wdiff rfc9733v1.txt rfc9733.txt

Internet Engineering Task Force (IETF) D. von Oheimb, Ed.
Request for Comments: 9733 S. Fries
Category: Standards Track H. Brockhaus
ISSN: 2070-1721 Siemens
February 2025

BRSKI-AE: Bootstrapping Remote Secure Key Infrastructure

BRSKI with Alternative Enrollment (BRSKI-AE) Protocol

Abstract

This document defines enhancements to the Bootstrapping Remote Secure
Key Infrastructure (BRSKI) protocol, known as BRSKI with Alternative
Enrollment (BRSKI-AE). BRSKI-AE extends BRSKI to support certificate
enrollment mechanisms instead of the originally specified use of
Enrollment over Secure Transport (EST). It supports certificate
enrollment protocols such as the Certificate Management Protocol
(CMP) that use authenticated self-contained signed objects for
certification messages, allowing for flexibility in network device
onboarding scenarios. The enhancements address use cases where the
existing enrollment mechanism may not be feasible or optimal,
providing a framework for integrating suitable alternative enrollment
protocols. This document also updates the BRSKI reference
architecture to accommodate these alternative methods, ensuring
secure and scalable deployment across a range of network
environments.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9733.

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

Table of Contents

1. Introduction
1.1. Supported Scenarios
2. Terminology and Abbreviations
3. Basic Requirements and Mapping to Solutions
3.1. Solution Options for Proof of Possession
3.2. Solution Options for Proof of Identity
4. Adaptations to BRSKI
4.1. Architecture
4.2. Message Exchange
4.2.1. Pledge - Registrar Discovery
4.2.2. Pledge - Registrar - MASA Voucher Exchange
4.2.3. Pledge - Registrar - MASA Voucher Status Telemetry
4.2.4. Pledge - Registrar - RA/CA Certificate Enrollment
4.2.5. Pledge - Registrar Enrollment Status Telemetry
4.3. Enhancements to the Endpoint Addressing Scheme of BRSKI
5. Instantiation with Existing Enrollment Protocols
5.1. BRSKI-CMP: BRSKI-AE Instantiated with CMP
5.2. Support of Other Enrollment Protocols
6. IANA Considerations
7. Security Considerations
8. Privacy Considerations
9. References
9.1. Normative References
9.2. Informative References
Appendix A. Application Examples
A.1. Rolling Stock
A.2. Building Automation
A.3. Substation Automation
A.4. Electric Vehicle Charging Infrastructure
A.5. Infrastructure Isolation Policy
A.6. Sites with Insufficient Levels of Operational Security
Acknowledgments
Contributors
Authors' Addresses

1. Introduction

Bootstrapping Remote Secure Key Infrastructure (BRSKI) [RFC8995] is
typically used with Enrollment over Secure Transport (EST) [RFC7030]
as the enrollment protocol for operator-specific device certificates,
employing HTTP over TLS for secure message transfer. BRSKI-AE is a
variant using alternative enrollment protocols with authenticated
self-contained objects for the device certificate enrollment.

This approach offers several distinct advantages. It allows for the
authentication of the origin of requests and responses independently
of message transfer mechanisms. This capability facilitates end-to-
end authentication (i.e., end-to-end proof of origin) across multiple
transport hops and supports the asynchronous operation of certificate
enrollment. Consequently, this provides architectural flexibility in
determining the location and timing for the ultimate authentication
and authorization of certification requests while ensuring that the
integrity and authenticity of the enrollment messages are maintained
with full cryptographic strength.

This specification carries over the main characteristics of BRSKI,
namely:

* The pledge is assumed to have received its Initial Device
Identifier
IDentifier (IDevID) [IEEE_802.1AR-2018] credentials during its
manufacturing. It uses them to authenticate itself to the
Manufacturer Authorized Signing Authority (MASA) [RFC8995] and [RFC8995], to the
registrar (which is the access point of the target domain) domain), and to
possibly further components of the domain where it will be
operated.

* The pledge first obtains via the voucher exchange [RFC8366] exchange a
trust anchor for authenticating entities in the domain such as the
domain registrar.

* The pledge then obtains its Local Locally Significant Device Identifier IDentifier
(LDevID) [IEEE_802.1AR-2018]. To this end, the pledge generates a
private key, called an "LDevID secret". Then, it requests via the
domain registrar from the PKI of its new domain a domain-specific
device certificate, called an "LDevID certificate". On success,
it receives the LDevID certificate along with its certificate
chain.

The objectives of BRSKI-AE are to enhance BRSKI by enabling LDevID
certificate enrollment through the use of an alternative protocol to
EST that:

* supports end-to-end authentication over multiple transport hops
and

* facilitates secure message exchanges over any type of transfer
mechanism, including asynchronous delivery.

It may be observed that the BRSKI voucher exchange between the
pledge, registrar, and MASA involves the use of authenticated self-
contained objects, which inherently possess these properties.

The existing well-known URI structure used for BRSKI and EST messages
is extended by introducing an additional path element that specifies
the enrollment protocol being employed.

This specification allows the registrar to offer multiple enrollment
protocols, enabling pledges and their developers to select the most
appropriate one based on the defined overall approach and specific
endpoints.

It may be important to note that BRSKI [RFC8995] specifies the use of HTTP
over TLS, but variations such as Constrained BRSKI [cBRSKI], which
uses the Constrained Application Protocol (CoAP) over DTLS, are
possible as well. In this context, "HTTP" and "TLS" are used as
references to the most common implementation, though variants using
CoAP and/or DTLS are implied where applicable, as the distinctions
are not pertinent here.

This specification, together with its referenced documents, is
sufficient to support BRSKI with the Certificate Management Protocol
(CMP) [RFC9480] as profiled in the Lightweight CMP Profile (LCMPP)
[RFC9483]. Integrating BRSKI with an enrollment protocol or profile
other than the LCMPP will necessitate additional IANA registrations,
as detailed in this document. Furthermore, additional specifications
may be required for the details of the protocol or profile, which
fall outside the scope of this document.

