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Versions: 00 01 02

anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                       December 02, 2019
Expires: June 4, 2020


             Operational Considerations for BRSKI Registrar
           draft-richardson-anima-registrar-considerations-00

Abstract

   This document describes a number of operational modes that a BRSKI
   Registration Authority (Registrar) 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 Registrar is
   deployed into.  This document does not change any protocol
   mechanisms.

   This document includes operational advice about avoiding unwanted
   consequences.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 4, 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
   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



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Reference Network and Diagrams  . . . . . . . . . . . . .   4
       1.2.1.  Tier-1 Network  . . . . . . . . . . . . . . . . . . .   4
       1.2.2.  Enterprise Network  . . . . . . . . . . . . . . . . .   4
       1.2.3.  Home Network  . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Internal architectural view . . . . . . . . . . . . . . .   5
       1.3.1.  Pledge Interface (Southbound Interface) . . . . . . .   5
       1.3.2.  MASA client (Northbound Interface)  . . . . . . . . .   6
       1.3.3.  Join Proxy (Southbound Interface) . . . . . . . . . .   7
       1.3.4.  EST and BRSKI GRASP announcements . . . . . . . . . .   7
       1.3.5.  Certificate Authority . . . . . . . . . . . . . . . .   7
       1.3.6.  Management Interface  . . . . . . . . . . . . . . . .   8
   2.  Connecting the Autonomic Control Plane to the Network
       Operations Center (NOC) . . . . . . . . . . . . . . . . . . .   8
   3.  Public Key Infrastructure Recommendations for the Registrar .   8
     3.1.  PKI recommendations for Tier-1/ISP Networks . . . . . . .   9
       3.1.1.  Enterprise Network  . . . . . . . . . . . . . . . . .  10
       3.1.2.  Home Network  . . . . . . . . . . . . . . . . . . . .  11
   4.  Architecture Considerations for the Registrar . . . . . . . .  11
     4.1.  Completely Synchronous Registrar  . . . . . . . . . . . .  12
     4.2.  Partially Synchronous Registrar . . . . . . . . . . . . .  13
     4.3.  Asynchronous Registrar  . . . . . . . . . . . . . . . . .  13
   5.  Certificates needed for the Registrar . . . . . . . . . . . .  13
     5.1.  TLS Server Certificate for BRSKI-EST  . . . . . . . . . .  13
     5.2.  TLS Client Certificate for BRSKI-MASA . . . . . . . . . .  14
     5.3.  Certificate for signing of Voucher-Requests . . . . . . .  14
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
     7.1.  Denial of Service Attacks against the Registrar . . . . .  15
     7.2.  Loss of Keys/Corruption of Infrastructure . . . . . . . .  16
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  16
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  16
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  18





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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.

   A key aspect of this is that there has to be a thing for the pledge
   to join!  [I-D.ietf-anima-bootstrapping-keyinfra] refers to this
   thing as the "Domain", identified technically by the "DomainID".  The
   domain is embodied by the Registrar component.  Membership in the
   domain is proven by possession of a valid Local DeviceID, a form of
   [ieee802-1AR] certificate.

   The Registrar is the component that implements the domain,
   authorizing new devices (pledges) to join.  Proper and efficient
   operation of the Registrar is key aspect for the Autonomic
   mechanisms, and for enabling secure onboarding.

   This document provides implementation, deployment and operational
   guidance for the BRSKI Registrar.

   There are however several classes of operator of a local domain: ISP
   and large managed multi-side Enterprises are the primary target for
   this document.  Medium sized single site Enterprises and Industrial
   Plant users are a secondary target for this document.  Unmanaged
   small enterprises and home users are addressed in a separate section
   at the end as special case.

   This document first introduces the different scales of deployment as
   a reference for further discussion and contrasts, and then provides
   analyses some consequences of architectural choices that may be
   appropriate for different scales of deployments.

   The document includes security best practices for the management of
   the certificates and the certificate authorities.

1.1.  Terminology

   ::boilerplate bcp14

   This document, while a Best Current Practices, makes use of BCP14
   language to indicate which practices are mandatory, and which ones
   are just recommendations.








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1.2.  Reference Network and Diagrams

   In order to deal with the full complexity and generality of
   operations, the reference network described herein is a bit more
   complicated than many networks actually are.

