BRSKI-AE: Alternative Enrollment Protocols in BRSKI
draft-ietf-anima-brski-ae-11
| Document | Type | Active Internet-Draft (anima WG) | |
|---|---|---|---|
| Authors | David von Oheimb , Steffen Fries , Hendrik Brockhaus | ||
| Last updated | 2024-06-19 (Latest revision 2024-06-03) | ||
| Replaces | draft-ietf-anima-brski-async-enroll | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
| Formats | |||
| Reviews |
GENART Last Call review
by Meral Shirazipour
Ready w/nits
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-03)
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OPSDIR Last Call Review due 2024-06-19
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Document shepherd | Toerless Eckert | ||
| Shepherd write-up | Show Last changed 2023-12-21 | ||
| IESG | IESG state | Waiting for AD Go-Ahead | |
| Action Holder | |||
| Consensus boilerplate | Yes | ||
| Telechat date | (None) | ||
| Responsible AD | Mahesh Jethanandani | ||
| Send notices to | tte@cs.fau.de | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-anima-brski-ae-11
ANIMA WG D. von Oheimb, Ed.
Internet-Draft S. Fries
Intended status: Standards Track H. Brockhaus
Expires: 5 December 2024 Siemens
3 June 2024
BRSKI-AE: Alternative Enrollment Protocols in BRSKI
draft-ietf-anima-brski-ae-11
Abstract
This document defines an enhancement of Bootstrapping Remote Secure
Key Infrastructure (BRSKI, RFC 8995). It supports alternative
certificate enrollment protocols, such as CMP, that use authenticated
self-contained signed objects for certification messages.
About This Document
This note is to be removed before publishing as an RFC.
Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-ietf-anima-brski-ae/.
Source for this draft and an issue tracker can be found at
https://github.com/anima-wg/anima-brski-ae.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 5 December 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Supported Scenarios . . . . . . . . . . . . . . . . . . . 4
2. Terminology and abbreviations . . . . . . . . . . . . . . . . 5
3. Basic Requirements and Mapping to Solutions . . . . . . . . . 8
3.1. Solution Options for Proof of Possession . . . . . . . . 8
3.2. Solution Options for Proof of Identity . . . . . . . . . 9
4. Adaptations to BRSKI . . . . . . . . . . . . . . . . . . . . 10
4.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Message Exchange . . . . . . . . . . . . . . . . . . . . 15
4.2.1. Pledge - Registrar Discovery . . . . . . . . . . . . 15
4.2.2. Pledge - Registrar - MASA Voucher Exchange . . . . . 15
4.2.3. Pledge - Registrar - MASA Voucher Status Telemetry . 15
4.2.4. Pledge - Registrar - RA/CA Certificate Enrollment . . 16
4.2.5. Pledge - Registrar Enrollment Status Telemetry . . . 19
4.3. Enhancements to the Endpoint Addressing Scheme of
BRSKI . . . . . . . . . . . . . . . . . . . . . . . . . . 20
5. Instantiation with Existing Enrollment Protocols . . . . . . 21
5.1. BRSKI-CMP: BRSKI-AE instantiated with CMP . . . . . . . . 21
5.2. Support of Other Enrollment Protocols . . . . . . . . . . 23
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
7. Security Considerations . . . . . . . . . . . . . . . . . . . 24
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 25
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Normative References . . . . . . . . . . . . . . . . . . 25
9.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Application Examples . . . . . . . . . . . . . . . . 28
A.1. Rolling Stock . . . . . . . . . . . . . . . . . . . . . . 28
A.2. Building Automation . . . . . . . . . . . . . . . . . . . 29
A.3. Substation Automation . . . . . . . . . . . . . . . . . . 29
A.4. Electric Vehicle Charging Infrastructure . . . . . . . . 30
A.5. Infrastructure Isolation Policy . . . . . . . . . . . . . 30
A.6. Sites with Insufficient Level of Operational Security . . 30
Appendix B. History of Changes TBD RFC Editor: please delete . . 31
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
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1. Introduction
BRSKI [RFC8995] is typically used with Enrollment over Secure
Transport (EST, [RFC7030]) as the enrollment protocol for device
certificates employing HTTP over TLS for its message transfer.
BRSKI-AE is a variant using alternative enrollment protocols with
authenticated self-contained objects for the device certificate
enrollment.
This approach provides the following advantages. The origin of
requests and responses can be authenticated independent of message
transfer. This supports end-to-end authentication (proof of origin)
also over multiple hops, as well as asynchronous operation of
certificate enrollment. This in turn provides architectural
flexibility where and when to ultimately authenticate and authorize
certification requests while retaining full-strength integrity and
authenticity of certification requests.
This specification carries over the main characteristics of BRSKI,
namely:
* The pledge is assumed to have received its Initial Device
IDentifier (IDevID, [IEEE_802.1AR-2018]) credentials during its
production. It uses them to authenticate itself to the
Manufacturer Authorized Signing Authority (MASA, [RFC8995]), and
to the registrar, which is the access point of the target domain,
and to possibly further components of the domain where it will be
operated.
