draft-ietf-anima-brski-ae-01.txt

ANIMA WG                                              D. von Oheimb, Ed.
Internet-Draft                                                  S. Fries
Intended status: Standards Track                            H. Brockhaus
Expires: 8 October 2022                                          Siemens
                                                                 E. Lear
                                                           Cisco Systems
                                                            6 April 2022


          BRSKI-AE: Alternative Enrollment Protocols in BRSKI
                      draft-ietf-anima-brski-ae-01

Abstract

   This document enhances Bootstrapping Remote Secure Key Infrastructure
   (BRSKI, RFC 8995) to allow employing alternative enrollment
   protocols, such as CMP.

   Using self-contained signed objects, the origin of enrollment
   requests and responses can be authenticated independently of message
   transfer.  This supports end-to-end security and asynchronous
   operation of certificate enrollment and provides flexibility where to
   authenticate and authorize certification requests.

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 8 October 2022.

Copyright Notice

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






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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Supported Environment . . . . . . . . . . . . . . . . . .   5
     1.3.  List of Application Examples  . . . . . . . . . . . . . .   6
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Requirements and Mapping to Solutions . . . . . . . . . . . .   7
     3.1.  Basic Requirements  . . . . . . . . . . . . . . . . . . .   7
     3.2.  Solution Options for Proof-of-possession  . . . . . . . .   8
     3.3.  Solution Options for Proof-of-identity  . . . . . . . . .   9
   4.  Adaptations to BRSKI  . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Architecture  . . . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Message Exchange  . . . . . . . . . . . . . . . . . . . .  13
     4.3.  Enhancements to Addressing Scheme . . . . . . . . . . . .  16
     4.4.  Domain Registrar Support of Alternative Enrollment
           Protocols . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Instantiation to Existing Enrollment Protocols  . . . . . . .  17
     5.1.  BRSKI-EST-fullCMC: Instantiation to EST (informative) . .  17
     5.2.  BRSKI-CMP: Instantiation to CMP (normative if CMP is
           chosen) . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  20
   Appendix A.  Using EST for Certificate Enrollment . . . . . . . .  22
   Appendix B.  Application Examples . . . . . . . . . . . . . . . .  23
     B.1.  Rolling Stock . . . . . . . . . . . . . . . . . . . . . .  23
     B.2.  Building Automation . . . . . . . . . . . . . . . . . . .  23
     B.3.  Substation Automation . . . . . . . . . . . . . . . . . .  24
     B.4.  Electric Vehicle Charging Infrastructure  . . . . . . . .  24
     B.5.  Infrastructure Isolation Policy . . . . . . . . . . . . .  24
     B.6.  Sites with Insufficient Level of Operational Security . .  25
   Appendix C.  History of Changes TBD RFC Editor: please delete . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  29





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1.  Introduction

1.1.  Motivation

   BRSKI, as defined in [RFC8995], specifies a solution for secure
   automated zero-touch bootstrapping of new devices, so-called pledges.
   This includes the discovery of the registrar in the target domain,
   time synchronization, and the exchange of security information
   necessary to establish mutual trust between pledges and the target
   domain.

   A pledge gains trust in the target domain via the domain registrar as
   follows.  It obtains security information about the domain,
   specifically a domain certificate to be trusted, by requesting a
   voucher object defined in [RFC8366].  Such a voucher is a self-
   contained signed object originating from a Manufacturer Authorized
   Signing Authority (MASA).  Therefore, the voucher may be provided in
   online mode (synchronously) or offline mode (asynchronously).  The
   pledge can authenticate the voucher because it is shipped with a
   trust anchor of its manufacturer such that it can validate signatures
   (including related certificates) by the MASA.

   Trust by the target domain in a pledge is established by providing
   the pledge with a domain-specific LDevID certificate.  The
   certification request of the pledge is signed using its IDevID secret
   and can be validated by the target domain using the trust anchor of
   the pledge manufacturer, which needs to pre-installed in the domain.

   For enrolling devices with LDevID certificates, BRSKI typically
   utilizes Enrollment over Secure Transport (EST) [RFC7030].  EST has
   its specific characteristics, detailed in Appendix A.  In particular,
   it requires online or on-site availability of the RA for performing
   the data origin authentication and final authorization decision on
   the certification request.  This type of enrollment can be called
   'synchronous enrollment'.  For various reasons, it may be preferable
   to use alternative enrollment protocols such as the Certificate
   Management Protocol (CMP) [RFC4210] profiled in
   [I-D.ietf-lamps-lightweight-cmp-profile] or Certificate Management
   over CMS (CMC) [RFC5272]. that are more flexible and independent of
   the transfer mechanism because they represent certification request
   messages as authenticated self-contained objects.

   Depending on the application scenario, the required RA/CA components
   may not be part of the registrar.  They even may not be available on-
   site but rather be provided by remote backend systems.  The registrar
   or its deployment site may not have an online connection with them or
   the connectivity may be intermittent.  This may be due to security
   requirements for operating the backend systems or due to site



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   deployments where on-site or always-online operation may be not
   feasible or too costly.  In such scenarios, the authentication and
   authorization of certification requests will not or can not be
   performed on-site at enrollment time.  In this document, enrollment
   that is not performed in a (time-wise) consistent way is called
   'asynchronous enrollment'.  Asynchronous enrollment requires a store-
   and-forward transfer of certification requests along with the
   information needed for authenticating the requester.  This allows
   offline processing the request.

   Application scenarios may also involve network segmentation, which is
   utilized in industrial systems to separate domains with different
   security needs.  Such scenarios lead to similar requirements if the
   TLS connection carrying the requester authentication is terminated
   and thus request messages need to be forwarded on further channels
   before the registrar/RA can authorize the certification request.  In
   order to preserve the requester authentication, authentication
   information needs to be retained and ideally bound directly to the
   certification request.

   There are basically two approaches for forwarding certification
   requests along with requester authentication information:

   *  A trusted component (e.g., a local RA) in the target domain is
      needed that forwards the certification request combined with the
      validated identity of the requester (e,g., its IDevID certificate)
      and an indication of successful verification of the proof-of-
      possession (of the corresponding private key) in a way preventing
      changes to the combined information.  When connectivity is
      available, the trusted component forwards the certification
      request together with the requester information (authentication
      and proof-of-possession) for further processing.  This approach
      offers only hop-by-hop security.  The backend PKI must rely on the
      local pledge authentication result provided by the local RA when
      performing the authorization of the certification request.  In
      BRSKI, the EST server is such a trusted component, being co-
      located with the registrar in the target domain.

