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Support of Asynchronous Enrollment in BRSKI (BRSKI-AE)

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Steffen Fries , Hendrik Brockhaus , David von Oheimb , Eliot Lear
Last updated 2021-10-25 (Latest revision 2021-06-24)
Replaces draft-fries-anima-brski-async-enroll
Replaced by draft-ietf-anima-brski-ae
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ANIMA WG                                                        S. Fries
Internet-Draft                                              H. Brockhaus
Intended status: Standards Track                           D. von Oheimb
Expires: 28 April 2022                                           Siemens
                                                                 E. Lear
                                                           Cisco Systems
                                                         25 October 2021

         Support of Asynchronous Enrollment in BRSKI (BRSKI-AE)


   This document describes enhancements of bootstrapping a remote secure
   key infrastructure (BRSKI, [RFC8995] ) to also operate in domains
   featuring no or only timely limited connectivity between involved
   components.  To support such use cases, BRSKI-AE relies on the
   exchange of authenticated self-contained objects (signature-wrapped
   objects) also for requesting and distributing of domain specific
   device certificates.  The defined approach is agnostic regarding the
   utilized enrollment protocol allowing the application of existing and
   potentially new certificate management protocols.

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 28 April 2022.

Copyright Notice

   Copyright (c) 2021 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 (
   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 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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Scope of solution . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Supported environment . . . . . . . . . . . . . . . . . .   6
     3.2.  Application Examples  . . . . . . . . . . . . . . . . . .   6
       3.2.1.  Rolling stock . . . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Building automation . . . . . . . . . . . . . . . . .   7
       3.2.3.  Substation automation . . . . . . . . . . . . . . . .   7
       3.2.4.  Electric vehicle charging infrastructure  . . . . . .   8
       3.2.5.  Infrastructure isolation policy . . . . . . . . . . .   8
       3.2.6.  Less operational security in the target domain  . . .   8
   4.  Requirement discussion and mapping to solution elements . . .   9
   5.  Architectural Overview and Communication Exchanges  . . . . .  11
     5.1.  Support of off-site PKI service . . . . . . . . . . . . .  11
       5.1.1.  Behavior of a pledge  . . . . . . . . . . . . . . . .  14
       5.1.2.  Pledge - Registrar discovery and voucher exchange . .  15
       5.1.3.  Registrar - MASA voucher exchange . . . . . . . . . .  15
       5.1.4.  Pledge - Registrar - RA/CA certificate enrollment . .  15
       5.1.5.  Addressing Scheme Enhancements  . . . . . . . . . . .  18
     5.2.  Domain registrar support of different enrollment
           options . . . . . . . . . . . . . . . . . . . . . . . . .  18
   6.  Example for signature-wrapping using existing enrollment
           protocols . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  EST Handling  . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  CMP Handling  . . . . . . . . . . . . . . . . . . . . . .  20
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  21
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     10.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Appendix A.  History of changes TBD RFC Editor: please delete . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

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1.  Introduction

   BRSKI as defined in [RFC8995] specifies a solution for secure zero-
   touch (automated) bootstrapping of devices (pledges) in a (customer)
   site domain.  This includes the discovery of network elements in the
   target domain, time synchronization, and the exchange of security
   information necessary to establish trust between a pledge and the
   domain.  Security information about the target domain, specifically
   the target domain certificate, is exchanged utilizing voucher objects
   as defined in [RFC8366].  These vouchers are authenticated self-
   contained (signed) objects, which may be provided online
   (synchronous) or offline (asynchronous) via the domain registrar to
   the pledge and originate from a Manufacturer's Authorized Signing
   Authority (MASA).

   For the enrollment of devices BRSKI relies on EST [RFC7030] to
   request and distribute target domain specific device certificates.
   EST in turn relies on a binding of the certification request to an
   underlying TLS connection between the EST client and the EST server.
   According to BRSKI the domain registrar acts as EST server and is
   also acting as registration authority (RA) or local registration
   authority (LRA).  The binding to TLS is used to protect the exchange
   of a certification request (for a LDevID EE certificate) and to
   provide data origin authentication (client identity information), to
   support the authorization decision for processing the certification
   request.  The TLS connection is mutually authenticated and the
   client-side authentication utilizes the pledge's manufacturer issued
   device certificate (IDevID certificate).  This approach requires an
   on-site availability of a local asset or inventory management system
   performing the authorization decision based on tuple of the
   certification request and the pledge authentication using the IDevID
   certificate, to issue a domain specific certificate to the pledge.
   The EST server (the domain registrar) terminates the security
   association with the pledge and thus the binding between the
   certification request and the authentication of the pledge via TLS.
   This type of enrollment utilizing an online connection to the PKI is
   considered as synchronous enrollment.

