Internet DRAFT - draft-ietf-anima-brski-async-enroll
draft-ietf-anima-brski-async-enroll
ANIMA WG D. von Oheimb, Ed.
Internet-Draft S. Fries
Intended status: Standards Track H. Brockhaus
Expires: 8 September 2022 Siemens
E. Lear
Cisco Systems
7 March 2022
BRSKI-AE: Alternative Enrollment Protocols in BRSKI
draft-ietf-anima-brski-async-enroll-05
Abstract
This document enhances Bootstrapping Remote Secure Key Infrastructure
(BRSKI, [RFC8995]) to allow employing alternative enrollment
protocols, such as CMP.
Using self-contained signed objects, the origin of enrollment
requests and responses can be authenticated independently of message
transfer. This supports end-to-end security and asynchronous
operation of certificate enrollment and provides flexibility where to
authenticate and authorize certification requests.
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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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 8 September 2022.
Copyright Notice
Copyright (c) 2022 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Supported environment . . . . . . . . . . . . . . . . . . 5
1.3. List of application examples . . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Requirements discussion and mapping to solution elements . . 7
4. Adaptations to BRSKI . . . . . . . . . . . . . . . . . . . . 10
4.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Message exchange . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Pledge - Registrar discovery and voucher exchange . . 13
4.2.2. Registrar - MASA voucher exchange . . . . . . . . . . 13
4.2.3. Pledge - Registrar - RA/CA certificate enrollment . . 13
4.2.4. Pledge - Registrar - enrollment status telemetry . . 16
4.2.5. Addressing scheme enhancements . . . . . . . . . . . 16
4.3. Domain registrar support of alternative enrollment
protocols . . . . . . . . . . . . . . . . . . . . . . . . 16
5. Examples for signature-wrapping using existing enrollment
protocols . . . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Instantiation to EST (informative) . . . . . . . . . . . 17
5.2. Instantiation to CMP (normative if CMP is chosen) . . . . 18
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 20
Appendix A. Using EST for certificate enrollment . . . . . . . . 21
Appendix B. Application examples . . . . . . . . . . . . . . . . 22
B.1. Rolling stock . . . . . . . . . . . . . . . . . . . . . . 23
B.2. Building automation . . . . . . . . . . . . . . . . . . . 23
B.3. Substation automation . . . . . . . . . . . . . . . . . . 24
B.4. Electric vehicle charging infrastructure . . . . . . . . 24
B.5. Infrastructure isolation policy . . . . . . . . . . . . . 24
B.6. Sites with insufficient level of operational security . . 25
Appendix C. History of changes TBD RFC Editor: please delete . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
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1. Introduction
1.1. Motivation
BRSKI, as defined in [RFC8995], specifies a solution for secure
automated zero-touch bootstrapping of new devices, so-called pledges.
This includes the discovery of the registrar in the target domain,
time synchronization, and the exchange of security information
necessary to establish mutual trust between pledges and the target
domain.
A pledge gains trust in the target domain via the domain registrar as
follows. It obtains security information about the domain,
specifically a domain certificate to be trusted, by requesting a
voucher object defined in [RFC8366]. Such a voucher is a self-
contained signed object originating from a Manufacturer Authorized
Signing Authority (MASA). Therefore, the voucher may be provided in
online mode (synchronously) or offline mode (asynchronously). The
pledge can authenticate the voucher because it is shipped with a
trust anchor of its manufacturer such that it can validate signatures
(including related certificates) by the MASA.
Trust by the target domain in a pledge is established by providing
the pledge with a domain-specific LDevID certificate. The
certification request of the pledge is signed using its IDevID secret
and can be validated by the target domain using the trust anchor of
the pledge manufacturer, which needs to pre-installed in the domain.
For enrolling devices with LDevID certificates, BRSKI typically
utilizes Enrollment over Secure Transport (EST) [RFC7030]. EST has
its specific characteristics, detailed in Appendix A. In particular,
it requires online or on-site availability of the RA for performing
the data origin authentication and final authorization decision on
the certification request. This type of enrollment can be called
'synchronous enrollment'. For various reasons, it may be preferable
to use alternative enrollment protocols such as the Certificate
Management Protocol (CMP) [RFC4210] profiled in
[I-D.ietf-lamps-lightweight-cmp-profile] or Certificate Management
over CMS (CMC) [RFC5272]. that are more flexible and independent of
the transfer mechanism because they represent certification request
messages as authenticated self-contained objects.