   *  Involved components use authenticated self-contained objects for
      the enrollment, directly binding the certification request and the
      requester authentication in a cryptographic way.  This approach
      supports end-to-end security, without the need to trust in
      intermediate domain components.  Manipulation of the request and
      the requester identity information can be detected during the
      validation of the self-contained signed object.






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   Focus of this document is the support of alternative enrollment
   protocols that allow using authenticated self-contained objects for
   device credential bootstrapping.  This enhancement of BRSKI is named
   BRSKI-AE, where AE stands for alternative enrollment protocols and
   for asynchronous enrollment.  This specification carries over the
   main characteristics of BRSKI, namely that the pledge obtains trust
   anchor information for authenticating the domain registrar and other
   target domain components as well as a domain-specific X.509 device
   certificate (the LDevID certificate) along with the corresponding
   private key (the LDevID secret) and certificate chain.

   The goals are to enhance BRSKI to

   *  support alternative enrollment protocols,

   *  support end-to-end security for enrollment, and

   *  make it applicable to scenarios involving asynchronous enrollment.

   This is achieved by

   *  extending the well-known URI approach with an additional path
      element indicating the enrollment protocol being used, and

   *  defining a certificate waiting indication and handling, for the
      case that the certifying component is (temporarily) not available.

   This specification can be applied to both synchronous and
   asynchronous enrollment.

   In contrast to BRSKI, this specification supports offering multiple
   enrollment protocols on the infrastructure side, which enables
   pledges and their developers to pick the preferred one.

1.2.  Supported Environment

   BRSKI-AE is intended to be used in domains that may have limited
   support of on-site PKI services and comprises application scenarios
   like the following.

   *  There are requirements or implementation restrictions that do not
      allow using EST for enrolling an LDevID certificate.

   *  Pledges and/or the target domain already have an established
      certificate management approach different from EST that shall be
      reused (e.g., in brownfield installations).





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   *  There is no registration authority available on site in the target
      domain.  Connectivity to an off-site RA is intermittent or
      entirely offline.  A store-and-forward mechanism is used for
      communicating with the off-site services.

   *  Authoritative actions of a local RA are limited and may not be
      sufficient for authorizing certification requests by pledges.
      Final authorization is done by an RA residing in the operator
      domain.

1.3.  List of Application Examples

   Bootstrapping can be handled in various ways, depending on the
   application domains.  The informative Appendix B provides
   illustrative examples from various industrial control system
   environments and operational setups.  They motivate the support of
   alternative enrollment protocols, based on the following examples of
   operational environments:

   *  Rolling stock

   *  Building automation

   *  Electrical substation automation

   *  Electric vehicle charging infrastructures

   *  Infrastructure isolation policy

   *  Sites with insufficient level of operational security

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   This document relies on the terminology defined in [RFC8995] and
   [IEEE.802.1AR_2009].The following terms are defined in addition:

   EE:  End entity, in the BRSKI context called pledge.  It is the
      entity that is bootstrapped to the target domain.  It holds a
      public-private key pair, for which it requests a public-key
      certificate.  An identifier for the EE is given as the subject
      name of the certificate.




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   RA:  Registration authority, an optional system component to which a
      CA delegates certificate management functions such as
      authenticating requesters and performing authorization checks on
      certification requests.

   CA:  Certification authority, issues certificates and provides
      certificate status information.

   target domain:  The set of entities that share a common local trust
      anchor, independent of where the entities are deployed.

   site:  Describes the locality where an entity, e.g., pledge,
      registrar, RA, CA, is deployed.  Different sites can belong to the
      same target domain.

   on-site:  Describes a component or service or functionality available
      in the target deployment site.

   off-site:  Describes a component or service or functionality
      available in an operator site different from the target deployment
      site.  This may be a central site or a cloud service, to which
      only a temporary connection is available.

   asynchronous communication:  Describes a time-wise interrupted
      communication between a pledge (EE) and a registrar or PKI
      component.

   synchronous communication:  Describes a time-wise uninterrupted
      communication between a pledge (EE) and a registrar or PKI
      component.

   authenticated self-contained object:  Describes in this context an
      object that is cryptographically bound to the IDevID certificate
      of a pledge.  The binding is assumed to be provided through a
      digital signature of the actual object using the IDevID secret.

3.  Requirements and Mapping to Solutions

3.1.  Basic Requirements

   There were two main drivers for the definition of BRSKI-AE:

   *  The solution architecture may already use or require a certificate
      management protocol other than EST.  Therefore, this other
      protocol should be usable for requesting LDevID certificates.






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   *  The domain registrar may not be the (final) point that
      authenticates and authorizes certification requests and the pledge
      may not have a direct connection to it.  Therefore, certification
      requests should be self-contained signed objects.

   Based on the intended target environment described in Section 1.2 and
   the application examples described in Appendix B, the following
   requirements are derived to support authenticated self-contained
   objects as containers carrying certification requests.

   At least the following properties are required:

   *  proof-of-possession: demonstrates access to the private key
      corresponding to the public key contained in a certification
      request.  This is typically achieved by a self-signature using the
      corresponding private key.

   *  proof-of-identity: provides data origin authentication of the
      certification request.  This typically is achieved by a signature
      using the IDevID secret of the pledge.

   The rest of this section gives an incomplete list of solution
   examples, based on existing technology described in IETF documents:

3.2.  Solution Options for Proof-of-possession

   Certification request objects: Certification requests are data
   structures protecting only the integrity of the contained data and
   providing proof-of-possession for a (locally generated) private key.
   Examples for certification request data structures are:

   *  PKCS#10 [RFC2986].  This certification request structure is self-
      signed to protect its integrity and prove possession of the
      private key that corresponds to the public key included in the
      request.

   *  CRMF [RFC4211].  Also this certificate request message format
      supports integrity protection and proof-of-possession, typically
      by a self-signature generated over (part of) the structure with
      the private key corresponding to the included public key.  CRMF
      also supports further proof-of-possession methods for types of
      keys that do not support any signature algorithm.