   For certain use cases on-site support of a RA/CA component and/or an
   asset management is not available and rather provided by an
   operator's backend and may be provided timely limited or completely
   through offline interactions.  This may be due to higher security
   requirements for operating the certification authority or for
   optimization of operation for smaller deployments to avoid the always
   on-site operation.  The authorization of a certification request
   based on an asset management in this case will not / can not be
   performed on-site at enrollment time.  Enrollment, which cannot be
   performed in a (timely) consistent fashion is considered as

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   asynchronous enrollment in this document.  It requires the support of
   a store and forward functionality of certification request together
   with the requester authentication (and identity) information.  This
   enables processing of the request at a later point in time.  A
   similar situation may occur through network segmentation, which is
   utilized in industrial systems to separate domains with different
   security needs.  Here, a similar requirement arises if the
   communication channel carrying the requester authentication is
   terminated before the RA/CA authorization handling of the
   certification request.  If a second communication channel is opened
   to forward the certification request to the issuing RA/ CA, the
   requester authentication information needs to be retained and ideally
   bound to the certification request.  This uses case is independent
   from timely limitations of the first use case.  For both cases, it is
   assumed that the requester authentication information is utilized in
   the process of authorization of a certification request.  There are
   different options to perform store and forward of certification
   requests including the requester authentication information:

   *  Providing a trusted component (e.g., an LRA) in the target domain,
      which stores the certification request combined with the requester
      authentication information (based on the IDevID) and potentially
      the information about a successful proof of possession (of the
      corresponding private key) in a way prohibiting changes to the
      combined information.  Note that the assumption is that the
      information elements may not be cryptographically bound together.
      Once connectivity to the backend is available, the trusted
      component forwards the certification request together with the
      requester information (authentication and proof of possession) to
      the off-site PKI for further processing.  It is assumed that the
      off-site PKI in this case relies on the local pledge
      authentication result and thus performs the authorization and
      issues the requested certificate.  In BRSKI the trusted component
      may be the EST server residing co-located with the registrar in
      the target domain.

   *  Utilization of authenticated self-contained objects for the
      enrollment, binding the certification request and the requester
      authentication in a cryptographic way.  This approach reduces the
      necessary trust in a domain component to storage and delivery.
      Unauthorized modifications of the requester information (request
      and authentication) can be detected during the verification of the
      authenticated self-contained object.

   Focus of this document the support of handling authenticated self-
   contained objects for bootstrapping.  As it is intended to enhance
   BRSKI it is named BRSKI-AE, where AE stands for asynchronous
   enrollment.  As BRSKI, BRSKI-AE results in the pledge storing an

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   X.509 domain certificate and sufficient information for verifying the
   domain registrar / proxy identity (LDevID CA Certificate) as well as
   domain specific X.509 device certificates (LDevID EE certificate).

   The goal is to enhance BRSKI to be applicable to the additional use
   cases.  This is addressed by

   *  enhancing the well-known URI approach with an additional path for
      the utilized enrollment protocol.

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

   Note that in contrast to BRSKI, BRSKI-AE assumes support of multiple
   enrollment protocols on the infrastructure side, allowing the pledge
   manufacturer to select the most appropriate.  Thus, BRSKI-AE can be
   applied for both, asynchronous and synchronous enrollment.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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].  The
   following terms are defined additionally:

   CA:  Certification authority, issues certificates.

   RA:  Registration authority, an optional system component to which a
      CA delegates certificate management functions such as
      authorization checks.

   LRA:  Local registration authority, an optional RA system component
      with proximity to end entities.

   IED:  Intelligent Electronic Device (in essence a pledge).

