anima Working Group M. Richardson
Internet-Draft Sandelman Software Works
Intended status: Standards Track December 04, 2019
Expires: June 6, 2020
Operational Considerations for Manufacturer Authorized Signing Authority
draft-richardson-anima-masa-considerations-01
Abstract
This document describes a number of operational modes that a BRSKI
Manufacturer Authorized Signing Authority (MASA) may take on.
Each mode is defined, and then each mode is given a relevance within
an over applicability of what kind of organization the MASA is
deployed into. This document does not change any protocol
mechanisms.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Operational Considerations for Manufacturer IDevID Public Key
Infrastructure . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. On-device private key generation . . . . . . . . . . . . 4
2.2. Off-device private key generation . . . . . . . . . . . . 4
2.3. Public Key infrastructure for IDevID . . . . . . . . . . 5
3. Operational Considerations for Manufacturer Authorized
Signing Authority (MASA) . . . . . . . . . . . . . . . . . . 5
3.1. Self-contained multi-product MASA . . . . . . . . . . . . 6
3.2. Self-contained per-product MASA . . . . . . . . . . . . . 7
3.3. Per-product MASA keys intertwined with IDevID PKI . . . . 7
3.4. Rotating MASA authorization keys . . . . . . . . . . . . 8
4. Operational Considerations for Constrained MASA . . . . . . . 9
5. Operational Considerations for creating Nonceless vouchers . 9
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 9
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
11.1. Normative References . . . . . . . . . . . . . . . . . . 9
11.2. Informative References . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
[I-D.ietf-anima-bootstrapping-keyinfra] introduces a mechanism for
new devices (called pledges) to be onboarded into a network without
intervention from an expert operator.
This mechanism leverages the pre-existing relationship between a
device and the manufacturer that built the device. There are two
aspects to this relationship: the provision of an identity for the
device by the manufacturer (the IDevID), and a mechanism which
convinces the device to trust the new owner (the [RFC8366] voucher).
The manufacturer, or their designate, is involved in both aspects of
this process. This requires the manufacturer to operate a
significant process for each aspect.
This document offers a number of operational considerations for each
aspect.
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The first aspect is the device identity in the form of an
[ieee802-1AR] certificate that is installed at manufacturing time in
the device. The first section of this document deals with
operational considerations of building this public key
infrastructure.
The second aspect is the use of the Manufacturer Authorized Signing
Authority (MASA), as described in
[I-D.ietf-anima-bootstrapping-keyinfra] section 2.5.4. The device
needs to have the MASA anchor built in; the exact nature of the
anchor is subject to a many possibilities. The second section of
this document deals with a number of options for architecting the
security of the MASA relationship.
There are some additional considerations for a MASA that deals with
constrained vouchers as described in
[I-D.ietf-anima-constrained-voucher]. In particular in the COSE
signed version, there are no PKI structure included in the voucher
mechanism, so cryptographic hygiene needs a different set of
tradeoffs.
2. Operational Considerations for Manufacturer IDevID Public Key
Infrastructure
The manufacturer has the responsability to provision a keypair 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 two 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.
There is a third 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. Or the mechanisms to install the
certificate into the TPM will use TPM APIs.
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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.
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. This asset database needs to be
shared with the MASA.
One way to do the transmission is during a manufacturing during a Bed
of Nails (see [BedOfNails]) or Boundary Scan. There are other ways
that could be used where a certificate signing request is sent over a
special network channel after the system has been started the first
time. There are risks with these methods, as an attacker with
physical access may be able to put device back into this mode
afterwards.
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. A serial number for the
device can 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.
The major downside to generating the private key off-device is that
it could be seen by the manufacturing infrastructure. This makes the
manufacturing infrastructure a high-value attack target, so
mechanisms that keep the private key secure within the manufacturing
process, yet allow the device to get access to the private key would
be adventageous.
Per-device keys would be ideal, but that would require that an
individual per-device key be provisioned in seperately. An alternate
mechanism would be that a constant decryption key is kept in the
firmware, but this provides little protection against an attack on
the manufacturing infrastructure unless the firmware is loaded in a
completely different place than the keypair.
