anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                                  W. Pan
Expires: 12 January 2023                             Huawei Technologies
                                                            11 July 2022

  Operatonal Considerations for Voucher infrastructure for BRSKI MASA


   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

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   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Operational Considerations for Manufacturer Authorized Signing
           Authority (MASA)  . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Deflecting unwanted TLS traffic with Client
           Certificates  . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Web framework architecture  . . . . . . . . . . . . . . .   4
     2.3.  Self-contained multi-product MASA, no PKI . . . . . . . .   5
     2.4.  Self-contained multi-product MASA, with one-level PKI . .   6
     2.5.  Self-contained per-product MASA . . . . . . . . . . . . .   7
     2.6.  Per-product MASA keys intertwined with IDevID PKI . . . .   7
     2.7.  Rotating MASA authorization keys  . . . . . . . . . . . .   8
   3.  Operational Considerations for Constrained MASA . . . . . . .   9
   4.  Operational Considerations for creating Nonceless vouchers  .   9
   5.  Business Continuity and Escow Considerations  . . . . . . . .   9
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     11.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

   [RFC8995] introduces a mechanism for new devices (called pledges) to
   be onboarded into a network without intervention from an expert

   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).

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   The manufacturer, or their designate, is involved in both aspects of
   this process.  This requires the manufacturer (or designate) to
   maintain on online presence.

   This document offers a number of operational considerations
   recommendations for operating this online presence.

   The first aspect is the device identity in the form of an
   [ieee802-1AR] certificate that is installed at manufacturing time in
   the device.  Some of the background for the operational
   considerations of building this public key infrastructure is
   described in [I-D.richardson-t2trg-idevid-considerations].

   The second aspect is the use of the Manufacturer Authorized Signing
   Authority (MASA), as described in [RFC8995] section 2.5.4.  The
   device needs to have the MASA anchor built in; the exact nature of
   the anchor is open to a number of possibilities which are explained
   in this document.  This document primarily 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 may be no PKI structure included in the voucher
   mechanism, so cryptographic hygiene needs a different set of

2.  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.

   The MASA is a public facing web system.  It will be subject to
   network load from legitimate users when a network is bootstrapped for
   the first time.  The legitimate load will be proportional to sales.

   The MASA will also be subject to a malicious load.

2.1.  Deflecting unwanted TLS traffic with Client Certificates

   One way to deflect unwanted users from the application framework
   backend is to require TLS Client Certificates for all connections.
   As described in Section 5.5.4 of [RFC8995], the Registrar may be
   authenticated with a TLS Client Certificate.

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   This offloads much of the defense to what is typically a hardware TLS
   termination system.  This can be effective even if the hardware is
   unable to do the actual validation of the TLS Client Certificate, as
   validation of the certificate occurs prior to any communication with
   the application server.

   This increases the effort requires for attackers, and if they repeat
   the same certificate then it becomes easier to reject such attackers
   if a list of invalid/unwanted clients is cached.

   The use of a client certificate forces attackers to generate new key
   pairs and certificates for each attack.

2.2.  Web framework architecture

   Web framework three-tier mechanisms are a very common architecture.
   See [threetier] for an overview.  There are Internet scale frameworks
   exist for Ruby (RubyOnRails), Python (Django), Java (J2EE), GO, PHP
   and others.  The methods of deploying them and dealing with expected
   scale are common in most enterprise IT departments.

   Consideration should be made to deploying the presentation layer into
   multiple data centers in order to provide resiliency against
   distributed denial of service (DDoS) attacks that affect all tenants
   of that data center.

   Consideration should also be given to the use of a cloud front end to
   mitigate attacks, however, such a system needs to be able to securely
   transmit the TLS Client Certificates, if the MASA wants to identify
   Registrars at the TLS connection time.

