Support of asynchronous Enrollment in BRSKI (BRSKI-AE)
draft-ietf-anima-brski-async-enroll-03

Document Type Active Internet-Draft (anima WG)
Authors Steffen Fries  , Hendrik Brockhaus  , Eliot Lear  , Thomas Werner 
Last updated 2021-06-25 (latest revision 2021-06-24)
Replaces draft-fries-anima-brski-async-enroll
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ANIMA WG                                                        S. Fries
Internet-Draft                                              H. Brockhaus
Intended status: Standards Track                                 Siemens
Expires: December 26, 2021                                       E. Lear
                                                           Cisco Systems
                                                               T. Werner
                                                                 Siemens
                                                           June 24, 2021

         Support of asynchronous Enrollment in BRSKI (BRSKI-AE)
                 draft-ietf-anima-brski-async-enroll-03

Abstract

   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.  Further enhancements are provided to perform the BRSKI
   approach in environments, in which the role of the pledge changes
   from a client to a server . This changes the interaction model from a
   pledge-initiator-mode to a pledge-responder-mode.  To support both
   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on December 26, 2021.

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Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Scope of solution . . . . . . . . . . . . . . . . . . . . . .   7
     3.1.  Supported environment . . . . . . . . . . . . . . . . . .   7
     3.2.  Application Examples  . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Rolling stock . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Building automation . . . . . . . . . . . . . . . . .   8
       3.2.3.  Substation automation . . . . . . . . . . . . . . . .   8
       3.2.4.  Electric vehicle charging infrastructure  . . . . . .   9
       3.2.5.  Infrastructure isolation policy . . . . . . . . . . .   9
       3.2.6.  Less operational security in the target domain  . . .   9
   4.  Requirement discussion and mapping to solution elements . . .  10
   5.  Architectural Overview and Communication Exchanges  . . . . .  12
     5.1.  Use Case 1 (pledge-initiator-mode): Support of off-site
           PKI service . . . . . . . . . . . . . . . . . . . . . . .  13
       5.1.1.  Behavior of a pledge  . . . . . . . . . . . . . . . .  16
       5.1.2.  Pledge - Registrar discovery and voucher exchange . .  16
       5.1.3.  Registrar - MASA voucher exchange . . . . . . . . . .  16
       5.1.4.  Pledge - Registrar - RA/CA certificate enrollment . .  16
       5.1.5.  Addressing Scheme Enhancements  . . . . . . . . . . .  19
     5.2.  Use Case 2 (pledge-responder-mode): Registrar-agent
           communication with Pledges  . . . . . . . . . . . . . . .  19
       5.2.1.  Behavior of a pledge in pledge-responder-mode . . . .  23
       5.2.2.  Behavior of a registrar-agent . . . . . . . . . . . .  23
       5.2.3.  Bootstrapping objects and corresponding exchanges . .  25
     5.3.  Domain registrar support of different enrollment options   47
   6.  YANG Extensions to Voucher Request  . . . . . . . . . . . . .  48
   7.  Example for signature-wrapping using existing enrollment
       protocols . . . . . . . . . . . . . . . . . . . . . . . . . .  51
     7.1.  EST Handling  . . . . . . . . . . . . . . . . . . . . . .  51
     7.2.  CMP Handling  . . . . . . . . . . . . . . . . . . . . . .  52

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   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  52
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  53
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  53
     10.1.  Exhaustion attack on pledge  . . . . . . . . . . . . . .  53
     10.2.  Misuse of acquired voucher and enrollment responses  . .  53
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  53
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  53
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  54
     12.2.  Informative References . . . . . . . . . . . . . . . . .  55
   Appendix A.  History of changes [RFC Editor: please delete] . . .  56
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

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.

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   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
   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:

   o  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.

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   o  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
   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).

   Based on the proposed approach, a second set of scenarios can be
   addressed, in which the pledge has either no direct communication
   path to the domain registrar, e.g., due to missing network
   connectivity or a different technology stack.  In such scenarios the
   pledge is expected to act as a server rather than a client.  The
   pledge will be triggered to generate request objects to be onboarded
   in the registrar's domain.  For this, an additional component is
   introduced acting as an agent for the domain registrar (registrar-
   agent) towards the pledge.  This could be a functionality of a
   commissioning tool or it may be even co-located with the registrar.
   In contrast to BRSKI the registrar-agent performs the object exchange
   with the pledge and provides/retrieves data objects to/from the
   domain registrar.  For the interaction with the domain registrar the
   registrar agent will use existing BRSKI endpoints.

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

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

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

   o  allowing to utilize credentials different from the pledge's IDevID
      to establish a TLS connection to the domain registrar, which is
      necessary in case of using a registrar-agent.

   o  defining the interaction (dta exchange and data objects) between a
      pledge acting as server an a registrar-agent and the domain
      registrar.

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   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",
   "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].  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.

   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.

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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:

   o  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.

   o  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.

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

   In addition, the solution is intended to be applicable in domains in
   which pledges have no direct connection to the domain registrar, but
   are expected to be managed by the registrar.  This can be motivated
   by pledges featuring a different technology stack or by pledges
   without an existing connection to the domain registrar during
   bootstrapping.  These pledges are likely to act in a server role.
   Therefore, the pledge has to offer endpoints on which it can be
   triggered for the generation of voucher-request objects and
   certification objects as well as to provide the response objects to
   the pledge.

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
   operation.  The preparation may comprise the generation of

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   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
   backend.

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
   case.

   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:

   o  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.

   o  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
      updated.

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

   o  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
         signing.

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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.

   o  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
         transport.

      *  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 the two target
   use cases.

   o  Use case 1 (Pledge-initiator-mode): the pledge requests
      certificates from a PKI operated off-site via the domain
      registrar.  The communication model follows the BRSKI model in
      which the pledge initiates the communication.

   o  Use case 2 (Pledge-responder-mode): allows delegated bootstrapping
      using a registrar-agent instead a direct connection from the
      pledge to the domain registrar.  The communication model between
      registrar-agent and pledge assumes that the pledge is acting as
      server and responds to requests.

   Both use cases are described in the next subsections.  They utilize
   the existing BRSKI architecture elements as much as possible.
   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 interactions.

   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
   above.

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5.1.  Use Case 1 (pledge-initiator-mode): 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
   leads to changes in the placement or 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      |  .
   | 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

   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:

   o  Join Proxy: same functionality as described in BRSKI.

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   o  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).

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

   o  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
      nonce).

   o  Ownership tracker: as defined in [RFC8995].

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

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   o  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.

   o  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.

   o  Discovery phase (same as BRSKI)

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

   o  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 7 discusses potential enrollment protocols and
   options applicable.

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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   o  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
      voucher).

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

   o  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.

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

   o  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.

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

   o  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.

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

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

   o  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 7.

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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 7).

   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:

   o  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.

   o  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 7).

5.2.  Use Case 2 (pledge-responder-mode): Registrar-agent communication
      with Pledges

   To support mutual trust establishment of pledges, not directly
   connected to the domain registrar.  It relies on the exchange of
   authenticated self-contained objects (the voucher request/response
   objects as known from BRSKI and the enrollment request/response
   objects as introduced by BRSKI-AE).  This approach has also been
   applied also for the use case 1.  This allows independence of a
   potential protection provided by the used transport protocol.

   In contrast to BRSKI, the object exchanges performed with the help of
   a registrar-agent component, supporting the interaction of the pledge
   with the domain registrar.  It may be an integrated functionality of
   a commissioning tool.  This leads to enhancements of the logical
   elements in the BRSKI architecture as shown in Figure 3.  The
   registrar-agent interacts with the pledge to acquire and to supply
   the required data objects for bootstrapping, which are also exchanged
   between the registrar-agent and the domain registrar.  Moreover, the

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   addition of the registrar-agent also influences the sequences for the
   data exchange between the pledge and the domain registrar described
   in [RFC8995].  The general goal for the registrar-agent application
   is the reuse of already defined endpoints of the domain registrar
   side.  The functionality of the already existing registrar endpoints
   may need small enhancements.