Depending on the application scenario, the required RA/CA components
may not be part of the registrar. They even may not be available on-
site but rather be provided by remote backend systems. The registrar
or its deployment site may not have an online connection with them or
the connectivity may be intermittent. This may be due to security
requirements for operating the backend systems or due to site
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deployments where on-site or always-online operation may be not
feasible or too costly. In such scenarios, the authentication and
authorization of certification requests will not or can not be
performed on-site at enrollment time. In this document, enrollment
that is not performed in a (time-wise) consistent way is called
_asynchronous enrollment_. Asynchronous enrollment requires a store-
and-forward transfer of certification requests along with the
information needed for authenticating the requester. This allows
offline processing the request.
Application scenarios may also involve network segmentation, which is
utilized in industrial systems to separate domains with different
security needs. Such scenarios lead to similar requirements if the
TLS connection carrying the requester authentication is terminated
and thus request messages need to be forwarded on further channels
before the registrar/RA can authorize the certification request. In
order to preserve the requester authentication, authentication
information needs to be retained and ideally bound directly to the
certification request.
There are basically two approaches for forwarding certification
requests along with requester authentication information:
* A trusted component (e.g., a local RA) in the target domain is
needed that forwards the certification request combined with the
validated identity of the requester (e,g., its IDevID certificate)
and an indication of successful verification of the proof-of-
possession (of the corresponding private key) in a way preventing
changes to the combined information. When connectivity is
available, the trusted component forwards the certification
request together with the requester information (authentication
and proof-of-possession) for further processing. This approach
offers only hop-by-hop security. The backend PKI must rely on the
local pledge authentication result provided by the local RA when
performing the authorization of the certification request. In
BRSKI, the EST server is such a trusted component, being co-
located with the registrar in the target domain.
* Involved components use authenticated self-contained objects for
the enrollment, directly binding the certification request and the
requester authentication in a cryptographic way. This approach
supports end-to-end security, without the need to trust in
intermediate domain components. Manipulation of the request and
the requester identity information can be detected during the
validation of the self-contained signed object.
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Focus of this document is the support of alternative enrollment
protocols that allow using authenticated self-contained objects for
device credential bootstrapping. This enhancement of BRSKI is named
BRSKI-AE, where AE stands for alternative enrollment protocols and
for asynchronous enrollment. This specification carries over the
main characteristics of BRSKI, namely that the pledge obtains trust
anchor information for authenticating the domain registrar and other
target domain components as well as a domain-specific X.509 device
certificate (the LDevID certificate) along with the corresponding
private key (the LDevID secret) and certificate chain.
The goals are to enhance BRSKI to
* support alternative enrollment protocols,
* support end-to-end security for enrollment, and
* make it applicable to scenarios involving asynchronous enrollment.
This is achieved by
* extending the well-known URI approach with an additional path
element indicating the enrollment protocol being used, and
* defining a certificate waiting indication and handling, for the
case that the certifying component is (temporarily) not available.
This specification can be applied to both synchronous and
asynchronous enrollment.
In contrast to BRSKI, this specification supports offering multiple
enrollment protocols on the infrastructure side, which enables
pledges and their developers to pick the preferred one.
1.2. Supported environment
BRSKI-AE is intended to be used in domains that may have limited
support of on-site PKI services and comprises application scenarios
like the following.
* There are requirements or implementation restrictions that do not
allow using EST for enrolling an LDevID certificate.
* Pledges and/or the target domain already have an established
certificate management approach different from EST that shall be
reused (e.g., in brownfield installations).
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* There is no registration authority available on site in the target
domain. Connectivity to an off-site RA is intermittent or
entirely offline. A store-and-forward mechanism is used for
communicating with the off-site services.
* Authoritative actions of a local RA are limited and may not be
sufficient for authorizing certification requests by pledges.