* The pledge first obtains via the voucher [RFC8366] exchange a
trust anchor for authenticating entities in the domain such as the
domain registrar.
* The pledge then obtains its Locally significant Device IDentifier
(IDevID, [IEEE_802.1AR-2018]). To this end, the pledge generates
a private key, called LDevID secret, and requests via the domain
registrar from the PKI of its new domain a domain-specific device
certificate, called LDevID certificate. On success, it receives
the LDevID certificate along with its certificate chain.
The goals of BRSKI-AE are to provide an enhancement of BRSKI for
LDevID certificate enrollment using, alternatively to EST, a protocol
that
* supports end-to-end authentication over multiple hops
* enables secure message exchange over any kind of transfer,
including asynchronous delivery.
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Note that the BRSKI voucher exchange of the pledge with the registrar
and MASA uses authenticated self-contained objects, so the voucher
exchange already has these properties.
The well-known URI approach of BRSKI and EST messages is extended
with an additional path element indicating the enrollment protocol
being used.
Based on the definition of the overall approach and specific
endpoints, this specification enables the registrar to offer multiple
enrollment protocols, from which pledges and their developers can
then pick the most suitable one.
It may be noted that BRSKI (RFC 8995) specifies how to use HTTP over
TLS, but further variants are known, such as Constrained BRSKI
[I-D.ietf-anima-constrained-voucher] using CoAP over DTLS. In the
sequel, 'HTTP' and 'TLS' are just references to the most common case,
where variants such as using CoAP and/or DTLS are meant to be
subsumed - the differences are not relevant here.
This specification is sufficient together with its references to
support BRSKI with the Certificate Management Protocol (CMP,
[RFC9480]) profiled in the Lightweight CMP Profile (LCMPP,
[RFC9483]). Combining BRSKI with a protocol or profile other than
LCMPP will require additional IANA registrations based on the rules
specified in this document. It may also require additional
specifications for details of the protocol or profile (similar to
[RFC9483]), which are outside the scope of this document.
1.1. Supported Scenarios
BRSKI-AE is intended to be used in situations like the following.
* Pledges and/or the target domain reuse an already established
certificate enrollment protocol different from EST, such as CMP.
* The application scenario indirectly excludes the use of EST for
certificate enrollment, for reasons like these:
- The Registration Authority (RA) is not co-located with the
registrar while it requires end-to-end authentication of
requesters. Yet EST does not support end-to-end authentication
over multiple hops.
- The RA or certification authority (CA) operator requires
auditable proof of origin for Certificate Signing Requests
(CSRs). This is not possible with TLS because it provides only
transient source authentication.
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- Certificates are requested for types of keys, such as Key
Encapsulation Mechanism (KEM) keys, that do not support signing
(nor alternative forms of single-shot proof of possession like
those described in [RFC6955]). This is not supported by EST
because it uses CSRs in PKCS #10 [RFC2986] format, which can
only use proof-of-possession methods that need just a single
message, while proof of possession for KEM keys, for instance,
requires receiving a fresh challenge value beforehand.
- The Pledge implementation uses security libraries not providing
EST support or uses a TLS library that does not support
providing the so-called tls-unique value [RFC5929], which is
needed by EST for strong binding of the source authentication.
* No full RA functionality is available on-site in the target
domain, while connectivity to an off-site RA may be intermittent
or entirely offline.
* Authoritative actions of a local RA at the registrar is not
sufficient for fully and reliably authorizing pledge certification
requests. This may be due to missing data access or due to an
insufficient level of security, for instance regarding the local
storage of private keys.
Bootstrapping can be handled in various ways, depending on the
application domains. The informative Appendix A provides
illustrative examples from various industrial control system
environments and operational setups motivating 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]. The following terms are
described partly in addition.
asynchronous communication: time-wise interrupted delivery of
messages, here between a pledge and the registrar or an RA
authenticated self-contained object: a data structure that is
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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: placement of a domain component separately from the domain
registrar; may be on-site or off-site
BRSKI: Bootstrapping Remote Secure Key Infrastructure [RFC8995]
BRSKI-AE: BRSKI with *A*lternative *E*nrollment, a variation of
BRSKI [RFC8995] in which BRSKI-EST, the enrollment protocol
between 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).
CMP: Certificate Management Protocol [RFC9480]
CSR: Certificate Signing Request
EST: Enrollment over Secure Transport [RFC7030]
IDevID: Initial Device IDentifier of a pledge, provided by the
manufacturer and comprising a private key and the related X.509
certificate with its chain
LDevID: Locally significant Device IDentifier of a pledge, provided
by its target domain and comprising a private key and the related
X.509 certificate with its chain
local RA (LRA): 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, and in this case, the LRA is co-
located with the registrar.