                                             +------------------------+
      +--------------Drop Ship---------------| Vendor Service         |
      |                                      +------------------------+
      |                                      | M anufacturer|         |
      |                                      | A uthorized  |Ownership|
      |                                      | S igning     |Tracker  |
      |                                      | A uthority   |         |
      |                                      +--------------+---------+
      |                                                     ^
      |                                                     |  BRSKI-
      V                                                     |   MASA
   +-------+     +---------+   .............................|.........
   |       |     |         |   .                            |        .
   |       |     |         |   .  +-----------+       +-----v-----+  .
   |       |     |Registrar|   .  |           |       |           |  .
   |Pledge |     |Agent    |   .  |   Join    |       | Domain    |  .
   |       |     |         |   .  |   Proxy   |       | Registrar |  .
   |       <----->.........<------>...........<-------> (PKI RA)  |  .
   |       |     |         |   .  |       BRSKI-AE    |           |  .
   |       |     |         |   .  |           |       +-----+-----+  .
   |IDevID |     | LDevID  |   .  +-----------+             |        .
   |       |     |         |   .         +------------------+-----+  .
   +-------+     +---------+   .         | Key Infrastructure     |  .
                               .         | (e.g., PKI Certificate |  .
                               .         |       Authority)       |  .
                               .         +------------------------+  .
                               .......................................
                                         "Domain" components

           Figure 3: Architecture overview using registrar-agent

   The architecture overview in Figure 3 utilizes the same logical
   components as BRSKI with the registrar-agent component in addition.

   For authentication towards the domain registrar, the registrar-agent
   uses its LDevID.  The provisioning of the registrar-agent LDevID may
   be done by a separate BRSKI run or other means in advance.  It is
   recommended to use short lived registrar-agent LDevIDs in the range
   of days or weeks.

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   If a registrar detects a request originates from a registrar-agent it
   is able to switch the operational mode from BRSKI to BRSKI-AE.

   In addition, the domain registrar may authenticate the user operating
   the registrar-agent to perform additional authorization of a pledge
   enrollment action.  Examples for such user level authentication are
   the application of HTTP authentication or the usage of authorization
   tokens or other.  This is out of scope of this document.

   The following list describes the components in a (customer) site
   domain:

   o  Pledge: The pledge is expected to respond with the necessary data
      objects for bootstrapping to the registrar-agent.  The transport
      protocol used between the pledge and the registrar-agent is
      assumed to be HTTP in the context of this document.  Other
      transport protocols may be used but are out of scope of this
      document.  As the pledge is acting as a server during
      bootstrapping it leads to some differences to BRSKI:

      *  Discovery of the domain registrar by the pledge is not needed
         as the pledge will be triggered by the registrar-agent.

      *  Discovery of the pledge by the registrar-agent must be
         possible.

      *  As the registrar-agent must be able to request data objects for
         bootstrapping of the pledge, the pledge must offer
         corresponding endpoints.

      *  The registrar-agent may provide additional data to the pledge,
         in the context of the triggering request.

      *  Order of exchanges in the call flow may be different as the
         registrar-agent collects both objects, pledge-voucher-request
         objects and pledge-enrollment-request objects, at once and
         provides them to the registrar.  This approach may also be used
         to perform a bulk bootstrapping of several devices.

      *  The data objects utilized for the data exchange between the
         pledge and the registrar are self-contained authenticated
         objects (signature-wrapped objects) as in use case 1
         Section 5.1.

   o  Registrar-agent: provides a communication path to exchange data
      objects between the pledge and the domain registrar.  The
      registrar-agent facilitates situations, in which the domain
      registrar is not directly reachable by the pledge, either due to a

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      different technology stack or due to missing connectivity.  The
      registrar-agent triggers the pledge to create bootstrapping
      information such as voucher request objects and enrollment request
      objects from one or multiple pledges at once and performs a bulk
      bootstrapping based on the collected data.  The registrar-agent is
      expected to possess information of the domain registrar, either by
      configuration or by using the discovery mechanism defined in
      [RFC8995].  There is no trust assumption between the pledge and
      the registrar-agent as only authenticated self-contained objects
      are applied, which are transported via the registrar-agent and
      provided either by the pledge or the registrar.  The trust
      assumption between the registrar-agent and the registrar bases on
      an own LDevID of the registrar-agent, acting as registrar
      component.  This allows the registrar-agent to authenticate
      towards the registrar.  The registrar can utilize this
      authentication to distinguish communication with a pledge from a
      registrar-agent based on the exchanged objects.

   o  Join Proxy: same functionality as described in [RFC8995].  Note
      that it may be used by the registrar-agent instead of the pledge
      to find the registrar, if not configured.

   o  Domain Registrar: In general the domain registrar fulfills the
      same functionality regarding the bootstrapping of the pledge in a
      (customer) site domain by facilitating the communication of the
      pledge with the MASA service and the domain PKI service.  In
      contrast to [RFC8995], the domain registrar does not interact with
      a pledge directly but through the registrar-agent.  The registrar
      detects if the bootstrapping is performed by the pledge directly
      or by the registrar-agent.  The manufacturer provided components/
      services (MASA and Ownership tracker) are used as defined in
      [RFC8995].  For issuing a voucher, the MASA may perform additional
      checks on voucher-request objects, to issue a voucher indicating
      agent-proximity instead of registrar-proximity.

   [RFC Editor: please delete] /*

   Open Issues: The voucher defined in [RFC8366] defines the leaf
   assertion as enum, which cannot be extended.  To define an additional
   assertion RFC 8366 may be revised. */

   "Agent-proximity" is a weaker assertion then "proximity".  In case of
   "agent-proximity" it is a statement, that the proximity-registrar-
   certificate was provided via the registrar-agent and not directly.
   This can be verified by the registrar and also by the MASA through
   voucher-request processing.  Note that at the time of creating the
   voucher-request, the pledge cannot verify the LDevID(Reg) EE
   certificate and has no proof-of-possession of the corresponding

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   private key for the certificate.  Trust handover to the domain is
   established via the "pinned-domain-certificate" in the voucher.

   In contrast, "proximity" provides a statement, that the pledge was in
   direct contact with the registrar and was able to verify proof-of-
   possession of the private key in the context of the TLS handshake.
   The provisionally accepted LDevID(Reg) EE certificate can be verified
   after the voucher has been processed by the pledge.

5.2.1.  Behavior of a pledge in pledge-responder-mode

   In contrast to use case 1 Section 5.1 the pledge acts as a server
   component if data is triggered by the registrar-agent for the
   generation of pledge-voucher-request and pledge-enrollment-request
   objects as well as for the processing of the response objects and the
   generation of status information.  Due to the use of the registrar-
   agent, the interaction with the domain registrar is changed as shown
   in Figure 5.  To enable interaction with the registrar-agent, the
   pledge provides endpoints using the BRSKI interface based on the
   "/.well-known/brski" URI tree.  The following endpoints are defined
   for the pledge in this document:

   o  /.well-known/brski/pledge-voucher-request: trigger pledge to
      create voucher request.  It returns the pledge-voucher-request.

   o  /.well-known/brski/pledge-enrollment-request: trigger pledge to
      create enrollment request. it returns the pledge-enrollment-
      request.

   o  /.well-known/brski/pledge-voucher: supply MASA provided voucher to
      pledge.  It returns the pledge-voucher-status.

   o  /.well-known/brski/pledge-enrollment: supply enroll response
      (certificate) to pledge.  It returns the pledge-enrollment-status.

   o  /.well-known/brski/pledge-CACerts: supply CACerts to pledge
      (optional).

5.2.2.  Behavior of a registrar-agent

   The registrar-agent is a new component in the BRSKI context.  It
   provides connectivity between the pledge and the domain registrar and
   reuses the endpoints of the domain registrar side already specified
   in [RFC8995].  It facilitates the exchange of data objects between
   the pledge and the domain registrar, which are the voucher request/
   response objects, the enrollment request/response objects, as well as
   related status objects.  For the communication the registrar-agent
   utilizes communication endpoints provided by the pledge.  The

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   transport in this specification is based on HTTP but may also be done
   using other transport mechanisms.  This new component changes the
   general interaction between the pledge and the domain registrar as
   shown in Figure 9.

   The registrar-agent is expected to already possess an LDevID(RegAgt)
   to authenticate towards the domain registrar.  The registrar-agent
   will use this LDevID(RegAgt) when establishing the TLS session with
   the domain registrar in the context of for TLS client-side
   authentication.  The LDevID(RegAgt) certificate MUST include a
   SubjectKeyIdentifier (SKID), which is used as reference in the
   context of an agent-signed-data object.  Note that this is an
   additional requirement for issuing the certificate, as [IEEE-802.1AR]
   only requires the SKID to be included for intermediate CA
   certificates.  In the specific application of BRSKI-AE, it is used in
   favor of a certificate fingerprint to avoid additional computations.