Final authorization is done by an RA residing in the operator
domain.
1.3. List of application examples
Bootstrapping can be handled in various ways, depending on the
application domains. The informative Appendix B provides
illustrative examples from various industrial control system
environments and operational setups. They motivate the support of
alternative enrollment protocols, based on the following examples of
operational environments:
* Rolling stock
* Building automation
* Electrical substation automation
* Electric vehicle charging infrastructures
* Infrastructure isolation policy
* Sites with insufficient level of operational security
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This document relies on the terminology defined in [RFC8995] and
[IEEE.802.1AR_2009].The following terms are defined in addition:
EE: End entity, in the BRSKI context called pledge. It is the
entity that is bootstrapped to the target domain. It holds a
public-private key pair, for which it requests a public-key
certificate. An identifier for the EE is given as the subject
name of the certificate.
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RA: Registration authority, an optional system component to which a
CA delegates certificate management functions such as
authenticating requesters and performing authorization checks on
certification requests.
CA: Certification authority, issues certificates and provides
certificate status information.
target domain: The set of entities that share a common local trust
anchor, independent of where the entities are deployed.
site: Describes the locality where an entity, e.g., pledge,
registrar, RA, CA, is deployed. Different sites can belong to the
same target domain.
on-site: Describes a component or service or functionality available
in the target deployment site.
off-site: Describes a component or service or functionality
available in an operator site different from the target deployment
site. This may be a central site or a cloud service, to which
only a temporary connection is available.
asynchronous communication: Describes a time-wise interrupted
communication between a pledge (EE) and a registrar or PKI
component.
synchronous communication: Describes a time-wise uninterrupted
communication between a pledge (EE) and a registrar or PKI
component.
authenticated self-contained object: Describes in this context an
object that is cryptographically bound to the IDevID certificate
of a pledge. The binding is assumed to be provided through a
digital signature of the actual object using the IDevID secret.
3. Requirements discussion and mapping to solution elements
There were two main drivers for the definition of BRSKI-AE:
* The solution architecture may already use or require a certificate
management protocol other than EST. Therefore, this other
protocol should be usable for requesting LDevID certificates.
* The domain registrar may not be the (final) point that
authenticates and authorizes certification requests and the pledge
may not have a direct connection to it. Therefore, certification
requests should be self-contained signed objects.
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Based on the intended target environment described in Section 1.2 and
the application examples described in Appendix B, the following
requirements are derived to support authenticated self-contained
objects as containers carrying certification requests.
At least the following properties are required:
* 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.
* proof-of-identity: provides data origin authentication of the
certification request. This typically is achieved by a signature
using the IDevID secret of the pledge.
Here is an incomplete list of solution examples, based on existing
technology described in IETF documents:
* Certification request objects: Certification requests are data
structures protecting only the integrity of the contained data and
providing proof-of-possession for a (locally generated) private
key. Examples for certification request data structures are:
- PKCS#10 [RFC2986]. This certification request structure is
self-signed to protect its integrity and prove possession of
the private key that corresponds to the public key included in
the request.
- CRMF [RFC4211]. Also this certificate request message format
supports integrity protection and proof-of-possession,
typically by a self-signature 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 support any signature
algorithm.
The integrity protection of certification request fields includes
the public key because it is part of the data signed by the
corresponding private key. Yet note that for the above examples
this is not sufficient to provide data origin authentication,
i.e., proof-of-identity. This extra property can be achieved by
an additional binding to the IDevID of the pledge. This binding
to source authentication supports the authorization decision for
the certification request. The binding of data origin
authentication to the certification request may be delegated to
the protocol used for certificate management.