LCMPP: Lightweight CMP Profile [RFC9483]
MASA: Manufacturer Authorized Signing Authority
on-site: locality of a component or service or functionality at the
site of the registrar
off-site: locality of component or service or functionality, such as
RA or CA, not at the site of the registrar. This may be a central
site or a cloud service, to which connection may be intermittent.
pledge: device that is to be bootstrapped into a target domain. It
requests an LDevID using IDevID credentials installed by its
manufacturer.
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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 communication: time-wise uninterrupted delivery of
messages, here between a pledge and a registrar or PKI component
target domain: the domain that a pledge is going to be bootstrapped
into
Note that this document utilizes a more generic terminology
regarding PKI management operations to be independent of a
specific enrollment protocol terminology
certification request: describes the request of a certificate with
proof of identity
certification response: :describes the answer to the certification
request
attribute request: describes the request of content to be included
in the certification request
attribute response: describes the describes the answer to the
attribute request
CA Certs request: describes the request of CA certificates.
CA Certs response: describes the describes the answer to the CA
Certs request
certificate confirm: describes a confirmation message to be used if
a backend PKI requires a confirmation of the certificate
acceptance by a pledge.
PKI/registrar confirm: describes an acknowledgement of the PKI to
the certificate confirm
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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: CSRs 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
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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 BRSKI context, 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], also its variant EST-coaps [RFC9148], utilizes PKCS
#10 to encode Certificate Signing Requests (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 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
to authenticate and pre-authorize the pledge and to indicate this
to the (second) RA by signing the forwarded certification request
with its private key and a related certificate that has the id-kp-
cmcRA extended key usage attribute.
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[RFC7030], Section 2.5 sketches wrapping PKCS #10-formatted CSRs
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.
* SCEP [RFC8894] supports using a shared secret (passphrase) or an
existing certificate to protect CSRs based on SCEP Secure Message
Objects using CMS wrapping ([RFC5652]). Note that the wrapping
using an existing IDevID in SCEP is referred to as 'renewal'.
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.
* CMC [RFC5272] also supports utilizing a shared secret (passphrase)
or an existing certificate to protect certification requests,
which can be either in CRMF or PKCS #10 structure. The proof of
identity can be provided as part of a FullCMCRequest, based on CMS
[RFC5652] and signed with an existing IDevID secret. Thus, CMC
does not rely on the security of the underlying 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 needed 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
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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.
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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 w/ | .
| | . |.......| | LRA or RA | .
| IDevID | . +-------+ +--------------+ .
| | BRSKI-AE over TLS ^ .
+--------+ using, e.g., LCMPP | .
. | .
...............................|.........
on-site (local) domain components |
| e.g., LCMPP
|
.............................................|..................
. Public-Key Infrastructure v .
. +---------+ +------------------------------------------+ .
. | |<----+ Registration Authority | .
. | CA +---->| RA (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 part of them.
The following list describes the on-site components in the target
domain of the pledge shown in Figure 1.
* Join Proxy: same requirements as in BRSKI, see [RFC8995],
Section 4
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* 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 facilitating 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, the registrar can not use an enrollment
protocol, which is different from the enrollment protocol used
between the pledge and the registrar, for its communication
with the backend PKI.
3. The use of a certificate enrollment protocol with
authenticated self-contained objects gives freedom 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
guarantees this also in case that the RA is not part of the
registrar, even if the further message transfer is unprotected
and involves further transport hops. See Section 7 for
details on how this can be achieved.
Despite the above generalizations to the enrollment phase, the final
step of BRSKI, namely the enrollment status telemetry, is kept as it
is.
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The following list describes the components provided by the vendor or
manufacturer outside the target domain.
* MASA: functionality as described in BRSKI [RFC8995]. The voucher
exchange with the MASA via the domain registrar is performed as
described in BRSKI.
Note: From the definition of the interaction with the MASA in
[RFC8995], Section 5 follows that it may be synchronous (using
voucher request with nonces) or asynchronous (using nonceless
voucher requests).
* Ownership tracker: as defined in BRSKI.
The following list describes backend target domain components, which
maybe located on-site or off-site in the target domain.
* RA: performs centralized certificate management functions as a
public-key infrastructure for the domain operator. As far as not
already done by the domain registrar, it performs the final
validation and authorization of certification requests.
Otherwise, the RA co-located with the domain registrar directly
connects to the CA.
* CA, also called domain CA: 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 to the original
approach as follows.
* Discovery phase: mostly as in BRSKI step (1). For details see
Section 4.2.1.
* Identification phase: same as in BRSKI step (2).
* Voucher exchange phase: same as in BRSKI steps (3) and (4).
* Voucher status telemetry: same as in BRSKI directly after step
(4).
* 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.
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For transporting the certificate enrollment request and response
messages, the (D)TLS channel established between pledge and registrar
is MANDATORY to use. To this end, the enrollment protocol, the
pledge, and the registrar MUST support the usage of the existing
channel for certificate enrollment. Due to this architecture, the
pledge SHOULD NOT establish additional connections for certificate
enrollment and the registrar MUST retain full control over the
certificate enrollment traffic.
* Enrollment status telemetry: the final exchange of BRSKI step (5).