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

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

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   asynchronous communication:  Describes a timely interrupted
      communication between an end entity and a PKI component.

   synchronous communication:  Describes a timely uninterrupted
      communication between an end entity and a PKI component.

   authenticated self-contained object:  Describes an object, which is
      cryptographically bound to the EE certificate (IDevID certificate
      or LDEVID certificate) of a pledge.  The binding is assumed to be
      provided through a digital signature of the actual object using
      the corresponding private key of the EE certificate.

3.  Scope of solution

3.1.  Supported environment

   This solution is intended to be used in domains with limited support
   of on-site PKI services and comprises use cases in which:

   *  there is no registration authority available in the target domain.
      The connectivity to an off-site RA in an operator's network may
      only be available temporarily.  A local store and forward device
      is used for the communication with the off-site services.

   *  authoritative actions of a LRA are limited and may not comprise
      authorization of certification requests of pledges.  Final
      authorization is done at the RA residing in the operator domain.

   *  the target deployment domain already has an established
      certificate management approach that shall be reused to (e.g., in
      brownfield installations).

3.2.  Application Examples

   The following examples are intended to motivate the support of
   different enrollment approaches in general and asynchronous
   enrollment specifically, by introducing industrial applications
   cases, which could leverage BRSKI as such but also require support of
   asynchronous operation as intended with BRSKI-AE.

3.2.1.  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,
   or with a backend.  These devices are typically unaware of backend
   connectivity.  Managing certificates may be done during maintenance
   cycles of the railroad car, but can already be prepared during

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   operation.  The preparation may comprise the generation of
   certification requests by the components which are collected and
   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

3.2.2.  Building automation

   In building automation, a use case can be described by a detached
   building or the basement of a building equipped with sensor,
   actuators, and controllers connected, but with only limited or no
   connection to the centralized building management system.  This
   limited connectivity may be during the installation time but also
   during operation time.  During the installation in the basement, a
   service technician collects the necessary information from the
   basement network and provides them to the central building management
   system, e.g., using a laptop or even a mobile phone to transport the
   information.  This information may comprise parameters and settings
   required in the operational phase of the sensors/actuators, like a
   certificate issued by the operator to authenticate against other
   components and services.

   The collected information may be provided by a domain registrar
   already existing in the installation network.  In this case
   connectivity to the backend PKI may be facilitated by the service
   technician's laptop.  Contrary, the information can also be collected
   from the pledges directly and provided to a domain registrar deployed
   in a different network.  In this cases connectivity to the domain
   registrar may be facilitated by the service technician's laptop.

3.2.3.  Substation automation

   In electrical substation automation a control center typically hosts
   PKI services to issue certificates for Intelligent Electronic Devices
   (IED)s operated in a substation.  Communication between the
   substation and control center is done 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 different enrollment
   protocols to facilitate the capabilities of IEDs from different
   vendors.  The IEC standard IEC62351-9 [IEC-62351-9] specifies the
   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.

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3.2.4.  Electric vehicle charging infrastructure

   For the electric vehicle charging infrastructure protocols have been
   defined for the interaction between the electric vehicle (EV) 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 the context of a
   TLS connection between the EV and the charging point.  The management
   of this certificate depends (beyond others) on the selected backend
   connectivity protocol.  Specifically, in case of OCPP it is intended
   as single 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 is intended to be handled in-band of OCPP.
   This requires to be able to encapsulate the certificate management
   exchanges in a transport independent way.  Authenticated self-
   containment will ease this by allowing the transport without a
   separate enrollment protocol.  This provides a binding of the
   exchanges to the identity of the communicating endpoints.

3.2.5.  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
   resources will be allowed in carefully controlled short periods of
   time, for example when a batch of new devices are deployed, but
   impossible at other times.

3.2.6.  Less operational security in the target domain

   The registration point performing the authorization of a certificate
   request is a critical PKI component and therefore implicates higher
   operational security than other 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.
   There may be the situation that the target domain does not offer
   enough security to operate a registration point and therefore wants
   to transfer this service to a backend that offers a higher level of
   operational security.

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4.  Requirement discussion and mapping to solution elements

   For the requirements discussion it is assumed that the domain
   registrar receiving a certification request as authenticated self-
   contained object is not the authorization point for this
   certification request.  If the domain registrar is the authorization
   point and the pledge has a direct connection to the registrar, BRSKI
   can be used directly.  Note that BRSKI-AE could also be used in this

   Based on the intended target environment described in Section 3.1 and
   the motivated application examples described in Section 3.2 the
   following base requirements are derived to support authenticated
   self-contained objects as container carrying the certification
   request and further information to support asynchronous operation.