1.2.1.  Tier-1 Network

   In this guide one target is a world-wide Tier-1 ISP.  It has three
   network operations centers (NOC), the two major ones in Frankfurt and
   Denver, with an secondary center located in Perth, Australia.  The
   exact location of these NOCs is not important: the locations have
   been chosen to have an hour overlap in their 8-6 daytime shift,
   typical of world-wide operations.  This overlap is also not
   important, it just adds a degree of realism to this discussion.  The
   use of actual names makes subsequent discussion about failures
   easier.


                         .---------.    .-----.
                         | Denver  |    | NYC |
                    .----|---------|----| rtr |--.  .-----------.
                   /     | NOC/JRC |    '-----'   \ | Frankfurt |
                  '      '---------'               '|-----------|
             .---------.                            | NOC/JRC   |
             | SanJose |                            '-----------'
             | rtr     |                                  .
             '---------'                                 /
                 .-                                     /
                '                                      /
          .-----------.                               /
          |   Perth   |              .-------.       /
          |-----------|              | Tokyo |      /
          | NOC/EST   |--------------| rtr   |-----'
          |           |              '-------'
          '-----------'                  '

                       Reference Tier-1 ISP network

1.2.2.  Enterprise Network

   A second target is a medium Enterprise that has a single (probably
   on-premise) data center.  The Enterprise has Information Technology
   (IT) operations that include the routers and systems supporting it's
   office staff in it's buildings.  It has Building Operations which
   integrates the IoT devices found in the buildings that it owns, and
   it has Operations Technology (OT) that manages the automated systems
   in it's on-site manufacturing facilities.



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   INSERT DIAGRAM

1.2.3.  Home Network

   A third target is a resident with a single CPE device.  The home
   owner has a few medium sized devices (a home NAS) as well as a few
   IoT devices (light bulbs, clothes washing machine).

1.3.  Internal architectural view

   A Registrar will have four major interfaces, connected together by a
   common database.

                             .------------.
                             |    MASA    |
                             |   client   |
                             | BRSKI-MASA |
                             '------------'
                                    ^
                                    |
            .------------.          |           .-------------.
            | management |    .----------.      | certificate |
            | interface  |<---| database |----->| authority   |
            '------------'    '----------'      '-------------'
                                    |
                                    |
                                    v
           .------------.     .-----------.       .------------.
           | Join Proxy |     |  Pledge   |--.    | EST/BRSKI  |
           |------------|     | Interface |  |    |------------|
           | GRASP      |     | BRSKI-EST |e |    | full-GRASP |
           | (DULL)     |     '-----------'T |    '------------'
           '------------'        '-----------'

          Figure 1: Reference Internal Architecture for Registrar

1.3.1.  Pledge Interface (Southbound Interface)

   The pledge interface is the southbound interface.  This interface
   runs the BRSKI-EST protocol.  This interface faces into the
   operator's network, receiving requests from devices to join the
   network.

   For [I-D.ietf-anima-bootstrapping-keyinfra] use, the pledge interface
   is an HTTPS interface.  It may run on arbitrary port numbers and due
   to the way that the voucher interface pins the associated certificate
   it does not need to have a specific DNS name, as it will not be
   verified by the pledge.



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   For [I-D.ietf-anima-constrained-voucher] use, the pledge interface is
   a CoAP or CoAPS interface over UDP.  For use with CoAP/EDHOC, then a
   plain CoAP interface is used, and the security (EDHOC and OSCORE)
   lives above CoAP.  For CoAP/DTLS (CoAPS) then there is DTLS layer
   below the CoAP layer.

   [I-D.richardson-anima-state-for-joinrouter] offers some additional
   mechanisms, one of which involves dynamically created IPIP tunnels.
   If these mechanisms are in use, then the southbound interface would
   need to support these options as well.

   The Pledge Interface requires a TLS ServerCertificate, and
   Section 5.1 discusses option for creating this certificate.

   The Pledge Inteface does not require a public IP address, nor does it
   have have to run on port 443.

   In an ACP application ([I-D.ietf-anima-autonomic-control-plane]), the
   Pledge Interface SHOULD have an IPv6 ULA address from the prefix
   allocated to the ACP.  Section 2 provides some options for how the
   Pledge Interface can be best connected to the ACP.

   Outside of the ACP context, running the Pledge interface on an IP
   address that has a FQDN that resolves to that IP address (if only
   internally), and operating it on port 443 may have operational
   advantages.