   The middle (application) tier needs to be scalable, but it is
   unlikely that it needs to scale very much on a per-minute or even
   per-hour basis.  It is probably easier and more reliable to have
   application tiers do database operations across the Internet or via
   VPN to a single location database cluster than it is to handle
   asynchronous database operations resulting from geographically
   dispersed multi-master database systems.

   But, these are local design decisions which web deployment make on a
   regular basis.  The MASA functionality is not different than other
   public facing systems.

   The database tables that the MASA uses scale linearly with the number
   of products sold, but as they are mostly read-only, they could be
   easily replicated in a read-only manner from a sales database.

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   Direct integration with a sales system could be considered, but would
   involve a more significant security impact analysis, so a process
   where the sales data is extracted to a less sensitive system is

   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!  While the
   case of the manufacturer going out of business is discussed in
   Section 5, there are more happy events which should be prepared for.
   For instance, if a manufacturer goes through a merge or acquisition
   and it makes sense to consolidate the Signing Authority in another
   part of the organization.

   Business continuity plan should include backing up the voucher
   signing keys.  This may involve multiple Hardware Security Modules,
   and secret splitting mechanisms SHOULD be employed.  For large value
   items, customers are going to need to review the plan as part of
   their contingency audits.  The document
   [I-D.richardson-t2trg-idevid-considerations] can provide some common
   basis for this kind of evaluation.

   The trust anchors needs to validate [RFC8366] vouchers will typically
   be part of the firmware loaded inot the devie firmware.

   There are many models to manage these trust anchors, but in order
   having only a single key, a PKI infrastructure is appropriate, but
   not required.

   On constrained devices without code space to parse and validate a
   public key certificate chain require different considerations, a
   single key may be necessary.  This document does not (yet) provide
   appropriate considerations for that case.

   What follows are a number of ways to construct a resilient PKI to
   sign vouchers.

2.3.  Self-contained multi-product MASA, no PKI

   The simplest situation is to create a self-signed End Entity
   certificate.  That is, a public/private key pair.  The certificate/
   public key is embedded in the products to validate vouchers, and the
   private part is kept online to sign vouchers.

   This situation has very low security against theft of a key from the
   MASA.  Such a theft would result in recall of all products that have
   not yet been onboarded.  It is very simple to operate.

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2.4.  Self-contained multi-product MASA, with one-level PKI

   A simple way is to create an new offline certification authority
   (CA), have it periodically sign a new End-Entity (EE) identity's
   certificate.  This End-Entity identity has a private key kept online,
   and it uses that to sign voucher requests.  Note that the entity used
   to sign [RFC8366] format vouchers does not need to be a certificate

   If the public key of this offline CA is then built-in to the firmware
   of the device, then the devices do not need any further anchors.

   There is no requirement for this CA to be signed by any other
   certification authority.  That is, it may be a root CA.  There is
   also no prohibition against it.

   If this offline CA signs any other certificates, then it is important
   that the device know which End-Entity certificates may sign vouchers.
   This is an authorization step, and it may be accomplished it a number
   of ways:

   1.  the Distinguished Name (DN) of the appropriate End-Entity
       certificate can be built-in to the firmware

   2.  a particular policy OID may be included in certificates intended
       to sign vouchers

   A voucher created for one product could be used to sign a voucher for
   another product.  This situation is also mitigated by never repeating
   serialNumbers across product lines.

   An 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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2.5.  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:

   *  if the private keys are kept separately (under different
      encryption keys), then compromise of a single product lines MASA
      does not compromise all products.

   *  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.

   *  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 will go 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.

2.6.  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, could they be used as the MASA trust
   anchor as well?

   In order to do this there is an attack that needs to mitigated.
   Since the root-CA that creates IDevIDs and the root-CA that creates
   vouchers are the same, when validating a voucher, a pledge needs to
   make sure that it is signed by a key authorized to sign vouchers.  In
   other scenarios any key signed by the voucher-signing-root-CA would
   be valid, but in this scenario that would also include any IDevID,
   such as would be installed in any other device.  Without an
   additional signal as to which keys can sign vouchers, and which keys
   are just IDevID keys, then it would be possible to sign vouchers with
   any IDevID private key, rather than just the designated voucher-

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   signing key.  An attacker that could extract a private key from even
   one instance of a product, could use that to sign vouchers, and
   impersonate the MASA.

