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draft-ietf-anima-brski-prm.xml
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draft-ietf-anima-brski-prm.xml
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<?xml version='1.0' encoding='utf-8'?>
<!DOCTYPE rfc [
<!ENTITY nbsp " ">
<!ENTITY zwsp "​">
<!ENTITY nbhy "‑">
<!ENTITY wj "⁠">
]>
<?xml-stylesheet type="text/xsl" href="rfc2629.xslt" ?>
<!-- generated by https://github.com/cabo/kramdown-rfc version 1.7.21 (Ruby 3.0.2) -->
<?rfc compact="yes"?>
<?rfc iprnotified="no"?>
<?rfc strict="yes"?>
<rfc xmlns:xi="http://www.w3.org/2001/XInclude" ipr="trust200902" docName="draft-ietf-anima-brski-prm-16" category="std" consensus="true" submissionType="IETF" tocInclude="true" sortRefs="true" symRefs="true" version="3">
<!-- xml2rfc v2v3 conversion 3.25.0 -->
<front>
<title abbrev="BRSKI-PRM">BRSKI with Pledge in Responder Mode (BRSKI-PRM)</title>
<seriesInfo name="Internet-Draft" value="draft-ietf-anima-brski-prm-16"/>
<author initials="S." surname="Fries" fullname="Steffen Fries">
<organization abbrev="Siemens">Siemens AG</organization>
<address>
<postal>
<street>Otto-Hahn-Ring 6</street>
<city>Munich</city>
<code>81739</code>
<country>Germany</country>
</postal>
<email>steffen.fries@siemens.com</email>
<uri>https://www.siemens.com/</uri>
</address>
</author>
<author initials="T." surname="Werner" fullname="Thomas Werner">
<organization abbrev="Siemens">Siemens AG</organization>
<address>
<postal>
<street>Otto-Hahn-Ring 6</street>
<city>Munich</city>
<code>81739</code>
<country>Germany</country>
</postal>
<email>thomas-werner@siemens.com</email>
<uri>https://www.siemens.com/</uri>
</address>
</author>
<author initials="E." surname="Lear" fullname="Eliot Lear">
<organization>Cisco Systems</organization>
<address>
<postal>
<street>Richtistrasse 7</street>
<city>Wallisellen</city>
<code>8304</code>
<country>Switzerland</country>
</postal>
<phone>+41 44 878 9200</phone>
<email>lear@cisco.com</email>
</address>
</author>
<author initials="M." surname="Richardson" fullname="Michael C. Richardson">
<organization>Sandelman Software Works</organization>
<address>
<email>mcr+ietf@sandelman.ca</email>
<uri>http://www.sandelman.ca/</uri>
</address>
</author>
<date year="2025"/>
<area>Operations and Management</area>
<workgroup>ANIMA WG</workgroup>
<abstract>
<?line 134?>
<t>This document defines enhancements to Bootstrapping a Remote Secure Key Infrastructure (BRSKI, RFC8995) to enable bootstrapping in domains featuring no or only limited connectivity between a pledge and the domain registrar.
It specifically changes the interaction model from a pledge-initiated mode, as used in BRSKI, to a pledge-responding mode, where the pledge is in server role.
For this, BRSKI with Pledge in Responder Mode (BRSKI-PRM) introduces new endpoints for the Domain Registrar and pledge, and a new component, the Registrar-Agent, which facilitates the communication between pledge and registrar during the bootstrapping phase.
To establish the trust relation between pledge and registrar, BRSKI-PRM relies on object security rather than transport security.
The approach defined here is agnostic to the enrollment protocol that connects the domain registrar to the Key Infrastructure (e.g., domain CA).</t>
</abstract>
<note removeInRFC="true">
<name>About This Document</name>
<t>
Status information for this document may be found at <eref target="https://datatracker.ietf.org/doc/draft-ietf-anima-brski-prm/"/>.
</t>
<t>Source for this draft and an issue tracker can be found at
<eref target="https://github.com/anima-wg/anima-brski-prm"/>.</t>
</note>
</front>
<middle>
<?line 142?>
<section anchor="introduction">
<name>Introduction</name>
<t>BRSKI as defined in <xref target="RFC8995"/> specifies a solution for secure zero-touch (automated) bootstrapping of devices (pledges) in a customer domain, which may be associated with a specific installation location.
This includes the discovery of the BRSKI registrar in the customer domain and the exchange of security information necessary to establish trust between a pledge and the domain.</t>
<t>Security information about the customer domain, specifically the customer domain certificate, are exchanged and authenticated utilizing signed data objects, the voucher artifacts as defined in <xref target="RFC8995"/>.
In response to a voucher-request, the Manufacturer Authorized Signing Authority (MASA) issues the voucher and provides it via the domain registrar to the pledge.
<xref target="I-D.ietf-anima-rfc8366bis"/> specifies the format of the voucher artifacts, including the voucher-request artifact.</t>
<t>For the certificate enrollment of devices, BRSKI relies on EST <xref target="RFC7030"/> to request and distribute customer domain specific device certificates.
EST in turn relies for the authentication and authorization of the certification request on the credentials used by the underlying TLS between the EST client and the EST server.</t>
<t>BRSKI addresses scenarios in which the pledge initiates the bootstrapping acting as client (referred to as initiator mode by this document).
BRSKI with Pledge in Responder Mode (BRSKI-PRM) defined in this document allows the pledge to act as server, so that it can be triggered externally and at a specific time to generate bootstrapping requests in the customer domain.
