Support of asynchronous Enrollment in BRSKI
draft-fries-anima-brski-async-enroll-01

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ANIMA WG                                                        S. Fries
Internet-Draft                                              H. Brockhaus
Intended status: Standards Track                                 Siemens
Expires: January 9, 2020                                         E. Lear
                                                           Cisco Systems
                                                            July 8, 2019

              Support of asynchronous Enrollment in BRSKI
                draft-fries-anima-brski-async-enroll-01

Abstract

   This document discusses an enhancement of automated bootstrapping of
   a remote secure key infrastructure (BRSKI) to operate in domains
   featuring no or only timely limited connectivity to backend services
   offering enrollment functionality, specifically a Public Key
   Infrastructure (PKI).  In the context of deploying new devices the
   design of BRSKI allows for online (synchronous object exchange) and
   offline interactions (asynchronous object exchange) with a
   manufacturer's authorization service.  For this it utilizes a self-
   contained voucher to transport the domain credentials as a signed
   object to establish an initial trust between the pledge and the
   deployment domain.  The currently supported enrollment protocol for
   request and distribution of deployment domain specific device
   certificates provides only limited support for asynchronous PKI
   interactions.  This memo motivates the enhancement of supporting
   self-contained objects for certificate management by using an
   abstract notation.  This allows off-site operation of PKI services
   outside the deployment domain of the pledge.  This addresses
   specifically scenarios, in which the final authorization of
   certification request of a pledge cannot be made in the deployment
   domain and is therefore delegated to a operator backend.  The goal is
   to enable the usage of existing and potentially new PKI protocols
   supporting self-containment for certificate management.

Status of This Memo

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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on January 9, 2020.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  History of changes  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Scope of solution . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Supported environment . . . . . . . . . . . . . . . . . .   6
     4.2.  Application Examples  . . . . . . . . . . . . . . . . . .   7
       4.2.1.  Rolling stock . . . . . . . . . . . . . . . . . . . .   7
       4.2.2.  Building automation . . . . . . . . . . . . . . . . .   7
       4.2.3.  Substation automation . . . . . . . . . . . . . . . .   8
       4.2.4.  Electric vehicle charging infrastructure  . . . . . .   8
       4.2.5.  Infrastructure isolation policy . . . . . . . . . . .   8
       4.2.6.  Less operational security in the deployment domain  .   9
     4.3.  Requirement discussion and mapping to solution elements .   9
   5.  Architectural Overview  . . . . . . . . . . . . . . . . . . .  11
     5.1.  Behavior of a pledge  . . . . . . . . . . . . . . . . . .  14
     5.2.  Secure Imprinting using Vouchers  . . . . . . . . . . . .  14
     5.3.  Addressing  . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Protocol Flows  . . . . . . . . . . . . . . . . . . . . . . .  15
     6.1.  Pledge - Registrar discovery and voucher exchange . . . .  15
     6.2.  Registrar - MASA voucher exchange . . . . . . . . . . . .  16
     6.3.  Pledge - Registrar - RA/CA certificate enrollment . . . .  17
   7.  Mapping to exisitng enrollment protocols  . . . . . . . . . .  19
     7.1.  EST Handling  . . . . . . . . . . . . . . . . . . . . . .  19
     7.2.  CMP Handling  . . . . . . . . . . . . . . . . . . . . . .  20
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   9.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  20

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   10. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     12.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   BRSKI as defined in [I-D.ietf-anima-bootstrapping-keyinfra] specifies
   a solution for secure zero-touch (automated) bootstrapping of devices
   (pledges) in a target deployment domain.  This includes the discovery
   of network elements in the deployment domain, time synchronization,
   and the exchange of security information necessary to establish trust
   between a pledge and the domain and to adopt a pledge as new network
   and application element.  Security information about the deployment
   domain, specifically the deployment domain certificate (domain root
   certificate), is exchanged utilizing vouchers as defined in
   [RFC8366].  These vouchers are self-contained objects, which may be
   provided online (synchronous) or offline (asynchronous) via the
   domain registrar to the pledge and originate from a manufacturer's
   authorization service (MASA).  The manufacturer signed voucher
   contains the target domain certificate and can be verified by the
   pledge due to the possession of a manufacturer root certificate.  It
   facilitates the enrollment of the pledge in the deployment domain and
   is used to establish trust.