   At least the following properties are required:

   *  Proof of Possession: proves to possess and control the private key
      corresponding to the public key contained in the certification
      request, typically by adding a signature using the private key.

   *  Proof of Identity: provides data-origin authentication of a data
      object, e.g., a certificate request, utilizing an existing IDevID.
      Certificate updates may utilize the certificate that is to be

   Solution examples (not complete) based on existing technology are
   provided with the focus on existing IETF documents:

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

      -  PKCS#10 [RFC2986]: Defines a structure for a certification
         request.  The structure is signed to ensure integrity
         protection and proof of possession of the private key of the
         requester that corresponds to the contained public key.

      -  CRMF [RFC4211]: Defines a structure for the certification
         request message.  The structure supports integrity protection
         and proof of possession, through a signature generated over
         parts of the structure by using the private key corresponding
         to the contained public key.  CRMF also supports further proof-
         of-possession methods for key pairs not capable to be used for

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      Note that the integrity of the certification request is bound to
      the public key contained in the certification request by
      performing the signature operation with the corresponding private
      key.  In the considered application examples, this is not
      sufficient to provide data origin authentication and needs to be
      bound to the existing credential of the pledge (IDevID)
      additionally.  This binding supports the authorization decision
      for the certification request through the provisioning of a proof
      of identity.  The binding of data origin authentication to the
      certification request may be delegated to the protocol used for
      certificate management.

   *  Proof of Identity options: The certification request should be
      bound to an existing credential (here IDevID) to enable a proof of
      identity and based on it an authorization of the certification
      request.  The binding may be realized through security options in
      an underlying transport protocol if the authorization of the
      certification request is done at the next communication hop.
      Alternatively, this binding can be done by a wrapping signature
      employing an existing credential (initial: IDevID, renewal:
      LDevID).  This requirement is addressed by existing enrollment
      protocols in different ways, for instance:

      -  EST [RFC7030]: Utilizes PKCS#10 to encode the certification
         request.  The Certificate Signing Request (CSR) may contain a
         binding to the underlying TLS by including the tls-unique value
         in the self-signed CSR structure.  The tls-unique value is one
         result of the TLS handshake.  As the TLS handshake is performed
         mutually authenticated and the pledge utilized its IDevID for
         it, the proof of identity can be provided by the binding to the
         TLS session.  This is supported in EST using the simpleenroll
         endpoint.  To avoid the binding to the underlying
         authentication in the transport layer, EST offers the support
         of a wrapping the CSR with an existing certificate by using
         Full PKI Request messages.

      -  SCEP [RFC8894]: Provides the option to utilize either an
         existing secret (password) or an existing certificate to
         protect the CSR based on SCEP Secure Message Objects using CMS
         wrapping ([RFC5652]).  Note that the wrapping using an existing
         IDevID credential in SCEP is referred to as renewal.  SCEP
         therefore does not rely on the security of an underlying

      -  CMP [RFC4210] Provides the option to utilize either an existing
         secret (password) or an existing certificate to protect the
         PKIMessage containing the certification request.  The
         certification request is encoded utilizing CRMF.  PKCS#10 is

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         optionally supported.  The proof of identity of the PKIMessage
         containing the certification request can be achieved by using
         IDevID credentials to a PKIProtection carrying the actual
         signature value.  CMP therefore does not rely on the security
         of an underlying transport protocol.

      -  CMC [RFC5272] Provides the option to utilize either an existing
         secret (password) or an existing certificate to protect the
         certification request (either in CRMF or PKCS#10) based on CMS
         [RFC5652]).  Here a FullCMCRequest can be used, which allows
         signing with an existing IDevID credential to provide a proof
         of identity.  CMC therefore does not rely on the security of an
         underlying transport.

   Note that besides the already existing enrollment protocols there is
   ongoing work in the ACE WG to define an encapsulation of EST messages
   in OSCORE to result in a TLS independent way of protecting EST.  This
   approach [I-D.selander-ace-coap-est-oscore] may be considered as
   further variant.