For this approach, this document:</t>
<ul spacing="normal">
<li>
<t>defines additional endpoints for the domain registrar and new endpoints for the pledge to enable responder mode.</t>
</li>
<li>
<t>introduces the Registrar-Agent as new component to facilitate the communication between the pledge and the domain registrar.
The Registrar-Agent may be implemented as an integrated functionality of a commissioning tool or be co-located with the domain registrar itself.
BRSKI-PRM supports the identification of the Registrar-Agent that was performing the bootstrapping allowing for accountability of the pledges installation, when the Registrar-Agent is a component used by an installer and not co-located with the domain registrar.</t>
</li>
<li>
<t>specifies additional artifacts for the exchanges between a pledge acting as server, the Registrar-Agent acting as client, and the domain registrar acting as server toward the Registrar-Agent.</t>
</li>
<li>
<t>allows the application of Registrar-Agent credentials to establish TLS connections to the domain registrar; these are different from the pledge IDevID credentials.</t>
</li>
<li>
<t>also enables the usage of alternative transports, both IP-based and non-IP, between the pledge and the domain registrar via the Registrar-Agent;
security is addressed at the application layer through object security with an additional signature wrapping the exchanged artifacts.</t>
</li>
</ul>
<t>The term endpoint used in the context of this document is equivalent to resource in HTTP <xref target="RFC9110"/> and CoAP <xref target="RFC7252"/>; it is not used to describe a device.
Endpoints are accessible via Well-Known URIs <xref target="RFC8615"/>.</t>
<t>To utilize EST <xref target="RFC7030"/> for enrollment, the domain registrar performs pre-processing of the wrapping signature before actually using EST as defined in <xref target="RFC7030"/>.</t>
<t>There may be pledges that can support both modes, initiator and responder mode.
In these cases BRSKI-PRM can be combined with BRSKI as defined in <xref target="RFC8995"/> or BRSKI-AE <xref target="I-D.ietf-anima-brski-ae"/> to allow for more bootstrapping flexibility.</t>
</section>
<section anchor="terminology">
<name>Terminology</name>
<t>The key words "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>", "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>", "<bcp14>SHALL
NOT</bcp14>", "<bcp14>SHOULD</bcp14>", "<bcp14>SHOULD NOT</bcp14>", "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
"<bcp14>MAY</bcp14>", and "<bcp14>OPTIONAL</bcp14>" in this document are to be interpreted as
described in BCP 14 <xref target="RFC2119"/> <xref target="RFC8174"/> when, and only when, they
appear in all capitals, as shown here.</t>
<?line -18?>
<t>This document relies on the terminology defined in <xref section="1.2" sectionFormat="of" target="RFC8995"/>.
The following terms are defined in addition:</t>
<dl>
<dt>authenticated self-contained object:</dt>
<dd>
<t>Describes a data object, which is cryptographically bound to the end entity (EE) certificate.
The binding is assumed to be provided through a digital signature of the actual object using the corresponding private key of the certificate.</t>
</dd>
<dt>CA:</dt>
<dd>
<t>Certification Authority, issues certificates.</t>
</dd>
<dt>Commissioning tool:</dt>
<dd>
<t>Tool to interact with devices to provide configuration data.</t>
</dd>
<dt>CSR:</dt>
<dd>
<t>Certificate Signing Request.</t>
</dd>
<dt>EE:</dt>
<dd>
<t>End entity, as defined in <xref target="RFC9483"/>.
Typically, a device or service that owns a public-private key pair for which it manages a public key certificate.</t>
</dd>
<dt>EE certificate:</dt>
<dd>
<t>the certificate of the EE signed by its owner (e.g., CA).
For domain components, the EE certificate is signed by the domain owner.
For the pledge, the EE certificate is either the IDevID certificate signed by the manufacturer or the LDevID certificate signed by the domain owner or an application-specific EE certificate signed by the domain owner.</t>
</dd>
<dt>endpoint:</dt>
<dd>
<t>Term equivalent to resource in HTTP <xref target="RFC9110"/> and CoAP <xref target="RFC7252"/>.
Endpoints are accessible via Well-Known URIs <xref target="RFC8615"/>.</t>
</dd>
<dt>IDevID:</dt>
<dd>
<t>An Initial Device Identifier X.509 certificate installed by the vendor on new equipment.
This is a term from 802.1AR <xref target="IEEE-802.1AR"/>.</t>
</dd>
<dt>LDevID:</dt>
<dd>
<t>A Local Device Identifier X.509 certificate installed by the owner of the equipment.
This is a term from 802.1AR <xref target="IEEE-802.1AR"/>.</t>
</dd>
<dt>mTLS:</dt>
<dd>
<t>mutual Transport Layer Security.</t>
</dd>
<dt>PER:</dt>
<dd>
<t>Pledge Enroll-Request is a signature-wrapped CSR, signed by the pledge that requests enrollment to a domain via the Registrar-Agent.</t>
</dd>
<dt>POI:</dt>
<dd>
<t>Proof-of-Identity, as defined in <xref target="RFC5272"/>.</t>
</dd>
<dt>POP:</dt>
<dd>
<t>Proof-of-Possession (of a private key), as defined in <xref target="RFC5272"/>.</t>
</dd>
<dt>PVR:</dt>
<dd>
<t>Pledge Voucher-Request is a signature-wrapped voucher-request, signed by the pledge that sends it to the domain registrar via the Registrar-Agent.</t>
</dd>
<dt>RA:</dt>
<dd>
<t>Registration Authority, an optional system component to which a CA delegates certificate management functions such as authorization checks.