   The integrity protection of certification request fields includes the
   public key because it is part of the data signed by the corresponding
   private key.  Yet note that for the above examples this is not
   sufficient to provide data origin authentication, i.e., proof-of-
   identity.  This extra property can be achieved by an additional



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   binding to the IDevID of the pledge.  This binding to source
   authentication supports the authorization decision for the
   certification request.  The binding of data origin authentication to
   the certification request may be delegated to the protocol used for
   certificate management.

3.3.  Solution Options for Proof-of-identity

   The certification request should be bound to an existing
   authenticated credential (here, the IDevID certificate) to enable a
   proof of identity and, based on it, an authorization of the
   certification request.  The binding may be achieved through security
   options in an underlying transport protocol such as TLS if the
   authorization of the certification request is (completely) done at
   the next communication hop.  This binding can also be done in a
   transport-independent way by wrapping the certification request with
   signature employing an existing IDevID.  the BRSKI context, this will
   be the IDevID.  This requirement is addressed by existing enrollment
   protocols in various ways, such as:

   *  EST [RFC7030] utilizes PKCS#10 to encode the certification
      request.  The Certificate Signing Request (CSR) optionally
      provides a binding to the underlying TLS session by including the
      tls-unique value in the self-signed PKCS#10 structure.  The tls-
      unique value results from the TLS handshake.  Since the TLS
      handshake includes client authentication and the pledge utilizes
      its IDevID for it, the proof-of-identity is provided by such a
      binding to the TLS session.  This can be supported using the EST
      /simpleenroll endpoint.  Note that the binding of the TLS
      handshake to the CSR is optional in EST.  As an alternative to
      binding to the underlying TLS authentication in the transport
      layer, [RFC7030] sketches wrapping the CSR with a Full PKI Request
      message using an existing certificate.

   *  SCEP [RFC8894] supports using a shared secret (passphrase) or an
      existing certificate to protect CSRs based on SCEP Secure Message
      Objects using CMS wrapping ([RFC5652]).  Note that the wrapping
      using an existing IDevID in SCEP is referred to as renewal.  Thus
      SCEP does not rely on the security of the underlying transfer.

   *  CMP [RFC4210] supports using a shared secret (passphrase) or an
      existing certificate, which may be an IDevID credential, to
      authenticate certification requests via the PKIProtection
      structure in a PKIMessage.  The certification request is typically
      encoded utilizing CRMF, while PKCS#10 is supported as an
      alternative.  Thus CMP does not rely on the security of the
      underlying transfer protocol.




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   *  CMC [RFC5272] also supports utilizing a shared secret (passphrase)
      or an existing certificate to protect certification requests,
      which can be either in CRMF or PKCS#10 structure.  The proof-of-
      identity can be provided as part of a FullCMCRequest, based on CMS
      [RFC5652] and signed with an existing IDevID secret.  Thus CMC
      does not rely on the security of the underlying transfer protocol.

4.  Adaptations to BRSKI

   In order to support alternative enrollment protocols, asynchronous
   enrollment, and more general system architectures, BRSKI-AE lifts
   some restrictions of BRSKI [RFC8995].  This way, authenticated self-
   contained objects such as those described in Section 3 above can be
   used for certificate enrollment.

   The enhancements needed are kept to a minimum in order to ensure
   reuse of already defined architecture elements and interactions.  In
   general, the communication follows the BRSKI model and utilizes the
   existing BRSKI architecture elements.  In particular, the pledge
   initiates communication with the domain registrar and interacts with
   the MASA as usual.

4.1.  Architecture

   The key element of BRSKI-AE is that the authorization of a
   certification request MUST be performed based on an authenticated
   self-contained object.  The certification request is bound in a self-
   contained way to a proof-of-origin based on the IDevID.
   Consequently, the authentication and authorization of the
   certification request MAY be done by the domain registrar and/or by
   other domain components.  These components may be offline or reside
   in some central backend of the domain operator (off-site) as
   described in Section 1.2.  The registrar and other on-site domain
   components may have no or only temporary (intermittent) connectivity
   to them.  The certification request MAY also be piggybacked on
   another protocol.

   This leads to generalizations in the placement and enhancements of
   the logical elements as shown in Figure 1.












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                                              +------------------------+
      +--------------Drop-Ship--------------->| Vendor Service         |
      |                                       +------------------------+
      |                                       | M anufacturer|         |
      |                                       | A uthorized  |Ownership|
      |                                       | S igning     |Tracker  |
      |                                       | A uthority   |         |
      |                                       +--------------+---------+
      |                                                      ^
      |                                                      |
      V                                                      |
   +--------+     .........................................  |
   |        |     .                                       .  | BRSKI-
   |        |     .  +------------+       +------------+  .  | MASA
   | Pledge |     .  |   Join     |       | Domain     <-----+
   |        |     .  |   Proxy    |       | Registrar/ |  .
   |        <-------->............<-------> Enrollment |  .
   |        |     .  |        BRSKI-AE    | Proxy/LRA  |  .
   | IDevID |     .  |            |       +------^-----+  .
   |        |     .  +------------+              |        .
   |        |     .                              |        .
   +--------+     ...............................|.........
                   on-site "domain" components   |
                                                 | e.g., RFC 4210,
                                                 |       RFC 7030, ...
    .............................................|.....................
    . +---------------------------+     +--------v------------------+ .
    . | Public-Key Infrastructure <-----+ Registration Authority    | .
    . | PKI CA                    +-----> PKI RA                    | .
    . +---------------------------+     +---------------------------+ .
    ...................................................................
            off-site or central "domain" components

       Figure 1: Architecture Overview Using Off-site PKI Components

   The architecture overview in Figure 1 has the same logical elements
   as BRSKI, but with more flexible placement of the authentication and
   authorization checks on certification requests.  Depending on the
   application scenario, the registrar MAY still do all of these checks
   (as is the case in BRSKI), or part of them, or none of them.

   The following list describes the on-site components in the target
   domain of the pledge shown in Figure 1.

   *  Join Proxy: same functionality as described in BRSKI [RFC8995].