5.  Architectural Overview and Communication Exchanges

   To support asynchronous enrollment, the base system architecture
   defined in BRSKI [RFC8995] is enhanced to facilitate support of
   alternative enrollment protocols.  In general, the communication
   follows the BRSKI model and utilizes the existing BRSKI architecture
   elements.  The pledge initiates the communication with the domain
   registrar.  Necessary enhancements to support authenticated self-
   contained objects for certificate enrollment are kept on a minimum to
   ensure reuse of already defined architecture elements and

   For the authenticated self-contained objects used for the
   certification request, BRSKI-AE relies on the defined message
   wrapping mechanisms of the enrollment protocols stated in Section 4

5.1.  Support of off-site PKI service

   One assumption of BRSKI-AE is that the authorization of a
   certification request is performed based on an authenticated self-
   contained object, binding the certification request to the
   authentication using the IDevID.  This supports interaction with off-
   site or off-line PKI (RA/CA) components.  In addition, the
   authorization of the certification request may not be done by the
   domain registrar but by a PKI residing in the backend of the domain
   operator (off-site) as described in Section 3.1.  Also, the
   certification request may be piggybacked by another protocol.  This

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   leads to changes in the placement or enhancements of the logical
   elements as shown in Figure 1.

      +--------------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      |  .
   | IDevID |     .  |            |       +------^-----+  .
   |        |     .  +------------+              |        .
   |        |     .                              |        .
   +--------+     ...............................|.........
                   "on-site domain" components   |
                                                 |e.g., RFC 7030,
                                                 |      RFC 4210, ...
    . +---------------------------+     +--------v------------------+ .
    . | Public Key Infrastructure |<----+ PKI RA                    | .
    . | PKI CA                    |---->+                           | .
    . +---------------------------+     +---------------------------+ .
            "off-site domain" components

       Figure 1: Architecture overview using off-site PKI components

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   The architecture overview in Figure 1 utilizes the same logical
   elements as BRSKI but with a different placement in the deployment
   architecture for some of the elements.  The main difference is the
   placement of the PKI RA/CA component, which is performing the
   authorization decision for the certification request message.  It is
   placed in the off-site domain of the operator (not the deployment
   site directly), which may have no or only temporary connectivity to
   the deployment or on-site domain of the pledge.  This is to underline
   the authorization decision for the certification request in the
   backend rather than on-site.  The following list describes the
   components in the target domain:

   *  Join Proxy: same functionality as described in BRSKI.

   *  Domain Registrar / Enrollment Proxy: In general the domain
      registrar proxy has a similar functionality regarding the
      imprinting of the pledge in the deployment domain to facilitate
      the communication of the pledge with the MASA and the PKI.
      Different is the authorization of the certification request.
      BRSKI-AE allows to perform this in the operator's backend (off-
      site), and not directly at the domain registrar.

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

      -  Certificate enrollment: For the pledge enrollment the domain
         registrar in the deployment domain supports the adoption of the
         pledge in the domain based on the voucher request.
         Nevertheless, it may not have sufficient information for
         authorizing the certification request.  If the authorization of
         the certification request is done in the off-site domain, the
         domain registrar forwards the certification request to the RA
         to perform the authorization.  Note that this requires, that
         the certification request object is enhanced with a proof-of-
         identity to allow the authorization based on the bound identity
         information of the pledge.  As stated above, this can be done
         by an additional signature using the IDevID.  The domain
         registrar here acts as an enrollment proxy or local
         registration authority.  It is also able to handle the case
         having no connection temporarily to an off-site PKI, by storing
         the authenticated certification request and forwarding it to
         the RA upon reestablished connectivity.  As authenticated self-
         contained objects are used, it requires an enhancement of the
         domain registrar.  This is done by supporting alternative
         enrollment approaches (protocol options, protocols, encoding)
         by enhancing the addressing scheme to communicate with the
         domain registrar (see Section 5.1.5).

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   The following list describes the vendor related components/service
   outside the deployment domain:

   *  MASA: general functionality as described in [RFC8995].  Assumption
      is that the interaction with the MASA may be synchronous (voucher
      request with nonce) or asynchronous (voucher request without

   *  Ownership tracker: as defined in [RFC8995].