In BRSKI-PRM, this is a functionality of the domain registrar, as in BRSKI <xref target="RFC8995"/>.</t>
</dd>
<dt>Registrar_Agent:</dt>
<dd>
<t>Component facilitating the data exchange between a pledge in responder mode and a domain registrar.</t>
</dd>
<dt>RVR:</dt>
<dd>
<t>Registrar Voucher-Request is a signature-wrapped voucher-request, signed by the domain registrar that sends it to the MASA.
For BRSKI-PRM, it contains a copy of the original PVR received from the pledge.</t>
</dd>
</dl>
<t>This document uses the following encoding notations in the given JWS-signed artifact examples:</t>
<dl>
<dt>BASE64(OCTETS):</dt>
<dd>
<t>Denotes the base64 encoding of an octet sequence using the character set defined in <xref section="4" sectionFormat="of" target="RFC4648"/> and without the inclusion of any line breaks, whitespace, or other additional characters.
Note that the base64 encoding of the empty octet sequence is the empty string.</t>
</dd>
<dt>BASE64URL(OCTETS):</dt>
<dd>
<t>Denotes the base64url encoding of an octet sequence, per <xref section="2" sectionFormat="of" target="RFC7515"/>.</t>
</dd>
<dt>UTF8(STRING):</dt>
<dd>
<t>Denotes the octet sequence of the UTF-8 <xref target="RFC3629"/> representation of STRING, per <xref section="1" sectionFormat="of" target="RFC7515"/>.</t>
</dd>
</dl>
<t>This document includes many examples that would contain many long sequences of base64-encoded objects with no content directly comprehensible to a human reader.
In order to keep those examples short, they use the token <tt>base64encodedvalue==</tt> as a placeholder for base64 data.
The full base64 data is included in the appendices of this document.</t>
</section>
<section anchor="scope-of-solution">
<name>Scope of Solution</name>
<section anchor="sup-env">
<name>Supported Environments and Use Case Examples</name>
<t>BRSKI-PRM is applicable to scenarios where pledges may have no direct connection to the domain registrar, may have no continuous connection, or require coordination of the pledge requests to be provided to a domain registrar.</t>
<t>This can be motivated by pledges deployed in environments not yet connected to the operational customer domain network, e.g., at a building construction site, or environments intentionally disconnected from the Internet, e.g., critical industrial facilities.
Another example is the assembly of electrical cabinets, which are prepared in advance before the installation at a customer domain.</t>
<section anchor="building-automation">
<name>Building Automation</name>
<t>In building automation a typical use case exists where a detached building or the basement is equipped with sensors, actuators, and controllers, but with only limited or no connection to the central building management system.
This limited connectivity may exist during installation time or also during operation time.</t>
<t>During the installation, for instance, a service technician collects the device-specific information from the basement network and provides them to the central building management system.
This could be done using a laptop, common mobile device, or dedicated commissioning tool to transport the information.
The service technician may successively collect device-specific information in different parts of the building before connecting to the domain registrar for bulk bootstrapping.</t>
<t>A domain registrar may be part of the central building management system and already be operational in the installation network.
The central building management system can then provide operational parameters for the specific devices in the basement or other detached areas.
These operational parameters may comprise values and settings required in the operational phase of the sensors/actuators, among them a certificate issued by the operator to authenticate against other components and services.
These operational parameters are then provided to the devices in the basement facilitated by the service technician's laptop.
The Registrar-Agent, defined in this document, may be run on the technician's laptop to interact with pledges.</t>
</section>
<section anchor="infrastructure-isolation-policy">
<name>Infrastructure Isolation Policy</name>
<t>This refers to any case in which the 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 a domain registrar may be allowed in carefully controlled short periods of time, for example when a batch of new devices are deployed, but prohibited at other times.</t>
</section>
<section anchor="less-operational-security-in-the-target-domain">
<name>Less Operational Security in the Target-Domain</name>
<t>The registration authority (RA) performing the authorization of a certificate request is a critical PKI component and therefore requires higher operational security than other components utilizing the issued certificates.
CAs may also require higher security in the registration procedures.
There may be situations in which the customer domain does not offer enough physical security to operate an RA/CA and therefore this service is transferred to a backend that offers a higher level of operational security.</t>
</section>
</section>
<section anchor="potential-limitations">
<name>Potential Limitations</name>
<t>The mechanism described in this document presumes the ability of the pledge and the Registrar-Agent to communicate with one another.
This may not be possible in constrained environments where, in particular, power must be conserved.
In these situations, it is anticipated that the transceiver will be powered down most of the time.
This presents a rendezvous problem: the pledge is unavailable for certain periods of time, and the Registrar-Agent is similarly presumed to be unavailable for certain periods of time.
To overcome this situation, the pledges may need to be powered on, either manually or by sending a trigger signal.</t>
</section>
</section>
<section anchor="req-sol">
<name>Requirements Discussion and Mapping to Solution-Elements</name>
<t>Based on the intended target environment described in <xref target="sup-env"/>, the following requirements are derived to support bootstrapping of pledges in responder mode (acting as server):</t>
<ul spacing="normal">
<li>
<t>To facilitate the communication between a pledge in responder mode and the registrar, additional functionality is needed either on the registrar or as a stand-alone component.
This new functionality is defined as Registrar-Agent and acts as an agent of the registrar to trigger the pledge to generate requests for voucher and enrollment.
These requests are then provided by the Registrar-Agent to the registrar.
This requires the definition of pledge endpoints to allow interaction with the Registrar-Agent.</t>
</li>
<li>
<t>The security of communication between the Registrar-Agent and the pledge must not rely on Transport Layer Security (TLS) to enable application of BRSKI-PRM in environments, in which the communication between the Registrar-Agent and the pledge is done over other technologies like BTLE or NFC, which may not support TLS protected communication.