   Using an LDevID for TLS client-side authentication is a deviation
   from [RFC8995], in which the pledge's IDevID credential is used to
   perform TLS client authentication.  The use of the LDevID(RegAgt)
   allows the domain registrar to distinguish, if bootstrapping is
   initiated from a pledge or from a registrar-agent and adopt the
   internal handling accordingly.  As BRSKI-AE uses authenticated self-
   contained data objects between the pledge and the domain registrar,
   the binding of the pledge identity to the request object is provided
   by the data object signature employing the pledge's IDevID.  The
   objects exchanged between the pledge and the domain registrar used in
   the context of this specifications are JOSE objects

   In addition to the LDevID(RegAgt), the registrar-agent is provided
   with the product-serial-numbers of the pledges to be bootstrapped.
   This is necessary to allow the discovery of pledges by the registrar-
   agent using mDNS.  The list may be provided by administrative means
   or the registrar agent may get the information via an interaction
   with the pledge, like scanning of product-serial-number information
   using a QR code or similar.

   According to [RFC8995] section 5.3, the domain registrar performs the
   pledge authorization for bootstrapping within his domain based on the
   pledge voucher-request object.

   The following information is therefore available at the registrar-
   agent:

   o  LDevID(RegAgt): own operational key pair.

   o  LDevID(reg) certificate: certificate of the domain registrar.

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   o  Serial-number(s): product-serial-number(s) of pledge(s) to be
      bootstrapped.

5.2.2.1.  Registrar discovery by the registrar-agent

   The discovery of the domain registrar may be done as specified in
   [RFC8995] with the deviation that it is done between the registrar-
   agent and the domain registrar.  Alternatively, the registrar-agent
   may be configured with the address of the domain registrar and the
   certificate of the domain registrar.

5.2.2.2.  Pledge discovery by the registrar-agent

   The discovery of the pledge by registrar-agent should be done by
   using DNS-based Service Discovery [RFC6763] over Multicast DNS
   [RFC6762] to discover the pledge at "product-serial-number.brski-
   pledge._tcp.local."  The pledge constructs a local host name based on
   device local information (product-serial-number), which results in
   "product-serial-number.brski-pledge._tcp.local.".  It can then be
   discovered by the registrar-agent via mDNS.  Note that other
   mechanisms for discovery may be used.

   The registrar-agent is able to build the same information based on
   the provided list of product-serial-number.

5.2.3.  Bootstrapping objects and corresponding exchanges

   The interaction of the pledge with the registrar-agent may be
   accomplished using different transport means (protocols and or
   network technologies).  For this document the usage of HTTP is
   targeted as in BRSKI.  Alternatives may be CoAP, Bluetooth Low Energy
   (BLE), or Nearfield Communication (NFC).  This requires independence
   of the exchanged data objects between the pledge and the registrar
   from transport security.  Therefore, authenticated self-contained
   objects (here: signature-wrapped objects) are applied in the data
   exchange between the pledge and the registrar.

   The registrar-agent provides the domain-registrar certificate
   (LDevID(Reg) EE certificate) to the pledge to be included into the
   "agent-provided-proximity-registrar-certificate" leaf in the pledge-
   voucher-request object.  This enables the registrar to verify, that
   it is the target registrar for handling the request.  The registrar
   certificate may be configured at the registrar-agent or may be
   fetched by the registrar-agent based on a prior TLS connection
   establishment with the domain registrar.  In addition, the registrar-
   agent provides agent-signed-data containing the product-serial-number
   in the body, signed with the LDevID(RegAgt).  This enables the
   registrar to verify and log, which registrar-agent was in contact

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   with the pledge.  Optionally the registrar-agent may provide its
   LDevID(RegAgt) certificate to the pledge for inclusion into the
   pledge-voucher-request as "agent-sign-cert" leaf.  Note that this may
   be omitted in constraint environments to safe bandwidth between the
   registrar-agent and the pledge.  If not contained, the registrar-
   agent MUST fetch the LDevID(RegAgt) certificate based on the
   SubjectKeyIdentifier (SKID) in the header of the agent-signed-data.
   The registrar may include the LDevID(RegAgt) certificate information
   into the registrar-voucher-request.

   The MASA in turn verifies the LDevID(Reg) certificate is included in
   the pledge-voucher-request (prior-signed-voucher-request) in the
   "agent-provided-proximity-registrar-certificate" leaf and may assert
   in the voucher "verified" or "logged" instead of "proximity", as
   there is no direct connection between the pledge and the registrar.
   If the LDevID(RegAgt) certificate is included contained in the
   "agent-sign-cert" leave of the registrar-voucher-request, the MASA
   can verify the LDevID(RegAgt) certificate and the signature of the
   registrar-agent in the agent-signed-data provided in the prior-
   signed-voucher-request.  If both can be verified successfully, the
   MASA can assert "agent-proximity" in the voucher.  Otherwise, it may
   assert "verified" or "logged".  The voucher can then be supplied via
   the registrar to the registrar-agent.

   Figure 4 provides an overview of the exchanges detailed in the
   following sub sections.

   +--------+  +-----------+    +-----------+   +--------+   +---------+
   | Pledge |  | Registrar |    | Domain    |   | Domain |   | Vendor  |
   |        |  | Agent     |    | Registrar |   | CA     |   | Service |
   |        |  | (RegAgt)  |    |  (JRC)    |   |        |   | (MASA)  |
   +--------+  +-----------+    +-----------+   +--------+   +---------+
        |              |                  |              |   Internet |
   [discovery of pledge]
        | mDNS query   |                  |              |            |
        |<-------------|                  |              |            |
        |------------->|                  |              |            |
        |              |                  |              |            |
   [trigger pledge-voucher-request and
    pledge-enrollment-request generation]
        |<- vTrigger --|                  |              |            |
        |-Voucher-Req->|                  |              |            |
        |              |                  |              |            |
        |<- eTrigger --|                  |              |            |
        |- Enroll-Req->|                  |              |            |
        ~              ~                  ~              ~            ~
   [provide pledge-voucher-request to infrastructure]
        |              |<------ TLS ----->|              |            |

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        |              |-- Voucher-Req -->|              |            |
        |              |          [accept device?]       |            |
        |              |          [contact vendor]       |            |
        |              |                  |------- Voucher-Req ------>|
        |              |                  |           [extract DomainID]
        |              |                  |           [update audit log]
        |              |                  |<-------- Voucher ---------|
        |              |<---- Voucher ----|              |            |
        |              |                  |              |            |
   [provide pledge enrollment request to infrastructure]
        |              |-- Enroll-Req --->|              |            |
        |              |                  |- Cert-Req -->|            |
        |              |                  |<-Certificate-|            |
        |              |<-- Enroll-Resp --|              |            |
        ~              ~                  ~              ~            ~
   [provide voucher and certificate
    to pledge and collect status info]
        |<-- Voucher --|                  |              |            |
        |-- vStatus -->|                  |              |            |
        |<-Enroll-Resp-|                  |              |            |
        |-- eStatus -->|                  |              |            |
        ~              ~                  ~              ~            ~
   [provide voucher-status and enrollment status to registrar]
        |              |<------ TLS ----->|              |            |
        |              |----  vStatus --->|              |            |
        |              |                  |-- req. device audit log ->|
        |              |                  |<---- device audit log ----|
        |              |           [verify audit log]
        |              |                  |              |            |
        |              |----  eStatus --->|              |            |
        |              |                  |              |            |

            Figure 4: Overview pledge-responder-mode exchanges

   The following sub sections split the interactions between the
   different components into:

   o  Request objects acquisition targets exchanges and objects between
      the registrar-agent and the pledge.

   o  Request handling targets exchanges and objects between the
      registrar-agent and the registrar and also the interaction of the
      registrar with the MASA and the domain CA.

   o  Response object supply targets the exchanges and objects between
      the registrar-agent and the pledge including the status objects.

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   o  Status handling addresses the exchanges between the registrar-
      agent and the registrar.

5.2.3.1.  Request objects acquisition (registrar-agent - pledge)

   The following description assumes that the registrar-agent already
   discovered the pledge.  This may be done as described in
   Section 5.2.2.2 based on mDNS.

   The focus is on the exchange of signature-wrapped objects using
   endpoints defined for the pledge in Section 5.2.1.

   Preconditions:

   o  Pledge: possesses IDevID

   o  Registrar-agent: possesses IDevID CA certificate and an own
      LDevID(RegAgt) EE credential for the registrar domain.  In
      addition, the registrar-agent can be configured with the product-
      serial-number(s) of the pledge(s) to be bootstrapped.  Note that
      the product-serial-number may have been used during the pledge
      discovery already.

   o  Registrar: possesses IDevID CA certificate and an own LDevID/Reg)
      credential.

   o  MASA: possesses own credentials (voucher signing key, TLS server
      certificate) as well as IDevID CA certificate of pledge vendor /
      manufacturer and site-specific LDevID CA certificate.