1.1. Supported Scenarios

BRSKI-AE is designed for use in scenarios such as the following:

* When pledges and/or the target domain leverage an existing
certificate enrollment protocol other than EST, such as CMP.

* When the application context precludes the use of EST for
certificate enrollment due to factors such as when:

- The Registration Authority (RA) is not co-located with the
registrar and requires end-to-end authentication of requesters,
which EST does not support over multiple transport hops.

- The RA or Certification Authority (CA) operator mandates
auditable proof of origin for Certificate Signing Requests
(CSRs), which cannot be provided by TLS as it only offers
transient source authentication.

- Certificates are requested for key types, such as Key
Encapsulation Mechanism (KEM) keys, that do not support signing
or other single-shot proof-of-possession methods as those
described in [RFC6955]. EST, which relies on CSRs in PKCS #10
format [RFC2986], does not accommodate these key types because
it necessitates proof-of-possession methods that operate within
a single message, whereas proof of possession for KEM keys
requires prior receipt of a fresh challenge value.

- The pledge implementation employs security libraries that do
not support EST or uses a TLS library lacking support for the
"tls-unique" value [RFC5929], which EST requires for the strong
binding of source authentication.

* When full RA functionality is not available on-site within the
target domain, and connectivity to an off-site RA may be
intermittent or entirely offline.

* When authoritative actions by a local RA at the registrar are
insufficient for fully and reliably authorizing pledge
certification requests, potentially due to a lack of access to
necessary data or inadequate security measures, such as the local
storage of private keys.

Bootstrapping may be managed in various ways depending on the
application domain. Appendix A provides illustrative examples from
different industrial control system environments and operational
contexts that motivate the support of alternative enrollment
protocols.

2. Terminology and Abbreviations

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],
[RFC5280], and [IEEE_802.1AR-2018], which is partly repeated here.
Several further terms are also described here.

To be independent of the terminology of a specific enrollment
protocol, this document utilizes generic terminology regarding PKI
management operations.

The following terminology is used in this document:

asynchronous: the time-wise interrupted delivery of messages, here,
between a pledge and some backend system (e.g., an RA).

attribute request: a message requesting content to be included in
the certification request.

attribute response: a message providing the answer to the attribute
request.

authenticated self-contained object: a data structure that is
cryptographically bound to the identity of its originator by an
attached digital signature on the actual object, using a private
key of the originator such as the IDevID secret.

backend: the placement of a domain component separately from the
domain registrar; it may be on-site or off-site.

BRSKI: Bootstrapping Remote Secure Key Infrastructure [RFC8995]

BRSKI-AE: BRSKI with Alternative Enrollment. Refers to a variation
of BRSKI [RFC8995] in which BRSKI-EST, the enrollment protocol
between the pledge and the registrar, is replaced by enrollment
protocols that support end-to-end authentication of the pledge to
the RA, such as Lightweight CMP (see LCMPP).

CA: Certification Authority

CA certs request: a message requesting CA certificates.

CA certs response: a message providing the answer to a CA certs
request.

certificate confirm: a message stating to the backend PKI that the
requester of a certificate received the new certificate and
accepted it.

certification request: a message requesting a certificate with proof
of identity.

certification response: a message providing the answer to a
certification request.

CMP: Certificate Management Protocol [RFC4210] [RFC9480]

CSR: Certificate Signing Request

EST: Enrollment over Secure Transport [RFC7030]

IDevID: Initial Device Identifier (of a pledge, provided by the
manufacturer and comprising of a private key and the related X.509
certificate with its chain).

LCMPP: Lightweight CMP Profile [RFC9483]

LDevID: Local Device Identifier (of a pledge, provided by its target
domain and comprising of a private key and the related X.509
certificate with its chain).

LRA: Local Registration Authority. A subordinate RA that is close
to entities being enrolled and separate from a subsequent RA. In
BRSKI-AE, it is needed if a backend RA is used; in this case, the
LRA is co-located with

local RA: the registrar.

MASA: Manufacturer Authorized Signing Authority. Provides vouchers. same as LRA.

off-site: the locality of a component, service, or functionality
(such as RA or CA) that is not at the site of the registrar. This
may be a central site or a cloud service, to which connection may
be intermittent.

on-site: the locality of a component, service, or functionality at
the site of the registrar.

PKI/registrar confirm: an acknowledgment of the PKI on the
certificate confirm.

pledge: a device that is to be bootstrapped into a target domain.
It requests an LDevID using IDevID credentials installed by its
manufacturer.

RA: Registration Authority. The PKI component to which a CA
typically delegates certificate management functions such as
authenticating pledges and performing authorization checks on
certification requests.

registrar: short for domain registrar.

site: the locality where an entity such as a pledge, registrar, or
PKI component is deployed. The target domain may have multiple
sites.

synchronous: the time-wise uninterrupted delivery of messages, here,
between a pledge and a registrar or backend system (e.g., the
MASA).

target domain: the domain that a pledge is going to be bootstrapped
into.

The following abbreviations are used in this document:

BRSKI: Bootstrapping Remote Secure Key Infrastructure [RFC8995]

CA: Certification Authority

CMC: Certificate Management over CMS

CMP: Certificate Management Protocol [RFC4210] [RFC9480]

CMS: Cryptographic Message Syntax

CRMF: Certificate Request Message Format

CSR: Certificate Signing Request

EST: Enrollment over Secure Transport [RFC7030]

IDevID: Initial Device IDentifier (of a pledge, provided by the
manufacturer and comprising of a private key and the related X.509
certificate with its chain).

LCMPP: Lightweight CMP Profile [RFC9483]

LDevID: Locally Significant Device IDentifier (of a pledge, provided
by its target domain and comprising of a private key and the
related X.509 certificate with its chain).

MASA: Manufacturer Authorized Signing Authority. Provides vouchers.

