Support of asynchronous Enrollment in BRSKI (BRSKI-AE)
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|Authors||Steffen Fries , Hendrik Brockhaus , Eliot Lear , Thomas Werner|
|RFC stream||Internet Engineering Task Force (IETF)|
|Additional resources||Mailing list discussion|
|Stream||WG state||WG Document|
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ANIMA WG S. Fries Internet-Draft H. Brockhaus Intended status: Standards Track Siemens Expires: July 11, 2021 E. Lear Cisco Systems T. Werner Siemens January 7, 2021 Support of asynchronous Enrollment in BRSKI (BRSKI-AE) draft-ietf-anima-brski-async-enroll-01 Abstract This document describes enhancements of bootstrapping a remote secure key infrastructure (BRSKI) to also operate in domains featuring no or only timely limited connectivity between involved components. Moreover, newly introduced are methods to perform the BRSKI approach in environments, in which the role of the pledge changes to a server instead of the client. This changes the interaction model as the pledge is pushed to interact with the registrar instead of pulling information from the registrar. To support both, BRSKI-AE relies on the exchange of it authenticated self-contained objects (signature- wrapped objects) also for requesting and distributing of domain specific device certificates. The defined approach is agnostic regarding the utilized enrollment protocol allowing the application of existing and potentially new certificate management protocols. 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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on July 11, 2021. Fries, et al. Expires July 11, 2021 [Page 1] Internet-Draft BRSKI-AE January 2021 Copyright Notice Copyright (c) 2021 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Scope of solution . . . . . . . . . . . . . . . . . . . . . . 7 3.1. Supported environment . . . . . . . . . . . . . . . . . . 7 3.2. Application Examples . . . . . . . . . . . . . . . . . . 7 3.2.1. Rolling stock . . . . . . . . . . . . . . . . . . . . 7 3.2.2. Building automation . . . . . . . . . . . . . . . . . 8 3.2.3. Substation automation . . . . . . . . . . . . . . . . 8 3.2.4. Electric vehicle charging infrastructure . . . . . . 9 3.2.5. Infrastructure isolation policy . . . . . . . . . . . 9 3.2.6. Less operational security in the target domain . . . 9 4. Requirement discussion and mapping to solution elements . . . 9 5. Architectural Overview and Communication Exchanges . . . . . 12 5.1. Use Case 1 (PULL): Support of off-site PKI service . . . 12 5.1.1. Behavior of a pledge . . . . . . . . . . . . . . . . 15 5.1.2. Pledge - Registrar discovery and voucher exchange . . 16 5.1.3. Registrar - MASA voucher exchange . . . . . . . . . . 16 5.1.4. Pledge - Registrar - RA/CA certificate enrollment . . 16 5.1.5. Addressing Scheme Enhancements . . . . . . . . . . . 19 5.2. Use Case 2 (PUSH): pledge-agent . . . . . . . . . . . . . 19 5.2.1. Behavior of a pledge . . . . . . . . . . . . . . . . 23 5.2.2. Behavior of a pledge-agent . . . . . . . . . . . . . 24 5.2.3. Protocol Details (Pledge-Agent - Pledge) . . . . . . 25 5.2.4. Protocol flow . . . . . . . . . . . . . . . . . . . . 29 5.3. Domain registrar support of different enrollment options 31 6. Example for signature-wrapping using existing enrollment protocols . . . . . . . . . . . . . . . . . . . . . . . . . . 32 6.1. EST Handling . . . . . . . . . . . . . . . . . . . . . . 33 6.2. Lightweight CMP Handling . . . . . . . . . . . . . . . . 33 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34 8. Privacy Considerations . . . . . . . . . . . . . . . . . . . 34 Fries, et al. Expires July 11, 2021 [Page 2] Internet-Draft BRSKI-AE January 2021 9. Security Considerations . . . . . . . . . . . . . . . . . . . 34 9.1. Exhaustion attack on pledge . . . . . . . . . . . . . . . 34 9.2. PSK usage in TLS establishment . . . . . . . . . . . . . 34 9.3. Misuse of acquired voucher and enrollment responses . . . 35 10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 35 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 35 11.1. Normative References . . . . . . . . . . . . . . . . . . 35 11.2. Informative References . . . . . . . . . . . . . . . . . 36 Appendix A. History of changes [RFC Editor: please delete] . . . 38 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40 1. Introduction BRSKI as defined in [I-D.ietf-anima-bootstrapping-keyinfra] specifies a solution for secure zero-touch (automated) bootstrapping of devices (pledges) in a deployment domain. This includes the discovery of network elements in the target domain, time synchronization, and the exchange of security information necessary to establish trust between a pledge and the domain and to adopt a pledge as new network and application element. Security information about the target domain, specifically the target domain certificate, is exchanged utilizing voucher objects as defined in [RFC8366]. These vouchers are authenticated self-contained (signed) objects, which may be provided online (synchronous) or offline (asynchronous) via the domain registrar to the pledge and originate from a Manufacturer's Authorized Signing Authority (MASA). The MASA signed voucher contains the target domain certificate and can be verified by the pledge due to the possession of a manufacturer root certificate. It facilitates the enrollment of the pledge in the target domain and is used to establish trust from the pledge to the domain. For the enrollment of devices BRSKI relies on EST [RFC7030] to request and distribute target domain specific device certificates. EST in turn relies on a binding of the certification request to an underlying TLS connection between the EST client and the EST server. According to BRSKI the domain registrar acts as EST server and is also acting as registration authority (RA) or local registration authority (LRA). The binding to TLS is used to protect the exchange of a certification request (for an LDevID certificate) and to provide data origin authentication to support the authorization decision for processing the certification request. The TLS connection is mutually authenticated and the client side authentication utilizes the pledge's manufacturer issued device certificate (IDevID certificate). This approach requires an on-site availability of a local asset or inventory management system performing the authorization decision based on tuple of the certification request and the pledge authentication using the IDevID certificate, to issue a domain specific certificate to the pledge. The EST server (the domain Fries, et al. Expires July 11, 2021 [Page 3] Internet-Draft BRSKI-AE January 2021 registrar) terminates the security association with the pledge and thus the binding between the certification request and the authentication of the pledge via TLS. This type of enrollment utilizing an online connection to the PKI is considered as