D.ietf-anima-brski-async-enroll],
[I-D.ietf-anima-constrained-voucher],
[I-D.richardson-anima-voucher-delegation],
[I-D.friel-anima-brski-cloud] extend this in a number of ways.
* [I-D.ietf-rats-architecture] depends upon a key provisioned into
the Attesting Environment to sign Evidence.
* [I-D.ietf-suit-architecture] may depend upon a key provisioned
into the device in order to decrypt software updates. Both
symmetric and asymmetric keys are possible. In both cases, the
decrypt operation depends upon the device having access to a
private key provisioned in advance. The IDevID can be used for
this if algorithm choices permit. ECDSA keys do not directly
support encryption in the same way that RSA does, for instance,
but the addition of ECIES can solve this. There may be other
legal considerations why the IDevID might not be used, and a
second key provisioned.
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* TBD
4.1.1. Operational Considerations for Manufacturer IDevID Public Key
Infrastructure
The manufacturer has the responsibility to provision a key pair into
each device as part of the manufacturing process. There are a
variety of mechanisms to accomplish this, which this document will
overview.
There are three fundamental ways to generate IDevID certificates for
devices:
1. generating a private key on the device, creating a Certificate
Signing Request (or equivalent), and then returning a certificate
to the device.
2. generating a private key outside the device, signing the
certificate, and the installing both into the device.
3. deriving the private key from a previously installed secret seed,
that is shared with only the manufacturer.
There is a fourth situation where the IDevID is provided as part of a
Trusted Platform Module (TPM), in which case the TPM vendor may be
making the same tradeoffs.
The document [I-D.moskowitz-ecdsa-pki] provides some practical
instructions on setting up a reference implementation for ECDSA keys
using a three-tier mechanism.
4.1.2. Key Generation process
4.1.2.1. On-device private key generation
Generating the key on-device has the advantage that the private key
never leaves the device. The disadvantage is that the device may not
have a verified random number generator. [factoringrsa] is an example
of a successful attack on this scenario.
There are a number of options of how to get the public key securely
from the device to the certification authority.
This transmission must be done in an integral manner, and must be
securely associated with the assigned serial number. The serial
number goes into the certificate, and the resulting certificate needs
to be loaded into the manufacturer's asset database.
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One way to do the transmission is during a factory Bed of Nails test
(see [BedOfNails]) or Boundary Scan. When done via a physical
connection like this, then this is referred to as a _device-
generated_ / _mechanically-transferred_ method.
There are other ways that could be used where a certificate signing
request is sent over a special network channel when the device is
powered up in the factory. This is referred to as the _device-
generated_ / _network-transferred_ method.
Regardless of how the certificate signing request is sent from the
device to the factory, and how the certificate is returned to the
device, a concern from production line managers is that the assembly
line may have to wait for the certification authority to respond with
the certificate.
After the key generation, the device needs to set a flag such that it
no longer will generate a new key / will accept a new IDevID via the
factory connection. This may be a software setting, or could be as
dramatic as blowing a fuse.
The risk is that if an attacker with physical access is able to put
the device back into an unconfigured mode, then the attacker may be
able to substitute a new certificate into the device. It is
difficult to construct a rationale for doing this, unless the network
initialization also permits an attacker to load or replace trust
anchors at the same time.
Devices are typically constructed in a fashion such that the device
is unable to ever disclose the private key via an external interface.
This is usually done using a secure-enclave provided by the CPU
architecture in combination with on-chip non-volatile memory.
4.1.2.2. Off-device private key generation
Generating the key off-device has the advantage that the randomness
of the private key can be better analyzed. As the private key is
available to the manufacturing infrastructure, the authenticity of
the public key is well known ahead of time.
If the device does not come with a serial number in silicon, then one
should be assigned and placed into a certificate. The private key
and certificate could be programmed into the device along with the
initial bootloader firmware in a single step.
Aside from the change of origin for the randomness, a major advantage
of this mechanism is that it can be done with a single write to the
flash. The entire firmware of the device, including configuration of
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trust anchors and private keys can be loaded in a single write pass.
Given some pipelining of the generation of the keys and the creation
of certificates, it may be possible to install unique identities
without taking any additional time.
The major downside to generating the private key off-device is that
it could be seen by the manufacturing infrastructure. It could be
compromised by humans in the factory, or the equipment could be
compromised. The use of this method increases the value of attacking
the manufacturing infrastructure.