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* Solution options for proof-of-identity: The certification request
should be bound to an existing authenticated credential (here, the
IDevID certificate) to enable a proof of identity and, based on
it, an authorization of the certification request. The binding
may be achieved through security options in an underlying
transport protocol such as TLS if the authorization of the
certification request is (completely) done at the next
communication hop. This binding can also be done in a transport-
independent way by wrapping the certification request with
signature employing an existing IDevID. the BRSKI context, this
will be the IDevID. This requirement is addressed by existing
enrollment protocols in various ways, such as:
- EST [RFC7030] utilizes PKCS#10 to encode the certification
request. The Certificate Signing Request (CSR) optionally
provides a binding to the underlying TLS session by including
the tls-unique value in the self-signed PKCS#10 structure. The
tls-unique value results from the TLS handshake. Since the TLS
handshake includes client authentication and the pledge
utilizes its IDevID for it, the proof-of-identity is provided
by such a binding to the TLS session. This can be supported
using the EST /simpleenroll endpoint. Note that the binding of
the TLS handshake to the CSR is optional in EST. As an
alternative to binding to the underlying TLS authentication in
the transport layer, [RFC7030] sketches wrapping the CSR with a
Full PKI Request message using an existing certificate.
- 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. Thus SCEP does not rely on the security of the
underlying transfer.
- CMP [RFC4210] 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 transfer protocol.
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- 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 transfer protocol.
4. Adaptations to BRSKI
In order to support alternative enrollment protocols, asynchronous
enrollment, and more general system architectures, BRSKI-AE lifts
some restrictions of BRSKI [RFC8995]. This way, authenticated self-
contained objects such as those described in Section 3 above can be
used for certificate enrollment.
The enhancements needed are kept to a minimum in order to ensure
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.
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.
Consequently, the authentication and authorization of the
certification request MAY be done by the domain registrar and/or by
other domain components. These components may be offline or reside
in some central backend of the domain operator (off-site) as
described in Section 1.2. The registrar and other on-site domain
components may have no or only temporary (intermittent) connectivity
to them. The certification request MAY also be piggybacked on
another protocol.
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/ | .
| <-------->............<-------> Enrollment | .
| | . | BRSKI-AE | Proxy/LRA | .
| IDevID | . | | +------^-----+ .
| | . +------------+ | .
| | . | .
+--------+ ...............................|.........
on-site "domain" components |
| e.g., RFC 4210,
| RFC 7030, ...
.............................................|.....................
. +---------------------------+ +--------v------------------+ .
. | Public-Key Infrastructure <-----+ Registration Authority | .
. | PKI CA +-----> PKI RA | .
. +---------------------------+ +---------------------------+ .
...................................................................
off-site or central "domain" components
Figure 1: Architecture overview using off-site PKI components
The architecture overview in Figure 1 has the same logical elements
as BRSKI, but with 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, or none of them.
The following list describes the on-site components in the target
domain of the pledge shown in Figure 1.
* Join Proxy: same functionality as described in BRSKI [RFC8995].
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* Domain Registrar / Enrollment Proxy / LRA: in BRSKI-AE, the domain
registrar has mostly the same functionality as in BRSKI, namely to
facilitate the communication of the pledge with the MASA and the
PKI. Yet in contrast to BRSKI, the registrar offers different
enrollment protocols and MAY act as a local registration authority
(LRA) or simply as an enrollment proxy. In such cases, the domain
registrar forwards the certification request to some off-site RA
component, which performs at least part of the authorization.
This also covers the case that the registrar has only intermittent
connection and forwards the certification request to the RA upon
re-established connectivity.
Note: To support alternative enrollment protocols, the URI scheme
for addressing the domain registrar is generalized (see
Section 4.2.5).
The following list describes the components provided by the vendor or
manufacturer outside the target domain.
* MASA: general functionality as described in BRSKI [RFC8995]. The
voucher exchange with the MASA via the domain registrar is
performed as described in BRSKI.
Note: The interaction with the MASA may be synchronous (voucher
request with nonce) or asynchronous (voucher request without
nonce).
* Ownership tracker: as defined in BRSKI.
The following list describes the target domain components that can
optionally be operated in the off-site backend of the target domain.
* PKI RA: Performs certificate management functions for the domain
as a centralized 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.
* PKI CA: Performs certificate generation by signing the certificate
structure requested in already authenticated and authorized
certification requests.
Based on the diagram in Section 2.1 of BRSKI [RFC8995] and the
architectural changes, the original protocol flow is divided into
three phases showing commonalities and differences to the original
approach as follows.
* Discovery phase: same as in BRSKI steps (1) and (2)
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* Voucher exchange phase: same as in BRSKI steps (3) and (4).