synchronous enrollment. For certain use cases on-site support of a RA/CA component and/or an asset management is not available and rather provided by an operator's backend and may be provided timely limited or completely through offline interactions. This may be due to higher security requirements for operating the certification authority or for optimization of operation for smaller deployments to avoid the always on-site operation. The authorization of a certification request based on an asset management in this case will not / can not be performed on-site at enrollment time. Enrollment, which cannot be performed in a (timely) consistent fashion is considered as asynchronous enrollment in this document. It requires the support of a store and forward functionality of certification request together with the requester authentication information. This enables processing of the request at a later point in time. A similar situation may occur through network segmentation, which is utilized in industrial systems to separate domains with different security needs. Here, a similar requirement arises if the communication channel carrying the requester authentication is terminated before the RA/CA authorization handling of the certification request. If a second communication channel is opened to forward the certification request to the issuing RA/ CA, the requester authentication information needs to be retained and ideally bound to the certification request. This uses case is independent from timely limitations of the first use case. For both cases, it is assumed that the requester authentication information is utilized in the process of authorization of a certification request. There are different options to perform store and forward of certification requests including the requester authentication information: o Providing a trusted component (e.g., an LRA) in the target domain, which stores the certification request combined with the requester authentication information (based on the IDevID) and potentially the information about a successful proof of possession (of the corresponding private key) in a way prohibiting changes to the combined information. Note that the assumption is that the information elements may not be cryptographically bound together. Once connectivity to the backend is available, the trusted component forwards the certification request together with the requester information (authentication and proof of possession) to the off-site PKI for further processing. It is assumed that the off-site PKI in this case relies on the local pledge authentication result and thus performs the authorization and Fries, et al. Expires July 11, 2021 [Page 4] Internet-Draft BRSKI-AE January 2021 issues the requested certificate. In BRSKI the trusted component may be the EST server residing co-located with the registrar in the target domain. o Utilization of authenticated self-contained objects for the enrollment, binding the certification request and the requester authentication in a cryptographic way. This approach reduces the necessary trust in a domain component to storage and delivery. Unauthorized modifications of the requester information (request and authentication) can be detected during the verification of the authenticated self-contained object. This targets environments, in which connectivity to a PKI is only temporary or not directly available, by specifying support for handling authenticated self-contained objects for enrollment. As it is intended to enhance BRSKI it is named BRSKI-AE, where AE stands for asynchronous enrollment. As BRSKI, BRSKI-AE results in the pledge storing an X.509 root certificate and sufficient for verifying the domain registrar / proxy identity (LDevID CA Certificate) as well as a domain specific X.509 device certificate (LDevID EE certificate). Based on the proposed approach, a second set of scenarios can be addressed, in which the pledge has either no direct communication path to the domain registrar, e.g., due to missing network connectivity or a different technology stack. In such scenarios the pledge is likely to act as a server rather than a client. It will be pushed (triggered) by the registrar or an intermediate component to generate request objects to be onboarded in the registrar's domain. For this, an additional component is introduced acting as an agent for the pledge towards the domain registrar, e.g., a commissioning tool. In contrast to BRSKI here the objects to trigger a request generation and the responses are pushed to the pledge instead of being pulled as done in BRSKI. The goal is to enhance BRSKI to either allow other existing certificate management protocols supporting authenticated self- contained objects to be applied or to allow other types of encoding for the certificate management information exchange. This is addressed by o enhancing the well-known URI approach with an additional path for the utilized enrollment protocol. o defining a certificate waiting indication and handling, if the certifying component is (temporarily) not available. Fries, et al. Expires July 11, 2021 [Page 5] Internet-Draft BRSKI-AE January 2021 o allowing to utilize credentials different from the pledge's IDevID to establish a TLS connection to the domain registrar, which is necessary in case of using a pledge-agent. Note that in contrast to BRSKI, BRSKI-AE assumes support of multiple enrollment protocols on the infrastructure side, allowing the pledge manufacturer to select the most appropriate. Thus, BRSKI-AE can be applied for both, asynchronous and synchronous enrollment. 2. Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD 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. The following terms are defined additionally: CA: Certification authority, issues certificates. RA: Registration authority, an optional system component to which a CA delegates certificate management functions such as authorization checks. LRA: Local registration authority, an optional RA system component with proximity to end entities. IED: Intelligent Electronic Device (in essence a pledge). on-site: Describes a component or service or functionality available in the target deployment domain. off-site: Describes a component or service or functionality available in an operator domain different from the target deployment domain. This may be a central site, to which only a temporary connection is available, or which is in a different administrative domain. asynchronous communication: Describes a timely interrupted communication between an end entity and a PKI component. synchronous communication: Describes a timely uninterrupted communication between an end entity and a PKI component. authenticated self-contained object: Describes an object, which is cryptographically bound to the IDevID EE certificate of a pledge. The binding is assumed to be provided through a digital signature of the actual object using the