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.
An abstract overview of the BRSKI-AE protocol can be found at
[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 can
not 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. Future work such as [I-D.eckert-anima-brski-discovery]
may provide this.
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.
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4.2.4. Pledge - Registrar - RA/CA Certificate Enrollment
This 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 the below Figure 2
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.
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+--------+ +------------+ +------------+
| 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 Response ---|
|<-------- Certification Response (6) ----| |
| | |
| [OPTIONAL certificate confirmation] | |
|--------- Certificate Confirm (7) ------>| |
| | [OPTIONAL forwarding] |
| |----- Certificate Confirm ---->|
| |<---- PKI Confirm -------------|
|<-------- PKI/Registrar Confirm (8) -----| |
Figure 2: Certificate Enrollment
It may be noted that connections between the registrar and the PKI
components of the operator (RA, CA, etc.) may be intermittent or off-
line. Messages should be sent as soon as sufficient transfer
capacity is available.
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The label [OPTIONAL forwarding] in Figure 2 means that on receiving
from a pledge a request message of the given type, 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 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 is not properly authenticated or not
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.
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* 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
in the Certification Request (5) additional attributes that are
specific to the target domain. 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 field required for enrollment into an 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): This MUST contain on success the
requested certificate and MAY include further information, like
certificates of intermediate CAs and any additional trust anchors.
* Certificate Confirm (7): An optional confirmation 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): 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, described next.
4.2.5. Pledge - Registrar Enrollment Status Telemetry
The enrollment status telemetry is performed as specified in
[RFC8995], Section 5.9.4.
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In BRSKI this is described as part of the certificate enrollment
step, but due to the generalization on 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 provides generalizations to the addressing scheme defined in
BRSKI [RFC8995], Section 5 to accommodate alternative enrollment
protocols that use authenticated self-contained objects for
certification requests. In existing RAs/CAs supporting such an
enrollment protocol (see also Section 5), these generalizations can
be employed without modifications.
The addressing scheme in BRSKI for certification requests and the
related CA certificates and CSR attributes retrieval functions uses
the definition from EST [RFC7030]. Here is the example of simple
enrollment: "/.well-known/est/simpleenroll". This approach is
generalized to the following notation: "/.well-known/<enrollment-
protocol>/<request>" in which <enrollment-protocol> refers to a
certificate enrollment protocol. Note that enrollment is considered
here a message sequence that contains at least a certification
request and a certification response. The following conventions are
used to provide maximal compatibility with BRSKI:
* <enrollment-protocol>: MUST reference the protocol being used.
Existing values include 'est' [RFC7030] as in BRSKI and 'cmp' as
in [RFC9483] and Section 5.1 below. Values for other existing
protocols such as CMC and SCEP, as well as for newly defined
protocols are outside the scope of this document. For use of the
<enrollment-protocol> and <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>: if present, this path component MUST describe,
depending on the enrollment protocol being used, the operation
requested. Enrollment protocols are expected to define their
request endpoints, as done by existing protocols (see also
Section 5).
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 is understood
and supported by the domain registrar by sending the request to the
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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
Instead of referring to CMP as specified in [RFC4210] and [RFC9480],
this document refers to the Lightweight CMP Profile (LCMPP) [RFC9483]
because the subset of CMP defined there is sufficient for the
functionality needed here.
When using CMP, adherence to the LCMPP [RFC9483] is REQUIRED. In
particular, the following specific requirements apply (cf.
Figure 2).
* 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.
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* 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 contents of 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 [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 way 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 by the pledge
to a backend RA, the registrar is RECOMMENDED to wrap the original
certification request in a nested message signed with its own
credentials as described in [RFC9483], Section 5.2.2.1. This
explicitly conveys the consent by the registrar to the RA while
retaining the certification request with its proof of origin
provided by the pledge signature.
In case additional trust anchors (besides the pinned-domain-cert)
need to be conveyed to the pledge, this SHOULD be done in the
caPubs field of the certification response rather than in 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 independently of certificate confirmation enrollment
status telemetry with the registrar will be performed as described
in BRSKI [RFC8995], Section 5.9.4.
* If delayed delivery of responses is needed (for instance, to
support enrollment over an asynchronous channel), it SHALL be
performed as specified in Section 4.4 and Section 5.1.2 of
[RFC9483].
Since CMP is independent of message transfer, the transfer mechanism
can be freely chosen according to the needs of the application
scenario.
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BRSKI-AE with CMP can also be combined with Constrained BRSKI
[I-D.ietf-anima-constrained-voucher], using CoAP for enrollment
message transport as described by CoAP Transport for CMP [RFC9482].
In this scenario, the EST-specific parts of
[I-D.ietf-anima-constrained-voucher] do not apply.