* Enrollment phase: step (5) is changed to employing an alternative
enrollment protocol that uses authenticated self-contained
objects.
4.2. Message exchange
The behavior of a pledge described in Section 2.1 of BRSKI [RFC8995]
is kept with one exception. After finishing the Imprint step (4),
the Enroll step (5) MUST be performed with an enrollment protocol
utilizing authenticated self-contained objects. Section 5 discusses
selected suitable enrollment protocols and options applicable.
4.2.1. Pledge - Registrar discovery and voucher exchange
The discovery phase and voucher exchange are applied as specified in
[RFC8995].
4.2.2. Registrar - MASA voucher exchange
This voucher exchange is performed as specified in [RFC8995].
4.2.3. Pledge - Registrar - RA/CA certificate enrollment
As stated in Section 3, the enrollment MUST be performed using an
authenticated self-contained object providing not only proof-of-
possession but also proof-of-identity (source authentication).
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+--------+ +------------+ +------------+
| Pledge | | Domain | | Operator |
| | | Registrar | | RA/CA |
| | | (JRC) | | (OPKI) |
+--------+ +------------+ +------------+
/--> | |
[Optional request of CA certificates] | |
|---------- CA Certs Request ------------>| |
| [if connection to operator domain is available] |
| |-Request CA Certs ->|
| |<-CA Certs Response-|
|<-------- CA Certs Response--------------| |
/--> | |
[Optional request of attributes to be included in Cert Request] |
|---------- Attribute Request ----------->| |
| [if connection to operator domain is available] |
| |-Attribute Request->|
| |<- Attrib Response -|
|<--------- Attribute Response -----------| |
/--> | |
[Certification request] | |
|-------------- Cert Request ------------>| |
| [when connection to off-site components is unavailable] |
|<----- optional: Cert Waiting Response --| |
| | |
|-------optional: Cert Polling ---------->| |
| | |
| [when connection to off-site components is available] |
| |--- Cert Request -->|
| |<-- Cert Response --|
|<------------- Cert Response ------------| |
/--> | |
[Optional certificate confirmation] | |
|-------------- Cert Confirm ------------>| |
| |--- Cert Confirm -->|
| |<-- PKI Confirm ----|
|<------------- PKI/Registrar Confirm ----| |
Figure 2: Certificate enrollment
The following list provides an abstract description of the flow
depicted in Figure 2.
* CA Cert Request: 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 may be just the domain
registrar certificate).
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* CA Cert Response: It MUST contain the current root CA certificate,
which typically is the LDevID trust anchor, and any additional
certificates that the pledge may need to validate certificates.
* Attribute Request: Typically, the automated bootstrapping occurs
without local administrative configuration of the pledge.
Nevertheless, there are cases in which the pledge may also include
additional attributes specific to the target domain into the
certification request. To get these attributes in advance, the
attribute request can be used.
* Attribute Response: It MUST contain the attributes to be included
in the subsequent certification request.
* Cert Request: This certification request MUST contain the
authenticated self-contained object ensuring both proof-of-
possession of the corresponding private key and proof-of-identity
of the requester.
* Cert Response: The certification response message MUST contain on
success the requested certificate and MAY include further
information, like certificates of intermediate CAs.
* Cert Waiting Response: Optional waiting indication for the pledge,
which SHOULD poll for a Cert Response after a given time. To this
end, a request identifier is necessary. The request identifier
may be either part of the enrollment protocol or can be derived
from the certification request.
* Cert Polling: This SHOULD be used by the pledge in reaction to a
Cert Waiting Response to query the registrar whether the
certification request meanwhile has been processed. It MUST be
answered either by another Cert Waiting, or the Cert Response.
* Cert Confirm: An optional confirmation sent after the requested
certificate has been received and validated. It contains a
positive or negative confirmation by the pledge whether the
certificate was successfully enrolled and fits its needs.
* PKI/Registrar Confirm: An acknowledgment by the PKI or registrar
that MUST be sent on reception of the Cert Confirm.
The generic messages described above may be implemented using various
enrollment protocols supporting authenticated self-contained objects,
as described in Section 3. Examples are available in Section 5.