corresponding private key of the IDevID. Fries, et al. Expires July 11, 2021 [Page 6] Internet-Draft BRSKI-AE January 2021 3. Scope of solution 3.1. Supported environment This solution is intended to be used in domains with limited support of on-site PKI services and comprises use cases in which: o there is no registration authority available in the target domain. The connectivity to an off-site RA in an operator's network may only be available temporarily. A local store and forward device is used for the communication with the off-site services. o authoritative actions of a LRA are limited and may not comprise authorization of certification requests of pledges. Final authorization is done at the RA residing in the operator domain. o the target deployment domain already has an established certificate management approach that shall be reused to (e.g., in brownfield installations). In addition, the solution is intended to be applicable in domains in which pledges have no direct connection to the domain registrar, but are expected to be managed by the registrar. This can be motivated by pledges featuring a different technology stack or by pledges without an existing connection to the domain registrar during onboarding. These pledges are likely to act in a server role. Therefore, the pledge needs to provide endpoints on which it can be triggered for requesting the generation of voucher-request objects and certification objects as well as to provide the response objects to the pledge. here the pledge is not expected to start a communication with the domain registrar for onboarding, but is expected to be pushed for the interaction. 3.2. Application Examples The following examples are intended to motivate the support of different enrollment approaches in general and asynchronous enrollment specifically, by introducing industrial applications cases, which could leverage BRSKI as such but also require support of asynchronous operation as intended with BRSKI-AE. 3.2.1. 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, or with a backend. These devices are typically unaware of backend connectivity. Managing certificates may be done during maintenance Fries, et al. Expires July 11, 2021 [Page 7] Internet-Draft BRSKI-AE January 2021 cycles of the railroad car, but can already be prepared during operation. The preparation may comprise the generation of certification requests by the components which are collected and forwarded for processing, once the railroad car is connected to the operator backend. The authorization of the certification request is then done based on the operator's asset/inventory information in the backend. 3.2.2. Building automation In building automation, a use case can be described by a detached building or the basement of a building equipped with sensor, actuators, and controllers connected, but with only limited or no connection to the centralized building management system. This limited connectivity may be during the installation time but also during operation time. During the installation in the basement, a service technician collects the necessary information from the basement network and provides them to the central building management system, e.g., using a laptop or even a mobile phone to transport the information. This information may comprise parameters and settings required in the operational phase of the sensors/actuators, like a certificate issued by the operator to authenticate against other components and services. The collected information may be provided by a domain registrar already existing in the installation network. In this case connectivity to the backend PKI may be facilitated by the service technician's laptop. Contrary, the information can also be collected from the pledges directly and provided to a domain registrar deployed in a different network. In this cases connectivity to the domain registrar may be facilitated by the service technician's laptop. 3.2.3. Substation automation In electrical substation automation a control center typically hosts PKI services to issue certificates for Intelligent Electronic Devices (IED)s operated in a substation. Communication between the substation and control center is done through a proxy/gateway/DMZ, which terminates protocol flows. Note that [NERC-CIP-005-5] requires inspection of protocols at the boundary of a security perimeter (the substation in this case). In addition, security management in substation automation assumes central support of different enrollment protocols to facilitate the capabilities of IEDs from different vendors. The IEC standard IEC62351-9 [IEC-62351-9] specifies the mandatory support of two enrollment protocols, SCEP [RFC8894] and EST [RFC7030] for the infrastructure side, while the IED must only support one of the two. Fries, et al. Expires July 11, 2021 [Page 8] Internet-Draft BRSKI-AE January 2021 3.2.4. Electric vehicle charging infrastructure For the electric vehicle charging infrastructure protocols have been defined for the interaction between the electric vehicle (EV) and the charging point (e.g., ISO 15118-2 [ISO-IEC-15118-2]) as well as between the charging point and the charging point operator (e.g. OCPP [OCPP]). Depending on the authentication model, unilateral or mutual authentication is required. In both cases the charging point uses an X.509 certificate to authenticate itself in the context of a TLS connection between the EV and the charging point. The management of this certificate depends (beyond others) on the selected backend connectivity protocol. Specifically, in case of OCPP it is intended as single communication protocol between the charging point and the backend carrying all information to control the charging operations and maintain the charging point itself. This means that the certificate management is intended to be handled in-band of OCPP. This requires to be able to encapsulate the certificate management exchanges in a transport independent way. Authenticated self- containment will ease this by allowing the transport without a separate enrollment protocol. 3.2.5. Infrastructure isolation policy This refers to any case in which network infrastructure is normally isolated from the Internet as a matter of policy, most likely for security reasons. In such a case, limited access to external PKI resources will be allowed in carefully controlled short periods of time, for example when a batch of new devices are deployed, but impossible at other times. 3.2.6. Less operational security in the target domain The registration point performing the authorization of a certificate request is a critical PKI component and therefore implicates higher operational security than other components utilizing the issued certificates for their security features. CAs may also demand higher security in the registration procedures. Especially the CA/Browser forum currently increases the security requirements in the certificate issuance procedures for publicly trusted certificates. There may be the situation that the target domain does not offer enough security to operate a registration point and therefore wants to transfer this service to a backend. 