SCEP: Simple Certificate Enrolment Protocol

3. Basic Requirements and Mapping to Solutions

Based on the intended target scenarios described in Section 1.1 and
the application examples described in Appendix A, the following
requirements are derived to support authenticated self-contained
objects as containers carrying certification requests.

The following properties are required for a certification request:

* Proof of possession: demonstrates access to the private key
corresponding to the public key contained in a certification
request. This is typically achieved by a self-signature using the
corresponding private key but can also be achieved indirectly; see
[RFC4210], Section 4.3.

* Proof of identity (also called "proof of origin"): provides data
origin authentication of the certification request. Typically,
this is achieved by a signature using the pledge IDevID secret
over some data, which needs to include a sufficiently strong
identifier of the pledge, such as the device serial number
typically included in the subject of the IDevID certificate.

The remainder of this section gives a non-exhaustive list of solution
examples, based on existing technology described in IETF documents.

3.1. Solution Options for Proof of Possession

Certificate Signing Request (CSR) objects are data structures
protecting only the integrity of the contained data and providing
proof of possession for a (locally generated) private key. Important
types of CSR data structures are:

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

* Certificate Request Message Format (CRMF) [RFC4211]: This less
common but more general CSR format supports several ways of
integrity protection and proof of possession. Typically a self-
signature is used, which is generated over (part of) the structure
with the private key corresponding to the included public key.
CRMF also supports further proof-of-possession methods for types
of keys that do not have signing capability. For details, see
[RFC4211], Section 4.

It should be noted that the integrity protection of CSRs includes the
public key because it is part of the data signed by the corresponding
private key. Yet, this signature does not provide data origin
authentication, (i.e., proof of identity of the requester) because
the key pair involved is new and therefore does not yet have a
confirmed identity associated with it.

3.2. Solution Options for Proof of Identity

Binding a Certificate Signing Request (CSR) to an existing
authenticated credential (the (which in the BRSKI context, context is the IDevID
certificate) enables proof of origin, which in turn supports an
authorization decision on the CSR.

The binding of data origin authentication to the CSR is typically
delegated to the protocol used for certificate management. This
binding may be achieved through security options in an underlying
transport protocol such as TLS if the authorization of the
certification request is (sufficiently) done at the next
communication hop. Depending on the key type, the binding can also
be done in a stronger, transport-independent way by wrapping the CSR
with a signature.

This requirement is addressed by existing enrollment protocols in
various ways, such as:

* EST [RFC7030] and its variant EST-coaps [RFC9148] utilize PKCS #10
to encode CSRs. While such a CSR has not been designed to include
proof of origin, there is a limited, indirect way of binding it to
the source authentication of the underlying TLS session. This is
achieved by including in the CSR the tls-unique "tls-unique" value [RFC5929]
resulting from the TLS handshake. As this is optionally supported
by the EST "/simpleenroll" endpoint used in BRSKI, and the TLS
handshake employed in BRSKI includes certificate-based client
authentication of the pledge with its IDevID credentials, the
proof of pledge identity being an authenticated TLS client can be
bound to the CSR.

Yet, this binding is only valid in the context of the TLS session
established with the registrar acting as the EST server and
typically also as an RA. So even such a cryptographic binding of
the authenticated pledge identity to the CSR is not visible nor
verifiable to authorization points outside the registrar, such as
a (second) RA in the backend. What the registrar needs to do is
authenticate and pre-authorize the pledge and indicate this to the
(second) RA. This is done by signing the forwarded certification
request with its private key and a related certificate that has
the id-kp-cmcRA extended key usage attribute.

[RFC7030], Section 2.5 sketches wrapping PKCS #10-formatted CSRs formatted per PKCS
#10 with a Full PKI Request message sent to the "/fullcmc"
endpoint. This would allow for source authentication at the
message level, such that the registrar could forward it to
external RAs in a meaningful way. This approach is so far not
sufficiently described and likely has not been implemented.

* The Simple Certificate Enrolment Protocol (SCEP) [RFC8894]
supports using a shared secret (passphrase) or an existing
certificate to protect CSRs based on SCEP Secure Message Objects
using Cryptographic Message Syntax (CMS) wrapping [RFC5652]. Note
that the wrapping using an existing IDevID in SCEP is referred to
as "renewal". This way, SCEP does not rely on the security of the
underlying message transfer.

* CMP [RFC4210] [RFC9480] supports using a shared secret
(passphrase) or an existing certificate, which may be an IDevID
credential, to authenticate certification requests via the
PKIProtection structure in a PKIMessage. The certification
request is typically encoded utilizing CRMF, while PKCS #10 is
supported as an alternative. Thus, CMP does not rely on the
security of the underlying message transfer.

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

To sum up, EST does not meet the requirements for authenticated self-
contained objects, but SCEP, CMP, and CMC do. This document
primarily focuses on CMP as it has more industry adoption than CMC
and SCEP has issues not detailed here.

4. Adaptations to BRSKI

To enable using alternative certificate enrollment protocols
supporting end-to-end authentication, asynchronous enrollment, and
more general system architectures, BRSKI-AE provides some
generalizations on BRSKI [RFC8995]. This way, authenticated self-
contained objects such as those described in Section 3 above can be
used for certificate enrollment, and RA functionality can be deployed
freely in the target domain. Parts of the RA functionality can even
be distributed over several nodes.

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

4.1. Architecture

The key element of BRSKI-AE is that the authorization of a
certification request MUST be performed based on an authenticated
self-contained object. The certification request is bound in a self-
contained way to a proof of origin based on the IDevID credentials.
Consequently, the certification request MAY be transferred using any
mechanism or protocol. Authentication and authorization of the
certification request can be done by the domain registrar and/or by
backend domain components. As mentioned in Section 1.1, these
components may be offline or off-site. The registrar and other on-
site domain components may have no or only temporary (intermittent)
connectivity to them.