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4.2.4. Pledge - Registrar - enrollment status telemetry
The enrollment status telemetry is performed as specified in
[RFC8995]. In BRSKI this is described as part of the enrollment
phase, but due to the generalization on the enrollment protocol
described in this document it fits better as a separate step here.
4.2.5. Addressing scheme enhancements
BRSKI-AE provides generalizations to the addressing scheme defined in
BRSKI [RFC8995] to accommodate alternative enrollment protocols that
use authenticated self-contained objects for certification requests.
As this is supported by various existing enrollment protocols, they
can be directly employed (see also Section 5).
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 on 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 in order to provide maximal compatibility to BRSKI:
* <enrollment-protocol>: MUST reference the protocol being used,
which MAY be CMP, CMC, SCEP, EST [RFC7030] as in BRSKI, or a newly
defined approach.
Note: additional endpoints (well-known URIs) at the registrar may
need to be defined by the enrollment protocol being used.
* <request>: if present, the <request> 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).
4.3. Domain registrar support of alternative enrollment protocols
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 at the registrar. The pledge will recognize whether its
preferred enrollment option is supported by the domain registrar by
sending a request to its preferred enrollment endpoint and evaluating
the HTTP response status code.
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The following list of endpoints provides an illustrative example for
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 [I-D.ietf-lamps-cmp-updates] and the Lightweight CMP
profile [I-D.ietf-lamps-lightweight-cmp-profile].
</brski/voucherrequest>,ct=voucher-cms+json
</brski/voucher_status>,ct=json
</brski/enrollstatus>,ct=json
</est/cacerts>;ct=pkcs7-mime
</est/fullcmc>;ct=pkcs7-mime
</est/csrattrs>;ct=pkcs7-mime
</cmp/initialization>;ct=pkixcmp
</cmp/p10>;ct=pkixcmp
</cmp/getcacerts>;ct=pkixcmp
</cmp/getcertreqtemplate>;ct=pkixcmp
5. Examples for signature-wrapping using existing enrollment protocols
This section maps the requirements to support proof-of-possession and
proof-of-identity to selected existing enrollment protocols.
5.1. Instantiation to EST (informative)
When using EST [RFC7030], the following aspects and constraints need
to be considered and the given extra requirements need to be
fulfilled, which adapt Section 5.9.3 of BRSKI [RFC8995]:
* proof-of-possession is provided typically by using the specified
PKCS#10 structure in the request. Together with Full PKI
requests, also CRMF can be used.
* proof-of-identity needs to be achieved by signing the
certification request object using the Full PKI Request option
(including the /fullcmc endpoint). This provides sufficient
information for the RA to authenticate the pledge as the origin of
the request and to make an authorization decision on the received
certification request. Note: EST references CMC [RFC5272] for the
definition of the Full PKI Request. For proof-of-identity, the
signature of the SignedData of the Full PKI Request is performed
using the IDevID secret of the pledge.
Note: In this case the binding to the underlying TLS connection is
not necessary.
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* When the RA is temporarily not available, as per Section 4.2.3 of
[RFC7030], an HTTP status code 202 should be returned by the
registrar, and the pledge will repeat the initial Full PKI Request
5.2. Instantiation to CMP (normative if CMP is chosen)
Note: Instead of referring to CMP as specified in [RFC4210] and
[I-D.ietf-lamps-cmp-updates], this document refers to the Lightweight
CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile] because the
subset of CMP defined there is sufficient for the functionality
needed here.
When using CMP, the following requirements SHALL be fulfilled:
* For proof-of-possession, the approach defined in Section 4.1.1
(based on CRMF) or Section 4.1.4 (based on PKCS#10) of the
Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile]
SHALL be applied.
* proof-of-identity SHALL be provided by using signature-based
protection of the certification request message as outlined in
Section 3.2. of [I-D.ietf-lamps-lightweight-cmp-profile] using the
IDevID secret.
* When the Cert Response from the RA/CA is not available and if
polling is supported, the registrar SHALL a Cert Waiting Response
as specified in Sections 4.4 and 5.1.2 of
[I-D.ietf-lamps-lightweight-cmp-profile].