In addition, the pledge does not have a certificate that can easily be verified by <xref target="RFC9525"/> methods.</t>
</li>
<li>
<t>The use of authenticated self-contained objects addresses both, the TLS challenges and the technology stack challenge.</t>
</li>
<li>
<t>By contrast, the Registrar-Agent can be authenticated by the registrar as a component, acting on behalf of the registrar.
In addition, the registrar must be able to verify, which Registrar-Agent was in direct contact with the pledge.</t>
</li>
<li>
<t>It would be inaccurate for the voucher-request and voucher-response to use the assertion type <tt>proximity</tt> in the voucher, as the pledge was not in direct contact with the registrar for bootstrapping.
Therefore, a new assertion type is necessary for distinguishing assertions the MASA can state.</t>
</li>
</ul>
<t>At least the following properties are required for the voucher and enrollment processing:</t>
<ul spacing="normal">
<li>
<t>POI: provides data-origin authentication of an artifact, e.g., a voucher-request or an Enroll-Request, utilizing an existing IDevID.
Certificate updates may utilize the certificate that is to be updated.</t>
</li>
<li>
<t>POP: proves that an entity possesses and controls the private key corresponding to the public key contained in the certification request, typically by adding a signature computed using the private key to the certification request.</t>
</li>
</ul>
<t>Solution examples based on existing technology are provided with the focus on existing IETF RFCs:</t>
<ul spacing="normal">
<li>
<t>Voucher-Requests and Vouchers as used in <xref target="RFC8995"/> already provide both, POP and POI, through a digital signature to protect the integrity of the voucher, while the corresponding signing certificate contains the identity of the signer.</t>
</li>
<li>
<t>Enroll-Requests are data structures containing the information from a requester for a CA to create a certificate.
The certification request format in BRSKI is PKCS#10 <xref target="RFC2986"/>.
In PKCS#10, the structure is signed to ensure integrity protection and POP of the private key of the requester that corresponds to the contained public key.
In the application examples, this POP alone is not sufficient.
A POI is also required for the certification request and therefore the certification request needs to be additionally bound to the existing pledge IDevID credential.
This binding supports the authorization decision for the certification request and may be provided directly with the certification request.
While BRSKI uses the binding to TLS, BRSKI-PRM aims at an additional signature of the PKCS#10 using existing credentials on the pledge (IDevID). This allows the process to be independent of the selected transport.</t>
</li>
</ul>
</section>
<section anchor="architecture">
<name>Solution Architecture</name>
<section anchor="overview">
<name>Overview</name>
<t>For BRSKI with Pledge in Responder Mode (BRSKI-PRM), the base system architecture defined in BRSKI <xref target="RFC8995"/> is enhanced to facilitate new use cases in which the pledge acts as server.
The responder mode allows delegated bootstrapping using a Registrar-Agent instead of a direct connection between the pledge and the domain registrar.</t>
<t>Necessary enhancements to support authenticated self-contained objects for certificate enrollment are kept at a minimum to enable reuse of already defined architecture elements and interactions.
The format of the bootstrapping objects produced or consumed by the pledge is usually based on JSON Web Signature (JWS) <xref target="RFC7515"/> and further specified in <xref target="exchanges"/> to address the requirements stated in <xref target="req-sol"/> above.
In constrained environments, it may be based on COSE <xref target="RFC9052"/>.</t>
<t>An abstract overview of the BRSKI-PRM protocol can be found on slide 8 of <xref target="BRSKI-PRM-abstract"/>.</t>
<t>To support mutual trust establishment between the domain registrar and pledges not directly connected to the customer domain, this document specifies the exchange of authenticated self-contained objects with the help of the Registrar-Agent.</t>
<t>This leads to extensions of the logical components in the BRSKI architecture as shown in <xref target="uc2figure"/>.</t>
<t>Note that the Join Proxy is not shown in the figure.
In certain situations the Join Proxy may still be present and could be used by the Registrar-Agent to connect to the Registrar.
For example, a Registrar-Agent application on a smartphone often can connect to local Wi-Fi without giving up their cellular network connection <xref target="androidnsd"/>, but only can make link-local connections.</t>
<t>The Registrar-Agent interacts with the pledge to transfer the required data objects for bootstrapping, which are then also exchanged between the Registrar-Agent and the domain registrar.
The addition of the Registrar-Agent influences the sequences of the data exchange between the pledge and the domain registrar described in <xref target="RFC8995"/>.
To enable reuse of BRSKI defined functionality as much as possible, BRSKI-PRM:</t>
<ul spacing="normal">
<li>
<t>uses existing endpoints where the required functionality is provided.</t>
</li>
<li>
<t>enhances existing endpoints with new supported media types, e.g., for JWS voucher.</t>
</li>
<li>
<t>defines new endpoints where additional functionality is required, e.g., for wrapped certification request, wrapped CA certificates, and new status information.</t>
</li>
</ul>
<figure anchor="uc2figure">
<name>BRSKI-PRM architecture overview using Registrar-Agent</name>
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<text x="224" y="324">cert.</text>
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+---------------------------+
..... Drop Ship .....| Vendor Services |
: +---------------+-----------+
: | M anufacturer | |
: | A uthorized | Ownership |
: | S igning | Tracker |
: | A uthority | |
: +---------------+-----------+
: ^
: | BRSKI-
: | MASA
: ...............................|.........
V . v .
+--------+ . +------------+ +-----------+ .
| | . | | | | .
| Pledge | BRSKI- | Registrar- | BRSKI- | Domain | .
| | PRM | Agent | PRM | Registrar | .
| |<------>| |<------>| | .
| | . | EE cert. | | EE cert. | .
| | . +------------+ +-----+-----+ .
| IDevID | . | .
| | . +------------------+-----+ .