   For the enrollment of devices BRSKI relies on EST [RFC7030] to
   request and distribute deployment 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 an LDevID certificate)
   and to provide data origin authentication to support the
   authorization decision for processing the certification request.  The
   TLS connection is mutually authenticated and the client side
   authentication bases on the pledge's manufacturer issued device
   certificate (IDevID certificate).  This approach requires an on-site
   availability of the RA as PKI component and/or a local asset or
   inventory management system performing the authorization decision
   based on the certification request to issue a domain specific
   certificate to the pledge.  This is due to the EST server terminating
   the security association with the pledge and thus the binding between
   the certification request and the authentication of the pledge.  This
   type of enrollment utilizing an online connection to the PKI is
   considered as synchronous enrollment.

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   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 operators
   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.  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
   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.  If a second communication channel is
   opened to forward the certification request to the issuing CA, the
   requester authentication information needs to be bound to the
   certification request.  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 deployment
      domain, which stores the certification request combined with the
      requester authentication information (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 possesion) to
      the off-site PKI for further processing.  It is assumed that the
      off-site PKI in this case relies on the local 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
      deployment domain.

   o  Utilization of self-contained objects 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 requestor
      information (request and authentication) can be detected during

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      the verification of the cryptographic binding of the self-
      contained object in the off-site PKI.

   This document targets environments, in which connectivity to the PKI
   functionality is only temporary or not directly available by
   specifying support for handling self-contained objects supporting
   asynchronous enrollment.  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 a X.509 root
   certificate sufficient for verifying the domain registrar / proxy
   identity as well as an domain specific X.509 device certificate
   (LDevID certificate).

   The goal is to enhance BRSKI to either allow other existing
   certificate management protocols supporting self-contained objects to
   be applied or to allow other types of encoding for the certificate
   management information exchange.

   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.  History of changes

   From version 00 -> 01:

   o  Update of examples, specifically for building automation as well
      as two new application use cases in Section 4.2.

   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.

   o  Consideration of existing enrollment protocols in the context of
      mapping the requirements to existing solutions in Section 4.3.

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

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3.  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
   [RFC2119].

   This document relies on the terminology defined in
   [I-D.ietf-anima-bootstrapping-keyinfra].  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 side, to which only a
      temporarily 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.

4.  Scope of solution

4.1.  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 deployment
      domain.  The connectivity to the backend RA may only be
      temporarily available.  A local store and forward device is used
      for the communication with the backend services.

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   o  authoritive 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 backend operator
      domain.

   o  the target deployment domain already uses a certificate management
      approach that shall be reused to be consistent throughout the
      lifecycle.

4.2.  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.

4.2.1.  Rolling stock

   Rolling stock or railroad cars contain a variety of sensors,
   actuators, and controller, 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
   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 operators asset/inventory information.

4.2.2.  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.

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4.2.3.  Substation automation

   In substation automation a control center typically hosts PKI
   services to issue certificates for Intelligent Electronic Devices
   (IED)s 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 [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
   [I-D.gutmann-scep] and EST [RFC7030] for the infrastructure side,
   while the IED must only support one of the two.

4.2.4.  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
   authenticates uses an X.509 certificate to authenticate 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.
   Self-containment will ease this by allowing the transport without a
   separate communication protocol.  For the purpose of certificate
   management CMP [RFC4210]is intended to be used.

4.2.5.  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.

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4.2.6.  Less operational security in the deployment 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 deployment domain does not offer
   enough security to operate a registration point and therefore wants
   to transfer this service to a backend.

4.3.  Requirement discussion and mapping to solution elements

   For the requirements discussion it is assumed that the entity
   receiving the self-contained object in the deployment domain is not
   the authorization point for the certification request contained in
   the object.  If the entity is the authorization point, BRSKI can be
   used directly.  Note that BRSKI-AE could address both cases.