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   +--------+                             +-----------+
   | Pledge |                             | Registrar |
   |        |                             | Agent     |
   |        |                             | (RegAgt)  |
   +--------+                             +-----------+
       |                                        |-create
       |                                        | agent-signed-data
       |<--- trigger pledge-voucher-request ----|
       |-agent-provided-proximity-registrar-cert|
       |-agent-signed-data                      |
       |-agent-sign-cert (optional)             |
       |                                        |
       |----- pledge-voucher-request ---------->|-store
       |                                        | pledge-voucher-request
       |<----- trigger enrollment request ------|
       |       (empty)                          |
       |                                        |
       |------ pledge-enrollment-request ------>|-store
       |                                        | pledge-enrollment-req.

          Figure 5: Request collection (registrar-agent - pledge)

   Triggering the pledge to create the pledge-voucher-request is done
   using HTTPS POST on the defined pledge endpoint "/.well-known/brski/
   pledge-voucher-request".

   The registrar-agent pledge-voucher-request Content-Type header is:

   application/json: defines a JSON document to provide three parameter:

   o  agent-provided-proximity-registrar-cert: base64-encoded
      LDevID(Reg) TLS EE certificate.

   o  agent-sign-cert: base64-encoded LDevID(RegAgt) signing certificate
      (optional).

   o  agent-signed-data: base64-encoded JWS-object.

   Note that optionally including the agent-sign-cert enables the pledge
   to verify at least the signature of the agent-signed-data.  It may
   not verify the agent-sign-cert itself due to missing issuing CA
   information.

   The agent-signed-data is a JOSE object and contains the following
   information:

   The header of the agent-signed-data contains:

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   o  alg: algorithm used for creating the object signature.

   o  kid: contains the base64-encoded SubjectKeyIdentifier of the
      LDevID(RegAgt) certificate.

   The body of the agent-signed-data contains an ietf-voucher-
   request:agent-signed-data element (defined in Section 6):

   o  created-on: MUST contain the creation date and time in yang:date-
      and-time format.

   o  serial-number: MUST contain the product-serial-number as type
      string as defined in [RFC8995], section 2.3.1.  The serial-number
      corresponds with the product-serial-number contained in the
      X520SerialNumber field of the IDevID certificate of the pledge.

   {
       "alg": "ES256",
       "kid": "base64encodedvalue=="
   }
   {
     "ietf-voucher-request-trigger:agent-signed-data": {
       "created-on": "2021-04-16T00:00:01.000Z",
       "serial-number": "callee4711"
     }
   }
   {
       SIGNATURE
   }

                  Figure 6: Example of agent-signed-data

   Upon receiving the voucher-request trigger, the pledge SHOULD
   construct the body of the pledge-voucher-request object as defined in
   [RFC8995].  This object becomes a JSON-in-JWS object as defined in
   [I-D.richardson-anima-jose-voucher].

   The header of the pledge-voucher-request SHALL contain the following
   parameter as defined in [RFC7515]:

   o  alg: algorithm used for creating the object signature.

   o  x5c: contains the base64-encoded pledge IDevID certificate.

   The body of the pledge-voucher-request object MUST contain the
   following parameter as part of the ietf-voucher-request:voucher as
   defined in [RFC8995]:

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   o  created-on: contains the current date and time in yang:date-and-
      time format.

   o  nonce: contains a cryptographically strong random or pseudo-random
      number.

   o  serial-number: contains the base64-encoded pledge product-serial-
      number.

   o  assertion: contains the requested voucher assertion.

   The ietf-voucher-request:voucher is enhanced with additional
   parameters:

   o  agent-provided-proximity-registrar-cert: MUST be included and
      contains the base64-encoded LDevID(Reg) EE certificate (provided
      as trigger parameter by the registrar-agent).

   o  agent-signed-data: MUST contain the base64-encoded agent-signed-
      data (as defined in Figure 6) and provided as trigger parameter.

   o  agent-sign-cert: May contain the base64-encoded LDevID(RegAgt) EE
      certificate if provided as trigger parameter.

   The enhancements of the YANG module for the ietf-voucher-request with
   these new leafs are defined in Section 6.

   The object is signed using the pledges IDevID credential contained as
   x5c parameter of the JOSE header.

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   {
      "alg": "ES256",
      "x5c": ["MIIB2jCC...dA=="]
   }
   {
     "ietf-voucher-request:voucher": {
      "created-on": "2021-04-16T00:00:02.000Z",
      "nonce": "eDs++/FuDHGUnRxN3E14CQ==",
      "serial-number": "callee4711",
      "assertion": "agent-proximity",
      "agent-provided-proximity-registrar-cert": "base64encodedvalue==",
      "agent-signed-data": "base64encodedvalue==",
      "agent-sign-cert": "base64encodedvalue=="
     }
   }
   {
       SIGNATURE
   }

                Figure 7: Example of pledge-voucher-request

   The pledge-voucher-request Content-Type is defined in
   [I-D.richardson-anima-jose-voucher] as:

   application/voucher-jose+json

   The pledge SHOULD include an "Accept" header field indicating the
   acceptable media type for the voucher response.  The media type
   "application/voucher-jose+json" is defined in
   [I-D.richardson-anima-jose-voucher].

   Once the registrar-agent has received the pledge-voucher-request it
   can trigger the pledge to generate an enrollment-request object.  As
   in BRSKI the enrollment request object is a PKCS#10, additionally
   signed by the IDevID.  Note, as the initial enrollment aims to
   request a general certificate, no certificate attributes are provided
   to the pledge.

   Triggering the pledge to create the enrollment-request is done using
   HTTPS GET on the defined pledge endpoint "/.well-known/brski/pledge-
   enrollment-request".

   The registrar-agent pledge-enrollment-request Content-Type header is:

   application/json:

   with an empty body.

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   Upon receiving the enrollment-trigger, the pledge SHALL construct the
   pledge-enrollment-request as authenticated self-contained object.
   The CSR already assures proof of possession of the private key
   corresponding to the contained public key.  In addition, based on the
   additional signature using the IDevID, proof of identity is provided.
   Here, a JOSE object is being created in which the body utilizes the
   YANG module for the CSR as defined in [I-D.ietf-netconf-sztp-csr].

   [RFC Editor: please delete] /* Open Issues: Reuse of the sub-tree
   ietf-sztp-csr:csr may not be possible as it is not the complete
   module. */

   Depending on the capability of the pledge, it MAY construct the
   enrollment request as plain PKCS#10.  Note that the focus here is
   placed on PKCS#10 as PKCS#10 can be transmitted in different
   enrollment protocols like EST, CMP, CMS, and SCEP.  If the pledge is
   already implementing an enrollment protocol, it may leverage that
   functionality for the creation of the enrollment request object.
   Note also that [I-D.ietf-netconf-sztp-csr] also allows for inclusion
   of certificate request objects from CMP or CMC.

   The pledge SHOULD construct the pledge-enrollment-request as PKCS#10
   object and sign it additionally with its IDevID credential.  The
   pledge-enrollment-request should be encoded as JOSE object.

   [RFC Editor: please delete] /* Open Issues: Depending on target
   environment, it may be useful to assume that the pledge may already
   "know" its functional scope and therefore the number of certificates
   needed during operation.  As a result, multiple CSRs may be generated
   to provide achieve multiple certificates as a result of the
   enrollment.  This would need further description and potential
   enhancements also in the enrollment-request object to transport
   different CSRs. */

   [I-D.ietf-netconf-sztp-csr] considers PKCS#10 but also CMP and CMC as
   certificate request format.  Note that the wrapping signature is only
   necessary for plain PKCS#10 as other request formats like CMP and CMS
   support the signature wrapping as part of their own certificate
   request format.

   The registrar-agent enrollment-request Content-Type header for a
   wrapped PKCS#10 is:

   application/jose:

   The header of the pledge enrollment-request SHALL contain the
   following parameter as defined in [RFC7515]:

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   o  alg: algorithm used for creating the object signature.

   o  x5c: contains the base64-encoded pledge IDevID certificate.

   The body of the pledge enrollment-request object SHOULD contain a P10
   parameter (for PKCS#10) as defined for ietf-sztp-csr:csr in
   [I-D.ietf-netconf-sztp-csr]:

   o  P10: contains the base64-encoded PKCS#10 of the pledge.

   The JOSE object is signed using the pledge's IDevID credential, which
   corresponds to the certificate signaled in the JOSE header.