+--------+ . | Key Infrastructure | .
. | (e.g., PKI CA) | .
. +------------------------+ .
.........................................
Customer Domain
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<t><xref target="uc2figure"/> shows the relations between the following main components:</t>
<ul spacing="normal">
<li>
<t>Pledge: Is expected to respond with the necessary data objects for bootstrapping to the Registrar-Agent.
The protocol used between the pledge and the Registrar-Agent is assumed to be HTTP(S) in the context of this document.
Any other protocol can be used as long as it supports the exchange of the necessary artifacts.
This includes CoAP or protocols to be used over Bluetooth or NFC connections.
A pledge acting as server leads to the following differences compared to BRSKI <xref target="RFC8995"/>: </t>
<ul spacing="normal">
<li>
<t>The pledge no longer initiates bootstrapping, but is discovered and triggered by the Registrar-Agent as defined in <xref target="discovery_uc2_ppa"/>.</t>
</li>
<li>
<t>The pledge offers additional endpoints as defined in <xref target="pledge_component"/>, so that the Registrar-Agent can request data required for bootstrapping the pledge.</t>
</li>
<li>
<t>The pledge includes additional data in the PVR, which is provided and signed by the Registrar-Agent as defined in <xref target="tpvr"/>.
This allows the registrar to identify with which Registrar-Agent the pledge was in contact (see <xref target="agt_prx"/>).</t>
</li>
<li>
<t>The artifacts exchanged between the pledge and the registrar via the Registrar-Agent are authenticated self-contained objects (i.e., signature-wrapped artifacts).</t>
</li>
</ul>
</li>
<li>
<t>Registrar-Agent: Is a new component defined in <xref target="agent_component"/> that provides a store and forward communication path to exchange data objects between the pledge and the domain registrar.
This is for situations in which the domain registrar is not directly reachable by the pledge, which may be due to a different technology stacks or due to missing connectivity.
A Registrar-Agent acting as client leads to the following new aspects: </t>
<ul spacing="normal">
<li>
<t>The order of exchanges in the BRSKI-PRM call flow is different from that in BRSKI <xref target="RFC8995"/>, as the Registrar-Agent can trigger one or more pledges and collects the PVR and PER artifacts simultaneously as defined in <xref target="exchanges"/>.
This enables bulk bootstrapping of several devices.</t>
</li>
<li>
<t>There is no trust assumption between the pledge and the Registrar-Agent as only authenticated self-contained objects are used, which are transported via the Registrar-Agent and provided either by the pledge or the domain registrar.</t>
</li>
<li>
<t>The trust assumption between the Registrar-Agent and the domain registrar may be based on EE certificates that are both signed by the domain owner.</t>
</li>
<li>
<t>The Registrar-Agent may be realized as stand-alone component supporting nomadic activities of a service technician moving between different installation sites.</t>
</li>
<li>
<t>Alternatively, the Registrar-Agent may also be realized as co-located functionality for a registrar, to support pledges in responder mode.</t>
</li>
</ul>
</li>
<li>
<t>Join Proxy (not shown): Has the same functionality as described in <xref target="RFC8995"/> if needed.
Note that a Registrar-Agent may use a join proxy to facilitate the TLS connection to the registrar in the same way that a BRSKI pledge would use a join proxy. This is useful in cases where the Registrar-Agent does not have full IP connectivity via the domain network or cases where it has no other means to locate the registrar on the network.</t>
</li>
<li>
<t>Domain registrar: In general fulfills the same functionality regarding the bootstrapping of the pledge in a customer domain by facilitating the communication of the pledge with the MASA service and the domain key infrastructure (PKI).
However, there are also differences compared to BRSKI <xref target="RFC8995"/>: </t>
<ul spacing="normal">
<li>
<t>A BRSKI-PRM domain registrar does not interact with a pledge directly, but through the Registrar-Agent as defined in <xref target="exchanges"/>.</t>
</li>
<li>
<t>A BRSKI-PRM domain registrar offers additional endpoints as defined in <xref target="registrar_component"/> to support the signature-wrapped artifacts used by BRSKI-PRM.</t>
</li>
</ul>
</li>
<li>
<t>Vendor services: Encompass MASA and Ownership Tracker and are used as defined in <xref target="RFC8995"/>.
A MASA responsible for pledges that implement BRSKI-PRM is expected to support BRSKI-PRM extensions: </t>
<ul spacing="normal">
<li>
<t>The default format for voucher artifacts (incl. voucher-request) is JWS-signed JSON as defined in <xref target="I-D.ietf-anima-jws-voucher"/>.</t>
</li>
<li>
<t>The Agent Proximity Assertion (see <xref target="agt_prx"/>) requires additional validation steps as defined in <xref target="masa_interaction"/>.</t>
</li>
</ul>
</li>
</ul>
</section>
<section anchor="arch_nomadic">
<name>Nomadic Connectivity</name>
<t>In one example instance of the PRM architecture as shown in <xref target="uc3figure"/>, there is no connectivity between the location in which the pledge is installed and the location of the domain registrar.
This is often the case in the aforementioned building automation use case (<xref target="building-automation"/>).</t>
<figure anchor="uc3figure">
<name>Registrar-Agent nomadic connectivity example</name>
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+---------------------------+
..... Drop Ship .....| Vendor Services |
: +---------------------------+
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........................................ |
. v . |
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. | | BRSKI-PRM : Registrar- : . |
. | Pledge |<--------->: Agent : . |
. +--------+ L2 or L3 :-.-.-.-.-.-.-: . | BRSKI-
. connectivity ^ . | MASA
..........................!............. |
Pledge Installation ! |
Location ! Nomadic |
! connectivity |
! |
...........!....................|.........