   Based on the supported deployment environment described in
   Section 4.1 and the motivated application examples described in
   Section 4.2 the following base requirements are derived to support
   self-contained objects as container carrying the certification
   request and further information to support asynchronous operation.
   Moreover, potential solution examples (not complete) based on
   existing technology are provided with the focus on existing IETF
   standards track documents:

   o  Certification requests are structures protecting at least
      integrity of the contained data combined with a proof-of-private-
      key-possession for locally generated key pairs.  Examples for
      certification requests

      *  PKCS#10 [RFC2986]: Defines a structure for a certification
         request.  The structure must be signed to ensure integrity
         protection and proof-of-private-key-possession.  Hence, the
         signature is performed by using the private key of the
         requestor (corresponding to the contained public key).

      *  CRMF [RFC4211]: Defines a structure for the certification
         request.  The structure also typically contains an integrity
         protection and a proof of possession, in which a signature
         value is generated by using the corresponding private key to
         the contained public key.  This self-signature can also be
         replaced by the RA after verification, if the RA intends to
         update or alter the request message.

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      Note that the integrity is bound to the public key contained in
      the certification request.  In the considered application
      examples, this is not sufficient and needs to be bound to the data
      origin authentication (IDevID).  This binding also supports the
      authorization decision for the certification request.  The binding
      of data origin authentication to the certification request is
      delegated to the management protocol.

   o  The container carrying the certification request should support a
      binding to an existing credential known to the peer performing the
      authoriation of the certification request as proof of identity.
      The binding may be transport dependent if the endpoint at the next
      communication hop is authorizing the certification request.  This
      requirements 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) contains 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.

      *  SCEP [I-D.gutmann-scep]: 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
         ([RFC5652]).  Note that the wrapping using an existing IDevID
         credential is not specified directly in SCEP.

      *  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
         optionally supported.  The proof of identity of the PKIMessage
         containing the certification request can be achieved by using
         IDevID credentials to calculate a signature over the header and
         the body of the PKIMessage utilizing the protectionAlg signaled
         in the PKIMessage header and the PKIProtection carrying the
         actual signature value.

      *  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 would be used, which allows
         signing with an existing IDevID credential to provide a proof
         of identity.

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   o  The container carrying the certification request should support
      transport independent protection using an existing credential of
      the pledge verifiable at the authorization point of the
      certification request (typically the RA in conjunction with an
      inventory).  This requirements is addressed by existing enrollment
      protocols in different ways, for instance:

      *  EST [RFC7030]: Not supported.

      *  SCEP [I-D.gutmann-scep]: Not specified in SCEP, could be done
         using message wrapping with signature (based on CMS).

      *  CMP [RFC4210]: Message wrapping with signature.

      *  CMC [RFC5272]: Message wrapping with signature.

5.  Architectural Overview

   To support asynchronous enrollment, the base system architecture
   defined in BRSKI [I-D.ietf-anima-bootstrapping-keyinfra] is changed
   to allow for off-site operation of the PKI components.  The
   assumption for BRSKI-AE is that the authorization for a certification
   request is performed using an inventory or asset management system
   residing in the backend of the domain operator as described in
   Section 4.1.  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/ |  .
   |        <-------->............<-------> Proxy      |  .
   |        |     .  |        BRSKI-AE    |            |  .
   | IDevID |     .  |            |       +------^-----+  .
   |        |     .  +------------+              |        .
   |        |     .                              |        .
   +--------+     ...............................|.........
                   "on-site domain" components   |
                                                 |
                                                 |
    .............................................|.....................
    . +---------------------------+     +--------v------------------+ .
    . | Public Key Infrastructure |<----+ PKI RA                    | .
    . | PKI CA                    |---->+ [(Domain) Registrar (opt)]| .
    . +---------------------------+     +--------+--^---------------+ .
    .                                            |  |                 .
    .                                   +--------v--+---------------+ .
    .                                   | Inventory (Asset)         | .
    .                                   | Management                | .
    .                                   +---------------------------+ .
    ...................................................................
            "off-site domain" components

   Figure 1: Architecture overview of BRSKI-AE

   The architecture overview in Figure 1 utilizes the same logical
   elements as BRSKI but with a different placement in the architecture
   for some of the elements.  The main difference is the placement of
   the PKI RA/CA component, which is actually performing the
   authorization decision for the certification request message.  Also
   shown is the connectivity of the RA/CA with an inventory management
   system, which is expected to be utilized in the authorization