   The challenge with combining it into the IDevID PKI is making sure
   that only an authorized entity can sign the vouchers.  The solution
   is 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.

   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.

2.7.  Rotating MASA authorization keys

   As a variation of the scenario described in Section 2.5, 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:

   *  all of the MASA signing keys need to be online and available in
      order to respond to any voucher request

   *  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.

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   The major advantage comes with the COSE signed constrained-vouchers
   described in [I-D.ietf-anima-constrained-voucher].  In this context,
   where there isn't space in the voucher for a certificate chain, nor
   is there code in the device to validate a certificate chain, a raw
   public key can sign the voucher.  The (public) key used to sign is
   embedded directly in the firmware of each device without the benefit
   of any public key infrastructure, which would allow indirection of
   the key.

3.  Operational Considerations for Constrained MASA


4.  Operational Considerations for creating Nonceless vouchers


5.  Business Continuity and Escow Considerations

   A number of jurisdictions have legal requirements for businesses to
   have contingency plans in order to continue operating after an
   incident or disaster.  Specifications include [iso22301_2019], but
   the problem of continuity goes back over 40 years.

   The [holman2012] document defined an eight tier process to understand
   how data would be backed up.  Tier 0 is "no off-site data", and would
   be inappropriate for the MASA's signing key.  The question as to how
   much delay (downtime) is tolerable during a disaster for activating
   new devices.  The consideration should depend upon the type of the
   device, and what kind of disasters are being planned for.  Given
   current technologies for replicating databases online, a tier-4
   ("Point-in-time copies") or better solution may be quite economically

   A key aspect of the MASA is that it was designed as a component that
   can be outsourced to a third party, and this third party can leverage
   economies of scale to provide more resilient systems at much lower

   The PKI components that are used to provision the IDevID
   certificiates into new devices need to be operational only when the
   factory that produces the devices is active.  The business continuity
   planning needs to include provision for backing up the private keys
   used within the PKI.  It may be enough to backup just the root CA
   key: the rest of the levels of the PKI can be regenerated in another
   location if necessary.

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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

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,

              Richardson, M., "A Taxonomy of operational security
              considerations for manufacturer installed keys and Trust
              Anchors", Work in Progress, Internet-Draft, draft-
              richardson-t2trg-idevid-considerations-06, 3 February
              2022, <

              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-17, 7 April 2022,

   [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, <>.

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              Richardson, M. and W. Pan, "Operational Considerations for
              BRSKI Registrar", Work in Progress, Internet-Draft, draft-
              richardson-anima-registrar-considerations-05, 6 March
              2022, <

              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, <

              Wikipedia, "Multitier architecture", December 2019,

              IEEE Standard, "IEEE 802.1AR Secure Device Identifier",
              2009, <

11.2.  Informative References

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

              Wikipedia, "In-circuit test", 2019,

              Qualcomm press release, "Qualcomm Licenses Rambus
              CryptoManager Key and Feature Management Security
              Solution", 2014, <

              Verisign, "DNSSEC Practice Statement for the Root Zone ZSK
              Operator", 2017, <

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              Community, "Root Key Ceremony, Cryptography Wiki", April

              Digi-Sign, "SAS 70 Key Ceremony", April 2020,

              NIST, "SP 800-57 Part 1 Rev. 4 Recommendation for Key
              Management, Part 1: General", 1 January 2016,

              ISO, "ISO 22301: Societal security — Business continuity
              management systems — Requirements", 1 January 2019,

              Holman, E., "A Business Continuity Solution Selection
              Methodology", 13 March 2012,

Authors' Addresses

   Michael Richardson
   Sandelman Software Works

   Wei Pan
   Huawei Technologies

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