If private keys are generated by the manufacturing plant, and are
immediately installed, but never stored, then the window in which an
attacker can gain access to the private key is immensely reduced.
As in the previous case, the transfer may be done via physical
interfaces such as bed-of-nails, giving the _infrastructure-
generated_ / _mechanically-transferred_ method.
There is also the possibility of having a _infrastructure-generated_
/ _network-transferred_ key. There is a support for "server-
generated" keys in [RFC7030], [RFC8894], and [RFC4210]. All methods
strongly recommend encrypting the private key for transfer. This is
difficult to comply with here as there is not yet any private key
material in the device, so in many cases it will not be possible to
encrypt the private key.
4.1.2.3. Key setup based on 256 bit secret seed
A hybrid of the previous two methods leverages a symmetric key that
is often provided by a silicon vendor to OEM manufacturers.
Each CPU (or a Trusted Execution Environment
[I-D.ietf-teep-architecture], or a TPM) is provisioned at fabrication
time with a unique, secret seed, usually at least 256 bits in size.
This value is revealed to the OEM board manufacturer only via a
secure channel. Upon first boot, the system (probably within a TEE,
or within a TPM) will generate a key pair using the seed to
initialize a Pseudo-Random-Number-Generator (PRNG). The OEM, in a
separate system, will initialize the same PRNG and generate the same
key pair. The OEM then derives the public key part, signs it and
turns it into a certificate. The private part is then destroyed,
ideally never stored or seen by anyone. The certificate (being
public information) is placed into a database, in some cases it is
loaded by the device as its IDevID certificate, in other cases, it is
retrieved during the onboarding process based upon a unique serial
number asserted by the device.
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This method appears to have all of the downsides of the previous two
methods: the device must correctly derive its own private key, and
the OEM has access to the private key, making it also vulnerable.
The secret seed must be created in a secure way and it must also be
communicated securely.
There are some advantages to the OEM however: the major one is that
the problem of securely communicating with the device is outsourced
to the silicon vendor. The private keys and certificates may be
calculated by the OEM asynchronously to the manufacturing process,
either done in batches in advance of actual manufacturing, or on
demand when an IDevID is demanded. Doing the processing in this way
permits the key derivation system to be completely disconnected from
any network, and requires placing very little trust in the system
assembly factory. Operational security such as often incorrectly
presented fictionalized stories of a "mainframe" system to which only
physical access is permitted begins to become realistic. That trust
has been replaced with a heightened trust placed in the silicon
(integrated circuit) fabrication facility.
The downsides of this method to the OEM are: they must be supplied by
a trusted silicon fabrication system, which must communicate the set
of secrets seeds to the OEM in batches, and they OEM must store and
care for these keys very carefully. There are some operational
advantages to keeping the secret seeds around in some form, as the
same secret seed could be used for other things. There are some
significant downsides to keeping that secret seed around.
5. Public Key Infrastructures (PKI)
[RFC5280] describes the format for certificates, and numerous
mechanisms for doing enrollment have been defined (including: EST
[RFC7030], CMP [RFC4210], SCEP [RFC8894]).
[RFC5280] provides mechanisms to deal with multi-level certification
authorities, but it is not always clear what operating rules apply.
The certification authority (CA) that is central to [RFC5280]-style
public key infrastructures can suffer three kinds of failures:
1. disclosure of a private key,
2. loss of a private key,
3. inappropriate signing of a certificate from an unauthorized
source.
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A PKI which discloses one or more private certification authority
keys is no longer secure.
An attacker can create new identities, and forge certificates
connecting existing identities to attacker controlled public/private
keypairs. This can permit the attacker to impersonate any specific
device.
There is an additional kind of failure when the CA is convinced to
sign (or issue) a certificate which it is not authorized to do so.
See for instance [ComodoGate]. This is an authorization failure, and
while a significant event, it does not result in the CA having to be
re-initialized from scratch.
This is distinguished from when a loss as described above renders the
CA completely useless and likely requires a recall of all products
that have ever had an IDevID issued from this CA.
If the PKI uses Certificate Revocation Lists (CRL)s, then an attacker
that has access to the private key can also revoke existing
identities.
In the other direction, a PKI which loses access to a private key can
no longer function. This does not immediately result in a failure,
as existing identities remain valid until their expiry time
(notAfter). However, if CRLs or OCSP are in use, then the inability
to sign a fresh CRL or OCSP response will result in all identities
becoming invalid once the existing CRLs or OCSP statements expire.