4. Requirement discussion and mapping to solution elements For the requirements discussion it is assumed that the domain registrar receiving a certification request as authenticated self- contained object is not the authorization point for this Fries, et al. Expires July 11, 2021 [Page 9] Internet-Draft BRSKI-AE January 2021 certification request. If the domain registrar is the authorization point, BRSKI can be used directly. Note that BRSKI-AE could also be used in this case. Based on the intended target environment described in Section 3.1 and the motivated application examples described in Section 3.2 the following base requirements are derived to support authenticated self-contained objects as container carrying the certification request and further information to support asynchronous operation. At least the following properties are required: o Proof of Possession: proves to possess and control the private key corresponding to the public key contained in the certification request, typically by addind a signature using the private key. o Proof of Identity: provides data-origin authentication of a data object, e.g., a certificate request, utilizing an existing IDevID. Certificate updates may utilize the certificate that is to be updated. Solution examples (not complete) based on existing technology are provided with the focus on existing IETF documents: o Certification request objects: Certification requests are structures protecting only the integrity of the contained data providing a proof-of-private-key-possession for locally generated key pairs. Examples for certification requests are: * PKCS#10 [RFC2986]: Defines a structure for a certification request. The structure is signed to ensure integrity protection and proof of possession of the private key of the requester that corresponds to the contained public key. * CRMF [RFC4211]: Defines a structure for the certification request message. The structure supports integrity protection and proof of possession, through a signature generated over parts of the structure by using the private key corresponding to the contained public key. Note that the integrity of the certification request is bound to the public key contained in the certification request by performing the signature operation with the corresponding private key. In the considered application examples, this is not sufficient and needs to be bound to the existing credential of the pledge (IDevID) additionally. This binding supports the authorization decision for the certification request through the provisioning of a proof of identity. The binding of data origin Fries, et al. Expires July 11, 2021 [Page 10] Internet-Draft BRSKI-AE January 2021 authentication to the certification request may be delegated to the protocol used for certificate management. o Proof of Identity options: The certification request should be bound to an existing credential (here IDevID) to enable a proof of identity and based on it an authorization of the certification request. The binding may be realized through security options in an underlying transport protocol if the authorization of the certification request is done at the next communication hop. Alternatively, this binding can be done by a wrapping signature employing an existing credential (initial: IDevID, renewal: LDevID). This requirement is addressed by existing enrollment protocols in different ways, for instance: * EST [RFC7030]: Utilizes PKCS#10 to encode the certification request. The Certificate Signing Request (CSR) may contain a binding to the underlying TLS by including the tls-unique value in the self-signed CSR structure. The tls-unique value is one result of the TLS handshake. As the TLS handshake is performed mutually authenticated and the pledge utilized its IDevID for it, the proof of identity can be provided by the binding to the TLS session. This is supported in EST using simpleenroll. To avoid the binding to the underlying authentication in the transport layer, EST offers the support of a wrapping the CSR with an existing certificate by using Full PKI Request messages. * SCEP [RFC8894]: Provides the option to utilize either an existing secret (password) or an existing certificate to protect the CSR based on SCEP Secure Message Objects using CMS wrapping ([RFC5652]). Note that the wrapping using an existing IDevID credential in SCEP is referred to as renewal. SCEP therefore does not rely on the security of an underlying transport. * CMP [RFC4210] Provides the option to utilize either an existing secret (password) or an existing certificate to protect the PKIMessage containing the certification request. The certification request is encoded utilizing CRMF. PKCS#10 is optionally supported. The proof of identity of the PKIMessage containing the certification request can be achieved by using IDevID credentials to a PKIProtection carrying the actual signature value. CMP therefore does not rely on the security of an underlying transport protocol. * CMC [RFC5272] Provides the option to utilize either an existing secret (password) or an existing certificate to protect the certification request (either in CRMF or PKCS#10) based on CMS Fries, et al. Expires July 11, 2021 [Page 11] Internet-Draft BRSKI-AE January 2021 [RFC5652]). Here a FullCMCRequest can be used, which allows signing with an existing IDevID credential to provide a proof of identity. CMC therefore does not rely on the security of an underlying transport. Note that besides the already existing enrollment protocols there is ongoing work in the ACE WG to define an encapsulation of EST messages in OSCORE to result in a TLS independent way of protecting EST. This approach [I-D.selander-ace-coap-est-oscore] may be considered as further variant. 5. Architectural Overview and Communication Exchanges To support asynchronous enrollment, the base system architecture defined in BRSKI [I-D.ietf-anima-bootstrapping-keyinfra] is enhanced to facilitate the two target use cases. o Use case 1 (PULL case): the pledge requests certificates from a PKI operated off-site via the domain registrar. o Use case 2 (PUSH case): allows delayed (delegated) onboarding using a pledge-agent instead a direct connection to the domain registrar. The communication model between pledge-agent and pledge is intended to use a PUSH approach in which the pledge acts as a server. Note that the terminology PUSH and PULL relates to the pledge behavior. In PULL the pledge requests data objects as in BRSKI, while in the PUSH case the pledge is in the server role and will be provided with the data objects. The pledge-agent, as it represents the pledge, is expected to act in a PULL mode towards the domain registrar. Both use cases are described in the next subsections. They utilize the existing BRSKI architecture elements as much as possible. Necessary enhancements to support authenticated self- contained objects for certificate enrollment are kept on a minimum to ensure reuse of already defined architecture elements and interactions. For the authenticated self-contained objects used for the certification request, BRSKI-AE relies on the defined message wrapping mechanisms of the enrollment protocols stated in Section 4 above. 