1.3.2.  MASA client (Northbound Interface)

   The MASA client interface connects outward to the Internet to speak
   to the Manufacturer Authorized Signing Authority (MASA).  This is a
   TLS Client interface.

   Use of a TLSClientCertificate is RECOMMENDED as this may be the best
   way for a manufacturer to identify clients.  Section 5.2 discusses
   options for signing this certificate.

   The MASA client interface is outgoing only and does not require any
   special connectivity.  It may be placed behind a typical enterprise
   or residential NAT44 gateway.  IPv6 connectivity is RECOMMENDED.  It
   does need access to DNS, and the DNS lookups SHOULD be validated with
   DNSSEC.

   The MASA client interface will need to validate the server
   certificates of the MASA, and to do this it will need access to the
   common public WebPKI ([WebPKI]) trust anchors to validate the MASA.
   The MASA client MAY also require access to a database of pinned




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   certificates to validate specific manufacturers as called out for in
   [I-D.ietf-anima-bootstrapping-keyinfra] section 2.8 and section 5.4.

1.3.3.  Join Proxy (Southbound Interface)

   In the ACP context, the Registrar is expected to have a Join Proxy
   operating on the Southbound Interface in order to announce the
   existence of the Registrar to the local network, for the benefit of
   directly connected devices.  This permits the machines in the NOC
   itself to autonomically join the domain.

   The Join Proxy MAY announce the IP address (ULA) and port of the
   actual Pledge Interface, rather than announcing a link-local address
   and then performing a proxy operation.

1.3.4.  EST and BRSKI GRASP announcements

   As specified in [I-D.ietf-anima-bootstrapping-keyinfra] section 4.3,
   in an ACP context, the Registrar MUST announce itself inside the ACP
   using GRASP.  The Registrar MUST incorporate enough of a GRASP daemon
   in order to perform the M_FLOOD announcements.

   As specified in [I-D.ietf-anima-autonomic-control-plane] section
   6.1.2, in an ACP context, if the Registrar will also be providing for
   renewal of certificates using EST, then it SHOULD announce itself
   inside the ACP using GRASP.  Unless made impossible due to loading
   concerns, it is RECOMMENDED that all Registrar instances offer
   certificate renewal services in this fashion.

   The use of [I-D.ietf-acme-star] Short-Term Automatically-Renewed
   Certificates is RECOMMENDED.  This mandates that the EST server be
   highly available.  If STAR-style renewals are not used, then the
   Certificate Authority will need to make OCSP or CRL Distribution
   points available.

1.3.5.  Certificate Authority

   If the Enterprise/ISP has an existing certificate authority system
   that it wishes to use, then an interface to it has to be enabled.
   This may run protocols like EST, CMP or ACME.

   Smaller Enterprises and Residential uses of BRSKI are encouraged to
   use an internal (private) certificate authority.  See Section 3 for a
   discussion of securing this CA.







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1.3.6.  Management Interface

   The Registrar will require a management interface.  As is the trend,
   this will often be a web-based single page application using AJAX API
   calls to perform communications.  This interface SHOULD be made
   available on the Southbound NOC interface only, and it MUST be on a
   different IP address and port number then the BRSKI-EST interface.
   It should be secured with HTTPS, and use of a public ([WebPKI])
   anchor is reasonable as it may be that the internal certificate
   authority may be unavailable or require maintenance.

   An entirely separate process is justified with the only connection to
   the other procesess being the database.  (This does not mean it can
   not share code modules)

2.  Connecting the Autonomic Control Plane to the Network Operations
    Center (NOC)

   [I-D.ietf-anima-autonomic-control-plane] section 8.1 describes a
   mechanism to connect non-ACP capable systems to the ACP.  The use of
   this mechanism is critical to incremental deployment of ANIMA and
   BRSKI in operators.

   The deployment of BRSKI capable equipment would ideally occur in a
   concentric ring outward from the NOC.  This would start by an upgrade
   of the router that connects the NOC to the production network.  This
   device needs to support the ACP connect functionality.

   It is possible, but beyond the scope of this document, to do initial
   connectivity of the ACP and of multiple NOCs by manually configured
   IPsec tunnels.  This is likely an important step for incremental
   initial deployment.

   The Registrar described in the next section either needs to be
   connected via one of the above mentioned tunnels, or it must be
   located on a network with ACP Connect, or it must itself be part of
   an automatically configured ACP.  It is quite reasonable for the
   Registrar to be part of a larger appliance that also includes an ACP
   Connect functionality.