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

+------------------------+
+--------------Drop-Ship--------------| Vendor Service |
| +------------------------+
| | M anufacturer| |
| | A uthorized |Ownership|
| | S igning |Tracker |
| | A uthority | |
| +--------------+---------+
| ^
| |
V | BRSKI-
+--------+ ......................................... | MASA
| | . . |
| | . +-------+ +--------------+ . |
| Pledge | . | Join | | Domain |<----+
| |<------>| Proxy |<-------->| Registrar | .
| | . | | | w/ LRA or RA | .
| IDevID | . +-------+ +--------------+ .
| | BRSKI-AE over TLS ^ .
+--------+ using, e.g., LCMPP | .
. | .
...............................|.........
on-site (local) domain components |
|
| e.g., LCMPP
|
.............................................|...............
. Public-Key Infrastructure (PKI) v .
. +---------+ +---------------------------------------+ .
. | |<----+ Registration Authority RA | .
. | CA +---->| (unless part of Domain Registrar) | .
. +---------+ +---------------------------------------+ .
.............................................................
backend (central or off-site) domain components

Figure 1: Architecture Overview Using Backend PKI Components

The architecture overview in Figure 1 has the same logical elements
as BRSKI but with a more flexible placement of the authentication and
authorization checks on certification requests. Depending on the
application scenario, the registrar MAY still do all of these checks
(as is the case in BRSKI) or only do part of them.

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

* Join Proxy: This has the same requirements as in BRSKI [RFC8995] (see
[RFC8995], Section 4).

* Domain Registrar (including LRA or RA functionality): In BRSKI-AE,
the domain registrar has mostly the same functionality as in
BRSKI, namely to act as the gatekeeper of the domain for
onboarding new devices and to facilitate the communication of
pledges with their MASA and the domain PKI. Yet, there are some
generalizations and specific requirements:

1. The registrar MUST support at least one certificate enrollment
protocol with authenticated self-contained objects for
certification requests. To this end, the URI scheme for
addressing endpoints at the registrar is generalized (see
Section 4.3).

2. Rather than having full RA functionality, the registrar MAY
act as a Local Registration Authority (LRA) and delegate part
of its involvement in certificate enrollment to a backend RA.
In such scenarios, the registrar optionally checks
certification requests it receives from pledges and forwards
them to the backend RA, which performs the remaining parts of
the enrollment request validation and authorization. Note
that to this end, the backend RA may need information
regarding the authorization of pledges from the registrar or
from other sources. On the way back, the registrar forwards
responses by the PKI to the pledge on the same channel.

To support end-to-end authentication of the pledge across the
registrar to the backend RA, the certification request signed
by the pledge needs to be upheld and forwarded by the
registrar. Therefore, for its communication with the PKI, the
registrar cannot use an enrollment protocol that is different
from the enrollment protocol used between the pledge and the
registrar.

3. The use of a certificate enrollment protocol with
authenticated self-contained objects gives freedom with how to
transfer enrollment messages between the pledge and an RA.
BRSKI demands that the RA accept certification requests for
LDevIDs only with the consent of the registrar. BRSKI-AE also
guarantees this in the case that the RA is not part of the
registrar, even if the message exchange with backend systems
is unprotected and involves further transport hops. See
Section 7 for details on how this can be achieved.

Despite the above generalizations of the enrollment phase, the final
step of BRSKI, namely the enrollment status telemetry, is kept as it
is.

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

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

| Note: From the The definition of the interaction with the MASA in
[RFC8995],
| Section 5 follows of [RFC8995] implies that it may be synchronous
| (using voucher requests with nonces) or asynchronous (using
| nonceless voucher requests).

* Ownership Tracker: This is as defined in BRSKI. [RFC8995].

The following list describes backend target domain components, which
may be located on-site or off-site in the target domain.

* RA: This performs centralized certificate management functions as
a public-key infrastructure PKI for the domain operator. As far as In case these functions are not already done
entirely performed by the domain registrar, it performs the final
validation and authorization of certification requests.
Otherwise, the RA co-located with the domain registrar directly
connects to the CA.

* CA (also called "domain CA"): This generates domain-specific
certificates according to certification requests that have been
authenticated and authorized by the registrar and/or an extra RA.

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

* Discovery phase: Discover: This is mostly as in step (1) of [RFC8995]. For
details, see Section 4.2.1.

* Identification phase: Identify: This is the same as in step (2) of [RFC8995].

* Voucher exchange phase: exchange: This is the same as in steps (3) and (4) of
[RFC8995].

* Voucher status telemetry: This is the same as directly after step
(4) in [RFC8995].

* Certificate enrollment phase: The use of EST in step (5) is
changed to employing a certificate enrollment protocol that uses
an authenticated self-contained object for requesting the LDevID
certificate.

For transporting the certificate enrollment request and response
messages,

It is REQUIRED to use the (D)TLS channel established between the
pledge and registrar is REQUIRED to use. transport the certificate enrollment
request and response messages. To this end, the enrollment
protocol, the pledge, and the registrar need to support the use of
this existing channel for certificate enrollment. Due to this
architecture, the pledge does not need to establish additional
connections for certificate enrollment and the registrar retains
full control over the certificate enrollment traffic.

* Enrollment status telemetry: This is the final exchange of step
(5) of [RFC8995].

4.2. Message Exchange

The behavior of a pledge described in BRSKI [RFC8995], Section 2.1 is kept,
with one major exception. After finishing the Imprint step (4), the
Enroll step (5) MUST be performed with an enrollment protocol
utilizing authenticated self-contained objects, as explained in
Section 3. Section 5 discusses selected suitable enrollment
protocols and options applicable. applicable options.

An abstract overview of the BRSKI-AE protocol can be found at
[BRSKI-AE-OVERVIEW]. in the
graphics on slide 4 of [BRSKI-AE-overview].