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   decision.  Note that this may also be an integrated functionality of
   the RA.  Both components are 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 domain of the pledge.
   This is to underline the authorization decision for the certification
   request in the backend rather than in the deployment domain itself.
   The following list describes the components in the deployment domain:

   o  Join Proxy: same functionality as described in BRSKI

   o  Domain Registrar / 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 operators backend (off-site), even if the deployment domain
      has only temporary or no connectivity to an operator domain.

      *  Voucher exchange: The voucher exchange with the MASA via the
         domain registrar is performed as described in BRSKI
         [I-D.ietf-anima-bootstrapping-keyinfra] .

      *  Certificate enrollment: For the pledge enrollment the domain
         registrar in the deployment domain support the authorization of
         the pledge to be part of the domain, but not necessarily to
         authorize the certification request provided during enrollment.
         This may be due to lack of authorization information in the
         deployment domain.  If the authorization is done in the
         operator domain, the domain registrar is used to forward the
         certification request to the RA.  Thus it basically works as a
         proxy.  In the case of no connectivity, the domain registrar
         stores the certification request and forwards it to the RA upon
         connectivity.  As this requires the certification request to be
         self-contained, the domain registrar needs functionality
         enhancements with respect to the support of alternative
         enrollment approaches supporting self-containment.  To support
         alternative enrollment approaches (protocols, encodings), it is
         necessary to enhance the addressing scheme at the domain
         registrar.  The communication channel between the pledge and
         the domain registrar may be similarly described as in BRSKI
         within the same "/.well-known" tree and may result in "/.well-
         known/enrollment-variant/request".

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

   o  MASA: general functionality as described in BRSKI.  Assumption
      that the interaction with the MASA may be synchronous (voucher

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      request with nonce) or asynchronous (voucher request without
      nonce).

   o  Ownership tracker: as defined in BRSKI.

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

   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 operator.

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

   o  Inventory (asset) management: contains information about the known
      devices belonging to the operator.  Specifically, the inventory is
      used to provide the information to authorize issuing a certificate
      based on the certification request of the pledge.  Note: the
      communication between the inventory (asset) management and the PKI
      components (RA/CA) are out of scope of this document.

   o  (Domain) registrar: Optional component if the deployment domain
      does not feature a domain registrar but only a proxy.  In this
      case it is involved in the certification request processing and is
      assumed to be co-located with the PKI RA.

5.1.  Behavior of a pledge

   The behavior of a pledge as described in
   [I-D.ietf-anima-bootstrapping-keyinfra] is kept with one exception.
   After finishing the imprinting phase (4) the enrollment phase (5) is
   performed with a method supporting self-contained objects.  Using
   simpleenroll with EST as taken in BRSKI cannot be applied here, as it
   binds the pledge authentication with the existing IDevID using the
   transport channel.  This authentication is not visible / verifiable
   at the authorization point in the off-site domain.  /* mapping to
   existing protocols based on the outcome of the discussion */

5.2.  Secure Imprinting using Vouchers

   The described approach in [I-D.ietf-anima-bootstrapping-keyinfra] is
   kept as is.

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5.3.  Addressing

   For the provisioning of different enrollment options at the domain
   registrar, the addressing approach of BRSKI using a "/.well-known"
   tree from [RFC5785] is enhanced.

   /* to be done: Description of "/.well-known/enrollment-protocol/
   request" in which enrollment-protocol may be an already existing
   protocol like "est" or "scep" or "cmp" or "cms" or a newly defined
   protocol.  */

6.  Protocol Flows

   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 accompany the application of self-
      contained objects for the enrollment).