5.1. Use Case 1 (PULL): Support of off-site PKI service One assumption of BRSKI-AE is that the authorization of a certification request is performed based on an authenticated self- contained object, binding the certification request to the Fries, et al. Expires July 11, 2021 [Page 12] Internet-Draft BRSKI-AE January 2021 authentication using the IDevID. This supports interaction with off- site or off-line PKI (RA/CA) components. In addition, the authorization of the certification request may not be done by the domain registrar but by a PKI residing in the backend of the domain operator (off-site) as described in Section 3.1. This leads to changes in the placement or enhancements of the logical elements as shown in Figure 1. +------------------------+ +--------------Drop Ship--------------->| Vendor Service | | +------------------------+ | | M anufacturer| | | | A uthorized |Ownership| | | S igning |Tracker | | | A uthority | | | +--------------+---------+ | ^ | | V | +--------+ ......................................... | | | . . | BRSKI- | | . +------------+ +------------+ . | MASA | Pledge | . | Join | | Domain <-----+ | | . | Proxy | | Registrar/ | . | <-------->............<-------> Enrollment | . | | . | BRSKI-AE | Proxy | . | IDevID | . | | +------^-----+ . | | . +------------+ | . | | . | . +--------+ ...............................|......... "on-site domain" components | |e.g., RFC 7030, | RFC 4210, ... .............................................|..................... . +---------------------------+ +--------v------------------+ . . | Public Key Infrastructure |<----+ PKI RA | . . | PKI CA |---->+ | . . +---------------------------+ +---------------------------+ . ................................................................... "off-site domain" components Figure 1: Architecture overview using off-site PKI components The architecture overview in Figure 1 utilizes the same logical elements as BRSKI but with a different placement in the deployment architecture for some of the elements. The main difference is the placement of the PKI RA/CA component, which is performing the authorization decision for the certification request message. It is Fries, et al. Expires July 11, 2021 [Page 13] Internet-Draft BRSKI-AE January 2021 placed in the off-site domain of the operator (not the deployment site directly), which may have no or only temporary connectivity to the deployment or on-site domain of the pledge. This is to underline the authorization decision for the certification request in the backend rather than on-site. The following list describes the components in the target domain: o Join Proxy: same functionality as described in BRSKI. o Domain Registrar / Enrollment Proxy: In general the domain registrar proxy has a similar functionality regarding the imprinting of the pledge in the deployment domain to facilitate the communication of the pledge with the MASA and the PKI. Different is the authorization of the certification request. BRSKI-AE allows to perform this in the operator's backend (off- site), and not directly at the domain registrar. * Voucher exchange: The voucher exchange with the MASA via the domain registrar is performed as described in BRSKI [I-D.ietf-anima-bootstrapping-keyinfra] . * Certificate enrollment: For the pledge enrollment the domain registrar in the deployment domain supports the adoption of the pledge in the domain based on the voucher request. Nevertheless, it may not have sufficient information for authorizing the certification request. If the authorization of the certification request is done in the off-site domain, the domain registrar forwards the certification request to the RA to perform the authorization. Note that this requires, that the certification request object is enhanced with a proof-of- identity to allow the authorization based on the bound identity information of the pledge. As stated above, this can be done by an additional signature using the IDevID. The domain registrar here acts as an enrollment proxy or local registration authority. It is also able to handle the case having no connection temporarily to an off-site PKI, by storing the authenticated certification request and forwarding it to the RA upon reestablished connectivity. As authenticated self- contained objects are used, it requires an enhancement of the domain registrar. This is done by supporting alternative enrollment approaches (protocol options, protocols, encoding) by enhancing the addressing scheme to communicate with the domain registrar (see Section 5.1.5). The following list describes the vendor related components/service outside the deployment domain: Fries, et al. Expires July 11, 2021 [Page 14] Internet-Draft BRSKI-AE January 2021 o MASA: general functionality as described in [I-D.ietf-anima-bootstrapping-keyinfra]. Assumption is that the interaction with the MASA may be synchronous (voucher request with nonce) or asynchronous (voucher request without nonce). o Ownership tracker: as defined in [I-D.ietf-anima-bootstrapping-keyinfra]. The following list describes the operator related components/service operated in the backend: o PKI RA: Performs certificate management functions (validation of certification requests, interaction with inventory/asset management for authorization of certification requests, etc.) for issuing, updating, and revoking certificates for a domain as a centralized infrastructure for the domain operator. The inventory (asset) management may be a separate component or integrated into the RA directly. o PKI CA: Performs certificate generation by signing the certificate structure provided in the certification request. Based on BRSKI and the architectural changes the original protocol flow is divided into three phases showing commonalities and differences to the original approach as depicted in the following. o Discovery phase (same as BRSKI) o Voucher exchange with deployment domain registrar (same as BRSKI). o Enrollment phase (changed to support the application of authenticated self-contained objects). 