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   *  Domain Registrar / Enrollment Proxy / LRA: in BRSKI-AE, the domain
      registrar has mostly the same functionality as in BRSKI, namely to
      facilitate the communication of the pledge with the MASA and the
      PKI.  Yet in contrast to BRSKI, the registrar offers different
      enrollment protocols and MAY act as a local registration authority
      (LRA) or simply as an enrollment proxy.  In such cases, the domain
      registrar forwards the certification request to some off-site RA
      component, which performs at least part of the authorization.
      This also covers the case that the registrar has only intermittent
      connection and forwards the certification request to the RA upon
      re-established connectivity.

      Note: To support alternative enrollment protocols, the URI scheme
      for addressing the domain registrar is generalized (see
      Section 4.3).

   The following list describes the components provided by the vendor or
   manufacturer outside the target domain.

   *  MASA: general functionality as described in BRSKI [RFC8995].  The
      voucher exchange with the MASA via the domain registrar is
      performed as described in BRSKI.

      Note: The interaction with the MASA may be synchronous (voucher
      request with nonce) or asynchronous (voucher request without
      nonce).

   *  Ownership tracker: as defined in BRSKI.

   The following list describes the target domain components that can
   optionally be operated in the off-site backend of the target domain.

   *  PKI RA: Performs certificate management functions for the domain
      as a centralized public-key infrastructure for the domain
      operator.  As far as not already done by the domain registrar, it
      performs the final validation and authorization of certification
      requests.

   *  PKI CA: Performs certificate generation by signing the certificate
      structure requested in already authenticated and authorized
      certification requests.

   Based on the diagram in Section 2.1 of BRSKI [RFC8995] and the
   architectural changes, the original protocol flow is divided into
   three phases showing commonalities and differences to the original
   approach as follows.

   *  Discovery phase: same as in BRSKI steps (1) and (2)



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   *  Voucher exchange phase: same as in BRSKI steps (3) and (4).

   *  Enrollment phase: step (5) is changed to employing an alternative
      enrollment protocol that uses authenticated self-contained
      objects.

4.2.  Message Exchange

   The behavior of a pledge described in Section 2.1 of BRSKI [RFC8995]
   is kept with one exception.  After finishing the Imprint step (4),
   the Enroll step (5) MUST be performed with an enrollment protocol
   utilizing authenticated self-contained objects.  Section 5 discusses
   selected suitable enrollment protocols and options applicable.

   [
    Cannot render SVG graphics - please view
    https://raw.githubusercontent.com/anima-wg/anima-brski-ae/main/o.png
   ]

               Figure 2: BRSKI-AE Abstract Protocol Overview

   *Pledge - registrar discovery and voucher exchange*

   The discovery phase and voucher exchange are applied as specified in
   [RFC8995].

   *Registrar - MASA voucher exchange*

   This voucher exchange is performed as specified in [RFC8995].

   *Pledge - registrar - RA/CA certificate enrollment*

   As stated in Section 3, the enrollment MUST be performed using an
   authenticated self-contained object providing not only proof-of-
   possession but also proof-of-identity (source authentication).
















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   +--------+                        +------------+       +------------+
   | Pledge |                        | Domain     |       | Operator   |
   |        |                        | Registrar  |       | RA/CA      |
   |        |                        |  (JRC)     |       | (PKI)      |
   +--------+                        +------------+       +------------+
    /-->                                      |                       |
   [Optional request of CA certificates]      |                       |
    |---------- CA Certs Request ------------>|                       |
    |                 [if connection to operator domain is available] |
    |                                         |-- CA Certs Request -->|
    |                                         |<- CA Certs Response --|
    |<--------- CA Certs Response ------------|                       |
    /-->                                      |                       |
   [Optional request of attributes to include in Certificate Request] |
    |---------- Attribute Request ----------->|                       |
    |                 [if connection to operator domain is available] |
    |                                         |- Attribute Request -->|
    |                                         |<- Attribute Response -|
    |<--------- Attribute Response -----------|                       |
    /-->                                      |                       |
   [Mandatory certificate request]            |                       |
    |---------- Certificate Request --------->|                       |
    |                 [if connection to operator domain is available] |
    |                                         |-Certificate Request ->|
    |                                         |<- Certificate Resp. --|
    |<--------- Certificate Response ---------|                       |
    /-->                                      |                       |
   [Optional certificate confirmation]        |                       |
    |---------- Certificate Confirm --------->|                       |
    |                 [if connection to operator domain is available] |
    |                                         |-Certificate Confirm ->|
    |                                         |<---- PKI Confirm -----|
    |<--------- PKI/Registrar Confirm --------|                       |

                      Figure 3: Certificate Enrollment

   The following list provides an abstract description of the flow
   depicted in Figure 3.

   *  CA Certs Request: The pledge optionally requests the latest
      relevant CA certificates.  This ensures that the pledge has the
      complete set of current CA certificates beyond the pinned-domain-
      cert (which is contained in the voucher and may be just the domain
      registrar certificate).







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   *  CA Certs Response: It MUST contain the current root CA
      certificate, which typically is the LDevID trust anchor, and any
      additional certificates that the pledge may need to validate
      certificates.

   *  Attribute Request: Typically, the automated bootstrapping occurs
      without local administrative configuration of the pledge.
      Nevertheless, there are cases in which the pledge may also include
      additional attributes specific to the target domain into the
      certification request.  To get these attributes in advance, the
      attribute request can be used.

   *  Attribute Response: It MUST contain the attributes to be included
      in the subsequent certification request.

   *  Certificate Request: This certification request MUST contain the
      authenticated self-contained object ensuring both proof-of-
      possession of the corresponding private key and proof-of-identity
      of the requester.

   *  Certificate Response: The certification response message MUST
      contain on success the requested certificate and MAY include
      further information, like certificates of intermediate CAs.

   *  Certificate Confirm: An optional confirmation sent after the
      requested certificate has been received and validated.  It
      contains a positive or negative confirmation by the pledge whether
      the certificate was successfully enrolled and fits its needs.

   *  PKI/Registrar Confirm: An acknowledgment by the PKI or registrar
      that MUST be sent on reception of the Cert Confirm.

   The generic messages described above may be implemented using various
   enrollment protocols supporting authenticated self-contained objects,
   as described in Section 3.  Examples are available in Section 5.

   *Pledge - registrar - enrollment status telemetry*

   The enrollment status telemetry is performed as specified in
   [RFC8995].  In BRSKI this is described as part of the enrollment
   phase, but due to the generalization on the enrollment protocol
   described in this document it fits better as a separate step here.