6.1.  Pledge - Registrar discovery and voucher exchange

   The discovery phase is applied as specified in
   [I-D.ietf-anima-bootstrapping-keyinfra].  /* for discussion: is a
   reference to BRSKI sufficient here or is it helpful to provide
   additional information and the figure? */

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   +--------+         +---------+    +------------+     +------------+
   | Pledge |         | Circuit |    | Domain     |     | Vendor     |
   |        |         | Join    |    | Registrar  |     | Service    |
   |        |         | Proxy   |    |  (JRC)     |     | (MASA)     |
   +--------+         +---------+    +------------+     +------------+
     |                     |                   |           Internet |
     |<-RFC4862 IPv6 addr  |                   |                    |
     |<-RFC3927 IPv4 addr  | Appendix A        |  Legend            |
     |-------------------->|                   |  C - circuit       |
     | optional: mDNS query| Appendix B        |      join proxy    |
     | RFC6763/RFC6762     |                   |  P - provisional   |
     |<--------------------|                   |    TLS connection  |
     | GRASP M_FLOOD       |                   |                    |
     |   periodic broadcast|                   |                    |
     |<------------------->C<----------------->|                    |
     |              TLS via the Join Proxy     |                    |
     |<--Registrar TLS server authentication---|                    |
   [PROVISIONAL accept of server cert]         |                    |
     P---X.509 client authentication---------->|                    |
     P                     |                   |                    |
     P--Voucher Request (w/nonce for voucher)->|                    |
     P                     |       /--->       |                    |
     P                     |       |      see Figure 3 below        |
     P                     |       \---->      |                    |
     P<------voucher---------------------------|                    |
   [verify voucher, imprint]                   |                    |
     |---------------------------------------->|                    |
     |      [voucher status telemetry]         |<-device audit log--|
     |                     |       [verify audit log and voucher]   |
     |<--------------------------------------->|                    |

   Figure 2: Pledge discovery of domain registrar discovery and voucher
   exchange

6.2.  Registrar - MASA voucher exchange

   The voucher exchange is performed as specified in
   [I-D.ietf-anima-bootstrapping-keyinfra].  /* for discussion: is a
   reference to BRSKI sufficient here or is it helpful to provide
   additional information and the figure? */

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   +--------+         +---------+    +------------+     +------------+
   | Pledge |         | Circuit |    | Domain     |     | Vendor     |
   |        |         | Join    |    | Registrar  |     | Service    |
   |        |         | Proxy   |    |  (JRC)     |     | (MASA)     |
   +--------+         +---------+    +------------+     +------------+
     P                     |       /--->       |                    |
     P                     |       |      [accept device in domain] |
     P                     |       |      [contact Vendor]          |
     P                     |       |           |--Pledge ID-------->|
     P                     |       |           |--Domain ID-------->|
     P                     |       |           |--optional:nonce--->|
     P                     |       |           |     [extract DomainID]
     P                     |    optional:      |     [update audit log]
     P                     |      can occur in advance if nonceless |

   Figure 3: Domain registrar - MASA voucher exchange

6.3.  Pledge - Registrar - RA/CA certificate enrollment

   The enrollment for BRSKI-AE will be performed using a self-contained
   object.  According to the abstract requirements from
   [I-D.ietf-anima-bootstrapping-keyinfra].  This object shall at least
   contain the following information:

   o  Proof of Possession: utilizing the private key corresponding to
      the public key contained in the certification request.

   o  Proof of Identity: utilizing the existing IDevID credential to
      generate a signature of the certification request.

   o  /* further parameter to be specified */.

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   +--------+         +---------+    +------------+     +------------+
   | Pledge |         | Circuit |    | Domain     |     | Operator   |
   |        |         | Join    |    | Registrar  |     | RA/CA      |
   |        |         | Proxy   |    |  (JRC)     |     | (OPKI)     |
   +--------+         +---------+    +------------+     +------------+
     /-->                                      |                    |
     |---------- Request CA Certs ------------>|                    |
     |              [if connection to operator domain is available] |
     |                                         |-Request CA Certs ->|
     |                                         |<- CA Certs Response|
     |<-------- CA Certs Response--------------|                    |
     |---------- Attribute Request ----------->|                    |
     |              [if connection to operator domain is available] |
     |                                         |Attribute Request ->|
     |                                         |<-Attribute Response|
     |<--------- Attribute Response -----------|                    |
     /-->                                      |                    |
     |-------------- Cert Request ------------>|                    |
     |              [if connection to operator domain is available] |
     |                                         |--- Cert Request -->|
     |                                         |<-- Cert Response --|
     /-->                                      |                    |
     |          [if connection to operator domain is not available] |
     |                                         |                    |
     |<---------- Cert Waiting ----------------|                    |
     |-- Cert Polling (with orig request ID) ->|                    |
     |              [if connection to operator domain is available] |
     |                                         |--- Cert Request -->|
     |                                         |<-- Cert Response --|
     /-->                                      |                    |
     |<------------- Cert Response ------------|                    |
     |-------------- Cert Confirm ------------>|                    |
     |                                         /-->                 |
     |                                         |[optional]          |
     |                                         |--- Cert Confirm -->|
     |                                         |<-- PKI Confirm ----|
     |<------------- PKI/Registrar Confirm ----|                    |