   The following list describes the operator related components/service
   operated in the backend:

   *  PKI RA: Performs certificate management functions (validation of
      certification requests, interaction with inventory/asset
      management for authorization of certification requests, etc.) for
      issuing, updating, and revoking certificates for a domain as a
      centralized infrastructure for the domain operator.  The inventory
      (asset) management may be a separate component or integrated into
      the RA directly.

   *  PKI CA: Performs certificate generation by signing the certificate
      structure provided in the certification request.

   Based on BRSKI and the architectural changes the original protocol
   flow is divided into three phases showing commonalities and
   differences to the original approach as depicted in the following.

   *  Discovery phase (same as BRSKI)

   *  Voucher exchange with deployment domain registrar (same as BRSKI).

   *  Enrollment phase (changed to support the application of
      authenticated self-contained objects).

5.1.1.  Behavior of a pledge

   The behavior of a pledge as described in [RFC8995] is kept with one
   exception.  After finishing the imprinting phase (4) the enrollment
   phase (5) is performed with a method supporting authenticated self-
   contained objects.  Using EST with simple-enroll cannot be applied
   here, as it binds the pledge authentication with the existing IDevID
   to the transport channel (TLS) rather than to the certification
   request object directly.  This authentication in the transport layer
   is not visible / verifiable at the authorization point in the off-
   site domain.  Section 6 discusses potential enrollment protocols and
   options applicable.

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5.1.2.  Pledge - Registrar discovery and voucher exchange

   The discovery phase is applied as specified in [RFC8995].

5.1.3.  Registrar - MASA voucher exchange

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

5.1.4.  Pledge - Registrar - RA/CA certificate enrollment

   As stated in Section 4 the enrollment shall be performed using an
   authenticated self-contained object providing proof of possession and
   proof of identity.

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   +--------+         +---------+    +------------+     +------------+
   | Pledge |         | Circuit |    | Domain     |     | Operator   |
   |        |         | Join    |    | Registrar  |     | RA/CA      |
   |        |         | Proxy   |    |  (JRC)     |     | (OPKI)     |
   +--------+         +---------+    +------------+     +------------+
     /-->                                      |                    |
   [Request of CA Certificates]                |                    |
     |---------- CA Certs Request ------------>|                    |
     |              [if connection to operator domain is available] |
     |                                         |-Request CA Certs ->|
     |                                         |<- CA Certs Response|
     |<-------- CA Certs Response--------------|                    |
     /-->                                      |                    |
   [Request of Certificate Attributes to be included]               |
     |---------- Attribute Request ----------->|                    |
     |              [if connection to operator domain is available] |
     |                                         |Attribute Request ->|
     |                                         |<-Attribute Response|
     |<--------- Attribute Response -----------|                    |
     /-->                                      |                    |
   [Certification request]                     |                    |
     |-------------- Cert Request ------------>|                    |
     |              [if connection to operator domain is available] |
     |                                         |--- Cert Request -->|
     |                                         |                    |
   [Optional Certificate waiting indication]   |                    |
     /-->                                      |                    |
     |<----- Cert Response (with Waiting) -----|                    |
     |-- Cert Polling (with orig request ID) ->|                    |
     |                                         |                    |
     /-->                                      |                    |
     |                                         |<-- Cert Response --|
     |                                         |                    |
     |<-- Cert Response (with Certificate) ----|                    |
     /-->                                      |                    |
   [Certificate confirmation]                  |                    |
     |-------------- Cert Confirm ------------>|                    |
     |                                         /-->                 |
     |                                         |[optional]          |
     |                                         |--- Cert Confirm -->|
     |                                         |<-- PKI Confirm ----|
     |<------------- PKI/Registrar Confirm ----|                    |

                      Figure 2: Certificate enrollment

   The following list provides an abstract description of the flow
   depicted in Figure 2.

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   *  CA Cert Request: The pledge SHOULD request the full distribution
      of CA Certificates.  This ensures that the pledge has the complete
      set of current CA certificates beyond the pinned-domain-cert
      (which may be the domain registrar certificate contained in the

   *  CA Cert Response: Contains at least one CA certificate of the
      issuing CA.

   *  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 deployment domain
      into the certification request.  To get these attributes in
      advance, the attribute request SHOULD be used.

   *  Attribute Response: Contains the attributes to be included in the
      certification request message.