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4.3.  Enhancements to Addressing Scheme

   BRSKI-AE provides generalizations to the addressing scheme defined in
   BRSKI [RFC8995] to accommodate alternative enrollment protocols that
   use authenticated self-contained objects for certification requests.
   As this is supported by various existing enrollment protocols, they
   can be directly employed (see also Section 5).

   The addressing scheme in BRSKI for certification requests and the
   related CA certificates and CSR attributes retrieval functions uses
   the definition from EST [RFC7030]; here on the example of simple
   enrollment: "/.well-known/est/simpleenroll".  This approach is
   generalized to the following notation: "/.well-known/<enrollment-
   protocol>/<request>" in which <enrollment-protocol> refers to a
   certificate enrollment protocol.  Note that enrollment is considered
   here a message sequence that contains at least a certification
   request and a certification response.  The following conventions are
   used in order to provide maximal compatibility to BRSKI:

   *  <enrollment-protocol>: MUST reference the protocol being used,
      which MAY be CMP, CMC, SCEP, EST [RFC7030] as in BRSKI, or a newly
      defined approach.

      Note: additional endpoints (well-known URIs) at the registrar may
      need to be defined by the enrollment protocol being used.

   *  <request>: if present, the <request> path component MUST describe,
      depending on the enrollment protocol being used, the operation
      requested.  Enrollment protocols are expected to define their
      request endpoints, as done by existing protocols (see also
      Section 5).

4.4.  Domain Registrar Support of Alternative Enrollment Protocols

   Well-known URIs for various endpoints on the domain registrar are
   already defined as part of the base BRSKI specification or indirectly
   by EST.  In addition, alternative enrollment endpoints MAY be
   supported at the registrar.  The pledge will recognize whether its
   preferred enrollment option is supported by the domain registrar by
   sending a request to its preferred enrollment endpoint and evaluating
   the HTTP response status code.










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   The following list of endpoints provides an illustrative example for
   a domain registrar supporting several options for EST as well as for
   CMP to be used in BRSKI-AE.  The listing contains the supported
   endpoints to which the pledge may connect for bootstrapping.  This
   includes the voucher handling as well as the enrollment endpoints.
   The CMP related enrollment endpoints are defined as well-known URIs
   in CMP Updates [I-D.ietf-lamps-cmp-updates] and the Lightweight CMP
   profile [I-D.ietf-lamps-lightweight-cmp-profile].

     </brski/voucherrequest>,ct=voucher-cms+json
     </brski/voucher_status>,ct=json
     </brski/enrollstatus>,ct=json
     </est/cacerts>;ct=pkcs7-mime
     </est/fullcmc>;ct=pkcs7-mime
     </est/csrattrs>;ct=pkcs7-mime
     </cmp/initialization>;ct=pkixcmp
     </cmp/p10>;ct=pkixcmp
     </cmp/getcacerts>;ct=pkixcmp
     </cmp/getcertreqtemplate>;ct=pkixcmp

5.  Instantiation to Existing Enrollment Protocols

   This section maps the requirements to support proof-of-possession and
   proof-of-identity to selected existing enrollment protocols handles
   provides further aspects of instantiating them in BRSKI-AE.

5.1.  BRSKI-EST-fullCMC: Instantiation to EST (informative)

   When using EST [RFC7030], the following aspects and constraints need
   to be considered and the given extra requirements need to be
   fulfilled, which adapt Section 5.9.3 of BRSKI [RFC8995]:

   *  proof-of-possession is provided typically by using the specified
      PKCS#10 structure in the request.  Together with Full PKI
      requests, also CRMF can be used.

   *  proof-of-identity needs to be achieved by signing the
      certification request object using the Full PKI Request option
      (including the /fullcmc endpoint).  This provides sufficient
      information for the RA to authenticate the pledge as the origin of
      the request and to make an authorization decision on the received
      certification request.  Note: EST references CMC [RFC5272] for the
      definition of the Full PKI Request.  For proof-of-identity, the
      signature of the SignedData of the Full PKI Request is performed
      using the IDevID secret of the pledge.

      Note: In this case the binding to the underlying TLS connection is
      not necessary.



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   *  When the RA is temporarily not available, as per Section 4.2.3 of
      [RFC7030], an HTTP status code 202 should be returned by the
      registrar, and the pledge will repeat the initial Full PKI Request

5.2.  BRSKI-CMP: Instantiation to CMP (normative if CMP is chosen)

   Note: Instead of referring to CMP as specified in [RFC4210] and
   [I-D.ietf-lamps-cmp-updates], this document refers to the Lightweight
   CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile] because the
   subset of CMP defined there is sufficient for the functionality
   needed here.

   When using CMP, the following specific implementation requirements
   apply (cf.  Figure 3).

   *  CA Certs Request

      -  Requesting CA certificates over CMP is OPTIONAL.
         If supported, it SHALL be implemented as specified in
         Section 4.3.1 of [I-D.ietf-lamps-lightweight-cmp-profile].

   *  Attribute Request

      -  Requesting certificate request attributes over CMP is OPTIONAL.
         If supported, it SHALL be implemented as specified in
         Section 4.3.3 of [I-D.ietf-lamps-lightweight-cmp-profile].
         Note that alternatively the registrar MAY modify the contents
         of requested certificate contents as specified in
         Section 5.2.3.2 of [I-D.ietf-lamps-lightweight-cmp-profile].

   *  Certificate Request

      -  Proof-of-possession SHALL be provided as defined in
         Section 4.1.1 (based on CRMF) or Section 4.1.4 (based on
         PKCS#10) of the Lightweight CMP Profile
         [I-D.ietf-lamps-lightweight-cmp-profile].
         The caPubs field of certificate response messages SHOULD NOT be
         used.

      -  Proof-of-identity SHALL be provided by using signature-based
         protection of the certification request message as outlined in
         Section 3.2. of [I-D.ietf-lamps-lightweight-cmp-profile] using
         the IDevID secret.

   *  Certificate Confirm






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      -  Explicit confirmation of new certificates to the RA MAY be used
         as specified in Section 4.1.1 of the Lightweight CMP Profile
         [I-D.ietf-lamps-lightweight-cmp-profile].
         Note that independently of certificate confirmation within CMP,
         enrollment status telemetry with the registrar will be
         performed as described in Section 5.9.4 of BRSKI [RFC8995].