4.2.1. Pledge - Registrar Discovery

Discovery as specified in BRSKI [RFC8995], Section 4 does not support the
discovery of registrars with enhanced feature sets. A pledge cannot
find out in this way whether discovered registrars support the
certificate enrollment protocol it expects, such as CMP.

As a more general solution, the BRSKI discovery mechanism can be
extended to provide up-front information on the capabilities of
registrars. For further discussion, see [BRSKI-DISCOVERY]. [BRSKI-discovery].

In the absence of such a generally applicable solution, BRSKI-AE
deployments may use their particular way of doing discovery.
Section 5.1 defines a minimalist approach that MAY be used for CMP.

4.2.2. Pledge - Registrar - MASA Voucher Exchange

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

4.2.3. Pledge - Registrar - MASA Voucher Status Telemetry

The voucher status telemetry is performed as specified in [RFC8995],
Section 5.7.

4.2.4. Pledge - Registrar - RA/CA Certificate Enrollment

This

The specification in this section replaces the EST integration for
PKI bootstrapping described in [RFC8995], Section 5.9 (while
[RFC8995], Section 5.9.4 remains as the final phase; see below).

The certificate enrollment phase may involve the transmission of
several messages. Details can depend on the application scenario,
the employed enrollment protocol, and other factors.

The only message exchange REQUIRED is for the actual certification
request and response. Further message exchanges MAY be performed as
needed.

| Note: The message exchanges marked OPTIONAL in Figure 2 below
| cover all those supported by the use of EST in BRSKI. The last
| OPTIONAL one, namely certificate confirmation, is not supported
| by EST but by CMP and other enrollment protocols.

+------+ +---------+ +--------+
|Pledge| |Domain | |Operator|
| | |Registrar| |RA/CA |
| | |(JRC) | |(PKI) |
+------+ +---------+ +--------+
| | |
|[OPTIONAL request of CA certificates]| |
|------- CA Certs Request (1) ------->| |
| | [OPTIONAL forwarding] |
| |--- CA Certs Request ----->|
| |<-- CA Certs Response -----|
|<------ CA Certs Response (2) -------| |
| | |
|[OPTIONAL request of attributes | |
| to include in Certification Request]| |
|------- Attribute Request (3) ------>| |
| | [OPTIONAL forwarding] |
| |--- Attribute Request ---->|
| |<-- Attribute Response ----|
|<------ Attribute Response (4) ------| |
| | |
|[REQUIRED certification request] | |
|------- Certification Request (5) -->| |
| | [OPTIONAL forwarding] |
| |---Certification Request-->|
| |<--Certification Resp. ---|
|<----- Certification Response (6) ---| |
| | |
|[OPTIONAL certificate confirmation] | |
|------- Certificate Confirm (7) ---->| |
| | [OPTIONAL forwarding] |
| |--- Certificate Confirm--->|
| |<-- PKI Confirm -----------|
|<------ PKI/Registrar Confirm (8) ---| |

Figure 2: Certificate Enrollment Message Flow

It may be noted that connections between the registrar and the PKI
components of the operator (RA, CA, etc.) may be intermittent or
offline. Messages should be sent as soon as sufficient transfer
capacity is available.

The label [OPTIONAL forwarding] '[OPTIONAL forwarding]' in Figure 2 means that on receiving
a request message of the given type from a pledge, the registrar MAY
answer the request directly. In this case, it MUST authenticate its
responses with the same credentials as used for authenticating itself
at the TLS level for the voucher exchange. Otherwise, the registrar
MUST forward the request to the RA and forward any resulting response
back to the pledge.

The decision of whether to forward a request or to answer it directly
can depend on various static and dynamic factors. They include the
application scenario, the capabilities of the registrar and registrar, the
capabilities of the local RA possibly co-located with the registrar,
the enrollment protocol being used, and the specific contents of the
request.

Note that there are several options for how the registrar could be
able to directly answer requests for CA certificates or for
certification request attributes. It could cache responses obtained
from the domain PKI and later use their contents for responding to
requests asking for the same data. The contents could also be
explicitly provisioned at the registrar.

Further note that certification requests typically need to be handled
by the backend PKI, but the registrar can answer them directly with
an error response in case it determines that such a request should be
rejected, for instance, because it is not properly authenticated or
authorized. Also, certificate confirmation messages will usually be
forwarded to the backend PKI, but if the registrar knows that they
are not needed or wanted there, it can acknowledge such messages
directly.

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

* CA Certs Request (1): The pledge optionally requests the latest
relevant CA certificates. This ensures that the pledge has the
complete set of current CA certificates beyond the pinned-domain-
cert (which is contained in the voucher and which may be just the
domain registrar certificate).

* CA Certs Response (2): This MUST contain any intermediate CA
certificates that the pledge may need to validate certificates and
MAY contain the LDevID trust anchor.

* Attribute Request (3): 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 that are specific to the target domain in
the Certification Request (5). To get these attributes in
advance, the attribute request may be used.

* Attribute Response (4): This MUST contain the attributes requested
in (3) to be included in the subsequent Certification Request (5).

For example, [RFC8994], Section 6.11.7.2 specifies how the
attribute request is used to signal to the pledge the acp-node-
name 'acp-node-
name' field required for enrollment into an Autonomic Control
Plane (ACP) domain.

* Certification Request (5): This MUST contain the authenticated
self-contained object ensuring both the proof of possession of the
corresponding private key and the proof of identity of the
requester.

* Certification Response (6): On success, this MUST contain the
requested certificate and MAY include further information, like
certificates of intermediate CAs and any additional trust anchors.

* Certificate Confirm (7): This is an optional confirmation that is
sent after the requested certificate has been received and
validated. If sent, it MUST contain a positive or negative
confirmation by the pledge to the PKI whether the certificate was
successfully enrolled and fits its needs.

* PKI/Registrar Confirm (8): This is an acknowledgment by the PKI
that MUST be sent on reception of the Certificate Confirm.