This section details some nomenclature about the structure of
certification authorities.
5.1. Number of levels of certification authorities
Section 6.1 of [RFC5280] provides a Basic Path Validation. In the
formula, the certificates are arranged into a list.
The certification authority (CA) starts with a Trust Anchor (TrAnc).
This is counted as the first level of the authority.
In the degenerate case of a self-signed certificate, then this a one
level PKI.
.----------.<-.
|Issuer= X | |
|Subject=X |--'
'----------'
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The private key associated with the Trust Anchor signs one or more
certificates. When this first level authority trusts only End-Entity
(EE) certificates, then this is a two level PKI.
.----------.<-.
|Issuer= X | | root
|Subject=X +--' CA
'--+-----+-'
| |
| '-------.
| |
v v
.----EE----. .----EE----.
|Issuer= X | |Issuer= X |
|Subject=Y1| |Subject=Y2|
'----------' '----------'
When this first level authority signs subordinate certification
authorities, and those certification authorities sign End-Entity
certificates, then this is a three level PKI.
.----------.<-.
root |Issuer= X | |
CA |Subject=X +--'
'--+-----+-'
| |
.-----------' '------------.
| |
v v
.----------. .----------.
|Issuer= X | subordinate |Issuer= X |
|Subject=Y1| CA |Subject=Y2|
'--+---+---' '--+----+--'
| | | |
.--' '-------. .---' '------.
| | | |
v v v v
.----EE----. .----EE----. .----EE----. .----EE----.
|Issuer= Y1| |Issuer= Y1| |Issuer= Y2| |Issuer= Y2|
|Subject=Z1| |Subject=Z1| |Subject=Z3| |Subject=Z4|
'----------' '----------' '----------' '----------'
In general, when arranged as a tree, with the End-Entity certificates
at the bottom, and the Trust Anchor at the top, then the level is
where the deepest EE certificates are, counting from one.
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It is quite common to have a three-level PKI, where the root of the
CA is stored in a Hardware Security Module, while the level one
subordinate CA is available in an online form.
5.2. Protection of CA private keys
The private key for the certification authorities must be protected
from disclosure. The strongest protection is afforded by keeping
them in a offline device, passing Certificate Signing Requests (CSRs)
to the offline device by human process.
For examples of extreme measures, see [kskceremony]. There is
however a wide spectrum of needs, as exampled in [rootkeyceremony].
The SAS70 audit standard is usually used as a basis for the Ceremony,
see [keyceremony2].
This is inconvenient, and may involve latencies of days, possibly
even weeks to months if the offline device is kept in a locked
environment that requires multiple keys to be present.
There is therefore a tension between protection and convenience.
This is often mitigated by having some levels of the PKI be offline,
and some levels of the PKI be online.
There is usually a need to maintain backup copies of the critical
keys. It is often appropriate to use secret splitting technology
such as Shamir Secret Sharing among a number of parties [shamir79]
This mechanism can be setup such that some threshold k (less than the
total n) of shares are needed in order to recover the secret.
5.3. Supporting provisioned anchors in devices
IDevID-type Identity (or Birth) Certificates which are provisioned
into devices need to be signed by a certification authority
maintained by the manufacturer. During the period of manufacture of
new product, the manufacturer needs to be be able to sign new
Identity Certificates.
During the anticipated lifespan of the devices the manufacturer needs
to maintain the ability for third parties to validate the Identity
Certificates. If there are Certificate Revocation Lists (CRLs)
involved, then they will need to re-signed during this period. Even
for devices with a short active lifetime, the lifespan of the device
could very long if devices are kept in a warehouse for many decades
before being activated.
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Trust anchors which are provisioned in the devices will have
corresponding private keys maintained by the manufacturer. The trust
anchors will often anchor a PKI which is going to be used for a
particular purpose. There will be End-Entity (EE) certificates of
this PKI which will be used to sign particular artifacts (such as
software updates), or messages in communications protocols (such as
TLS connections). The private keys associated with these EE
certificates are not stored in the device, but are maintained by the
manufacturer. These need even more care than the private keys stored
in the devices, as compromise of the software update key compromises
all of the devices, not just a single device.
6. Evaluation Questions
This section recaps the set of questions that may need to be
answered. This document does not assign valuation to the answers.