For BRSKI-AE scenarios where a general solution (cf. Section 4.2.1)
for discovering registrars with CMP support is not available, the
following minimalist approach MAY be used. Perform discovery as
defined in BRSKI [RFC8995], Appendix B but using the service name
"brski-registrar-cmp" (defined in Section 6) instead of "brski-
registrar" (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
This document requires one IANA action: register in the Service Name
and Transport Protocol Port Number Registry
(https://www.iana.org/assignments/service-names-port-numbers/service-
names-port-numbers.xhtml) the following service name.
*Service Name:* brski-registrar-cmp
*Transport Protocol(s):* tcp
*Assignee:* IESG iesg@ietf.org (mailto:iesg@ietf.org)
*Contact:* IESG iesg@ietf.org (mailto:iesg@ietf.org)
*Description:* Bootstrapping Remote Secure Key Infrastructure
registrar with CMP capabilities according to the Lightweight CMP
Profile (LCMPP, [RFC9483])
*Reference:* [THISRFC]
Note: We chose here the suffix "cmp" rather than some other
abbreviation like "lcmpp" mainly because this document defines the
normative CMP instantiation of BRSKI-AE, which implies adherence to
LCMPP is necessary and sufficient.
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7. Security Considerations
The security considerations laid out in BRSKI [RFC8995] 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 pertains also to obtaining a valid
domain-specific certificate, it must be made sure that a pledge can
not circumvent the registrar in the decision of whether it is granted
an LDevID certificate by the CA. There are various ways how to
fulfill this, including:
* implicit consent
* the registrar signals 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 provides its consent using an extra message that is
transferred on the same channel as the enrollment messages,
possibly in a TLS tunnel.
* the registrar explicitly states its consent by signing, in
addition to the pledge, the authenticated self-contained
certificate enrollment request message.
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 extension.
When CMP is used, the security considerations laid out in the LCMPP
[RFC9483] apply.
Note that CMP messages are not encrypted. This may give
eavesdroppers insight into which devices are bootstrapped into the
domain, and this in turn might also be used to selectively block the
enrollment of certain devices. To prevent this, the underlying
message transport channel can be encrypted, for instance by employing
TLS. For the communication between the pledge and the registrar, the
use of TLS is already provided but needs to be obeyed for the further
transport from the registrar to a backend RA.
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8. 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), Michael
Richardson (ANIMA design team member), as well as Rajeev Ranjan,
Rufus Buschart, Andreas Reiter, and Szofia Fazekas-Zisch (Siemens
colleagues) for their reviews with suggestions for improvements.
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/rfc/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/rfc/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/rfc/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/rfc/rfc8995>.
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[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/rfc/rfc9483>.
9.2. Informative References
[BRSKI-AE-overview]
S. Fries and D. von Oheimb, "BRSKI-AE Protocol Overview",
March 2023,
<https://datatracker.ietf.org/meeting/116/materials/
slides-116-anima-update-on-brski-ae-alternative-
enrollment-protocols-in-brski-00>. Graphics on slide 4 of
the status update on the BRSKI-AE draft 04 at IETF 116.
[I-D.eckert-anima-brski-discovery]
Eckert, T. T., von Oheimb, D., and E. Dijk, "Discovery for
BRSKI variations", Work in Progress, Internet-Draft,
draft-eckert-anima-brski-discovery-01, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-eckert-anima-
brski-discovery-01>.
[I-D.ietf-anima-constrained-voucher]
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-24, 3 March
2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
anima-constrained-voucher-24>.
[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.
[ISO-IEC-15118-2]
International Standardization Organization / International
Electrotechnical Commission, "ISO/IEC 15118-2 Road
vehicles - Vehicle-to-Grid Communication Interface - Part
2: Network and application protocol requirements", ISO/
IEC 15118-2, April 2014.
[NERC-CIP-005-5]
North American Reliability Council, "Cyber Security -
Electronic Security Perimeter", CIP 005-5, December 2013.
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[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/rfc/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/rfc/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/rfc/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/rfc/rfc5272>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/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/rfc/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/rfc/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/rfc/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/rfc/rfc8366>.
[RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol",
RFC 8894, DOI 10.17487/RFC8894, September 2020,
<https://www.rfc-editor.org/rfc/rfc8894>.
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[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/rfc/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/rfc/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/rfc/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/rfc/rfc9482>.
[UNISIG-Subset-137]
UNISIG, "Subset-137; ERTMS/ETCS On-line Key Management
FFFIS; V1.0.0", 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>.
http://www.kmc-subset137.eu/index.php/download/
Appendix A. Application Examples
This informative annex provides some detail about application
examples.
A.1. Rolling Stock
Rolling stock or railroad cars contain a variety of sensors,
actuators, and controllers, which communicate within the railroad car
but also exchange information between railroad cars forming a train,
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
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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 specifying 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 operated in a substation. Communication between
the substation and control center is performed through a
proxy/gateway/DMZ, which terminates protocol flows. Note that
[NERC-CIP-005-5] requires inspection of protocols at the boundary of
a security perimeter (the substation in this case). In addition,
security management in substation automation assumes central support
of several enrollment protocols to support the various capabilities
of IEDs from different vendors. The IEC standard IEC62351-9
[IEC-62351-9] specifies for the infrastructure side mandatory support
of two enrollment protocols: SCEP [RFC8894] and EST [RFC7030], while
an Intelligent Electronic Device may support only one of them.