3.  Public Key Infrastructure Recommendations for the Registrar

   The Registrar requires access to, or must contain a Certificate
   Authority (CA).

   This section deals with the situation where the CA is provided
   internally.  [I-D.friel-acme-integrations] deals with the case where
   the CA is provided by an external service, and the CA trust anchors



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   are public.  These use ACME ([RFC8555]) is used as the interface.
   That is out of scope for this document.

   There are also a number of commercial offerings where a private CA is
   operated by an external entity using a wide variety of protocols,
   including proprietary ones.  Those are also out of scope for this
   document.

   The requirements for the PKI depends upon what kind of network is
   being managed.

3.1.  PKI recommendations for Tier-1/ISP Networks

   A three-tier PKI infrastructure is appropriate for an ISP.  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.

   Whether the root private key is secured by applying secret-splitting,
   and then storing the results on multiple USBs key kept in multiple
   safes, or via Hardware Security Module is a local decision informed
   by best current practices.

   The root CA is then used to sign a number of intermediate entities:
   this will include an intermediate CA for the Registrar that is
   deployed into each redundant NOC location.  Multiple intermediate CAs
   with a common root provides significantly more security and
   operational flexibility than attempts to share a private key among
   locations.

   While the root CA should have a longevity of at least 5 years, after
   which it can be resigned rather than re-generated, the intermediate
   CA keys need only have a 1-2 year duration, and at the end of their
   lifetime, a new private key should be generated.  Shorter periods are
   possible, but until there is more experience with them, not
   recommended.  The intermediate CA key should be regenerated because
   the intermediate CA private key will need to be online, available to
   the Registrar CA system.  There are many more opportunities for the
   key to leak, such as into backups.

   The intermediate CA is then used to sign End-Entity certificates
   which are returned as part of the BRSKI-EST mechanism.

   The Registrar needs client and server certificates for it's BRSKI-EST
   and BRSKI-MASA connections.  It is recommended that an additional
   intermediate CA be created for manually issued certificates such as
   these.  This intermediate CA could be called the NOC Infrastructure
   CA, and could be used to issue certificates for all manner of



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   infrastructure such as web-based monitoring tools.  The private root
   CA certificate should be installed into the browsers of NOC
   personnel.

   The document [I-D.moskowitz-ecdsa-pki] provides some practical
   instructions on setting up this kind of system.  This document
   recommends the use of ECDSA keys for the root and intermediate CAs,
   but there may be operational reasons why an RSA intermediate CA will
   be required for some legacy equipment.

3.1.1.  Enterprise Network

   Enterprises that have multiple Network Operations Center should
   consider the recommendations above for an ISP.

   This section applies to Enterprises that have all NOC operations/
   personel into a single location.  This is not a hard rule for
   scaling, but the intent is that physical security for the ACP Connect
   network is rather easy, that only a single legal jurisdiction will
   apply, and that it is possible to get people together easily to do
   things like resign keys.

   A three-tier PKI infrastructure is still recommended for the reason
   that it provides operational continuity options not available with a
   two-level system.  The recommendation is to have a root CA mechanism
   installed on a Virtual Machine which is not connected to a network.
   The root CA private key is kept offline, secret split among a number
   of USB keys, kept in the possession of key personnel.

   The secret split should have at least five components, of which at
   least three are required to reconstruct the key.  See
   [I-D.hallambaker-mesh-udf] section 4.5 for one such mechanism, there
   are many others, and there are no interoperability requirements for
   the secret split.

   The essential point is that the Enterprise is able to recover the
   root CA key even without some number of personnel and is able to
   continue operating it's network.

   As in the ISP case, the intermediate CA is then used to sign End-
   Entity certificates which are returned as part of the BRSKI-EST
   mechanism.  One intermediate CA key suffices as there is only one NOC
   location with a Registrar.  Incidental keys for internal operations,
   and for the BRSKI-EST server certificate can be done with this single
   intermediate CA.

   The BRSKI-MASA TLS Client Certificate key for an enterprise may not
   be needed; it depends upon the policies of the manufacturers which



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   are involved.  It may be simpler to use a certificate produced by a
   public CA (such as LetsEncrypt), as this makes it easier for
   manufacturers to validate the provided certificate.