   Figure 4: Certificate enrollment

   The following list provides an abstract description of the flow
   depicted in Figure 4.

   o  CA Cert Request: The pledge SHOULD request the full distribution
      of CA Certificates message.  This ensures that the pledge has the
      complete set of current CA certificates beyond the pinned-domain-
      cert.

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   o  Attribute Request: Typically, the automated bootstrapping occurs
      without local administrative configuration of the pledge.
      Nevertheless, there are cases, in which the pledge should 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  Cert Request: certification request message (to be done: reference
      to PKCS#10 or CRMF, proof of possession, pledge authentication)

   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 bei either part of the enrollment protocol
      or build based on the certification request.

   o  Cert Poling: 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.

   /* to be done: - investigation into handling of certificate request
   retries - message exchange description - confirmation message
   (necessary? optional? from Registrar and/or PKI?) */

7.  Mapping to exisitng enrollment protocols

   This sections maps the requirements and the approach described in
   Section 6.3 to already exisitng enrollment protocols.

7.1.  EST Handling

   When using the EST protocol [RFC7030], the following constrains
   should be observed:

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

   o  For proof of identit only the /fullcmc endpoint should be used
      with a fullcmc request.  This contains sufficient information for

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      the RA/CA to make an authorization decision on the received
      certification request.  Note that EST references CMC [RFC5272] for
      the definition of the full PKI request.  For proof of identity,
      the signature of the SignedData of the Full PKI Request would be
      calculated using the IDEVID credential of the pledge.  /*TBD: in
      this case the binding to the underlying TLS connection may not be
      necessary */

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

7.2.  CMP Handling

   When using the CMP protocol [RFC4210], the following constrains
   should be observed:

   o  For proof of posession, the defined approach in CMP [RFC4210]
      section 4.3 should be supported.  This can be achieved by using
      either CRMF or PKCS#10 to specify the certification request.

   o  Proof of identity can be provided by using the MSG_SIG_ALG to
      protect the certificate request message with signatures as
      outlined in section D.5.

   o  When the CA/CA is not available, as per [RFC4210] Section 5.2.3, a
      waiting indication should be returned in the PKIStatus by the Join
      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 5.3.22.

8.  IANA Considerations

   This document requires the following IANA actions:

   /* to be done: clarification necessary */

9.  Privacy Considerations

   /* to be done: clarification necessary */

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10.  Security Considerations

   /* to be done: clarification necessary */

11.  Acknowledgements

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

12.  References

12.1.  Normative References

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-22 (work in progress), June 2019.

   [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>.

   [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>.

   [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>.

12.2.  Informative References

   [I-D.gutmann-scep]
              Gutmann, P., "Simple Certificate Enrolment Protocol",
              draft-gutmann-scep-14 (work in progress), June 2019.

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

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   [ISO-IEC-15118-2]
              International Standardization Organization / International
              Electrotechnical Commission, "ISO/IEC 15118-2 Road
              vehicles - Vehicle-to-Grid Communication Interface - Part
              2: Network and application protocol requirements", ISO/
              IEC 15118 , 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",
              April 2018.

   [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>.

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

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <https://www.rfc-editor.org/info/rfc5785>.

Authors' Addresses

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

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

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

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

   Eliot Lear
   Cisco Systems
   Richtistrasse 7
   Wallisellen  CH-8304
   Switzerland

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

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