   *  Cert Request: Depending on the utilized enrollment protocol, this
      certification request contains the authenticated self-contained
      object ensuring both, proof-of-possession of the corresponding
      private key and proof-of-identity of the requester.

   *  Cert Response: certification response message containing the
      requested certificate and potentially further information like
      certificates of intermediary CAs on the certification path.

   *  Cert Waiting: waiting indication for the pledge to retry after a
      given time.  For this a request identifier is necessary.  This
      request identifier may be either part of the enrollment protocol
      or build based on the certification request.

   *  Cert Polling: querying the registrar, if the certificate request
      was already processed; can be answered either with another Cert
      Waiting, or a Cert Response.

   *  Cert Confirm: confirmation message from pledge after receiving and
      verifying the certificate.

   *  PKI/Registrar Confirm: confirmation message from PKI/registrar
      about reception of the pledge's certificate confirmation.

   The generic messages described above can implemented using various
   protocols implementing authenticated self-contained objects, as
   described in Section 4.  Examples are available in Section 6.

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5.1.5.  Addressing Scheme Enhancements

   BRSKI-AE provides enhancements to the addressing scheme defined in
   [RFC8995] to accommodate the additional handling of authenticated
   self-contained objects for the certification request.  As this is
   supported by different enrollment protocols, they can be directly
   employed (see also Section 6).

   The addressing scheme in BRSKI for client certificate request and CA
   certificate distribution function during the enrollment uses the
   definition from EST [RFC7030], here on the example on simple enroll:
   "/.well-known/est/simpleenroll" This approach is generalized to the
   following notation: "/.well-known/enrollment-protocol/request" in
   which enrollment-protocol may be an already existing protocol or a
   newly defined approach.  Note that enrollment is considered here as a
   sequence of at least a certification request and a certification
   response.  In case of existing enrollment protocols the following
   notation is used proving compatibility to BRSKI:

   *  enrollment-protocol: references either EST [RFC7030] as in BRSKI
      or CMP, CMC, SCEP, or newly defined approaches as alternatives.
      Note: additional endpoints (well-known URI) at the registrar may
      need to be defined by the utilized enrollment protocol.

   *  request: depending on the utilized enrollment protocol, the
      request describes the required operation at the registrar side.
      Enrollment protocols are expected to define the request endpoints
      as done by existing protocols (see also Section 6).

5.2.  Domain registrar support of different enrollment options

   Well-known URIs for different endpoints on the domain registrar are
   already defined as part of the base BRSKI specification.  In
   addition, alternative enrollment endpoints may be supported at the
   domain registrar.  The pledge will recognize if its supported
   enrollment option is supported by the domain registrar by sending a
   request to its preferred enrollment endpoint.

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

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     </est/serverkeygen>;ct= pkcs7-mime

   TBD RFC Editor: please delete /*

   Open Issues:

   *  In addition to the current content types, we may specify that the
      response provide information about different content types as
      multiple values.  This would allow to further adopt the encoding
      of the objects exchanges (ASN.1, JSON, CBOR, ...). -> dependent on
      the utilized protocol. */

6.  Example for signature-wrapping using existing enrollment protocols

   This section map the requirements to support proof of possession and
   proof of identity to selected existing enrollment protocols.  Note
   that that the work in the ACE WG described in
   [I-D.selander-ace-coap-est-oscore] may be considered here as well, as
   it also addresses the encapsulation of EST in a way to make it
   independent from the underlying TLS using OSCORE resulting in an
   authenticated self-contained object.

6.1.  EST Handling

   When using EST [RFC7030], the following constraints should be

   *  Proof of possession is provided by using the specified PKCS#10
      structure in the request.

   *  Proof of identity is achieved by signing the certification request
      object, which is only supported when Full PKI Request (the
      /fullcmc endpoint) is used.  This contains sufficient information
      for the RA to make an authorization decision on the received

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      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 would be
      calculated using the IDevID credential of the pledge.

   *  TBD RFC Editor: please delete /* TBD: in this case the binding to
      the underlying TLS connection is not be necessary. */

   *  When the RA is not available, as per [RFC7030] Section 4.2.3, a
      202 return code should be returned by the Registrar.  The pledge
      in this case would retry a simpleenroll with a PKCS#10 request.
      Note that if the TLS connection is teared down for the waiting
      time, the PKCS#10 request would need to be rebuilt if it contains
      the unique identifier (tls_unique) from the underlying TLS
      connection for the binding.