. v v .
. .-.-.-.-.-.-.-. BRSKI- +-----------+ .
. : Registrar- : PRM | Domain | .
. : Agent :<------>| Registrar | .
. :-.-.-.-.-.-.-: +-----+-----+ .
. | .
. +-------------------+-----+ .
. | Key Infrastructure | .
. | (e.g., PKI CA) | .
. +-------------------------+ .
..........................................
Customer Domain
]]></artwork>
</artset>
</figure>
<t>PRM enables support of this case through nomadic connectivity of the Registrar-Agent.
To perform enrollment in this setup, multiple round trips of the Registrar-Agent between the pledge installation location and the domain registrar are required.</t>
<ol spacing="normal" type="1"><li>
<t>Connectivity to domain registrar: preparation tasks for pledge bootstrapping not part of the BRSKI-PRM protocol definition, like retrieval of list of pledges to enroll.</t>
</li>
<li>
<t>Connectivity to pledge installation location: retrieve information about available pledges (IDevID), collect request objects (i.e., Pledge Voucher-Requests and Pledge Enroll-Requests using the BRSKI-PRM approach described in <xref target="tpvr"/> and <xref target="tper"/>).</t>
</li>
<li>
<t>Connectivity to domain registrar, submit collected request information of pledges, retrieve response objects (i.e., Voucher and Enroll-Response) using the BRSKI-PRM approach described in <xref target="pvr"/> and <xref target="per"/>.</t>
</li>
<li>
<t>Connectivity to pledge installation location, provide retrieved objects to the pledges to enroll pledges and collect status using the BRSKI-PRM approach described in <xref target="voucher"/>, <xref target="cacerts"/>, and <xref target="enroll_response"/>.</t>
</li>
<li>
<t>Connectivity to domain registrar, submit Voucher Status and Enrollment Status using the BRSKI-PRM approach described in <xref target="vstatus"/> and <xref target="estatus"/>.</t>
</li>
</ol>
<t>Variations of this setup include cases where the Registrar-Agent uses for example WiFi to connect to the pledge installation network, and mobile network connectivity to connect to the domain registrar.
Both connections may also be possible in a single location at the same time, based on installation building conditions.</t>
</section>
<section anchor="co-located-registrar-agent-and-domain-registrar">
<name>Co-located Registrar-Agent and Domain Registrar</name>
<t>Compared to <xref target="RFC8995"/> BRSKI, pledges supporting BRSKI-PRM can be completely passive and only need to react when being requested to react by a Registrar-Agent.
In <xref target="RFC8995"/>, pledges instead need to continuously request enrollment from a domain registrar, which may result in undesirable communications pattern and possible overload of a domain registrar.</t>
<figure anchor="uc4figure">
<name>Registrar-Agent integrated into Domain Registrar example</name>
<artset>
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<g class="text">
<text x="48" y="52">.....</text>
<text x="92" y="52">Drop</text>
<text x="132" y="52">Ship</text>
<text x="176" y="52">.....</text>
<text x="236" y="52">Vendor</text>
<text x="296" y="52">Service</text>
<text x="32" y="68">:</text>
<text x="32" y="84">:</text>
<text x="32" y="100">:</text>
<text x="420" y="100">BRSKI-MASA</text>
<text x="32" y="116">:</text>
<text x="240" y="116">...............................</text>
<text x="408" y="116">.........</text>
<text x="32" y="132">:</text>
<text x="120" y="132">.</text>
<text x="440" y="132">.</text>
<text x="32" y="148">v</text>
<text x="120" y="148">.</text>
<text x="440" y="148">.</text>
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<text x="156" y="164">BRSKI-</text>
<text x="268" y="164">..............</text>
<text x="440" y="164">.</text>
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<text x="268" y="180">Registrar-</text>
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<text x="320" y="196">.</text>
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<text x="440" y="196">.</text>
<text x="100" y="212">L2</text>
<text x="124" y="212">or</text>
<text x="148" y="212">L3</text>
<text x="268" y="212">..............</text>
<text x="440" y="212">.</text>
<text x="140" y="228">connectivity</text>
<text x="440" y="228">.</text>
<text x="120" y="244">.</text>
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</artwork>
<artwork type="ascii-art" align="center"><![CDATA[
+---------------------------+
..... Drop Ship .....| Vendor Service |
: +---------------------------+
: ^
: | BRSKI-MASA
: ...............................|.........
: . v .
v . +-------------------------+ .
+--------+ . BRSKI- |.............. | .
| | . PRM |. Registrar- . Domain | .
| Pledge |<------------->|. Agent . Registrar | .
+--------+ L2 or L3 |.............. | .
connectivity +-------------------+-----+ .
. | .
. +------------------+-----+ .
. | Key Infrastructure | .
. +------------------------+ .
.........................................
Customer Domain
]]></artwork>
</artset>
</figure>
<t>The benefits of BRSKI-PRM can be achieved even without the operational complexity of stand-alone Registrar-Agents by integrating the necessary functionality of the Registrar-Agent as a module into the domain registrar as shown in <xref target="uc4figure"/> so that it can support the BRSKI-PRM communications to the pledge.</t>
</section>
<section anchor="agt_prx">
<name>Agent Proximity Assertion</name>
<t>"Agent proximity" is a statement in the PVR and the voucher that the registrar communicates via the Registrar-Agent as defined in <xref target="exchanges"/> and not directly to the pledge.