The generic messages described above may be implemented using any
certificate enrollment protocol that supports authenticated self-
contained objects for the certification request as described in
Section 3. Examples are available in Section 5.

| Note that the optional certificate confirmation by the pledge
| to the PKI described above is independent of the mandatory
| enrollment status telemetry done between the pledge and the
| registrar in the final phase of BRSKI-AE, which is described
| next.

4.2.5. Pledge - Registrar Enrollment Status Telemetry

The enrollment status telemetry is performed as specified in
[RFC8995], Section 5.9.4.

In BRSKI, [RFC8995], this is described as part of the certificate enrollment
step, but due to the generalization of the enrollment protocol
described in this document, it is regarded as a separate phase here.

4.3. Enhancements to the Endpoint Addressing Scheme of BRSKI

BRSKI-AE extends the addressing scheme outlined in [RFC8995],
Section 5 to support alternative enrollment protocols that utilize
authenticated, self-contained objects for certification requests
(also see Section 5). These extensions are designed to be compatible
with existing Registration Authorities (RAs) and Certification
Authorities (CAs) that already support such enrollment protocols,
enabling their use without requiring any modifications.

The addressing scheme in BRSKI [RFC8995] for certification requests,
related CA certificates, and CSR attributes retrieval functions uses
the definition from EST [RFC7030]. An example of simple enrollment
is: "/.well-known/est/simpleenroll". This approach is generalized to
the following notation: "/.well-known/<enrollment-protocol>/<request>" "/.well-known/<enrollment-
protocol>/<request>" in which <enrollment-protocol> "<enrollment-protocol>" refers to a
certificate enrollment protocol. Note that here, enrollment is
considered a message sequence that contains at least a certification
request and a certification response. The following conventions are
used to provide maximal compatibility with BRSKI:

* <enrollment-protocol>: "<enrollment-protocol>": This MUST reference the protocol being
used. Existing values include 'est' [RFC7030] as in BRSKI [RFC8995] and
'cmp' as in [RFC9483] and Section 5.1 below. Values for other
existing protocols such as CMC and SCEP, as well as newly defined
protocols, are outside the scope of this document. For use of the
<enrollment-protocol>
"<enrollment-protocol>" and <request> "<request>" URI components, they would
need to be specified in a suitable RFC and placed into the "Well-
Known URIs" registry, just as EST in [RFC7030].

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

Well-known URIs for various endpoints on the domain registrar are
already defined as part of the base BRSKI specification or indirectly
by EST. In addition, alternative enrollment endpoints MAY be
supported by the registrar.

A pledge SHOULD use the endpoints defined for the enrollment
protocol(s) that it can use. It will recognize whether the protocol
it uses and the specific request it wants to perform are understood
and supported by the domain registrar. This is done by sending the
request to the respective endpoint according to the above addressing
scheme and then evaluating the HTTP status code of the response. If
the pledge uses endpoints that are not standardized, it risks that
the registrar does not recognize a request and thus may reject it
even if the registrar supports the intended protocol and operation.

The following list of endpoints provides an illustrative example of a
domain registrar supporting several options for EST as well as for
CMP to be used in BRSKI-AE. The listing contains the supported
endpoints to which the pledge may connect for bootstrapping. This
includes the voucher handling as well as the enrollment endpoints.
The CMP-related enrollment endpoints are defined as well-known URIs
in CMP Updates [RFC9480] and the Lightweight CMP Profile [RFC9483].

* /.well-known/brski/voucherrequest

* /.well-known/brski/voucher_status

* /.well-known/brski/enrollstatus

* /.well-known/est/cacerts

* /.well-known/est/csrattrs

* /.well-known/est/fullcmc

* /.well-known/cmp/getcacerts

* /.well-known/cmp/getcertreqtemplate

* /.well-known/cmp/initialization

* /.well-known/cmp/pkcs10

5. Instantiation with Existing Enrollment Protocols

This section maps the generic requirements to support proof of
possession and proof of identity to selected existing certificate
enrollment protocols and specifies further aspects of using such
enrollment protocols in BRSKI-AE.

5.1. BRSKI-CMP: BRSKI-AE Instantiated with CMP

In this document, references to CMP follow the Lightweight CMP
Profile (LCMPP) from [RFC9483] rather than [RFC4210] and [RFC9480],
as the subset of CMP defined in the LCMPP sufficiently meets the
required functionality.

Adherence to the LCMPP [RFC9483] is REQUIRED when using CMP. The
following specific requirements apply (refer to Figure 2):

* The validation of server response messages performed by the CMP
client within the pledge MUST be based on the trust anchor
established beforehand via the BRSKI voucher, i.e., on the pinned-
domain-cert.

Note that the integrity and authenticity checks on the RA/CA by
the CMP client can be stronger than for EST because they do not
need to be performed hop-by-hop but are usually end-to-end.

* CA Certs Request (1) and Response (2): Requesting CA certificates
is OPTIONAL. If supported, it SHALL be implemented as specified
in [RFC9483], Section 4.3.1.

* Attribute Request (3) and Response (4): Requesting certification
request attributes is OPTIONAL. If supported, it SHALL be
implemented as specified in [RFC9483], Section 4.3.3.

Alternatively, the registrar MAY modify the requested certificate
contents as specified in [RFC9483], Section 5.2.3.2.

* Certification Request (5) and Response (6): Certificates SHALL be
requested and provided as specified in the LCMPP from [RFC9483],
Section 4.1.1 (based on CRMF) or [RFC9483], Section 4.1.4 (based
on PKCS #10).

Proof of possession SHALL be provided in a manner suitable for the
key type. Proof of identity SHALL be provided by signature-based
protection of the certification request message as outlined in
[RFC9483], Section 3.2 using the IDevID secret.

When the registrar forwards a certification request from the
pledge to a backend RA/CA, it is RECOMMENDED that the registrar
wraps the original certification request in a nested message
signed with its own credentials, as described in [RFC9483],
Section 5.2.2.1. This approach explicitly conveys the registrar's
consent to the RA while retaining the original certification
request with the proof of origin provided by the pledge's
signature.