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2.3. Public Key infrastructure for IDevID
A three-tier PKI infrastructure is appropriate. This entails having
a root CA created with the key kept offline, and a number of
intermediate CAs that have online keys that issue "day-to-day"
certificates.
The root private key should be kept offline, quite probably in a
Hardware Security Module if financially feasible. If not, then it
should be secret-split across seven to nine people, with a threshold
of four to five people. The split secrets should be kept in
geographically diverse places if the manufacturer has operations in
multiple places.
Ongoing access to the root-CA is important, but not as critical as
access to the MASA key.
The root CA is then used to sign a number of intermediate entities.
If manufacturing occurs in multiple factories, then an intermediate
CA for each factory is appropriate. It is also reasonable to use
different intermediate CAs for different product lines. It may also
be valuable to split IDevID certificates across intermediate CAs in a
round-robin fashion for products with high volumes.
Cycling the intermediate CAs after a period of a few months or so is
a quite reasonable strategy. The intermediate CAs private key may be
destroyed after it signed some number of IDevIDs, and a new key
generated. The IDevID certificates have very long (ideally infinite)
validity lifetimes for reasons that [ieee802-1AR] explains, but once
the certificates have been created the intermediate CA has no further
obligations as neither CRLs nor OCSP are appropriate.
In all cases the serialNumber embedded in the certificate must be
unique across all products produced by the manufacturer. This
suggests some amount of structure to the serialNumber, such that
different intermediate CAs do not need to coordinate when issuing
certificates.
3. Operational Considerations for Manufacturer Authorized Signing
Authority (MASA)
The manufacturer needs to make a Signing Authority available to new
owners so that they may obtain [RFC8366] format vouchers to prove
ownership. This section initially assumes that the manufacturer will
provide this Authority internally, but subsequent sections deal with
some adjustments when the authority is externally run.
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In any case, the manufacturer SHOULD plan for a situation where the
manufacturer is no longer able or interested in running the
Authority: this does not have to an unhappy situation of the
manufacturer going out of business. It could be a happy event where
the manufacturer goes through a merge or acquisition and it makes
sense to consolidate the Signing Authority in another part of the
organziation.
A plan to escrow the signing keys SHOULD be detailed, and it is
likely that customers will want to review it for high-value products.
The anchors for the MASA need to be "baked-in" to the device firmware
so that they are always available to validate vouchers. In order to
avoid locking down a single validation key, a PKI infrastructure is
appropriate. Note that constrained devices without code space to
parse and validate a public key certificate chain require different
considerations, and this document does not (yet) provide that
consideration.
There are many ways to construct a resilient PKI to sign vouchers.
3.1. Self-contained multi-product MASA
The most obvious is to just create a new offline CA, have it
periodically sign a new End-Entity (EE) Certificate with an online
private key, and use that to sign voucher requests. The entity used
to sign [RFC8366] format vouchers does not need to be a certificate
authority.
The public key of the offline CA is then built-in to the firmware of
the device, providing a trust anchor with with to validate vouchers.
The End-Entity certificate used to sign the voucher is included in
the certificate set in the CMS structure that is used to sign the
voucher. The root CA's trust anchor should _also_ be included, even
though it is self-signed, as this permits auditing elements in a
Registrar to validate the End-Entity Certificate.
The inclusion of the full chain also supports a Trust-on-First-Use
(TOFU) workflow for the manager of the Registrar: they can see the
trust anchor chain and can compare a fingerprint displayed on their
screen with one that could be included in packaging or other sales
channel information.
When building the MASA public key into a device, only the public key
contents matter, not the structure of the self-signed certificate
itself. Using only the public key enables a MASA architecture to
evolve from a single self-contained system into a more complex
architecture later on.
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3.2. Self-contained per-product MASA
A simple enhancement to the previous scenario is to have a unique
MASA offline key for each product line. This has a few advantages:
o if the private keys are kept separately (under different
encryption keys), then compromise of a single product lines MASA
does not compromise all products.
o if a product line is sold to another entity, or if it has to go
through an escrow process due to the product going out of
production, then the process affects only a single product line.
o it is safe to have serialNumber duplicated among different product
lines since a voucher for one product line would not validate on
another product line.