It is therefore a different assertion than "network proximity", which is defined in <xref section="3" sectionFormat="of" target="RFC8995"/>.
Hence, <xref target="I-D.ietf-anima-rfc8366bis"/> defines the additional assertion type <tt>agent-proximity</tt>.
This assertion type can be verified by the registrar and MASA during BRSKI-PRM voucher-request processing.</t>
<t>In BRSKI, the pledge verifies POP of the registrar end-entity (EE) credentials via the TLS handshake and pins that public key as the <tt>proximity-registrar-cert</tt> into the voucher request.
This allows the MASA to verify the proximity of the pledge and registrar, facilitating a decision to assign the pledge to that domain owner.
In BRSKI, the TLS session is considered provisional until the pledge receives the voucher to verify POI.</t>
<t>In contrast, in BRSKI-PRM the pledge has no direct connection to the registrar and <bcp14>MUST</bcp14> accept the supplied registrar EE certificate provisionally until it receives the voucher as described in <xref target="voucher"/> to verify both POP and POI.
The provisional registrar EE certificate is used for the object security along the authenticated self-contained objects that in BRSKI-PRM replace the direct TLS connection to the registrar available in BRSKI <xref target="RFC8995"/>.
See also <xref section="5" sectionFormat="of" target="RFC8995"/> on "provisional state".</t>
<t>For the Agent Proximity Assertion, the Registrar-Agent EE certificate and registrar EE certificate must be signed by the same domain owner, i.e., <bcp14>MUST</bcp14> possess a common domain trust anchor in their certificate chain.
Akin to the Network Proximity Assertion in BRSKI <xref target="RFC8995"/>, the Agent Proximity Assertion provides pledge proximity evidence to the MASA.
But additionally, the Agent Proximity Assertion allows the domain registrar to be sure that the PVR supplied by the Registrar-Agent was in fact collected by the Registrar-Agent to which the registrar is connected by utilizing an agent-signed data object.</t>
</section>
</section>
<section anchor="system-components">
<name>System Components</name>
<section anchor="agent_component">
<name>Registrar-Agent</name>
<t>The Registrar-Agent is a new component in BRSKI-PRM that provides a store and forward communication path with secure message passing between pledges in responder mode and the domain registrar.
It uses its own end-entity (EE) certificate and corresponding credentials (i.e., private key) for TLS client authentication and for signing agent-signed data objects.</t>
<t>The Registrar-Agent EE certificate <bcp14>MUST</bcp14> include a SubjectKeyIdentifier as defined in <xref section="4.2.1.2" sectionFormat="of" target="RFC5280"/>, which is used as a reference within agent-signed data objects as defined in <xref target="jws-asd"/>.
Note that this is an additional requirement for issuing the Registrar-Agent EE certificate.
<xref target="RFC8995"/> has a similar requirement for the registrar EE certificate.</t>
<t>The SubjectKeyIdentifier is used in favor of providing the complete Registrar-Agent EE certificate in agent-signed data objects to accommodate also constrained environments and reduce bandwidth needed for communication with the pledge.
In addition, it follows the recommendation from BRSKI to use SubjectKeyIdentifier in favor of a certificate fingerprint to avoid additional computations.</t>
<t>The provisioning of the Registrar-Agent EE certificate is out of scope for this document, but may be done using its own BRSKI run or by other means such as configuration.
It is <bcp14>RECOMMENDED</bcp14> to use short lived Registrar-Agent EE certificates in the range of days or weeks as outlined in <xref target="sec_cons_reg-agt"/>.</t>
<t>Further, the Registrar-Agent requires the registrar EE certificate to provide it to the pledge.
It <bcp14>MAY</bcp14> use the certificate verified during server authentication within an initial TLS session with the registrar;
in this case, the Registrar-Agent <bcp14>MUST</bcp14> possess the domain trust anchor (i.e., CA certificate) for the registrar EE certificate to verify the certificate chain.
Alternatively, the registrar EE certificate <bcp14>MAY</bcp14> be provided via configuration or a repository.
The registrar IP address or hostname is provided either by configuration or by using the discovery mechanism defined in <xref target="RFC8995"/> (see <xref target="discovery_uc2_reg"/>).</t>
<t>In addition to the certificates, the Registrar-Agent is provided with the product-serial-number(s) of the pledge(s) to be bootstrapped.
This is necessary to allow for the discovery of pledges by the Registrar-Agent using DNS-SD with mDNS (see <xref target="discovery_uc2_ppa"/>).
The list may be provided by prior administrative means or the Registrar-Agent may get the information via an (out-of-band) interaction with the pledge.
For instance, <xref target="RFC9238"/> describes scanning of a QR code, where the product-serial-number would be initialized from the 12N B005 Product Serial Number data record.</t>
<t>In summary, the following information <bcp14>MUST</bcp14> be available at the Registrar-Agent before the interaction with a pledge:</t>
<ul spacing="normal">
<li>
<t>Registrar-Agent EE certificate and corresponding private key: own operational credentials to authenticate and sign agent-signed data</t>
</li>
<li>
<t>Registrar EE certificate: certificate of the domain registrar to be provided to the pledge</t>
</li>
<li>
<t>Serial number(s): product-serial-number(s) of pledge(s) to be bootstrapped; used for discovery</t>
</li>
</ul>
<t>Further, the Registrar-Agent <bcp14>SHOULD</bcp14> have synchronized time.</t>
<t>Finally, the Registrar-Agent <bcp14>MAY</bcp14> possess the IDevID (root or issuing) CA certificate of the pledge manufacturer/vendor to validate the IDevID certificate on returned PVR or in case of optional TLS usage for pledge communication (see <xref target="pledgehttps"/>).