   The document [I-D.moskowitz-ecdsa-pki] provides some practical
   instructions on setting up this kind of system.  This document
   recommends the use of ECDSA keys for the root and intermediate CAs.
   In an Enterprise, there are likely many more legacy devices that
   might need to become involved in the secure domain.  It is
   recommended that an RSA root and intermediate CA be more strongly
   considered.

3.1.2.  Home Network

   Home networks and small offices that use residential class equipment
   are the most challenging situation.  The three-tier PKI architecture
   is not justified because the ability to keep the root CA offline has
   no operational value.

   The home network registrar should be initialized with a single key
   pair used as the certificate authority.

   Secret splitting is useful in order to save the generated key with a
   few neighbours.  It is recommended that the entire PKI system
   database (including CA private key) be encrypted with a symmetric key
   and the results made available regularly for download to a variety of
   devices.  The symmetric key is split among the neighbours.

   The most difficult part of the Home Network PKI and Registrar is
   where to locate it.  Generally it should be located on a device that
   is fully owned by the home user.  This is sometimes the Home Router,
   but in a lot of situations the Home Router is the ISP's CPE router.
   If the home has a Network Attached Storage (NAS) system, then running
   it there is probably better.

   A compromise for CPE devices owned by the ISP that can run containers
   is for the Registrar to be located on detachable storage that is
   inserted into the CPE.  The detachable storage is owned by the home
   owner, and can be removed from the CPE device if it is replaced.
   More experience will be necessary in order to determine if this is a
   workable solution.

4.  Architecture Considerations for the Registrar

   There are a number of ways to scale the Registrar.  Web framework
   three-tier mechanisms are the most obvious.  See [threetier] for an
   overview.  This architecture is very familiar and can work well for a




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   Registrar.  There are a few small issues that need to be addressed
   relating to the TLS connections.

   The BRSKI-EST connection uses TLS Client Certificate information, so
   it is necessary for the presentation tier to pass the entire
   certificate through to the application layer.  The presentation tier
   MUST accept all Client Certificates, many of which might it might not
   have anchors for.

   In addition, the Registrar Voucher-Request MUST be signed using the
   same key pair that is used to terminate the TLS connection, so the
   application layer will need access to the same keypair that the
   presentation tier uses.  This can be operationally challenging if the
   presentation tier is provided by a hardware-based TLS load balancer.

   For this reason, an alternate architecture where the front-end load
   balancer provides TCP level load balancing, leaving the TLS
   operations to software TLS implementations in the application layer
   may be simpler to build.  Given that the Registrar is an inward
   facing system, and is not subject to the Internet-scale loads typical
   of "Black Friday" web system, the same kind of extreme scaling is not
   necessary.

   The BRSKI-EST flow includes a back-end call to the BRSKI-MASA flow.
   That is, during the BRSKI-EST /voucherrequest call, a voucher will
   need to be fetched from the MASA using a BRSKI-MASA connection.
   There are three ways to do this.

4.1.  Completely Synchronous Registrar

   In this simplest version the Registrar operates as a single thread,
   processing the voucher-request from the Pledge, and then starting a
   BRSKI-MASA client session, while the connection from the Pledge
   waits.

   This flow is very simple to implement, but requires an entire
   processing thread to block while the BRSKI-MASA protocol executes.
   The Pledge may timeout on this request, disconnect and retry.
   Experience so far is that typical default timeouts work fine.

   It is recommended that the voucher-request be recorded in a database,
   and if a corresponding fresh voucher is also found in the database,
   that it be returned rather than fetching a new voucher from the MASA.
   This accomodates the situation where the Pledge did timeout, but the
   BRSKI-MASA protocol did complete.  This results in the Pledge
   receiving the voucher upon retrying without having to go through the
   process of getting a new voucher.  This only works if the Pledge
   retries with the same Nonce each time.



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4.2.  Partially Synchronous Registrar

   A slightly more complicated version is for the Registrar to look in a
   database for a matching voucher-request, and if none is found, to
   return a 202 code upon the voucher-request, asking the Pledge to
   retry.

   In the meantime the BRSKI-MASA connection can be performed, and the
   resulting voucher stored in a database.  The connection can be done
   in the same thread that just deferred the connection, or in another
   thread kicked off for this purpose.

4.3.  Asynchronous Registrar

   In the completely asynchronous architecture, things work as with the
   Partially Synchronous version.  The voucher request is placed into a
   database as before.