5.1.1. Behavior of a pledge The behavior of a pledge as described in [I-D.ietf-anima-bootstrapping-keyinfra] is kept with one exception. After finishing the imprinting phase (4) the enrollment phase (5) is performed with a method supporting authenticated self-contained objects. Using EST with simpleenroll cannot be applied here, as it binds the pledge authentication with the existing IDevID to the transport channel (TLS) rather than to the certification request object directly. This authentication in the transport layer is not visible / verifiable at the authorization point in the off-site domain. Section 6 discusses potential enrollment protocols and options applicable. Fries, et al. Expires July 11, 2021 [Page 15] Internet-Draft BRSKI-AE January 2021 5.1.2. Pledge - Registrar discovery and voucher exchange The discovery phase is applied as specified in [I-D.ietf-anima-bootstrapping-keyinfra]. 5.1.3. Registrar - MASA voucher exchange The voucher exchange is performed as specified in [I-D.ietf-anima-bootstrapping-keyinfra]. 5.1.4. Pledge - Registrar - RA/CA certificate enrollment As stated in Section 4 the enrollment shall be performed using an authenticated self-contained object providing proof of possession and proof of identity. Fries, et al. Expires July 11, 2021 [Page 16] Internet-Draft BRSKI-AE January 2021 +--------+ +---------+ +------------+ +------------+ | Pledge | | Circuit | | Domain | | Operator | | | | Join | | Registrar | | RA/CA | | | | Proxy | | (JRC) | | (OPKI) | +--------+ +---------+ +------------+ +------------+ /--> | | [Request of CA Certificates] | | |---------- CA Certs Request ------------>| | | [if connection to operator domain is available] | | |-Request CA Certs ->| | |<- CA Certs Response| |<-------- CA Certs Response--------------| | /--> | | [Request of Certificate Attributes to be included] | |---------- Attribute Request ----------->| | | [if connection to operator domain is available] | | |Attribute Request ->| | |<-Attribute Response| |<--------- Attribute Response -----------| | /--> | | [Certification request] | | |-------------- Cert Request ------------>| | | [if connection to operator domain is available] | | |--- Cert Request -->| | |<-- Cert Response --| [Optional Certificate waiting indication] | | /--> | | |<---------- Cert Waiting ----------------| | |-- Cert Polling (with orig request ID) ->| | | [if connection to operator domain is available] | | |--- Cert Request -->| | |<-- Cert Response --| /--> | | |<------------- Cert Response ------------| | /--> | | [Certificate confirmation] | | |-------------- Cert Confirm ------------>| | | /--> | | |[optional] | | |--- 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. Fries, et al. Expires July 11, 2021 [Page 17] Internet-Draft BRSKI-AE January 2021 o CA Cert Request: The pledge SHOULD request the full distribution of CA Certificates. This ensures that the pledge has the complete set of current CA certificates beyond the pinned-domain-cert (which may be the domain registrar certificate contained in the voucher). o CA Cert Response: Contains at least one CA certificate of the issuing CA. o 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 deployment domain into the certification request. To get these attributes in advance, the attribute request SHOULD be used. o Attribute Response: Contains the attributes to be included in the certification request message. o Cert Request: Depending on the utilized enrollment protocol, this certification request contains the authenticated self-contained object ensuring both, proof-of-possession of the corresponding private key and proof-of-identity of the requester. o Cert Response: certification response message containing the requested certificate and potentially further information like certificates of intermediary CAs on the certification path. o Cert Waiting: waiting indication for the pledge to retry after a given time. For this a request identifier is necessary. This request identifier may be either part of the enrollment protocol or build based on the certification request. o Cert Polling: querying the registrar, if the certificate request was already processed; can be answered either with another Cert Waiting, or a Cert Response. o Cert Confirm: confirmation message from pledge after receiving and verifying the certificate. o PKI/Registrar Confirm: confirmation message from PKI/registrar about reception of the pledge's certificate confirmation. [RFC Editor: please delete] /* Open Issues: o Description of certificate waiting and retries. Fries, et al. Expires July 11, 2021 [Page 18] Internet-Draft BRSKI-AE January 2021 o Message exchange description is expected to be done by the utilized enrollment protocol based on the addressing scheme (see also Section 6. o Handling of certificate/PKI confirmation message between pledge and domain registrar and PKI (treated optional?). */ 5.1.5. Addressing Scheme Enhancements BRSKI-AE requires enhancements to the addressing scheme defined in [I-D.ietf-anima-bootstrapping-keyinfra] to accommodate the additional handling of authenticated self-contained objects for the certification request. As this is supported by different enrollment protocols, they can be directly employed (see also Section 6). For the support of different enrollment options at the domain registrar, the addressing approach of BRSKI using a "/.well-known" tree from [RFC8615] is enhanced. The current addressing scheme in BRSKI for the client certificate request function during the enrollment is using the definition from EST [RFC7030], here on the example on simple enroll: "/.well- known/est/simpleenroll" This approach is generalized to the following notation: "/.well-known/enrollment-protocol/request" in which enrollment-protocol may be an already existing protocol or a newly defined approach. Note that enrollment is considered here as a sequence of at least a certification request and a certification response. In case of existing enrollment protocols the following notation is used proving compatibility to BRSKI: o enrollment-protocol: references either EST [RFC7030] as in BRSKI or CMP, CMC, SCEP, or newly defined approaches as alternatives. Note: the IANA registration of the well-known URI is expected to be done by the enrollment protocol. For CMP an update is defined, which provides the definition of the well-known URI in CMP Updates Lightweight CMP Profile [I-D.ietf-lamps-cmp-updates]. o request: depending on the utilized enrollment protocol, the request describes the required operation at the registrar side. Enrollment protocols are expected to define the request endpoints as done by existing protocols (see also Section 6). 