If additional trust anchors beyond the pinned-domain-cert need to
be conveyed to the pledge, this SHOULD be done in the caPubs 'caPubs'
field of the certification response rather than through a CA Certs
Response.

* Certificate Confirm (7) and PKI/Registrar Confirm (8): Explicit
confirmation of new certificates to the RA/CA MAY be used as
specified in [RFC9483], Section 4.1.1.

| Note that independent of the certificate confirmation within
| CMP, enrollment status telemetry with the registrar at the
| BRSKI level will be performed as described in [RFC8995],
| Section 5.9.4.

* If delayed delivery of CMP messages is needed (e.g., to support
enrollment over an asynchronous channel), it SHALL be performed as
specified in Sections 4.4 and 5.1.2 of [RFC9483].

The mechanisms for exchanging messages between the registrar and
backend PKI components (i.e., RA and/or CA) are outside the scope of
this document. CMP's independence from the message transfer
mechanism allows for flexibility in choosing the appropriate exchange
method based on the application scenario. For the applicable
security and privacy considerations, refer to Sections 7 and 8.
Further guidance can be found in [RFC9483], Section 6.

BRSKI-AE with CMP can also be combined with Constrained BRSKI
[cBRSKI], using CoAP for enrollment message transport as described by
CoAP Transfer for CMP [RFC9482]. In such scenarios, the EST-specific
parts of [cBRSKI] do not apply.

For BRSKI-AE scenarios where a general solution for discovering
registrars with CMP support is not available (cf. Section 4.2.1), the
following minimalist approach MAY be used: Perform discovery as
defined in BRSKI [RFC8995], Appendix B, but use the service name
"brski-reg-cmp" "brski-
reg-cmp" (as defined in Section 6) instead of "brski-
registrar" "brski-registrar" (as
defined in [RFC8995], Section 8.6). Note that this approach does not
support join proxies.

5.2. Support of Other Enrollment Protocols

Further instantiations of BRSKI-AE can be done. They are left for
future work.

In particular, CMC [RFC5272] (using its in-band source authentication
options) and SCEP [RFC8894] (using its 'renewal' option) could be
used.

The fullCMC variant of EST sketched in [RFC7030], Section 2.5 might
also be used here. For EST-fullCMC, further specification is
necessary.

6. IANA Considerations

IANA has registered the following service name in the "Service Name
and Transport Protocol Port Number Registry"
<https://www.iana.org/assignments/service-names-port-numbers/service-
names-port-numbers.xhtml>.

Service Name: brski-reg-cmp
Transport Protocol(s): tcp
Description: Bootstrapping Remote Secure Key Infrastructure
registrar with CMP capabilities according to the Lightweight CMP
Profile (LCMPP) [RFC9483]
Assignee: IESG iesg@ietf.org (mailto:iesg@ietf.org)
Contact: IETF chair@ietf.org (mailto:chair@ietf.org)
Reference: RFC 9733

| Note: We chose the suffix "cmp" here rather than some other
| abbreviation like "lcmpp" mainly because this document defines
| the normative CMP instantiation of BRSKI-AE, which implies
| adherence to the LCMPP is necessary and sufficient.

7. Security Considerations

The security considerations laid out in BRSKI [RFC8995], Section 11 apply
to the discovery and voucher exchange as well as for the status
exchange information.

In particular, even if the registrar delegates part or all of its RA
role during certificate enrollment to a separate system, it still
must be made sure that the registrar takes part in the decision on
accepting or declining a request to join the domain, as required in
[RFC8995], Section 5.3. As this also pertains to obtaining a valid
domain-specific certificate, it must be made sure that a pledge
cannot circumvent the registrar in the decision of whether it is
granted an LDevID certificate by the CA. There are various ways to
fulfill this, including:

* implicit consent;

* the registrar signaling its consent to the RA out-of-band before
or during the enrollment phase, for instance, by entering the
pledge identity in a database;

* the registrar providing its consent using an extra message that is
transferred on the same channel as the enrollment messages,
possibly in a TLS tunnel; and

* the registrar explicitly stating its consent by signing the
authenticated self-contained certificate enrollment request
message in addition to the pledge.

| Note: If EST was used, the registrar could give implicit
| consent on a certification request by forwarding the request to
| a PKI entity using a connection authenticated with a
| certificate containing an id-kp-
cmcRA id-kp-cmcRA extension.

When CMP is used, the security considerations laid out in the LCMPP
from [RFC9483] apply.

8. Privacy Considerations

The privacy considerations laid out in BRSKI [RFC8995], Section 10 apply as
well.

Note that CMP messages themselves are not encrypted. This may give
eavesdroppers insight into which devices are bootstrapped into the
domain. In turn, this might also be used to selectively block the
enrollment of certain devices.

To prevent such issues, the underlying message transport channel can
be encrypted. This is already provided by TLS between the pledge and
the registrar, and for the onward exchange with backend systems,
encryption may need to be added.

9. References

9.1. Normative References

[IEEE_802.1AR-2018]
IEEE, "IEEE Standard for Local and Metropolitan Area
Networks - Secure Device Identity", IEEE 802.1AR-2018,
DOI 10.1109/IEEESTD.2018.8423794, August 2018,
<https://ieeexplore.ieee.org/document/8423794>.

[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>.

[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>.

[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>.

[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>.

[RFC9483] Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", RFC 9483,
DOI 10.17487/RFC9483, November 2023,
<https://www.rfc-editor.org/info/rfc9483>.

9.2. Informative References

[BRSKI-AE-OVERVIEW]

[BRSKI-AE-overview]
von Oheimb, D., Ed., Fries, S., and H. Brockhaus, "Update
on BRSKI-AE: Alternative Enrollment Protocols in BRSKI",
IETF 116 - ANIMA Working Group Presentation, March 2023,
<https://datatracker.ietf.org/meeting/116/materials/
slides-116-anima-update-on-brski-ae-alternative-
enrollment-protocols-in-brski-00>.