   {
       "alg": "ES256",
       "x5c": ["MIIB2jCC...dA=="]
   }
   {
     "ietf-sztp-csr:csr": {
       "p10": "base64encodedvalue=="
     }
   }
   {
       SIGNATURE
   }

              Figure 8: Example of pledge-enrollment-request

   With the collected pledge-voucher-request object and the pledge-
   enrollment-request object, the registrar-agent starts the interaction
   with the domain registrar.

   [RFC Editor: please delete] /*

   Open Issues: further description necessary at least for */

   o  Values to be taken from the IDevID into the PKCS#10 (like product-
      serial-number or subjectName, or certificate template)

   Once the registrar-agent has collected the pledge-voucher-request and
   pledge-enrollment-request objects, it connects to the registrar and
   sends the request objects.  As the registrar-agent is intended to
   work between the pledge and the domain registrar, a collection of
   requests from more than one pledges is possible, allowing a bulk
   bootstrapping of multiple pledges using the same connection between
   the registrar-agent and the domain registrar.

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5.2.3.2.  Request handling (registrar-agent - infrastructure)

   The bootstrapping exchange between the registrar-agent and the domain
   registrar resembles the exchanges between the pledge and the domain
   registrar from BRSKI in the pledge-initiator-mode with some
   deviations.

   Preconditions:

   o  Registrar-agent: possesses IDevID CA certificate and own
      LDevID(RegAgt) EE credential of registrar domain.  It knows the
      address of the domain registrar through configuration or discovery
      by, e.g., mDNS/DNSSD.  The registrar-agent has acquired pledge-
      voucher-request and pledge-enrollment-request objects(s).

   o  Registrar: possesses IDevID CA certificate of pledge vendors /
      manufacturers and an own LDevID(Reg) EE credential.

   o  MASA: possesses own credentials (voucher signing key, TLS server
      certificate) as well as IDevID CA certificate of pledge vendor /
      manufacturer and site-specific LDevID CA certificate.

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   +-----------+    +-----------+   +--------+   +---------+
   | Registrar |    | Domain    |   | Domain |   | Vendor  |
   | Agent     |    | Registrar |   | CA     |   | Service |
   | (RegAgt)  |    |  (JRC)    |   |        |   | (MASA)  |
   +-----------+    +-----------+   +--------+   +---------+
       |                  |              |   Internet |
   [exchange between pledge and ]
   [registrar-agent done. ]
       |                  |              |            |
       |<------ TLS ----->|              |            |
       |                  |              |            |
       |-- Voucher-Req -->|              |            |
       |          [accept device?]       |            |
       |          [contact vendor]       |            |
       |                  |------------ TLS --------->|
       |                  |-- Voucher-Req ----------->|
       |                  |                   [extract DomainID]
       |                  |                   [update audit log]
       |<---- Voucher ----|<-------- Voucher ---------|
       |                  |              |            |
   [certification request handling registrar-agent]
   [and site infrastructure]
       |--- Enroll-Req -->|              |            |
       |                  |---- TLS ---->|            |
       |                  |- Enroll-Req->|            |
       |                  |<-Enroll-Resp-|            |
       |<-- Enroll-Resp---|              |            |
       |                  |              |            |

         Figure 9: Request processing between registrar-agent and
                   infrastructure bootstrapping services

   The registrar-agent establishes a TLS connection with the registrar.
   As already stated in [RFC8995], the use of TLS 1.3 (or newer) is
   encouraged.  TLS 1.2 or newer is REQUIRED on the registrar-agent
   side.  TLS 1.3 (or newer) SHOULD be available on the registrar, but
   TLS 1.2 MAY be used.  TLS 1.3 (or newer) SHOULD be available on the
   MASA, but TLS 1.2 MAY be used.

   In contrast to [RFC8995] client authentication is achieved by using
   the LDevID(RegAgt) of the registrar-agent instead of the IDevID of
   the pledge.  This allows the registrar to distinguish between pledge-
   initiator-mode and pledge-responder-mode.  In pledge-responder-mode
   the registrar has no direct connection to the pledge but via the
   registrar-agent.  The registrar can receive request objects in
   different forms as defined in [RFC8995].  Specifically, the registrar
   will receive JOSE objects from the pledge for voucher-request and

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   enrollment-request (instead of the objects for voucher-request (CMS-
   signed JSON) and enrollment-request (PKCS#10).

   The registrar-agent sends the pledge-voucher-request to the registrar
   with an HTTPS POST to the endpoint "/.well-known/brski/
   requestvoucher".

   The pledge-voucher-request Content-Type used in the pledge-responder-
   mode is defined in [I-D.richardson-anima-jose-voucher] as:

   application/voucher-jose+json (see Figure 7 for the content
   definition).

   The registrar-agent SHOULD include the "Accept" header field received
   during the communication with the pledge, indicating the pledge
   acceptable Content-Type for the voucher-response.  The voucher-
   response Content-Type "application/voucher-jose+json" is defined in
   [I-D.richardson-anima-jose-voucher].

   Upon reception of the pledge-voucher-request, the registrar SHALL
   perform the verification of the voucher-request parameter as defined
   in section 5.3 of [RFC8995].  In addition, the registrar shall verify
   the following parameters from the pledge-voucher-request:

   o  agent-provided-proximity-registrar-cert: MUST contain the own
      LDevID(Reg) EE certificate to ensure the registrar in proximity is
      the target registrar for the request.

   o  agent-signed-data: The registrar MUST verify that the data has
      been signed with the LDevID(RegAgt) credential indicated in the
      "kid" JOSE header parameter.  If the certificate is not contained
      in the agent-sign-cert component of the pledge-voucher-request, it
      must fetch the certificate from a repository.

   o  agent-sign-cert: May contain the base64-encoded LDevID(RegAgt)
      certificate.  If contained the registrar MUST verify that the
      connected credential used to sign the data was valid at signature
      creation time and that the corresponding registrar-agent was
      authorized to be involved in the bootstrapping.

   If validation fails the registrar SHOULD respond with the HTTP 404
   error code to the registrar-agent.  If the pledge-voucher-request is
   in an unknown format, then an HTTP 406 error code is more
   appropriate.

   If validation succeeds, the registrar will accept the pledge request
   to join the domain as defined in section 5.3 of [RFC8995].  The
   registrar then establishes a TLS connection with the MASA as

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   described in section 5.4 of [RFC8995] to obtain a voucher for the
   pledge.

   The registrar SHALL construct the body of the registrar-voucher-
   request object as defined in [RFC8995].  The encoding SHALL be done
   as JOSE object as defined in [I-D.richardson-anima-jose-voucher].

   The header of the registrar-voucher-request SHALL contain the
   following parameter as defined in [RFC7515]:

   o  alg: algorithm used for creating the object signature.

   o  x5c: contains the base64-encoded registrar LDevID certificate.

   The body of the registrar-voucher-request object MUST contain the
   following parameter as part of the ietf-voucher-request:voucher as
   defined in [RFC8995]:

   o  created-on: contains the current date and time in yang:date-and-
      time format for the registrar-voucher-request creation time.

   o  nonce: copied form the pledge-voucher-request

   o  serial-number: contains the base64-encoded product-serial-number.
      The registrar MUST verify that the product-serial-number contained
      in the IDevID certificate of the pledge matches the serial-number
      field in the pledge-voucher-request.  In addition, it MUST be
      equal to the serial-number field contained in the agent-signed
      data of pledge-voucher-request.

   o  assertion: contains the voucher assertion requested the pledge
      (agent-proximity).  The registrar provides this information to
      assure successful verification of agent proximity based on the
      agent-signed-data.

   The ietf-voucher-request:voucher can be optionally enhanced with the
   following additional parameter:

   o  agent-sign-cert: Contain the base64-encoded LDevID(RegAgt) EE
      certificate if MASA verification of agent-proximity is required to
      provide the assertion "agent-proximity".

   The object is signed using the registrar LDevID(Reg) credential,
   which corresponds to the certificate signaled in the JOSE header.

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   {
      "alg": "ES256",
      "x5c": ["MIIB2jCC...dA=="]
   }
   {
     "ietf-voucher-request:voucher": {
      "created-on": "2021-04-16T02:37:39.235Z",
      "nonce": "eDs++/FuDHGUnRxN3E14CQ==",
      "serial-number": "callee4711",
      "assertion": "agent-proximity",
      "prior-signed-voucher-request": "base64encodedvalue==",
      "agent-sign-cert": "base64encodedvalue=="
     }
   }
   {
       SIGNATURE
   }

              Figure 10: Example of registrar-voucher-request

   The registrar sends the registrar-voucher-request to the MASA with an
   HTTPS POST at the endpoint "/.well-known/brski/requestvoucher".