   *  If delayed delivery of responses (for instance, to support
      asynchronous enrollment) within CMP is needed, it SHALL be
      performed as specified in Sections 4.4 and 5.1.2 of
      [I-D.ietf-lamps-lightweight-cmp-profile].

6.  IANA Considerations

   This document does not require IANA actions.

7.  Security Considerations

   The security considerations as laid out in BRSKI [RFC8995] apply for
   the discovery and voucher exchange as well as for the status exchange
   information.

   The security considerations as laid out in the Lightweight CMP
   Profile [I-D.ietf-lamps-lightweight-cmp-profile] apply as far as CMP
   is used.

8.  Acknowledgments

   We would like to thank Brian E.  Carpenter, Michael Richardson, and
   Giorgio Romanenghi for their input and discussion on use cases and
   call flows.

9.  References

9.1.  Normative References

   [I-D.ietf-lamps-cmp-updates]
              Brockhaus, H., Oheimb, D. V., and J. Gray, "Certificate
              Management Protocol (CMP) Updates", Work in Progress,
              Internet-Draft, draft-ietf-lamps-cmp-updates-17, 12
              January 2022, <https://www.ietf.org/archive/id/draft-ietf-
              lamps-cmp-updates-17.txt>.









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   [I-D.ietf-lamps-lightweight-cmp-profile]
              Brockhaus, H., Oheimb, D. V., and S. Fries, "Lightweight
              Certificate Management Protocol (CMP) Profile", Work in
              Progress, Internet-Draft, draft-ietf-lamps-lightweight-
              cmp-profile-10, 1 February 2022,
              <https://www.ietf.org/archive/id/draft-ietf-lamps-
              lightweight-cmp-profile-10.txt>.

   [IEEE.802.1AR_2009]
              IEEE, "IEEE Standard for Local and metropolitan area
              networks - Secure Device Identity", IEEE 802.1AR-2009,
              DOI 10.1109/ieeestd.2009.5367679, 28 December 2009,
              <http://ieeexplore.ieee.org/servlet/
              opac?punumber=5367676>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4210]  Adams, C., Farrell, S., Kause, T., and T. Mononen,
              "Internet X.509 Public Key Infrastructure Certificate
              Management Protocol (CMP)", RFC 4210,
              DOI 10.17487/RFC4210, September 2005,
              <https://www.rfc-editor.org/info/rfc4210>.

   [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>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

9.2.  Informative References

   [IEC-62351-9]
              International Electrotechnical Commission, "IEC 62351 -
              Power systems management and associated information
              exchange - Data and communications security - Part 9:
              Cyber security key management for power system equipment",
              IEC 62351-9, May 2017.



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   [ISO-IEC-15118-2]
              International Standardization Organization / International
              Electrotechnical Commission, "ISO/IEC 15118-2 Road
              vehicles - Vehicle-to-Grid Communication Interface - Part
              2: Network and application protocol requirements", ISO/
              IEC 15118-2, April 2014.

   [NERC-CIP-005-5]
              North American Reliability Council, "Cyber Security -
              Electronic Security Perimeter", CIP 005-5, December 2013.

   [OCPP]     Open Charge Alliance, "Open Charge Point Protocol 2.0.1
              (Draft)", December 2019.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC4211]  Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              DOI 10.17487/RFC4211, September 2005,
              <https://www.rfc-editor.org/info/rfc4211>.

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,
              <https://www.rfc-editor.org/info/rfc5272>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC5929]  Altman, J., Williams, N., and L. Zhu, "Channel Bindings
              for TLS", RFC 5929, DOI 10.17487/RFC5929, July 2010,
              <https://www.rfc-editor.org/info/rfc5929>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [RFC8894]  Gutmann, P., "Simple Certificate Enrolment Protocol",
              RFC 8894, DOI 10.17487/RFC8894, September 2020,
              <https://www.rfc-editor.org/info/rfc8894>.

   [UNISIG-Subset-137]
              UNISIG, "Subset-137; ERTMS/ETCS On-line Key Management
              FFFIS; V1.0.0", December 2015,



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              <https://www.era.europa.eu/sites/default/files/filesystem/
              ertms/ccs_tsi_annex_a_-_mandatory_specifications/
              set_of_specifications_3_etcs_b3_r2_gsm-r_b1/index083_-
              _subset-137_v100.pdf>.
              http://www.kmc-subset137.eu/index.php/download/

Appendix A.  Using EST for Certificate Enrollment

   When using EST with BRSKI, pledges interact via TLS with the domain
   registrar, which acts both as EST server and as registration
   authority (RA).  The TLS connection is mutually authenticated, where
   the pledge uses its IDevID certificate issued by its manufacturer.

   In order to provide a strong proof-of-origin of the certification
   request, EST has the option to include in the certification request
   the so-called tls-unique value [RFC5929] of the underlying TLS
   channel.  This binding of the proof-of-identity of the TLS client,
   which is supposed to be the certificate requester, to the proof-of-
   possession for the private key is conceptually non-trivial and
   requires specific support by TLS implementations.

   The registrar terminates the security association with the pledge at
   TLS level and thus the binding between the certification request and
   the authentication of the pledge.  The EST server uses the
   authenticated pledge identity provided by the IDevID for checking the
   authorization of the pledge for the given certification request
   before issuing to the pledge a domain-specific certificate (LDevID
   certificate).  This approach typically requires online or on-site
   availability of the RA for performing the final authorization
   decision for the certification request.

   Using EST for BRSKI has the advantage that the mutually authenticated
   TLS connection established between the pledge and the registrar can
   be reused for protecting the message exchange needed for enrolling
   the LDevID certificate.  This strongly simplifies the implementation
   of the enrollment message exchange.

   Yet the use of TLS has the limitation that this cannot provide
   auditability nor end-to-end security for the certificate enrollment
   request because the TLS session is transient and terminates at the
   registrar.  This is a problem in particular if the enrollment is done
   via multiple hops, part of which may not even be network-based.

   A further limitation of using EST as the certificate enrollment
   protocol is that due to using PKCS#10 structures in enrollment
   requests, the only possible proof-of-possession method is a self-
   signature, which excludes requesting certificates for key types that
   do not support signing.