6.1. Integrity and Privacy of on-device data
initial-enclave-location: Is the location of the initial software
trust anchor internal to the CPU package? Some systems have a
software verification public key which is built into the CPU
package, while other systems store that initial key in a non-
volatile device external to the CPU.
initial-enclave-integrity-key: If the first-stage bootloader is
external to the CPU, and if it is integrity protected, where is
the key used to check the integrity?
initial-enclave-privacy-key: If the first-stage data is external to
the CPU, is it kept confidential by use of encryption?
first-stage-initialization: The number of people involved in the
first stage initialization. An entirely automated system would
have a number zero. A factory with three 8 hour shifts might have
a number that is a multiple of three. A system with humans
involved may be subject to bribery attacks, while a system with no
humans may be subject to attacks on the system which are hard to
notice.
first-second-stage-gap: If a board is initialized with a first-stage
bootloader in one location (factory), and then shipped to another
location, there may situations where the device can not be locked
down until the second step.
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6.2. Integrity and Privacy of device identify infrastructure
For IDevID provisioning, which includes a private key and matching
certificate installed into the device, the associated public key
infrastructure that anchors this identity must be maintained by the
manufacturer.
identity-pki-level: how deep are the IDevID certificates that are
issued?
identity-time-limits-per-subordinate: how long is each subordinate
CA maintained before a new subordinate CA key is generated? There
may be no time limit, only a device count limit.
identity-number-per-subordinate: how many identities are signed by a
particular subordinate CA before it is retired? There may be no
numeric limit, only a time limit.
identity-anchor-storage: how is the root CA key stored? How many
people are needed to recover the private key?
6.3. Integrity and Privacy of included trust anchors
For each trust anchor (public key) stored in the device, there will
be an associated PKI. For each of those PKI the following questions
need to be answered.
pki-level: how deep is the EE that will be evaluated (the trust root
is at level 1)
pki-algorithms: what kind of algorithms and key sizes will be
considered to valid
pki-level-locked: (a Boolean) is the level where the EE cert will be
found locked by the device, or can levels be added or deleted by
the PKI operator without code changes to the device.
pki-breadth: how many different non-expired EE certificates is the
PKI designed to manage?
pki-lock-policy: can any EE certificate be used with this trust
anchor to sign? Or, is there some kind of policy OID or Subject
restriction? Are specific subordinate CAs needed that lead to the
EE?
pki-anchor-storage: how is the private key associated with this
trust root stored? How many people are needed to recover it?
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7. Privacy Considerations
many yet to be detailed
8. Security Considerations
This entire document is about security considerations.
9. IANA Considerations
This document makes no IANA requests.
10. Acknowledgements
Robert Martin of MITRE provided some guidance about citing the SBOM
efforts. Carsten Borman provides many editorial suggestions.
11. Changelog
12. References
12.1. Normative References
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<https://www.rfc-editor.org/info/rfc5280>.
[ieee802-1AR]
IEEE Standard, "IEEE 802.1AR Secure Device Identifier",
2009, <http://standards.ieee.org/findstds/
standard/802.1AR-2009.html>.
12.2. Informative References
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
[I-D.richardson-anima-voucher-delegation]
Richardson, M. and W. Pan, "Delegated Authority for
Bootstrap Voucher Artifacts", Work in Progress, Internet-
Draft, draft-richardson-anima-voucher-delegation-03, 22
March 2021, <https://www.ietf.org/archive/id/draft-
richardson-anima-voucher-delegation-03.txt>.
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[I-D.friel-anima-brski-cloud]
Friel, O., Shekh-Yusef, R., and M. Richardson, "BRSKI
Cloud Registrar", Work in Progress, Internet-Draft, draft-
friel-anima-brski-cloud-04, 6 April 2021,
<https://www.ietf.org/archive/id/draft-friel-anima-brski-
cloud-04.txt>.
[I-D.ietf-anima-constrained-voucher]
Richardson, M., Stok, P. V. D., Kampanakis, P., and E.
Dijk, "Constrained Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", Work in Progress, Internet-Draft,
draft-ietf-anima-constrained-voucher-15, 7 December 2021,
<https://www.ietf.org/archive/id/draft-ietf-anima-
constrained-voucher-15.txt>.