5.2. Use Case 2 (PUSH): pledge-agent To support mutual trust establishment of pledges, not directly connected to the domain registrar, a similar approach is applied as discussed for the use case 1. It relies on the exchange of Fries, et al. Expires July 11, 2021 [Page 19] Internet-Draft BRSKI-AE January 2021 authenticated self-contained objects (the voucher request/response objects as known from BRSKI and the certification request/response objects as introduced by BRSKI-AE). This allows independence from the protection provided by the underlying transport. In contrast to BRSKI, the exchange of these objects is performed with the help of a pledge-agent, supporting the interaction of the pledge with the domain registrar. It may be an integrated functionality of a commissioning tool. This leads to enhancements of the logical elements in the BRSKI architecture as shown in Figure 3. The pledge- agent interfaces with the pledge to enable creation or consumption of required data objects, which are exchanged with the domain registrar. Moreover, the addition of the pledge-agent also influences the sequences for the data exchange between the pledge and the domain registrar described in [I-D.ietf-anima-bootstrapping-keyinfra]. The general goal for the pledge-agent application is the reuse of already defined endpoints on the domain registrar side. The behavior of the endpoint may need to be adapted. Fries, et al. Expires July 11, 2021 [Page 20] Internet-Draft BRSKI-AE January 2021 +------------------------+ +--------------Drop Ship---------------| Vendor Service | | +------------------------+ | | M anufacturer| | | | A uthorized |Ownership| | | S igning |Tracker | | | A uthority | | | +--------------+---------+ | ^ | | BRSKI- V | MASA +-------+ +-------+ .............................|......... | | | | . | . | | | | . +-----------+ +-----v-----+ . | | |Pledge | . | | | | . |Pledge | | Agent | . | Join | | Domain | . | | | | . | Proxy | | Registrar | . | <----->.......<-------->...........<-------> (PKI RA) | . | | | | . | BRSKI-AE | | . | | | | . | | +-----+-----+ . |IDevID | |opt. | . +-----------+ | . | | |IDevID | . +------------------+-----+ . | | |or | . | Key Infrastructure | . | | |LDevID | . | (e.g., PKI Certificate | . +-------+ +-------+ . | Authority) | . . +------------------------+ . ....................................... "Domain" components Figure 3: Architecture overview using a pledge-agent The architecture overview in Figure 3 utilizes the same logical components as BRSKI with the pledge-agent component as additional component. For authentication towards the domain registrar, the pledge-agent may use the IDevID or LDevID credentials if available, which are verified by the domain registrar as part of the TLS establishment. The provisioning of this credential to the pledge-agent is out of scope for this specification. Alternatively, the domain registrar may authenticate the user operating the pledge-agent to perform authorization of a pledge onboarding action. Examples for such user level authentication are the application of HTTP authentication or the usage of authorization tokens or the application of user related certificates in the TLS handshake or other. If the pledge-agent utilizes a certificate, the domain registrar must be able to verify the certificate by possessing the corresponding root certificate. Fries, et al. Expires July 11, 2021 [Page 21] Internet-Draft BRSKI-AE January 2021 The following list describes the components in the deployment domain: o Pledge: The pledge is expected to respond the necessary data objects for bootstrapping to the pledge-agent. The transport protocol used between the pledge and the pledge-agent in the context of this document is assumed to be HTTP. Other transport protocols may be used, such as CoAP, but their usage is out of scope for this document. As the pledge is triggered/PUSHED by the pledge-agent, it becomes a callee. This leads to some differences to BRSKI: * Discovery of the domain registrar by the pledge will be omitted as the pledge is expected to be triggered by the pledge-agent. * Discovery of the pledge by the pledge-agent must be possible to enable interaction. * As the pledge-agent must be able to trigger the pledge for bootstrapping, the pledge must offer communication endpoints. * The pledge-agent is expected to provide an option to trigger the bootstrapping by pushing data objects to the pledge. * Order of exchanges in the call flow is different as the pledge- agent collects both voucher request objects and certification request objects at once and provides them to the registrar. This approach may also be used to perform a bulk bootstrapping of several devices. * The data objects utilized for the data exchange between the pledge and the registrar are signature-wrapped objects as in use case 1 Section 5.1. o Pledge-Agent: provides a communication path to exchange data objects between the pledge and the domain registrar. The pledge- agent facilitates situations, in which the domain registrar is not directly reachable by the pledge, either due to a different technology stack or due to missing connectivity (e.g., if the domain registrar resides in the cloud and the pledge has no connectivity, yet). The pledge-agent collect the bootstrapping information such as voucher request objects and certification request objects from one or multiple pledges at once and performs a bulk bootstrapping based on the collected data. The pledge- agent triggers the pledge for generating these objects. The pledge-agent may be configured with the domain registrar information or may use the discovery mechanism defined in [I-D.ietf-anima-bootstrapping-keyinfra]. The trust assumptions on the connection between the pledge and the pledge-agent only Fries, et al. Expires July 11, 2021 [Page 22] Internet-Draft BRSKI-AE January 2021 require to ensure proximity between both to provide a minimum protection of arbitrary request to generate voucher request objects and/or enrollment request objects. The trust assumptions on the connection between the pledge-agent and the registrar only requires that the pledge-agent enables the exchange of signature wrapped objects between the pledge and the registrar. o Join Proxy: same functionality as described in [I-D.ietf-anima-bootstrapping-keyinfra]. Note that it may be used by the pledge-agent instead of the pledge. o Domain Registrar: In general the domain registrar fulfills the same functionality regarding the bootstrapping of the pledge in the deployment domain by facilitating the communication of the pledge with the MASA service and the domain PKI service. In contrast to [I-D.ietf-anima-bootstrapping-keyinfra], the domain registrar does not interact with a pledge directly but through the pledge-agent. This prohibits a pledge authentication using its IDevID during TLS establishment towards the registrar. If the pledge-agent has an IDevID or is already possessing a LDevID valid in the deployment domain, it is expected to use this authentication towards the domain registrar. The manufacturer provided components/services (MASA and Ownership tracker) are used as defined in [I-D.ietf-anima-bootstrapping-keyinfra]. 