The distribution of IDevID CA certificates to the Registrar-Agent is out of scope of this document and may be done by a manual configuration.</t>
<section anchor="discovery_uc2_reg">
<name>Discovery of the Registrar</name>
<t>While the Registrar-Agent requires the IP address of the domain registrar to initiate a TLS session, a separate discovery of the registrar is likely not needed and a configuration of the domain registrar IP address or hostname is assumed.
Registrar-Agent and registrar are domain components that already have a trust relation, as a Registrar-Agent acts as representative of the domain registrar towards the pledge or may even be collocated with the domain registrar.
Further, other communication (not part of this document) between the Registrar-Agent and the registrar is assumed, e.g., to exchange information about product-serial-number(s) of pledges to be discovered as outlined in <xref target="arch_nomadic"/>.</t>
<t>Moreover, the standard discovery described in <xref section="4" sectionFormat="of" target="RFC8995"/> and the <xref section="A.2" sectionFormat="of" target="RFC8995"/> does not support identification of registrars with an enhanced feature set (like the support of BRSKI-PRM), and hence this standard discovery is not applicable.</t>
<t>As a more general solution, the BRSKI discovery mechanism can be extended to provide upfront information on the capabilities of registrars, such as the mode of operation (pledge-responder-mode or registrar-responder-mode).
Defining discovery extensions is out of scope of this document.
For further discussion, see <xref target="I-D.ietf-anima-brski-discovery"/>.</t>
</section>
<section anchor="discovery_uc2_ppa">
<name>Discovery of the Pledge</name>
<t>The discovery of the pledge by the Registrar-Agent in the context of this document describes the minimum discovery approach that <bcp14>MUST</bcp14> be supported.
A more general discovery mechanism, also supporting GRASP besides DNS-SD with mDNS, is discussed in <xref target="I-D.ietf-anima-brski-discovery"/>.</t>
<t>Discovery in BRSKI-PRM uses DNS-based Service Discovery <xref target="RFC6763"/> over Multicast DNS <xref target="RFC6762"/> to discover the pledge.
Note that <xref section="9" sectionFormat="of" target="RFC6762"/> provides support for conflict resolution in situations when a DNS-SD with mDNS responder receives an mDNS response with inconsistent data.
Note that <xref target="RFC8990"/> does not support conflict resolution of mDNS, which may be a limitation for its application.</t>
<t>The pledge constructs a Service Instance Name based on device local information (manufacturer/vendor name and serial number), which results in <tt><product-serial-number>._brski-pledge._tcp.local</tt>.
The product-serial-number composition is manufacturer dependent and may contain information regarding the manufacturer, the product type, and further information specific to the product instance.
To allow distinction of pledges, the product-serial-number therefore needs to be sufficiently unique.</t>
<t>Note that this goes against the naming recommendation of <xref target="RFC6763"/>.
The <tt>_brski-pledge._tcp</tt> service, however, targets machine-to-machine discovery.</t>
<t>In the absence of a more general discovery as defined in <xref target="I-D.ietf-anima-brski-discovery"/> the Registrar-Agent <bcp14>MUST</bcp14> use</t>
<ul spacing="normal">
<li>
<t><tt><product-serial-number>._brski-pledge._tcp.local</tt>, to discover a specific pledge, e.g., when connected to a local network.</t>
</li>
<li>
<t><tt>_brski-pledge._tcp.local</tt> to get a list of pledges to be bootstrapped.</t>
</li>
</ul>
<t>A manufacturer may allow the pledge to react on DNS-SD with mDNS discovery without its product-serial-number contained.
This allows a commissioning tool to discover pledges to be bootstrapped in the domain.
The manufacturer supports this functionality as outlined in <xref target="sec_cons_mDNS"/>.</t>
<t>Establishing network connectivity of the pledge is out of scope of this document but necessary to apply DNS-SD with mDNS.
For Ethernet, it is provided by simply connecting the network cable.
For WiFi networks, connectivity can be provided by using a pre-agreed SSID for bootstrapping, e.g., as proposed in <xref target="I-D.richardson-emu-eap-onboarding"/>.
The same approach can be used by 6LoWPAN/mesh using a pre-agreed PAN ID.
How to gain network connectivity is out of scope of this document.</t>
</section>
</section>
<section anchor="pledge_component">
<name>Pledge in Responder Mode</name>
<t>In BRSKI-PRM, the pledge is triggered by the Registrar-Agent to create the PVR and PER.
It is also triggered for processing of the responses and the generation of status information once the Registrar-Agent has received the responses from the registrar later in the process.</t>
<t>To enable interaction as responder with the Registrar-Agent, pledges in responder mode <bcp14>MUST</bcp14> act as servers and <bcp14>MUST</bcp14> provide the endpoints defined in <xref target="pledge_ep_table"/> within the BRSKI-defined <tt>/.well-known/brski/</tt> URI path, except for the <bcp14>OPTIONAL</bcp14> endpoint "qps".
The endpoints are defined with short names to also accommodate for resource-constrained devices.</t>
<table anchor="pledge_ep_table">
<name>Well-Known Endpoints on a Pledge in Responder Mode</name>
<thead>
<tr>
<th align="left">Endpoint</th>
<th align="left">Operation</th>
<th align="left">Exchange and Artifacts</th>
</tr>
</thead>
<tbody>
<tr>
<td align="left">tpvr</td>
<td align="left">Trigger Pledge Voucher-Request</td>
<td align="left">
<xref target="tpvr"/></td>
</tr>
<tr>
<td align="left">tper</td>