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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 others, on the selected backend
connectivity protocol. In the case of OCPP, this protocol is meant
to be the only communication protocol between the charging point and
the backend, carrying all information to control the charging
operations and maintain the charging point itself. This means that
the certificate management needs to be handled in-band of OCPP. This
requires the ability to encapsulate the certificate management
messages in a transport-independent way. Authenticated self-
containment will support this by allowing the transport without a
separate enrollment protocol, binding the messages to the identity of
the communicating endpoints.
A.5. Infrastructure Isolation Policy
This refers to any case in which network infrastructure is normally
isolated from the Internet as a matter of policy, most likely for
security reasons. In such a case, limited access to external PKI
services will be allowed in carefully controlled short periods of
time, for example when a batch of new devices is deployed, and
forbidden or prevented at other times.
A.6. Sites with Insufficient Level 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.
Especially 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 can not 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.
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Appendix B. History of Changes TBD RFC Editor: please delete
List of reviewers:
* Toerless Eckert (document shepherd)
* Barry Leiba (SECdir)
* Mahesh Jethanandani (IETF area director)
* Michael Richardson (ANIMA design team)
* Rajeev Ranjan, Rufus Buschart, Szofia Fazekas-Zisch, etc.
(Siemens)
* YANGDOCTORS Early review of 2021-08-15
(https://datatracker.ietf.org/doc/review-ietf-anima-brski-async-
enroll-03-yangdoctors-early-rahman-2021-08-15/) referred to the
PRM aspect of draft-ietf-anima-brski-async-enroll-03
(https://datatracker.ietf.org/doc/draft-ietf-anima-brski-async-
enroll/03/). This has been carved out of the draft to a different
one and thus is no more applicable here.
IETF draft ae-10 -> ae-11:
* In response to AD review by Mahesh Jethanandani,
- replace most occurrences of 'Note:' by 'Note that' or the like
- move 2nd paragraph of abstract to the introduction
- remove section 1.2 and merge its first paragraph with the
preceding section
- reconsider normative language, replacing one 'may' by 'MAY' in
section 4.1
- fix several ambiguities and hard-to-read sentences by re-
phrasing them
- make wording more consistent, in particular: 'certification
request'
- fix a number of (mostly grammar) nits
* Improve item on limitations of PKCS#10 regarding keys that cannot
sign
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IETF draft ae-09 -> ae-10:
* Add reference to RFC 8633 at first occurrence of 'voucher' (fixes
#37)
* Update reference of CoAP Transfer for CMP from I-D to RFC 9482
* Move RFC 4210 and RFC 9480 references from normative to
informative
* Fix p10 vs. pkcs10 entry in list of example endpoints in
Section 4.3
* Minor fix in Figure 1 and few text tweaks due to Siemens-internal
review
* Extend the list of reviewers and acknowledgments by two Siemens
colleagues
IETF draft ae-08 -> ae-09:
* In response to review by Toerless,
- tweak abstract to make meaning of 'alternative enrollment' more
clear
- expand on first use not "well-known" abbreviations, such as
'EST',
adding also a references on their first use
- add summary and reason for choosing CMP at end of Section 3.2
- remove paragraph on optimistic discovery in controlled
environments
- mention role of reviewers also in acknowledgments section
* A couple of grammar and spelling fixes
IETF draft ae-07 -> ae-08:
* Update references to service names in Section 5.1
IETF draft ae-06 -> ae-07:
* Update subsections on discovery according to discussion in the
design team
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* In Section 5.1, replace 'mandatory' by 'REQUIRED' regarding
adherence to LCMPP,
in response to SECDIR Last Call Review of ae-06 by Barry Leiba
IETF draft ae-05 -> ae-06:
* Extend section on discovery according to discussion in the design
team
* Make explicit that MASA voucher status telemetry is as in BRSKI
* Add note that on delegation, RA may need info on pledge
authorization
IETF draft ae-04 -> ae-05:
* Remove entries from the terminology section that should be clear
from BRSKI
* Tweak use of the terms IDevID and LDevID and replace PKI RA/CA by
RA/CA
* Add the abbreviation 'LCMPP' for Lightweight CMP Profile to the
terminology section
* State clearly in Section 5.1 that LCMPP is mandatory when using
CMP
* Change URL of BRSKI-AE-overview graphics to slide on IETF 116
meeting material
IETF draft ae-03 -> ae-04:
* In response to SECDIR Early Review of ae-03 by Barry Leiba,
- replace 'end-to-end security' by the more clear 'end-to-end
authentication'
- restrict the meaning of the abbreviation 'AE' to 'Alternative
Enrollment'
- replace 'MAY' by 'may' in requirement on delegated registrar
actions
- re-phrase requirement on certification request exchange,
avoiding MANDATORY
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- mention that further protocol names need be put in Well-Known
URIs registry
- explain consequence of using non-standard endpoints, not
following SHOULD
- remove requirement that 'caPubs' field in CMP responses SHOULD
NOT be used
- add paragraph in security considerations on additional use of
TLS for CMP
* In response to further internal reviews and suggestions for
generalization,
- significantly cut down the introduction because the original
motivations and most explanations are no more needed and would
just make it lengthy to read
- sort out asynchronous vs. offline transfer, off-site vs.
backend components
- improve description of CSRs and proof of possession vs. proof
of origin
- clarify that the channel between pledge and registrar is not
restricted to TLS, but in connection with constrained BRSKI may
also be DTLS. Also move the references to Constrained BRSKI
and CoAPS to better contexts.
