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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


   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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on June 6, 2020.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

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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

   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

2.  Operational Considerations for Manufacturer IDevID Public Key

   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

   There are two fundamental ways to generate IDevID certificates for

   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

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"

   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

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

   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

   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

   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

   Since they are already present, why not use of them as the MASA trust

   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


5.  Operational Considerations for creating Nonceless vouchers


6.  Privacy Considerations


7.  Security Considerations


8.  IANA Considerations

   This document makes no IANA requests.

9.  Acknowledgements


10.  Changelog

11.  References

11.1.  Normative References

              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.

              Richardson, M., Stok, P., and P. Kampanakis, "Constrained
              Voucher Artifacts for Bootstrapping Protocols", draft-
              ietf-anima-constrained-voucher-05 (work in progress), July

              IEEE Standard, ., "IEEE 802.1AR Secure Device Identifier",
              2009, <http://standards.ieee.org/findstds/

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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,

11.2.  Informative References

              Wikipedia, "In-circuit test", 2019,

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,

Author's Address

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca

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