[BRSKI-DISCOVERY]

[BRSKI-discovery]
Eckert, T. T. and E. Dijk, "BRSKI discovery and
variations", Work in Progress, Internet-Draft, draft-ietf-
anima-brski-discovery-05, 21 October 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-anima-
brski-discovery-05>.

[cBRSKI] Richardson, M., Van der Stok, P., Kampanakis, P., and E.
Dijk, "Constrained Bootstrapping Remote Secure Key
Infrastructure (cBRSKI)", Work in Progress, Internet-
Draft, draft-ietf-anima-constrained-voucher-26, 8 January
2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
anima-constrained-voucher-26>.

[IEC-62351-9]
International Electrotechnical Commission, "Power systems
management and associated information exchange - Data and
communications security - Part 9: Cyber security key
management for power system equipment", IEC 62351-9:2017,
May 2017, <https://webstore.iec.ch/en/publication/30287>. 62351-9:2023,
June 2023, <https://webstore.iec.ch/en/publication/66864>.

[ISO-IEC-15118-2]
International Organization for Standardization, "Road
vehicles - Vehicle-to-Grid Communication Interface - Part
2: Network and application protocol requirements",
ISO 15118-2:2014, April 2014,
<https://www.iso.org/standard/55366.html>.

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

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

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

[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>.

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

[RFC6955] Schaad, J. and H. Prafullchandra, "Diffie-Hellman Proof-
of-Possession Algorithms", RFC 6955, DOI 10.17487/RFC6955,
May 2013, <https://www.rfc-editor.org/info/rfc6955>.

[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>.

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

[RFC8994] Eckert, T., Ed., Behringer, M., Ed., and S. Bjarnason, "An
Autonomic Control Plane (ACP)", RFC 8994,
DOI 10.17487/RFC8994, May 2021,
<https://www.rfc-editor.org/info/rfc8994>.

[RFC9148] van der Stok, P., Kampanakis, P., Richardson, M., and S.
Raza, "EST-coaps: Enrollment over Secure Transport with
the Secure Constrained Application Protocol", RFC 9148,
DOI 10.17487/RFC9148, April 2022,
<https://www.rfc-editor.org/info/rfc9148>.

[RFC9480] Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
Management Protocol (CMP) Updates", RFC 9480,
DOI 10.17487/RFC9480, November 2023,
<https://www.rfc-editor.org/info/rfc9480>.

[RFC9482] Sahni, M., Ed. and S. Tripathi, Ed., "Constrained
Application Protocol (CoAP) Transfer for the Certificate
Management Protocol", RFC 9482, DOI 10.17487/RFC9482,
November 2023, <https://www.rfc-editor.org/info/rfc9482>.

[UNISIG-Subset-137]
UNISIG, "ERTMS/ETCS On-line Key Management FFFIS", Subset-
137, Version 1.0.0, December 2015,
<https://www.era.europa.eu/sites/default/files/filesystem/
ertms/ccs_tsi_annex_a_-_mandatory_specifications/
set_of_specifications_3_etcs_b3_r2_gsm-r_b1/index083_-
_subset-137_v100.pdf>.
<https://www.era.europa.eu/system/files/2023-01/
sos3_index083_-_subset-137_v100.pdf>.

Appendix A. Application Examples

This informative annex provides some details about application
examples.

A.1. Rolling Stock

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

UNISIG has included a CMP profile for the enrollment of TLS client
and server X.509 certificates of on-board and track-side components
in the Subset-137, which specifies the ETRAM/ETCS online key
management for train control systems [UNISIG-Subset-137].

A.2. Building Automation

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

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

A.3. Substation Automation

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

A.4. Electric Vehicle Charging Infrastructure

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

A.5. Infrastructure Isolation Policy

This

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

A.6. Sites with Insufficient Levels of Operational Security

The RA performing (at least part of) the authorization of a
certification request is a critical PKI component and therefore
requires higher operational security than components utilizing the
issued certificates for their security features. CAs may also demand
higher security in the registration procedures from RAs, which domain
registrars with co-located RAs may not be able to fulfill. In
particular, the CA/Browser forum currently increases the security
requirements in the certificate issuance procedures for publicly
trusted certificates, i.e., those placed in trust stores of browsers,
which may be used to connect with devices in the domain. In case the
on-site components of the target domain cannot be operated securely
enough for the needs of an RA, this service should be transferred to
an off-site backend component that has a sufficient level of
security.

Acknowledgments

We thank Eliot Lear for his contributions as a co-author at an
earlier draft stage.

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

Moreover, we thank Toerless Eckert (document shepherd); Barry Leiba
(SECdir review); Mahesh Jethanandani (IETF area director); Meral
Shirazipour (Gen-ART reviewer); Reshad Rahman (YANGDOCTORS reviewer);
Deb Cooley, Gunter Van de Velde, John Scudder, Murray Kucherawy,
Roman Danyliw, and Éric Vyncke (IESG reviewers); Michael Richardson
(ANIMA design team member); and Rajeev Ranjan, Rufus Buschart,
Andreas Reiter, and Szofia Fazekas-Zisch (Siemens colleagues) for
their reviews with suggestions for improvements.

Contributors

Eliot Lear
Cisco Systems
Richtistrasse 7
CH-8304 Wallisellen
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com

Authors' Addresses

David von Oheimb (editor)
Siemens AG
Otto-Hahn-Ring 6
81739 Munich
Germany
Email: david.von.oheimb@siemens.com
URI: https://www.siemens.com/

Steffen Fries
Siemens AG
Otto-Hahn-Ring 6
81739 Munich
Germany
Email: steffen.fries@siemens.com
URI: https://www.siemens.com/

Hendrik Brockhaus
Siemens AG
Otto-Hahn-Ring 6
81739 Munich
Germany
Email: hendrik.brockhaus@siemens.com
URI: https://www.siemens.com/