- clarify that the registrar must not be circumvented in the
decision to grant and LDevID, and give hints and
recommendations how to make sure this
- clarify that the cert enrollment phase may involve additional
messages and that BRSKI-AE replaces [RFC8995], Section 5.9
(except Section 5.9.4)
- the certificate enrollment protocol needs to support transport
over (D)TLS only as far as its messages are transported between
pledge and registrar.
- the certificate enrollment protocol chosen between pledge and
registrar needs to be used also for the upstream enrollment
exchange with the PKI only if end-to-end authentication shall
be achieved across the registrar to the PKI.
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- add that with CMP, further trust anchors SHOULD be transported
via caPubs
- remove the former Appendix A: "Using EST for Certificate
Enrollment", moving relevant points to the list of scenarios in
Section 1.1: "Supported Scenarios",
- streamline the item on EST in Section 3.2: "Solution Options
for Proof of Identity",
- various minor editorial improvements like making the wording
more consistent
IETF draft ae-02 -> ae-03:
* In response to review by Toerless Eckert,
- many editorial improvements and clarifications as suggested,
such as the comparison to plain BRSKI, the description of
offline vs. synchronous message transfer and enrollment, and
better differentiation of RA flavors.
- clarify that for transporting certificate enrollment messages
between pledge and registrar, the TLS channel established
between these two (via the join proxy) is used and the
enrollment protocol MUST support this.
- clarify that the enrollment protocol chosen between pledge and
registrar MUST also be used for the upstream enrollment
exchange with the PKI.
- extend the description and requirements on how during the
certificate enrollment phase the registrar MAY handle requests
by the pledge itself and otherwise MUST forward them to the PKI
and forward responses to the pledge.
* Change "The registrar MAY offer different enrollment protocols" to
"The registrar MUST support at least one certificate enrollment
protocol ..."
* In response to review by Michael Richardson,
- slightly improve the structuring of the Message Exchange
Section 4.2 and add some detail on the request/response
exchanges for the enrollment phase
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- merge the 'Enhancements to the Addressing Scheme' Section 4.3
with the subsequent one: 'Domain Registrar Support of
Alternative Enrollment Protocols'
- add reference to SZTP (RFC 8572)
- extend venue information
- convert output of ASCII-art figures to SVG format
- various small other text improvements as suggested/provided
* Remove the tentative informative application to EST-fullCMC
* Move Eliot Lear from co-author to contributor, add Eliot to the
acknowledgments
* Add explanations for terms such as 'target domain' and 'caPubs'
* Fix minor editorial issues and update some external references
IETF draft ae-01 -> ae-02:
* Architecture: clarify registrar role including RA/LRA/enrollment
proxy
* CMP: add reference to CoAP Transport for CMPV2 and Constrained
BRSKI
* Include venue information
From IETF draft 05 -> IETF draft ae-01:
* Renamed the repo and files from 'anima-brski-async-enroll' to
'anima-brski-ae'
* Added graphics for abstract protocol overview as suggested by
Toerless Eckert
* Balanced (sub-)sections and their headers
* Added details on CMP instance, now called BRSKI-CMP
From IETF draft 04 -> IETF draft 05:
* David von Oheimb became the editor.
* Streamline wording, consolidate terminology, improve grammar, etc.
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* Shift the emphasis towards supporting alternative enrollment
protocols.
* Update the title accordingly - preliminary change to be approved.
* Move comments on EST and detailed application examples to
informative annex.
* Move the remaining text of section 3 as two new sub-sections of
section 1.
From IETF draft 03 -> IETF draft 04:
* Moved UC2-related parts defining the pledge in responder mode to a
separate document. This required changes and adaptations in
several sections. Main changes concerned the removal of the
subsection for UC2 as well as the removal of the YANG model
related text as it is not applicable in UC1.
* Updated references to the Lightweight CMP Profile (LCMPP).
* Added David von Oheimb as co-author.
From IETF draft 02 -> IETF draft 03:
* Housekeeping, deleted open issue regarding YANG voucher-request in
UC2 as voucher-request was enhanced with additional leaf.
* Included open issues in YANG model in UC2 regarding assertion
value agent-proximity and CSR encapsulation using SZTP sub
module).