   The registrar-voucher-request Content-Type is defined in
   [I-D.richardson-anima-jose-voucher] as:

   application/voucher-jose+json

   The registrar SHOULD include an "Accept" header field indicating the
   acceptable media type for the voucher-response.  The media type
   "application/voucher-jose+json" is defined in
   [I-D.richardson-anima-jose-voucher].

   Once the MASA receives the registrar-voucher-request it SHALL perform
   the verification of the contained components as described in section
   5.5 in [RFC8995].  In addition, the following additional processing
   SHALL be done for components contained in the prior-signed-voucher-
   request:

   o  agent-provided-proximity-registrar-cert: The MASA MAY verify that
      this field contains the LDevID(Reg) certificate.  If so, it MUST
      be consistent with the certificate used to sign the registrar-
      voucher-request.

   o  agent-signed-data: The MASA MAY verify this field to be able to
      provide an assertion "agent-proximity".  If so, the agent-signed-
      data MUST contain the product-serial-number of the pledge
      contained in the serial-number component of the prior-signed-

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      voucher and also in serial-number component of the registrar-
      voucher-request.  The LDevID(RegAgt) used to generate provide the
      signature is identified by the "kid" parameter of the JOSE header
      (agent-signed-data).  If the assertion "agent-proximity" is
      requested, the registrar-voucher-request MUST contain the
      corresponding LDevID(RegAgt) EE certificate in the agent-sign-
      cert, which can be verified by the MASA as issued by the same
      domain CA as the LDevID(Reg) EE certificate.  If the agent-sign-
      cert is not provided, the MASA MAY provide a lower level assertion
      "logged" or "verified"

   If validation fails, the MASA SHOULD respond with an HTTP error code
   to the registrar.  The error codes are kept as defined in section 5.6
   of [RFC8995].  and comprise the response codes 403, 404, 406, and
   415.

   The voucher response format is as indicated in the submitted Accept
   header fields or based on the MASA's prior understanding of proper
   format for this pledge.  Specifically for the pledge-responder-mode
   the "application/voucher-jose+json" as defined in
   [I-D.richardson-anima-jose-voucher] is applied.  The syntactic
   details of vouchers are described in detail in [RFC8366].  Figure 11
   shows an example of the contents of a voucher.

   {
       "alg": "ES256",
       "x5c": ["MIIBkzCCAT...dA=="]
   }
   {
     "ietf-voucher:voucher": {
       "assertion": "agent-proximity",
       "serial-number": "callee4711",
       "nonce": "eDs++/FuDHGUnRxN3E14CQ==",
       "created-on": "2021-04-17T00:00:02.000Z",
       "pinned-domain-cert": "MIIBpDCCA...w=="
     }
   }
   {
       SIGNATURE
   }

                 Figure 11: Example of MASA issued voucher

   The MASA sends the voucher in the indicated form to the registrar.
   After receiving the voucher the registrar may evaluate the voucher
   for transparency and logging purposes as outlined in section 5.6 of

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   [RFC8995].  The registrar forwards the voucher without changes to the
   registrar-agent.

   After receiving the voucher, the registrar-agent sends the pledge's
   enrollment-request to the registrar.  Deviating from BRSKI the
   enrollment-request is not a raw PKCS#10 request.  As the registrar-
   agent is involved in the exchange, the PKCS#10 is contained in the
   JOSE object.  The signature is created using the pledge's IDevID to
   provide proof-of-identity as outlined in Figure 8.

   When using EST, the registrar-agent sends the enrollment request to
   the registrar with an HTTPS POST at the endpoint "/.well-known/est/
   simpleenroll".

   The enrollment-request Content-Type is:

   application/jose

   If validation of the wrapping signature fails, the registrar SHOULD
   respond with the HTTP 404 error code.  If the voucher-request is in
   an unknown format, then an HTTP 406 error code is more appropriate.
   A situation that could be resolved with administrative action (such
   as adding a vendor/manufacturer IDevID CA as trusted party) MAY be
   responded with an 403 HTTP error code.

   This results in a deviation from the content types used in [RFC7030]
   and results in additional processing at the domain registrar as EST
   server as following.  Note that the registrar is already aware that
   the bootstrapping is performed in a pledge-responder-mode due to the
   use of the LDevID(RegAgt) certificate in the TLS establishment and
   the provided pledge-voucher-request in JOSE object.

   o  If registrar receives the enrollment-request with the Content Type
      application/jose, it MUST verify the signature using the
      certificate indicated in the JOSE header.

   o  The domain registrar verifies that the serial-number contained in
      the pledge's IDevID certificate contained in the JOSE header as
      being accepted to join the domain, based on the verification of
      the pledge-voucher-request.

   o  If both succeed, the registrar utilizes the PKCS#10 request
      contained in the JOSE body as "P10" parameter of "ietf-sztp-
      csr:csr" for further processing of the enrollment request with the
      domain CA.

   [RFC Editor: please delete] /*

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   Open Issues:

   o  The domain registrar may either enhance the PKCS#10 request or
      generate a structure containing the attributes to be included by
      the CA and sends both (the original PKCS#10 request and the
      enhancements) to the domain CA.  As enhancing the PKCS#10 request
      destroys the initial proof of possession of the corresponding
      private key, the CA would need to accept RA-verified requests.

   A successful interaction with the domain CA will result in the pledge
   LDevID EE certificate, which is then forwarded by the registrar to
   the registrar-agent using the content type "application/pkcs7-mime".

   The registrar-agent has now finished the exchanges with the domain
   registrar.  Now the registrar-agent can supply the voucher-response
   (from MASA via Registrar) and the enrollment-response (LDevID EE
   certificate) to the pledge.  It can close the TLS connection to the
   domain registrar and provide the objects to the pledge(s).  The
   content of the response objects is defined through the voucher
   [RFC8366] and the certificate [RFC5280].

5.2.3.3.  Response object supply (registrar-agent - pledge)

   The following description assumes that the registrar-agent has
   obtained the response objects from the domain registrar.  It will re-
   start the interaction with the pledge.  To contact the pledge, it may
   either discover the pledge as described in Section 5.2.2.2 or use
   stored information from the first contact with the pledge.

   Preconditions in addition to Section 5.2.3.2:

   o  Registrar-agent: possesses voucher and LDevID certificate.

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   +--------+                        +-----------+
   | Pledge |                        | Registrar |
   |        |                        | Agent     |
   |        |                        | (RegAgt)  |
   +--------+                        +-----------+
       |                                   |
       |<------- supply voucher -----------|
       |                                   |
       |--------- voucher-status --------->| - store
       |                                   |   pledge voucher-status
       |<--- supply enrollment response ---|
       |                                   |
       |--------- enroll-status ---------->| - store
       |                                   |   pledge enroll-status

   Figure 12: Response and status handling between pledge and registrar-
                                   agent

   The registrar-agent provides the information via two distinct
   endpoints to the pledge as following.

   The voucher response is provided with a HTTP POST using the operation
   path value of "/.well-known/brski/pledge-voucher".

   The registrar-agent voucher-response Content-Type header is
   "application/voucher-jose+json and contains the voucher as provided
   by the MASA.  An example if given in Figure 11.

   The pledge verifies the voucher as described in section 5.6.1 in
   [RFC8995].

   After successful verification the pledge MUST reply with a status
   telemetry message as defined in section 5.7 of [RFC8995].  As for the
   other objects, the defined object is provided with an additional
   signature using JOSE.  The pledge generates the voucher-status-object
   and provides it in the response message to the registrar-agent.

   The response has the Content-Type "application/jose", signed using
   the IDevID of the pledge as shown in Figure 13.  As the reason field
   is optional (see [RFC8995]), it MAY be omitted in case of success.

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   {
       "alg": "ES256",
       "x5c": ["MIIB2jCC...dA=="]
   {
       "version": 1,
       "status":true,
       "reason":"Informative human readable message",
       "reason-context": { "additional" : "JSON" }
   }
   {
       SIGNATURE
   }

           Figure 13: Example of pledge voucher-status telemetry

   The enrollment response is provided with a HTTP POST using the
   operation path value of "/.well-known/brski/pledge-enrollment".

   The registrar-agent enroll-response Content-Type header when using
   EST [RFC7030] as enrollment protocol, from the registrar-agent to the
   infrastructure is:

   application/pkcs7-mime: note that it only contains the LDevID
   certificate for the pledge, not the certificate chain.