   A completely separate system, probably with different network
   connectivity, but connected to the same database, performs the BRSKI-
   MASA processing, then fills the database with the answer.

   This version may have a noticeably higher latency as it requires a
   database operation and a database trigger to invoke the process.
   This architecture has the advantage that the internal facing
   Registrar never connects to the Internet.  Furthermore, the number of
   internal facing Registrar instances can be tuned independently from
   the number of outward facing clients.  This may be an advantage for
   networks that need to deal with a high number of malicious or lost
   internal clients.

5.  Certificates needed for the Registrar

   In addition to hosting a PKI root, the Registrar needs several other
   key pairs.  They are:

5.1.  TLS Server Certificate for BRSKI-EST

   A certificate to be used to answer TLS connections from new devices
   (pledges).  This must be of a type that expected pledges can
   understand.  Returning an RSA key to a client that can validate only
   ECDSA chains is a problem.  The constrained IoT ecosystem prefers
   ECDSA, and often does not have code that can verify RSA.  Meanwhile,
   older FIPS-140 validated libraries present in many router operating
   systems support only RSA!

   The recommendation is to use ECDSA keys, with a sensitiviity to when
   a majority of systems might support EdDSA.  There are well



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   established mechanisms in most TLS server libraries to permit
   multiple certificates to be loaded and to return an appropriate key
   based upon the client capabilities.  This should be used.

   The certificate used for the BRSKI-EST end point is not validated by
   the BRSKI pledge using public trust anchors, but rather it is pinned
   by the [RFC8366] voucher.  As such it can come from the private CA,
   as recommended above: either signed by a specific intermediate CA or
   via a root CA as appropriate for the environment.

5.2.  TLS Client Certificate for BRSKI-MASA

   A certificate may optionally be used for authentication of the
   Registrar to the MASA.  It is recommended to always include one.

   It can be the same certificate used by TLS Server Certificate above,
   and this is most appropriate in small Registrars which are not
   distributed, such as ones aimed as Residential/Home networks.

   In larger, distributed Registrars, cryptographic hygiene dictates
   that the private key not be distributed, so a unique certificate per
   Registrar client is appropriate.  They should all be signed by the
   same intermediate CA, with the intermediate and root CA certificates
   being supplied in the TLS connection.

5.3.  Certificate for signing of Voucher-Requests

   As part of the BRSKI voucher-request process the Pledge's Voucher-
   Request is wrapped by the Registrar in another voucher-request and
   signed.  It is this certificate which which pinned by MASA to
   validate the connection.

   The certificate used to sign the voucher-request MUST be the same as
   the one that is used for the TLS Server Connection.  This implies
   that the signed voucher-request MUST be constructed on the same
   machine that terminates the BRSKI-EST MASA connection.

6.  Privacy Considerations

   Section 10.2 of [I-D.ietf-anima-bootstrapping-keyinfra] details a
   number of things that are revealed by the BRSKI-EST protocol.  A
   multi-location Registrar with different TLS Server Certificates will
   have a different privacy profile than a Registrar that uses only a
   single certificate.

   Section 10.3 of [I-D.ietf-anima-bootstrapping-keyinfra] details what
   is revealed by the BRSKI-MASA protocol.  The operational




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   recommendations of this document do not affect or mitigate things at
   all.

7.  Security Considerations

   Section 11 of [I-D.ietf-anima-bootstrapping-keyinfra] does not deal
   with any attacks against the Registrar, as the Registrar is
   considered to be an internally facing system.

   In the context of the Autonomic Control Plane
   ([I-D.ietf-anima-bootstrapping-keyinfra] section 9, and
   [I-D.ietf-anima-autonomic-control-plane]) it is expected that the
   majority of equipment attached to a network are connected by wired
   ethernet.  The opportunity for a massive attack against the Registrar
   is considered low in an ISP, or multi-side Enterprise backbone
   network.

7.1.  Denial of Service Attacks against the Registrar

   However, there are some exposures which need to be taken into
   account, particular in the Enterprise or Institutional Campus
   network: typically these networks have large number of access ports,
   one for each desktop system.  Those systems can be infected with
   Malware, or may be located in student computer labs physically
   accessible with minimal authorization.  While an attack on the
   Registrar might be part of some kind of student protest, an attack by
   malware seems more likely.