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Appendix B.  Application Examples

   This informative annex provides some detail to the application
   examples listed in Section 1.3.

B.1.  Rolling Stock

   Rolling stock or railroad cars contain a variety of sensors,
   actuators, and controllers, which communicate within the railroad car
   but also exchange information between railroad cars building a train,
   with track-side equipment, and/or possibly with backend systems.
   These devices are typically unaware of backend system connectivity.
   Managing certificates may be done during maintenance cycles of the
   railroad car, but can already be prepared during operation.
   Preparation will include generating certification requests, which are
   collected and later forwarded for processing, once the railroad car
   is connected to the operator backend.  The authorization of the
   certification request is then done based on the operator's asset/
   inventory information in the backend.

   UNISIG has included a CMP profile for enrollment of TLS certificates
   of on-board and track-side components in the Subset-137 specifying
   the ETRAM/ETCS on-line key management for train control systems
   [UNISIG-Subset-137].

B.2.  Building Automation

   In building automation scenarios, a detached building or the basement
   of a building may be equipped with sensors, actuators, and
   controllers that are connected with each other in a local network but
   with only limited or no connectivity to a central building management
   system.  This problem may occur during installation time but also
   during operation.  In such a situation a service technician collects
   the necessary data and transfers it between the local network and the
   central building management system, e.g., using a laptop or a mobile
   phone.  This data may comprise parameters and settings required in
   the operational phase of the sensors/actuators, like a component
   certificate issued by the operator to authenticate against other
   components and services.

   The collected data may be provided by a domain registrar already
   existing in the local network.  In this case connectivity to the
   backend PKI may be facilitated by the service technician's laptop.
   Alternatively, the data can also be collected from the pledges
   directly and provided to a domain registrar deployed in a different
   network as preparation for the operational phase.  In this case,
   connectivity to the domain registrar may also be facilitated by the
   service technician's laptop.



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B.3.  Substation Automation

   In electrical substation automation scenarios, a control center
   typically hosts PKI services to issue certificates for Intelligent
   Electronic Devices (IEDs) operated in a substation.  Communication
   between the substation and control center is performed through a
   proxy/gateway/DMZ, which terminates protocol flows.  Note that
   [NERC-CIP-005-5] requires inspection of protocols at the boundary of
   a security perimeter (the substation in this case).  In addition,
   security management in substation automation assumes central support
   of several enrollment protocols in order to support the various
   capabilities of IEDs from different vendors.  The IEC standard
   IEC62351-9 [IEC-62351-9] specifies mandatory support of two
   enrollment protocols: SCEP [RFC8894] and EST [RFC7030] for the
   infrastructure side, while the IED must only support one of the two.

B.4.  Electric Vehicle Charging Infrastructure

   For electric vehicle charging infrastructure, protocols have been
   defined for the interaction between the electric vehicle and the
   charging point (e.g., ISO 15118-2 [ISO-IEC-15118-2]) as well as
   between the charging point and the charging point operator (e.g.
   OCPP [OCPP]).  Depending on the authentication model, unilateral or
   mutual authentication is required.  In both cases the charging point
   uses an X.509 certificate to authenticate itself in TLS connections
   between the electric vehicle and the charging point.  The management
   of this certificate depends, among others, on the selected backend
   connectivity protocol.  In the case of OCPP, this protocol is meant
   to be the only communication protocol between the charging point and
   the backend, carrying all information to control the charging
   operations and maintain the charging point itself.  This means that
   the certificate management needs to be handled in-band of OCPP.  This
   requires the ability to encapsulate the certificate management
   messages in a transport-independent way.  Authenticated self-
   containment will support this by allowing the transport without a
   separate enrollment protocol, binding the messages to the identity of
   the communicating endpoints.

B.5.  Infrastructure Isolation Policy

   This refers to any case in which network infrastructure is normally
   isolated from the Internet as a matter of policy, most likely for
   security reasons.  In such a case, limited access to external PKI
   services will be allowed in carefully controlled short periods of
   time, for example when a batch of new devices is deployed, and
   forbidden or prevented at other times.





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B.6.  Sites with Insufficient Level of Operational Security

   The registration authority performing (at least part of) the
   authorization of a certification request is a critical PKI component
   and therefore requires higher operational security than components
   utilizing the issued certificates for their security features.  CAs
   may also demand higher security in the registration procedures.
   Especially the CA/Browser forum currently increases the security
   requirements in the certificate issuance procedures for publicly
   trusted certificates.  In case the on-site components of the target
   domain cannot be operated securely enough for the needs of a
   registration authority, this service should be transferred to an off-
   site backend component that has a sufficient level of security.

Appendix C.  History of Changes TBD RFC Editor: please delete

   From IETF draft 06 -> IETF draft 06:

   *  Renamed the repo and files from anima-brski-async-enroll to anima-
      brski-ae

   *  Added graphics for abstract protocol overview as suggested by
      Toerless Eckert

   *  Balanced (sub-)sections and their headers

   *  Added details on CMP instance, now called BRSKI-CMP

   From IETF draft 04 -> IETF draft 05:

   *  David von Oheimb became the editor.

   *  Streamline wording, consolidate terminology, improve grammar, etc.

   *  Shift the emphasis towards supporting alternative enrollment
      protocols.

   *  Update the title accordingly - preliminary change to be approved.

   *  Move comments on EST and detailed application examples to
      informative annex.

   *  Move the remaining text of section 3 as two new sub-sections of
      section 1.

   From IETF draft 03 -> IETF draft 04:





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   *  Moved UC2 related parts defining the pledge in responder mode to a
      separate document.  This required changes and adaptations in
      several sections.  Main changes concerned the removal of the
      subsection for UC2 as well as the removal of the YANG model
      related text as it is not applicable in UC1.

   *  Updated references to the Lightweight CMP Profile.

   *  Added David von Oheimb as co-author.

   From IETF draft 02 -> IETF draft 03:

   *  Housekeeping, deleted open issue regarding YANG voucher-request in
      UC2 as voucher-request was enhanced with additional leaf.

   *  Included open issues in YANG model in UC2 regarding assertion
      value agent-proximity and CSR encapsulation using SZTP sub
      module).