   [RFC Editor: please delete] /*

   Open Issue: the enrollment response object may also be an
   application/jose object with a signature of the domain registrar.
   This may be used either to transport additional data which is bound
   to the LDevID or it may be considered for enrollment status to ensure
   that in an error case the registrar providing the certificate can be
   identified. */

   After successful verification the pledge MUST reply with a status
   telemetry message as defined in section 5.9.4 of [RFC8995].  As for
   the other objects, the defined object is provided with an additional
   signature using the JOSE.  The pledge generates the enrollment status
   and provides it in the response message to the registrar-agent.

   The response has the Content-Type "application/jose", signed using
   the LDevID of the pledge as shown in Figure 14.  As the reason field
   is optional, it MAY be omitted in case of success.

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   {
     "alg": "ES256",
     "x5c": ["MIIB56uz...dA=="]
   {
     "version": 1,
     "status":true,
     "reason":"Informative human readable message",
     "reason-context": { "additional" : "JSON" }
   }
   {
     SIGNATURE
   }

           Figure 14: Example of pledge enroll-status telemetry

   Once the registrar-agent has collected the information, it can
   connect to the registrar agent to provide the status responses to the
   registrar.

5.2.3.4.  Telemetry status handling (registrar-agent - domain registrar)

   The following description assumes that the registrar-agent has
   collected the status objects from the pledge.  It will provide the
   status objects to the registrar for further processing and audit log
   information of voucher-status for MASA.

   Preconditions in addition to Section 5.2.3.2:

   o  Registrar-agent: possesses voucher-status and enroll-status
      objects from pledge.

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   +-----------+    +-----------+   +--------+   +---------+
   | Registrar |    | Domain    |   | Domain |   | Vendor  |
   | Agent     |    | Registrar |   | CA     |   | Service |
   | RegAgt)   |    |  (JRC)    |   |        |   | (MASA)  |
   +-----------+    +-----------+   +--------+   +---------+
       |                  |              |   Internet |
       |                  |              |            |
       |<------ TLS ----->|              |            |
       |                  |              |            |
       |--Voucher-Status->|              |            |
       |                  |<---- device audit log ----|
       |           [verify audit log ]
       |                  |              |            |
       |--Enroll-Status-->|              |            |
       |                  |              |            |
       |                  |              |            |

                 Figure 15: Bootstrapping status handling

   The registrar-agent MUST provide the collected pledge voucher-status
   to the registrar.  This status indicates the pledge could process the
   voucher successfully or not.

   If the TLS connection to the registrar was closed, the registrar-
   agent establishes a TLS connection with the registrar as stated in
   Section 5.2.3.2.

   The registrar-agent sends the pledge voucher-status object without
   modification to the registrar with an HTTPS POST using the operation
   path value of "/.well-known/brski/voucher_status".  The Content-Type
   header is kept as "application/jose" as described in Figure 12 and
   depicted in the example in Figure 13.

   The registrar SHALL verify the signature of the pledge voucher-status
   and validate that it belongs to an accepted device in his domain
   based on the contained "serial-number" in the IDevID certificate
   referenced in the header of the voucher-status object.

   According to [RFC8995] section 5.7, the registrar SHOULD respond with
   an HTTP 200 but MAY simply fail with an HTTP 404 error.  The
   registrar-agent may use the response to signal success / failure to
   the service technician operating the registrar agent.  Within the
   server logs the server SHOULD capture this telemetry information.

   The registrar SHOULD proceed with the collecting and logging the
   status information by requesting the MASA audit-log from the MASA
   service as described in section 5.8 of [RFC8995].

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   The registrar-agent MUST provide the enroll-status object to the
   registrar.  The status indicates the pledge could process the enroll-
   response object and holds the corresponding private key.

   The registrar-agent sends the pledge enroll-status object without
   modification to the registrar with an HTTPS POST using the operation
   path value of "/.well-known/brski/enrollstatus".  The Content-Type
   header is kept as "application/jose" as described in Figure 12 and
   depicted in the example in Figure 14.

   The registrar SHALL verify the signature of the pledge enroll-status
   object and validate that it belongs to an accepted device in his
   domain based on the contained product-serial-number in the LDevID EE
   certificate referenced in the header of the enroll-status object.
   Note that the verification of a signature of the object is a
   deviation form the described handling in section 5.9.4 of [RFC8995].

   According to [RFC8995] section 5.9.4, the registrar SHOULD respond
   with an HTTP 200 but MAY simply fail with an HTTP 404 error.  The
   registrar-agent may use the response to signal success / failure to
   the service technician operating the registrar agent.  Within the
   server log the registrar SHOULD capture this telemetry information.

5.3.  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 / registrar-agent 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].

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     </brski/voucherrequest>,ct=voucher-cms+json
     </brski/voucher_status>,ct=json
     </brski/enrollstatus>,ct=json
     </est/cacerts>;ct=pkcs7-mime
     </est/simpleenroll>;ct=pkcs7-mime
     </est/simplereenroll>;ct=pkcs7-mime
     </est/fullcmc>;ct=pkcs7-mime
     </est/serverkeygen>;ct= pkcs7-mime
     </est/csrattrs>;ct=pkcs7-mime
     </cmp/initialization>;ct=pkixcmp
     </cmp/certification>;ct=pkixcmp
     </cmp/keyupdate>;ct=pkixcmp
     </cmp/p10>;ct=pkixcmp
     </cmp/getCAcert>;ct=pkixcmp
     </cmp/getCSRparam>;ct=pkixcmp

   [RFC Editor: please delete] /*

   Open Issues:

   o  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.  YANG Extensions to Voucher Request

   The following modules extends the [RFC8995] Voucher Request to
   include a signed artifact from the registrar-agent as well as the
   registrar-proximity-certificate and the agent-signing certificate.

   module ietf-async-voucher-request {
     yang-version 1.1;

     namespace
       "urn:ietf:params:xml:ns:yang:ietf-async-voucher-request";
     prefix "constrained";

     import ietf-restconf {
       prefix rc;
       description
         "This import statement is only present to access
          the yang-data extension defined in RFC 8040.";
       reference "RFC 8040: RESTCONF Protocol";
     }

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     import ietf-voucher-request {
       prefix ivr;
       description
         "This module defines the format for a voucher request,
             which is produced by a pledge as part of the RFC8995
             onboarding process.";
       reference
         "RFC 8995: Bootstrapping Remote Secure Key Infrastructure";
     }

     organization
      "IETF ANIMA Working Group";

     contact
      "WG Web:   <http://tools.ietf.org/wg/anima/>
       WG List:  <mailto:anima@ietf.org>
       Author:   Steffen Fries
                 <mailto:steffen.fries@siemens.com>
       Author:   Hendrik Brockhaus
                 <mailto: hendrik.brockhaus@siemens.com>
       Author:   Eliot Lear
                 <mailto: lear@cisco.com>";
       Author:   Thomas Werner
                 <mailto: thomas-werner@siemens.com>";
     description
      "This module defines an extension of the RFC8995 voucher
       request to permit a registrar-agent to convey the adjacency
       relationship from the registrar-agent to the registrar.

       The key words 'MUST', 'MUST NOT', 'REQUIRED', 'SHALL',
       'SHALL NOT', 'SHOULD', 'SHOULD NOT', 'RECOMMENDED', 'MAY',
       and 'OPTIONAL' in the module text are to be interpreted as
       described in RFC 2119.";
     revision "YYYY-MM-DD" {
       description
        "Initial version";
       reference
        "RFC XXXX: Voucher Request for Asynchronous Enrollment";
     }
     rc:yang-data voucher-request-async-artifact {
       // YANG data template for a voucher.
       uses voucher-request-async-grouping;
     }
     // Grouping defined for future usage
     grouping voucher-request-async-grouping {
       description
         "Grouping to allow reuse/extensions in future work.";
       uses ivr:voucher-request-grouping {

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         augment "voucher-request" {
           description "Base the constrained voucher-request upon the
             regular one";
           leaf agent-signed-data {
             type binary;
             description
               "The agent-signed-data field contains a JOSE [RFC7515]
                object provided by the Registrar-Agent to the Pledge.