   *  TBD RFC Editor: please delete /* TBD: clarification of retry for
      fullcmc is necessary as not specified in the context of EST */

6.2.  CMP Handling

   Instead of using general CMP [RFC4210], this specification refers to
   the Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile],
   as it restricts the full featured CMP to the functionality needed
   here.  For this, the following constrains should be observed:

   *  For proof of possession, the defined approach in Lightweight CMP
      Profile [I-D.ietf-lamps-lightweight-cmp-profile] section 4.1.1
      (based on CRMF) and 4.1.4 (based on PCKS#10) should be supported.

   *  Proof of identity can be provided by using the signatures to
      protect the certificate request message as outlined in section
      3.2. of [I-D.ietf-lamps-lightweight-cmp-profile].

   *  When the RA/CA is not available, a waiting indication should be
      returned in the PKIStatus by the Registrar as specified in
      sections 4.4 and 5.1.2 of [I-D.ietf-lamps-lightweight-cmp-profile]
      for delayed delivery.

   *  Requesting CA certificates and certificate request attributes
      should be implemented a specified in Lightweight CMP Profile
      sections 4.3.1 and 4.3.3 [I-D.ietf-lamps-lightweight-cmp-profile].

7.  IANA Considerations

   This document does not require IANA actions.

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8.  Security Considerations

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

9.  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.

10.  References

10.1.  Normative References

              Brockhaus, H. and D. V. Oheimb, "Certificate Management
              Protocol (CMP) Updates", Work in Progress, Internet-Draft,
              draft-ietf-lamps-cmp-updates-12, 9 July 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,

   [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,

   [RFC4211]  Schaad, J., "Internet X.509 Public Key Infrastructure
              Certificate Request Message Format (CRMF)", RFC 4211,
              DOI 10.17487/RFC4211, September 2005,

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

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

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

   [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, <>.

10.2.  Informative References

              Brockhaus, H., Fries, S., and D. V. Oheimb, "Lightweight
              Certificate Management Protocol (CMP) Profile", Work in
              Progress, Internet-Draft, draft-ietf-lamps-lightweight-
              cmp-profile-06, 9 July 2021,

              Selander, G., Raza, S., Furuhed, M., Vucinic, M., and T.
              Claeys, "Protecting EST Payloads with OSCORE", Work in
              Progress, Internet-Draft, draft-selander-ace-coap-est-
              oscore-05, 5 May 2021, <

              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.

              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.

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

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

   [RFC5272]  Schaad, J. and M. Myers, "Certificate Management over CMS
              (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008,

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,

   [RFC8894]  Gutmann, P., "Simple Certificate Enrolment Protocol",
              RFC 8894, DOI 10.17487/RFC8894, September 2020,

Appendix A.  History of changes TBD RFC Editor: please delete

   From IETF draft 03 -> IETF draft 04:

   *  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

   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.

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   *  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

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

   *  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 3.1 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 5.2 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

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   *  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 5.2 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

   *  Updated CMP example in Section 6 to use Lightweight CMP instead of
      CMP, as the draft already provides the necessary /.well-known

   *  Requirements discussion moved to separate section in Section 4.
      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 5.1 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

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      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 5.1.5.

   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 5.1.5.  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.

   *  Update of description of architecture elements and changes to
      BRSKI in Section 5.

   *  Enhanced consideration of existing enrollment protocols in the
      context of mapping the requirements to existing solutions in
      Section 4 and in Section 6.

   From individual version 00 -> 01:

   *  Update of examples, specifically for building automation as well
      as two new application use cases in Section 3.2.

   *  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 5.

   *  Enhancement of description of architecture elements and changes to
      BRSKI in Section 5.

   *  Consideration of existing enrollment protocols in the context of
      mapping the requirements to existing solutions in Section 4.

   *  New section starting Section 6 with the mapping to existing
      enrollment protocols by collecting boundary conditions.

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Authors' Addresses

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


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


   David von Oheimb
   Siemens AG
   Otto-Hahn-Ring 6
   81739 Munich


   Eliot Lear
   Cisco Systems
   Richtistrasse 7
   CH-8304 Wallisellen

   Phone: +41 44 878 9200

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