* As far as requesting CA certificates or certificate request
attributes is supported, they SHALL be implemented as specified in
Sections 4.3.1 and 4.3.3 of
[I-D.ietf-lamps-lightweight-cmp-profile].
TBD RFC Editor: please delete /* ToDo: The following aspects need to
be further specified: * Whether to use /getcacerts or the caPubs and
extraCerts fields to return trust anchor and CA Certificates *
Whether to use /getcertreqtemplate or modify the CRMF and use
raVerified * Whether to specify the usage of /p10 */
6. IANA Considerations
This document does not require IANA actions.
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7. Security Considerations
The security considerations as laid out in BRSKI [RFC8995] apply for
the discovery and voucher exchange as well as for the status exchange
information.
The security considerations as laid out in the Lightweight CMP
Profile [I-D.ietf-lamps-lightweight-cmp-profile] apply as far as CMP
is used.
8. Acknowledgments
We would like to thank Brian E. Carpenter, Michael Richardson, and
Giorgio Romanenghi for their input and discussion on use cases and
call flows.
9. References
9.1. Normative References
[I-D.ietf-lamps-cmp-updates]
Brockhaus, H., Oheimb, D. V., and J. Gray, "Certificate
Management Protocol (CMP) Updates", Work in Progress,
Internet-Draft, draft-ietf-lamps-cmp-updates-17, 12
January 2022, <https://www.ietf.org/archive/id/draft-ietf-
lamps-cmp-updates-17.txt>.
[I-D.ietf-lamps-lightweight-cmp-profile]
Brockhaus, H., Oheimb, D. V., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", Work in
Progress, Internet-Draft, draft-ietf-lamps-lightweight-
cmp-profile-10, 1 February 2022,
<https://www.ietf.org/archive/id/draft-ietf-lamps-
lightweight-cmp-profile-10.txt>.
[IEEE.802.1AR_2009]
IEEE, "IEEE Standard for Local and metropolitan area
networks - Secure Device Identity", IEEE 802.1AR-2009,
DOI 10.1109/ieeestd.2009.5367679, 28 December 2009,
<http://ieeexplore.ieee.org/servlet/
opac?punumber=5367676>.
[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>.
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[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>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"A Voucher Artifact for Bootstrapping Protocols",
RFC 8366, DOI 10.17487/RFC8366, May 2018,
<https://www.rfc-editor.org/info/rfc8366>.
[RFC8995] Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
and K. Watsen, "Bootstrapping Remote Secure Key
Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
May 2021, <https://www.rfc-editor.org/info/rfc8995>.
9.2. Informative References
[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.
[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>.
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[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>.
[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>.
[RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol",
RFC 8894, DOI 10.17487/RFC8894, September 2020,
<https://www.rfc-editor.org/info/rfc8894>.
[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. Using EST for certificate enrollment
When using EST with BRSKI, pledges interact via TLS with the domain
registrar, which acts both as EST server and as registration
authority (RA). The TLS connection is mutually authenticated, where
the pledge uses its IDevID certificate issued by its manufacturer.
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In order to provide a strong proof-of-origin of the certification
request, EST has the option to include in the certification request
the so-called tls-unique value [RFC5929] of the underlying TLS
channel. This binding of the proof-of-identity of the TLS client,
which is supposed to be the certificate requester, to the proof-of-
possession for the private key is conceptually non-trivial and
requires specific support by TLS implementations.
The registrar terminates the security association with the pledge at
TLS level and thus the binding between the certification request and
the authentication of the pledge. The EST server uses the
authenticated pledge identity provided by the IDevID for checking the
authorization of the pledge for the given certification request
before issuing to the pledge a domain-specific certificate (LDevID
certificate). This approach typically requires online or on-site
availability of the RA for performing the final authorization
decision for the certification request.
Using EST for BRSKI has the advantage that the mutually authenticated
TLS connection established between the pledge and the registrar can
be reused for protecting the message exchange needed for enrolling
the LDevID certificate. This strongly simplifies the implementation
of the enrollment message exchange.