The disadvantage is that it requires a private key to be stored per
product line, and most large OEMs have many dozens of product lines.
If the keys are stored in a single Hardware Security Module (HSM),
with the access to it split across the same parties, then some of the
cryptographic advantages of different private keys goes away, as a
compromise of one key likely compromises them all. Given a HSM, the
most likely way a key is compromised is by an attacker getting
authorization on the HSM through theft or coercion.
The use of per-product MASA signing keys is encouraged.
3.3. Per-product MASA keys intertwined with IDevID PKI
The IDevID certificate chain (the intermediate CA and root CA that
signed the IDevID certificate) should be included in the device
firmware so that they can be communicated during the BRSKI-EST
exchange.
Since they are already present, why not use of them as the MASA trust
anchor?
A voucher needs to be signed by recognized End-Entity, which has been
authorized to be a voucher signer. The challenge with combining it
into the IDevID PKI is making sure that only an authorized entity can
sign the vouchers. This suggests that it can not be the same
intermediate CA that is used to sign the IDevID, since that CA should
have the authority to sign vouchers. If it did, then the End-
Entities that it created, the IDevID for devices, would then be able
to sign vouchers, which would not be an appropriate authorization.
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The PKI root CA therefore needs to sign an intermediate CA, or End-
Entity certificate with an extension OID that is specific for Voucher
Authorization. This is easy to do as policy OIDs can be created from
Private Enterprise Numbers. There is no need for standardization, as
the entity doing the signing is also creating the verification code.
If the entire PKI operation was outsource, then there would be a
benefit for standardization.
3.4. Rotating MASA authorization keys
As a variation of the scenario described in Section 3.2, there could
be multiple Signing Authority keys per product line. They could be
rotated though in some deterministic order. For instance, serial
numbers ending in 0 would have MASA key 0 embedded in them at
manufacturing time. The asset database would have to know which key
that corresponded to, and it would have to produce vouchers using
that key.
There are significant downsides to this mechanism:
o all of the MASA signing keys need to be online and available in
order to respond to any voucher request
o it is necessary to keep track of which device trust which key in
the asset database
There is no obvious advantage to doing this if all the MASA signing
private keys are kept in the same device, under control of the same
managers. But if the keys are spread out to multiple locations and
are under control of different people, then there may be some
advantage. A single MASA signing authority key compromise does not
cause a recall of all devices, but only the portion that had that key
embedded in it.
The relationship between signing key and device could be temporal:
all devices made on Tuesday could have the same key, there could be
hundreds of keys, each one used only for a few hundred devices.
There are many variations possible.
The major advantage comes with the COSE signed constrained-vouchers
described in [I-D.ietf-anima-constrained-voucher]. In this context
there isn't space in the voucher for a certificate chain, nor is
there code space in the device to validate a certificate chain. The
(public) key used to sign is embedded directly in the firmware of
each device without the benefit of any public key infrastructure to
allow indirection of the key.
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4. Operational Considerations for Constrained MASA
TBD
5. Operational Considerations for creating Nonceless vouchers
TBD
6. Privacy Considerations
YYY
7. Security Considerations
ZZZ
8. IANA Considerations
This document makes no IANA requests.
9. Acknowledgements
Hello.
10. Changelog
11. References
11.1. Normative References
[I-D.ietf-anima-bootstrapping-keyinfra]
Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
keyinfra-30 (work in progress), November 2019.
[I-D.ietf-anima-constrained-voucher]
Richardson, M., Stok, P., and P. Kampanakis, "Constrained
Voucher Artifacts for Bootstrapping Protocols", draft-
ietf-anima-constrained-voucher-05 (work in progress), July
2019.
[ieee802-1AR]
IEEE Standard, ., "IEEE 802.1AR Secure Device Identifier",
2009, <http://standards.ieee.org/findstds/
standard/802.1AR-2009.html>.
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[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
11.2. Informative References
[BedOfNails]
Wikipedia, "In-circuit test", 2019,
<https://en.wikipedia.org/wiki/In-
circuit_test#Bed_of_nails_tester>.
[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>.
Author's Address
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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