[I-D.ietf-anima-brski-async-enroll]
Fries, S., Brockhaus, H., Oheimb, D. V., and E. Lear,
"Support of Asynchronous Enrollment in BRSKI (BRSKI-AE)",
Work in Progress, Internet-Draft, draft-ietf-anima-brski-
async-enroll-04, 25 October 2021,
<https://www.ietf.org/archive/id/draft-ietf-anima-brski-
async-enroll-04.txt>.
[I-D.moskowitz-ecdsa-pki]
Moskowitz, R., Birkholz, H., Xia, L., and M. C.
Richardson, "Guide for building an ECC pki", Work in
Progress, Internet-Draft, draft-moskowitz-ecdsa-pki-10, 31
January 2021, <https://www.ietf.org/archive/id/draft-
moskowitz-ecdsa-pki-10.txt>.
[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>.
[RFC5011] StJohns, M., "Automated Updates of DNS Security (DNSSEC)
Trust Anchors", STD 74, RFC 5011, DOI 10.17487/RFC5011,
September 2007, <https://www.rfc-editor.org/info/rfc5011>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/info/rfc8572>.
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[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<https://www.rfc-editor.org/info/rfc7030>.
[RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol",
RFC 8894, DOI 10.17487/RFC8894, September 2020,
<https://www.rfc-editor.org/info/rfc8894>.
[RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen,
"Internet X.509 Public Key Infrastructure Certificate
Management Protocol (CMP)", RFC 4210,
DOI 10.17487/RFC4210, September 2005,
<https://www.rfc-editor.org/info/rfc4210>.
[_3GPP.51.011]
3GPP, "Specification of the Subscriber Identity Module -
Mobile Equipment (SIM-ME) interface", 3GPP TS 51.011
4.15.0, 15 June 2005,
<http://www.3gpp.org/ftp/Specs/html-info/51011.htm>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <https://www.rfc-editor.org/info/rfc6024>.
[BedOfNails]
Wikipedia, "Bed of nails tester", 1 July 2020,
<https://en.wikipedia.org/wiki/In-
circuit_test#Bed_of_nails_tester>.
[pelionfcu]
ARM Pelion, "Factory provisioning overview", 28 June 2020,
<https://www.pelion.com/docs/device-management-
provision/1.2/introduction/index.html>.
[factoringrsa]
"Factoring RSA keys from certified smart cards:
Coppersmith in the wild", 16 September 2013,
<https://core.ac.uk/download/pdf/204886987.pdf>.
[kskceremony]
Verisign, "DNSSEC Practice Statement for the Root Zone ZSK
Operator", 2017, <https://www.iana.org/dnssec/dps/zsk-
operator/dps-zsk-operator-v2.0.pdf>.
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[rootkeyceremony]
Community, "Root Key Ceremony, Cryptography Wiki", 4 April
2020,
<https://cryptography.fandom.com/wiki/Root_Key_Ceremony>.
[keyceremony2]
Digi-Sign, "SAS 70 Key Ceremony", 4 April 2020,
<http://www.digi-sign.com/compliance/key%20ceremony/
index>.
[shamir79] Shamir, A., "How to share a secret.", 1979,
<https://www.cs.jhu.edu/~sdoshi/crypto/papers/
shamirturing.pdf>.
[nistsp800-57]
NIST, "SP 800-57 Part 1 Rev. 4 Recommendation for Key
Management, Part 1: General", 1 January 2016,
<https://csrc.nist.gov/publications/detail/sp/800-57-part-
1/rev-4/final>.
[fidotechnote]
FIDO Alliance, "FIDO TechNotes: The Truth about
Attestation", 19 July 2018, <https://fidoalliance.org/
fido-technotes-the-truth-about-attestation/>.
[ntiasbom] NTIA, "NTIA Software Component Transparency", 1 July 2020,
<https://www.ntia.doc.gov/SoftwareTransparency>.
[cisqsbom] CISQ/Object Management Group, "TOOL-TO-TOOL SOFTWARE BILL
OF MATERIALS EXCHANGE", 1 July 2020, <https://www.it-
cisq.org/software-bill-of-materials/index.htm>.
[ComodoGate]
"Comodo-gate hacker brags about forged certificate
exploit", 28 March 2011,
<https://www.theregister.com/2011/03/28/
comodo_gate_hacker_breaks_cover/>.
[openbmc] Linux Foundation/OpenBMC Group, "Defining a Standard
Baseboard Management Controller Firmware Stack", 1 July
2020, <https://www.openbmc.org/>.