Yet the use of TLS has the limitation that this cannot provide
auditability nor end-to-end security for the certificate enrollment
request because the TLS session is transient and terminates at the
registrar. This is a problem in particular if the enrollment is done
via multiple hops, part of which may not even be network-based.
A further limitation of using EST as the certificate enrollment
protocol is that due to using PKCS#10 structures in enrollment
requests, the only possible proof-of-possession method is a self-
signature, which excludes requesting certificates for key types that
do not support signing.
Appendix B. Application examples
This informative annex provides some detail to the application
examples listed in Section 1.3.
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B.1. Rolling stock
Rolling stock or railroad cars contain a variety of sensors,
actuators, and controllers, which communicate within the railroad car
but also exchange information between railroad cars building a train,
with track-side equipment, and/or possibly with backend systems.
These devices are typically unaware of backend system connectivity.
Managing certificates may be done during maintenance cycles of the
railroad car, but can already be prepared during operation.
Preparation will include generating certification requests, which are
collected and later forwarded for processing, once the railroad car
is connected to the operator backend. The authorization of the
certification request is then done based on the operator's asset/
inventory information in the backend.
UNISIG has included a CMP profile for enrollment of TLS certificates
of on-board and track-side components in the Subset-137 specifying
the ETRAM/ETCS on-line key management for train control systems
[UNISIG-Subset-137].
B.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 with 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 as preparation for the operational phase. In this case,
connectivity to the domain registrar may also be facilitated by the
service technician's laptop.
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B.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 (the substation in this case). In addition,
security management in substation automation assumes central support
of several enrollment protocols in order 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: SCEP [RFC8894] and EST [RFC7030] for the
infrastructure side, while the IED must only support one of the two.
B.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 connections
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.
B.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.
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B.6. Sites with insufficient level of operational security
The registration authority 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.
Especially the CA/Browser forum currently increases the security
requirements in the certificate issuance procedures for publicly
trusted certificates. In case the on-site components of the target
domain cannot be operated securely enough for the needs of a
registration authority, this service should be transferred to an off-
site backend component that has a sufficient level of security.
Appendix C. History of changes TBD RFC Editor: please delete
From IETF draft 04 -> IETF draft 05:
* David von Oheimb became the editor.
* Streamline wording, consolidate terminology, improve grammar, etc.
* 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.
* Added David von Oheimb as co-author.
From IETF draft 02 -> IETF draft 03:
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* 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.
* 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:
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* Update of scope in Section 1.2 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.
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.
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* Reworked abstract and introduction to be more crisp regarding the
targeted solution. Several structural changes in the document to
have a better distinction between requirements, use case
description, and solution description as separate sections.
History moved to appendix.
From individual version 02 -> 03:
* Update of terminology from self-contained to authenticated self-
contained object to be consistent in the wording and to underline
the protection of the object with an existing credential. Note
that the naming of this object may be discussed. An alternative
name may be attestation object.
* Simplification of the architecture approach for the initial use
case having an offsite PKI.
* Introduction of a new use case utilizing authenticated self-
contain objects to onboard a pledge using a commissioning tool
containing a pledge-agent. This requires additional changes in
the BRSKI call flow sequence and led to changes in the
introduction, the application example,and also in the related
BRSKI-AE call flow.
* Update of provided examples of the addressing approach used in
BRSKI to allow for support of multiple enrollment protocols in
Section 4.2.5.
From individual version 01 -> 02:
* Update of introduction text to clearly relate to the usage of
IDevID and LDevID.
* Definition of the addressing approach used in BRSKI to allow for
support of multiple enrollment protocols in Section 4.2.5. This
section also contains a first discussion of an optional discovery
mechanism to address situations in which the registrar supports
more than one enrollment approach. Discovery should avoid that
the pledge performs a trial and error of enrollment protocols.
* Update of description of architecture elements and changes to
BRSKI in Section 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:
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* Update of examples, specifically for building automation as well
as two new application use cases in Appendix B.
* 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.
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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Eliot Lear
Cisco Systems
Richtistrasse 7
CH-8304 Wallisellen
Switzerland
Phone: +41 44 878 9200
Email: lear@cisco.com
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