From IETF draft 01 -> IETF draft 02:
* Defined call flow and objects for interactions in UC2. Object
format based on draft for JOSE signed voucher artifacts and
aligned the remaining objects with this approach in UC2 .
* Terminology change: issue #2 pledge-agent -> registrar-agent to
better underline agent relation.
* Terminology change: issue #3 PULL/PUSH -> pledge-initiator-mode
and pledge-responder-mode to better address the pledge operation.
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* Communication approach between pledge and registrar-agent changed
by removing TLS-PSK (former section TLS establishment) and
associated references to other drafts in favor of relying on
higher layer exchange of signed data objects. These data objects
are included also in the pledge-voucher-request and lead to an
extension of the YANG module for the voucher-request (issue #12).
* Details on trust relationship between registrar-agent and
registrar (issue #4, #5, #9) included in UC2.
* Recommendation regarding short-lived certificates for registrar-
agent authentication towards registrar (issue #7) in the security
considerations.
* Introduction of reference to agent signing certificate using SKID
in agent signed data (issue #11).
* Enhanced objects in exchanges between pledge and registrar-agent
to allow the registrar to verify agent-proximity to the pledge
(issue #1) in UC2.
* Details on trust relationship between registrar-agent and pledge
(issue #5) included in UC2.
* Split of use case 2 call flow into sub sections in UC2.
From IETF draft 00 -> IETF draft 01:
* Update of scope in Section 1.1 to include in which the pledge acts
as a server. This is one main motivation for use case 2.
* Rework of use case 2 to consider the transport between the pledge
and the pledge-agent. Addressed is the TLS channel establishment
between the pledge-agent and the pledge as well as the endpoint
definition on the pledge.
* First description of exchanged object types (needs more work)
* Clarification in discovery options for enrollment endpoints at the
domain registrar based on well-known endpoints in Section 4.3 do
not result in additional /.well-known URIs. Update of the
illustrative example. Note that the change to /brski for the
voucher-related endpoints has been taken over in the BRSKI main
document.
* Updated references.
* Included Thomas Werner as additional author for the document.
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From individual version 03 -> IETF draft 00:
* Inclusion of discovery options of enrollment endpoints at the
domain registrar based on well-known endpoints in Section 4.3 as
replacement of section 5.1.3 in the individual draft. This is
intended to support both use cases in the document. An
illustrative example is provided.
* Missing details provided for the description and call flow in
pledge-agent use case UC2, e.g. to accommodate distribution of CA
certificates.
* Updated CMP example in Section 5 to use Lightweight CMP instead of
CMP, as the draft already provides the necessary /.well-known
endpoints.
* Requirements discussion moved to separate section in Section 3.
Shortened description of proof-of-identity binding and mapping to
existing protocols.
* Removal of copied call flows for voucher exchange and registrar
discovery flow from [RFC8995] in Section 4 to avoid doubling or
text or inconsistencies.
* Reworked abstract and introduction to be more crisp regarding the
targeted solution. Several structural changes in the document to
have a better distinction between requirements, use case
description, and solution description as separate sections.
History moved to appendix.
From individual version 02 -> 03:
* Update of terminology from self-contained to authenticated self-
contained object to be consistent in the wording and to underline
the protection of the object with an existing credential. Note
that the naming of this object may be discussed. An alternative
name may be attestation object.
* Simplification of the architecture approach for the initial use
case having an off-site PKI.
* Introduction of a new use case utilizing authenticated self-
contain objects to onboard a pledge using a commissioning tool
containing a pledge-agent. This requires additional changes in
the BRSKI call flow sequence and led to changes in the
introduction, the application example,and also in the related
BRSKI-AE call flow.
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* Update of provided examples of the addressing approach used in
BRSKI to allow for support of multiple enrollment protocols in
Section 4.3.
From individual version 01 -> 02:
* Update of introduction text to clearly relate to the usage of
IDevID and LDevID.
* Definition of the addressing approach used in BRSKI to allow for
support of multiple enrollment protocols in Section 4.3. This
section also contains a first discussion of an optional discovery
mechanism to address situations in which the registrar supports
more than one enrollment approach. Discovery should avoid that
the pledge performs a trial and error of enrollment protocols.
* Update of description of architecture elements and changes to
BRSKI in Section 4.1.
* Enhanced consideration of existing enrollment protocols in the
context of mapping the requirements to existing solutions in
Section 3 and in Section 5.
From individual version 00 -> 01:
* Update of examples, specifically for building automation as well
as two new application use cases in Appendix A.
* Deletion of asynchronous interaction with MASA to not complicate
the use case. Note that the voucher exchange can already be
handled in an asynchronous manner and is therefore not considered
further. This resulted in removal of the alternative path the
MASA in Figure 1 and the associated description in Section 4.1.
* Enhancement of description of architecture elements and changes to
BRSKI in Section 4.1.
* Consideration of existing enrollment protocols in the context of
mapping the requirements to existing solutions in Section 3.
* New section starting Section 5 with the mapping to existing
enrollment protocols by collecting boundary conditions.
Contributors
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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/
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