   From IETF draft 01 -> IETF draft 02:

   *  Defined call flow and objects for interactions in UC2.  Object
      format based on draft for JOSE signed voucher artifacts and
      aligned the remaining objects with this approach in UC2 .

   *  Terminology change: issue #2 pledge-agent -> registrar-agent to
      better underline agent relation.

   *  Terminology change: issue #3 PULL/PUSH -> pledge-initiator-mode
      and pledge-responder-mode to better address the pledge operation.

   *  Communication approach between pledge and registrar-agent changed
      by removing TLS-PSK (former section TLS establishment) and
      associated references to other drafts in favor of relying on
      higher layer exchange of signed data objects.  These data objects
      are included also in the pledge-voucher-request and lead to an
      extension of the YANG module for the voucher-request (issue #12).

   *  Details on trust relationship between registrar-agent and
      registrar (issue #4, #5, #9) included in UC2.

   *  Recommendation regarding short-lived certificates for registrar-
      agent authentication towards registrar (issue #7) in the security
      considerations.

   *  Introduction of reference to agent signing certificate using SKID
      in agent signed data (issue #11).




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   *  Enhanced objects in exchanges between pledge and registrar-agent
      to allow the registrar to verify agent-proximity to the pledge
      (issue #1) in UC2.

   *  Details on trust relationship between registrar-agent and pledge
      (issue #5) included in UC2.

   *  Split of use case 2 call flow into sub sections in UC2.

   From IETF draft 00 -> IETF draft 01:

   *  Update of scope in Section 1.2 to include in which the pledge acts
      as a server.  This is one main motivation for use case 2.

   *  Rework of use case 2 to consider the transport between the pledge
      and the pledge-agent.  Addressed is the TLS channel establishment
      between the pledge-agent and the pledge as well as the endpoint
      definition on the pledge.

   *  First description of exchanged object types (needs more work)

   *  Clarification in discovery options for enrollment endpoints at the
      domain registrar based on well-known endpoints in Section 4.4 do
      not result in additional /.well-known URIs.  Update of the
      illustrative example.  Note that the change to /brski for the
      voucher related endpoints has been taken over in the BRSKI main
      document.

   *  Updated references.

   *  Included Thomas Werner as additional author for the document.

   From individual version 03 -> IETF draft 00:

   *  Inclusion of discovery options of enrollment endpoints at the
      domain registrar based on well-known endpoints in Section 4.4 as
      replacement of section 5.1.3 in the individual draft.  This is
      intended to support both use cases in the document.  An
      illustrative example is provided.

   *  Missing details provided for the description and call flow in
      pledge-agent use case UC2, e.g. to accommodate distribution of CA
      certificates.

   *  Updated CMP example in Section 5 to use Lightweight CMP instead of
      CMP, as the draft already provides the necessary /.well-known
      endpoints.




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   *  Requirements discussion moved to separate section in Section 3.
      Shortened description of proof of identity binding and mapping to
      existing protocols.

   *  Removal of copied call flows for voucher exchange and registrar
      discovery flow from [RFC8995] in Section 4 to avoid doubling or
      text or inconsistencies.

   *  Reworked abstract and introduction to be more crisp regarding the
      targeted solution.  Several structural changes in the document to
      have a better distinction between requirements, use case
      description, and solution description as separate sections.
      History moved to appendix.

   From individual version 02 -> 03:

   *  Update of terminology from self-contained to authenticated self-
      contained object to be consistent in the wording and to underline
      the protection of the object with an existing credential.  Note
      that the naming of this object may be discussed.  An alternative
      name may be attestation object.

   *  Simplification of the architecture approach for the initial use
      case having an offsite PKI.

   *  Introduction of a new use case utilizing authenticated self-
      contain objects to onboard a pledge using a commissioning tool
      containing a pledge-agent.  This requires additional changes in
      the BRSKI call flow sequence and led to changes in the
      introduction, the application example,and also in the related
      BRSKI-AE call flow.

   *  Update of provided examples of the addressing approach used in
      BRSKI to allow for support of multiple enrollment protocols in
      Section 4.3.

   From individual version 01 -> 02:

   *  Update of introduction text to clearly relate to the usage of
      IDevID and LDevID.

   *  Definition of the addressing approach used in BRSKI to allow for
      support of multiple enrollment protocols in Section 4.3.  This
      section also contains a first discussion of an optional discovery
      mechanism to address situations in which the registrar supports
      more than one enrollment approach.  Discovery should avoid that
      the pledge performs a trial and error of enrollment protocols.




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   *  Update of description of architecture elements and changes to
      BRSKI in Section 4.1.

   *  Enhanced consideration of existing enrollment protocols in the
      context of mapping the requirements to existing solutions in
      Section 3 and in Section 5.

   From individual version 00 -> 01:

   *  Update of examples, specifically for building automation as well
      as two new application use cases in Appendix B.

   *  Deletion of asynchronous interaction with MASA to not complicate
      the use case.  Note that the voucher exchange can already be
      handled in an asynchronous manner and is therefore not considered
      further.  This resulted in removal of the alternative path the
      MASA in Figure 1 and the associated description in Section 4.1.

   *  Enhancement of description of architecture elements and changes to
      BRSKI in Section 4.1.

   *  Consideration of existing enrollment protocols in the context of
      mapping the requirements to existing solutions in Section 3.

   *  New section starting Section 5 with the mapping to existing
      enrollment protocols by collecting boundary conditions.

Authors' Addresses

   David von Oheimb (editor)
   Siemens AG
   Otto-Hahn-Ring 6
   81739 Munich
   Germany

   Email: david.von.oheimb@siemens.com
   URI:   https://www.siemens.com/


   Steffen Fries
   Siemens AG
   Otto-Hahn-Ring 6
   81739 Munich
   Germany

   Email: steffen.fries@siemens.com
   URI:   https://www.siemens.com/




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   Hendrik Brockhaus
   Siemens AG
   Otto-Hahn-Ring 6
   81739 Munich
   Germany

   Email: hendrik.brockhaus@siemens.com
   URI:   https://www.siemens.com/


   Eliot Lear
   Cisco Systems
   Richtistrasse 7
   CH-8304 Wallisellen
   Switzerland

   Phone: +41 44 878 9200
   Email: lear@cisco.com

































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