[JTAG] "Joint Test Action Group", 26 August 2020,
<https://en.wikipedia.org/wiki/JTAG>.
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[JTAGieee] IEEE Standard, "1149.7-2009 - IEEE Standard for Reduced-
Pin and Enhanced-Functionality Test Access Port and
Boundary-Scan Architecture",
DOI 10.1109/IEEESTD.2010.5412866, 2009,
<https://ieeexplore.ieee.org/document/5412866>.
[rootkeyrollover]
ICANN, "Proposal for Future Root Zone KSK Rollovers",
2019, <https://www.icann.org/en/system/files/files/
proposal-future-rz-ksk-rollovers-01nov19-en.pdf>.
[CABFORUM] CA/Browser Forum, "CA/Browser Forum Baseline Requirements
for the Issuance and Management of Publicly-Trusted
Certificates, v.1.7.3", October 2020,
<https://cabforum.org/wp-content/uploads/CA-Browser-Forum-
BR-1.7.3.pdf>.
[I-D.richardson-rats-usecases]
Richardson, M., Wallace, C., and W. Pan, "Use cases for
Remote Attestation common encodings", Work in Progress,
Internet-Draft, draft-richardson-rats-usecases-08, 2
November 2020, <https://www.ietf.org/archive/id/draft-
richardson-rats-usecases-08.txt>.
[I-D.ietf-suit-architecture]
Moran, B., Tschofenig, H., Brown, D., and M. Meriac, "A
Firmware Update Architecture for Internet of Things", Work
in Progress, Internet-Draft, draft-ietf-suit-architecture-
16, 27 January 2021, <https://www.ietf.org/archive/id/
draft-ietf-suit-architecture-16.txt>.
[I-D.ietf-emu-eap-noob]
Aura, T., Sethi, M., and A. Peltonen, "Nimble Out-of-Band
Authentication for EAP (EAP-NOOB)", Work in Progress,
Internet-Draft, draft-ietf-emu-eap-noob-06, 3 September
2021, <https://www.ietf.org/archive/id/draft-ietf-emu-eap-
noob-06.txt>.
[I-D.ietf-rats-architecture]
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
W. Pan, "Remote Attestation Procedures Architecture", Work
in Progress, Internet-Draft, draft-ietf-rats-architecture-
14, 9 December 2021, <https://www.ietf.org/archive/id/
draft-ietf-rats-architecture-14.txt>.
[I-D.birkholz-suit-coswid-manifest]
Birkholz, H., "A SUIT Manifest Extension for Concise
Software Identifiers", Work in Progress, Internet-Draft,
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draft-birkholz-suit-coswid-manifest-00, 17 July 2018,
<https://www.ietf.org/archive/id/draft-birkholz-suit-
coswid-manifest-00.txt>.
[I-D.birkholz-rats-mud]
Birkholz, H., "MUD-Based RATS Resources Discovery", Work
in Progress, Internet-Draft, draft-birkholz-rats-mud-00, 9
March 2020, <https://www.ietf.org/archive/id/draft-
birkholz-rats-mud-00.txt>.
[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>.
[I-D.ietf-sacm-coswid]
Birkholz, H., Fitzgerald-McKay, J., Schmidt, C., and D.
Waltermire, "Concise Software Identification Tags", Work
in Progress, Internet-Draft, draft-ietf-sacm-coswid-20, 26
January 2022, <https://www.ietf.org/archive/id/draft-ietf-
sacm-coswid-20.txt>.
[RFC7168] Nazar, I., "The Hyper Text Coffee Pot Control Protocol for
Tea Efflux Appliances (HTCPCP-TEA)", RFC 7168,
DOI 10.17487/RFC7168, April 2014,
<https://www.rfc-editor.org/info/rfc7168>.
[I-D.bormann-lwig-7228bis]
Bormann, C., Ersue, M., Keranen, A., and C. Gomez,
"Terminology for Constrained-Node Networks", Work in
Progress, Internet-Draft, draft-bormann-lwig-7228bis-07,
25 October 2021, <https://www.ietf.org/archive/id/draft-
bormann-lwig-7228bis-07.txt>.
[I-D.ietf-teep-architecture]
Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
"Trusted Execution Environment Provisioning (TEEP)
Architecture", Work in Progress, Internet-Draft, draft-
ietf-teep-architecture-15, 12 July 2021,
<https://www.ietf.org/archive/id/draft-ietf-teep-
architecture-15.txt>.
Author's Address
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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