                This artifact is signed by the Registrar-Agent
                and contains a copy of the pledge's serial-number.";
           }

           leaf agent-provided-proximity-registrar-cert {
             type binary;
             description
               "An X.509 v3 certificate structure, as specified by
                RFC 5280, Section 4, encoded using the ASN.1
                distinguished encoding rules (DER), as specified
                in ITU X.690.
                The first certificate in the registrar TLS server
                certificate_list sequence (the end-entity TLS
                certificate; see RFC 8446) presented by the
                registrar to the registrar-agent and provided to
                the pledge.
                This MUST be populated in a pledge's voucher-request
                when an agent-proximity assertion is requested.";
             reference
               "ITU X.690: Information Technology - ASN.1 encoding
                rules: Specification of Basic Encoding Rules (BER),
                Canonical Encoding Rules (CER) and Distinguished
                Encoding Rules (DER)
                RFC 5280: Internet X.509 Public Key Infrastructure
                Certificate and Certificate Revocation List (CRL)
                Profile
                RFC 8446: The Transport Layer Security (TLS)
                Protocol Version 1.3";
           }

           leaf agent-sign-cert {
             type binary;
             description
               "An X.509 v3 certificate structure, as specified by
                RFC 5280, Section 4, encoded using the ASN.1
                distinguished encoding rules (DER), as specified
                in ITU X.690.
                This certificate can be used by the pledge,
                the registrar, and the MASA to verify the signature

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                of agent-signed-data. It is an optional component
                for the pledge-voucher request.
                This MUST be populated in a registrar's
                voucher-request when an agent-proximity assertion
                is requested.";
             reference
               "ITU X.690: Information Technology - ASN.1 encoding
                rules: Specification of Basic Encoding Rules (BER),
                Canonical Encoding Rules (CER) and Distinguished
                Encoding Rules (DER)
                RFC 5280: Internet X.509 Public Key Infrastructure
                Certificate and Certificate Revocation List (CRL)
                Profile";
           }
         }
       }
     }
   }

7.  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.

7.1.  EST Handling

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

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

   o  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
      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.

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

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   o  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.

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

7.2.  CMP Handling

   Instead of using 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:

   o  For proof of possession, the defined approach in Lightweight CMP
      Profile section 4.1.1 (based on CRMF) and 4.1.5 (based on PCKS#10)
      should be supported.

   o  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].

   o  When the RA/CA is not available, a waiting indication should be
      returned in the PKIStatus by the Registrar.  The pledge in this
      case would retry using the PollReqContent with a request
      identifier certReqId provided in the initial CertRequest message
      as specified in section 5.2.4 of
      [I-D.ietf-lamps-lightweight-cmp-profile] with delayed enrollment.

8.  IANA Considerations

   This document requires the following IANA actions:

   IANA is requested to enhance the Registry entitled: "BRSKI well-
   known URIs" with the following:

    URI                       document  description
    pledge-voucher-request    [THISRFC] create pledge-voucher-request
    pledge-enrollment-request [THISRFC] create pledge-enrollment-request
    pledge-voucher            [THISRFC] supply voucher response
    pledge-enrollment         [THISRFC] supply enrollment response
    pledge-CACerts            [THISRFC] supply CA certs to pledge

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9.  Privacy Considerations

   The credential used by the registrar-agent to sign the data for the
   pledge in case of the pledge-initiator-mode should not contain
   personal information.  Therefore, it is recommended to use an LDevID
   certificate associated with the device instead of a potential service
   technician operating the device, to avoid revealing this information
   to the MASA.

10.  Security Considerations

10.1.  Exhaustion attack on pledge

   Exhaustion attack on pledge based on DoS attack (connection
   establishment, etc.)

10.2.  Misuse of acquired voucher and enrollment responses

   Registrar-agent that uses acquired voucher and enrollment response
   for domain 1 in domain 2: can be detected in Voucher Request
   processing on domain registrar side.  Requires domain registrar to
   verify the proximity-registrar-cert leaf in the pledge-voucher-
   request against his own as well as the association of the pledge to
   his domain based on the product-serial-number contained in the
   voucher.

   Misbinding of pledge by a faked domain registrar is countered as
   described in BRSKI security considerations (section 11.4).

   Misuse of registrar-agent LDevID may be addressed by utilizing short-
   lived certificates to be used for authenticating the registrar-agent
   against the registrar.  The LDevID certificate for the registrar-
   agent may be provided by a prior BRSKI execution based on an existing
   IDevID.  Alternatively, the LDevID may be acquired by a service
   technician after authentication against the issuing CA.

11.  Acknowledgments

   We would like to thank the various reviewers for their input, in
   particular Brian E.  Carpenter, Michael Richardson, Giorgio
   Romanenghi, Oskar Camenzind, for their input and discussion on use
   cases and call flows.

12.  References

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12.1.  Normative References

   [I-D.ietf-netconf-sztp-csr]
              Watsen, K., Housley, R., and S. Turner, "Conveying a
              Certificate Signing Request (CSR) in a Secure Zero Touch
              Provisioning (SZTP) Bootstrapping Request", draft-ietf-
              netconf-sztp-csr-03 (work in progress), June 2021.

   [I-D.richardson-anima-jose-voucher]
              Richardson, M. and T. Werner, "JOSE signed Voucher
              Artifacts for Bootstrapping Protocols", draft-richardson-
              anima-jose-voucher-01 (work in progress), June 2021.

   [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>.

   [RFC6762]  Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
              DOI 10.17487/RFC6762, February 2013,
              <https://www.rfc-editor.org/info/rfc6762>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [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>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/info/rfc7515>.

   [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>.

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12.2.  Informative References

   [I-D.ietf-lamps-cmp-updates]
              Brockhaus, H. and D. V. Oheimb, "Certificate Management
              Protocol (CMP) Updates", draft-ietf-lamps-cmp-updates-10
              (work in progress), May 2021.

   [I-D.ietf-lamps-lightweight-cmp-profile]
              Brockhaus, H., Fries, S., and D. V. Oheimb, "Lightweight
              Certificate Management Protocol (CMP) Profile", draft-
              ietf-lamps-lightweight-cmp-profile-05 (work in progress),
              February 2021.

   [I-D.selander-ace-coap-est-oscore]
              Selander, G., Raza, S., Furuhed, M., Vucinic, M., and T.
              Claeys, "Protecting EST Payloads with OSCORE", draft-
              selander-ace-coap-est-oscore-05 (work in progress), May
              2021.

   [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.

   [IEEE-802.1AR]
              Institute of Electrical and Electronics Engineers, "IEEE
              802.1AR Secure Device Identifier", IEEE 802.1AR , June
              2018.

   [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.

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   [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>.

   [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>.

   [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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

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

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

Appendix A.  History of changes [RFC Editor: please delete]

   From IETF draft 02 -> IETF draft 03:

   o  Housekeeping, deleted open issue regarding YANG voucher-request in
      Section 5.2.3.1 as voucher-request was enhanced with additional
      leaf.

   o  Included open issues in YANG model in Section 5.2 regarding
      assertion value agent-proximity and csr encapsulation using SZTP
      sub module).

   From IETF draft 01 -> IETF draft 02:

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   o  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 Section 5.2.3
      .

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

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

   o  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).

   o  Details on trust relationship between registrar-agent and
      registrar (issue #4, #5, #9) included in Section 5.2.

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

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

   o  Enhanced objects in exchanges between pledge and registrar-agent
      to allow the registrar to verify agent-proximity to the pledge
      (issue #1) in Section 5.2.3.

   o  Details on trust relationship between registrar-agent and pledge
      (issue #5) included in Section 5.2.

   o  Split of use case 2 call flow into sub sections in Section 5.2.3.

   From IETF draft 00 -> IETF draft 01:

   o  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.

   o  Rework of use case 2 in Section 5.2 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.

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

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   o  Clarification in discovery options for enrollment endpoints at the
      domain registrar based on well-known endpoints in Section 5.3 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.

   o  Updated references.

   o  Included Thomas Werner as additional author for the document.

   From individual version 03 -> IETF draft 00:

   o  Inclusion of discovery options of enrollment endpoints at the
      domain registrar based on well-known endpoints in Section 5.3 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.

   o  Missing details provided for the description and call flow in
      pledge-agent use case Section 5.2, e.g. to accommodate
      distribution of CA certificates.

   o  Updated CMP example in Section 7 to use lightweight CMP instead of
      CMP, as the draft already provides the necessary /.well-known
      endpoints.

   o  Requirements discussion moved to separate section in Section 4.
      Shortened description of proof of identity binding and mapping to
      existing protocols.

   o  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.

   o  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:

   o  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.

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

   o  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.

   o  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:

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

   o  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.

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

   o  Enhanced consideration of existing enrollment protocols in the
      context of mapping the requirements to existing solutions in
      Section 4 and in Section 7.

   From individual version 00 -> 01:

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

   o  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.

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

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

   o  New section starting Section 7 with the mapping to existing
      enrollment protocols by collecting boundary conditions.

Authors' Addresses

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

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

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

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

   Eliot Lear
   Cisco Systems
   Richtistrasse 7
   Wallisellen  CH-8304
   Switzerland

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

   Thomas Werner
   Siemens AG
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: thomas-werner@siemens.com
   URI:   https://www.siemens.com/

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