   The different architectures proposed in Section 4 of this document
   provides some recommendations on differing scales.  The use of a
   fully asynchronous design is recommended for Enterprise uses of BRSKI
   where there may be a large number of IoT devices that are expected to
   onboard.  The ability to scale the BRSKI-EST Pledge Interface without
   having the scale the rest of the system provides for resiliency of
   the Registrary.

   It bears repeating that the use of of a stateless technology in the
   Join Proxy moves the load due to attacking systems from the Join
   Proxy into the Registrar.  This increases the network bandwidth
   required from the Join Proxy to the Registrar with the benefit of
   simplifying the Join Proxy.

   This is an intentional design decision to centralize the impact into
   the purpose built Registrar system(s).







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7.2.  Loss of Keys/Corruption of Infrastructure

   In Home/Residential Network ("homenet") uses of
   [I-D.ietf-anima-bootstrapping-keyinfra] the biggest risk is likely
   that of loss of the Registrar's key pairs.  Not to a malicious entity
   that steals them with intent to cause damage, but from outright loss.

   This can be due to failure to backup the database followed by a CPE
   device failure, but the case where a CPE device is simply thrown away
   to be replaced by an uninformed technician is probably the most
   likely situation.

   This situation results in loss of control for all devices in the
   home, and much frustration from the home owner who has to go through
   an onboarding process for all the devices.  The use of a standardized
   onboarding protocol significantly mitigates the hassle; the current
   "state of the art" proccess involves a series of appliance-specific
   smartphone applications, which may or not not actually work on more
   recent devices.

   This is why the focus on saving of the database along with a secret
   splitting process to secure it.  At present there is no cross-vendor
   format for this database, so the saved data is only useable with a
   Registrar from the same vendor.  So this protects against device
   failure, where it is replaced by an identical device or an upward
   compatible device from the same manufacturer, but not against changes
   of vendor.

8.  IANA Considerations

   This document makes no IANA allocations.

9.  Acknowledgements

   Your name here.

10.  Changelog

11.  References

11.1.  Normative References

   [I-D.ietf-acme-star]
              Sheffer, Y., Lopez, D., Dios, O., Pastor, A., and T.
              Fossati, "Support for Short-Term, Automatically-Renewed
              (STAR) Certificates in Automated Certificate Management
              Environment (ACME)", draft-ietf-acme-star-11 (work in
              progress), October 2019.



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   [I-D.ietf-anima-autonomic-control-plane]
              Eckert, T., Behringer, M., and S. Bjarnason, "An Autonomic
              Control Plane (ACP)", draft-ietf-anima-autonomic-control-
              plane-21 (work in progress), November 2019.

   [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.

   [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

   [I-D.friel-acme-integrations]
              Friel, O., Barnes, R., and R. Shekh-Yusef, "ACME
              Integrations", draft-friel-acme-integrations-02 (work in
              progress), October 2019.

   [I-D.hallambaker-mesh-udf]
              Hallam-Baker, P., "Mathematical Mesh 3.0 Part II: Uniform
              Data Fingerprint.", draft-hallambaker-mesh-udf-07 (work in
              progress), October 2019.

   [I-D.moskowitz-ecdsa-pki]
              Moskowitz, R., Birkholz, H., Xia, L., and M. Richardson,
              "Guide for building an ECC pki", draft-moskowitz-ecdsa-
              pki-07 (work in progress), August 2019.

   [I-D.richardson-anima-state-for-joinrouter]
              Richardson, M., "Considerations for stateful vs stateless
              join router in ANIMA bootstrap", draft-richardson-anima-
              state-for-joinrouter-02 (work in progress), January 2018.




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   [ieee802-1AR]
              IEEE Standard, ., "IEEE 802.1AR Secure Device Identifier",
              2009, <http://standards.ieee.org/findstds/
              standard/802.1AR-2009.html>.

   [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>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [threetier]
              Wikipedia, ., "Multitier architecture", December 2019,
              <https://en.wikipedia.org/wiki/Multitier_architecture>.

   [WebPKI]   CA/Browser Forum, ., "CA/Browser Forum Baseline
              Requirements for the Issuance and Management of Publicly-
              Trusted Certificates, v.1.2.2", October 2014,
              <https://cabforum.org/wp-content/uploads/BRv1.2.2.pdf>.

Author's Address

   Michael Richardson
   Sandelman Software Works

   Email: mcr+ietf@sandelman.ca





















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