5.2.1. Behavior of a pledge In contrast to use case 1 Section 5.1 the pledge acts as a server component if data is pushed in the bootstrapping phase. It is triggered by the pledge-agent for the generation of voucher request and enrollment request objects as well as for the processing of the response objects and the generation of status information. Due to the use of the pledge-agent, the interaction with the domain registrar is changed as shown in Figure 4. To enable interaction with the pledge-agent, the pledge provides interfaces using the BRSKI REST interface based on the "/.well-known/brski" URI tree. The following endpoints are defined for the pledge: o /.well-known/brski/triggervoucherrequest: trigger pledge to create voucher request. o /.well-known/brski/triggerenrollrequest: trigger pledge to create enrollment request. o /.well-known/brski/supplyvoucherresponse: supply voucher response to pledge. Fries, et al. Expires July 11, 2021 [Page 23] Internet-Draft BRSKI-AE January 2021 o /.well-known/brski/supplyenrollresponse: supply enroll response to pledge. o /.well-known/brski/supplyCACerts: supply CACerts to pledge (optional). 5.2.2. Behavior of a pledge-agent The pledge-agent is a new component in the BRSKI context. It provides connectivity between the pledge and the domain registrar and utilizes the endpoints on the domain registrar side already specified in [I-D.ietf-anima-bootstrapping-keyinfra]. The pledge-agent is expected to interact with the pledge independent of the domain registrar. As stated before, the data exchange concerns the data objects exchanged between the pledge and the domain registrar, which are the voucher request/response objects, the enrollment request/ response objects, as well as status information. As the pledge acts as server, it has to provide endpoints to allow for request/response interaction. For the transport HTTPS is chosen in non-constraint environments, but may also be performed using other transport mechanisms. This changes the general interaction between the pledge and the domain registrar as shown in Figure 4. The pledge-agent may have an own IDevID or a deployment domain issued LDevID to be utilized in the TLS communication establishment towards the domain registrar. Note that the pledge-agent may also be used without TLS client-side authentication if no suitable credential is available. This is a deviation from BRSKI, in which the pledge's IDevID credential is used to perform TLS client authentication. As BRSKI-AE targets the use of authenticated self-contained data objects between the pledge and the domain registrar, the binding of the pledge identity to the requests can be achieved through the data object signature. Nevertheless, the authentication of the pledge- agent is recommended if the onboarding is only to be performed by an authorized pledge-agent. This authentication may be realized by a device (IDevID or LDevID), and if not available through user related credentials in the context of the HTTP based authentication, SAML tokens or other. Note that having no specific credentials on the pledge-agent allows to employ applications with low trust requirements. According to [I-D.ietf-anima-bootstrapping-keyinfra] section 5.3, the domain registrar performs the pledge authorization for onboarding within his domain based on the provided voucher request. Fries, et al. Expires July 11, 2021 [Page 24] Internet-Draft BRSKI-AE January 2021 188.8.131.52. Registrar discovery The discovery phase may be applied as specified in [I-D.ietf-anima-bootstrapping-keyinfra] with the deviation that it is done between the pledge-agent and the domain registrar. Alternatively, the pledge-agent may be configured with the address of the domain registrar. 184.108.40.206. Pledge/Pledge-agent discovery The discovery of the pledge by pledge-agent is done by mDNS. The pledge constructs a local host name based on device local information (device serial number), which results for instance in "serialnumber.brski-pledge._tcp.local.". This can then be discovered by the pledge-agent via mDNS. Note that other mechanisms for discovery may be used. 5.2.3. Protocol Details (Pledge-Agent - Pledge) The interaction of the pledge with the pledge-agent may be accomplished using different transport means (protocols and or network technologies). For this document the usage of HTTP is targeted as in BRSKI. Alternatives may be CoAP or Bluetooth Low Energy (BLE) or Nearfield Communication (NFC). This requires an independence of the security of the exchanged data objects between the pledge and the registrar from the security provided by the transport. Therefore, signature-wrapped objects build the base for the information exchange between the pledge and the registrar. Note that trigger messages from the pledge-agent may not be signed as the pledge has no means to verify them. Therefore, TLS support is required to ensure a secure transport based on a proximity information shared between the pledge-agent and the pledge. This is done to ensure that a technician had physical contact to the pledge. 220.127.116.11. TLS establishment The pledge and the pledge-agent establish a TLS secured communication channel. As the IDevID on the pledge cannot be used as TLS server certificate, a pre-shared key (PSK) is applied. TLS [RFC8446] allows to import externally provided PSKs. The use of an external PSK is defined based on the guidelines provided in [I-D.ietf-tls-external-psk-guidance] The PSK is derived from information known to the pledge and the pledge-agent, which are Fries, et al. Expires July 11, 2021 [Page 25] Internet-Draft BRSKI-AE January 2021 device serial-number: Device serial number stored in the pledge and also part of the X520SerialNumber (defined in [RFC5280]) as part of the IDevID. randomizer: additional value, which is not exposed on the communication interface of the pledge. This information may be provided through the bill of material or a QR code attached to the device and must have a length of at least 128 bits. The pledge and the pledge-agent construct the PSK to be used in TLS based on a HMAC-based Extract-and-Expand Key Derivation Function (HKDF, [RFC5869].). The PSK is derived following the external PSK importer [I-D.ietf-tls-external-psk-importer]. The interface takes an EPSK (External PSK) with an external identity and a base key (epsk) as input. The external identity is provided as part of the ImportedIdentity structure containing information: ImportedIdentity.external_identity = "onboarding" ImportedIdentity.context = "brski_proximity" ImportedIdentity.target_protocol = 0x0304 ImportedIdentity.target_kdf = 0x0001 The target protocol identified is TLS 1.3 (value 0x0304). The target KDF identified is HKDF_SHA256 (value 0x0001). The base key is determined as following: epsk = serial-number | randomizer epskx = HKDF-Extract(0, epsk) ipskx = HKDF-Expand-Label(epskx, "derived psk", Hash(ImportedIdentity), 32) The length value of the HKDF-Expand function is set to 32 octets corresponds to the output length of the selected KDF. TLS shall be used with the derived IPSK with TLS key agreement: "psk_dhe_ke" TLS cipher suite: TLS_AES_128_GCM_SHA256 Fries, et al. Expires July 11, 2021 [Page 26] Internet-Draft BRSKI-AE January 2021 18.104.22.168. Object exchange The pledge-agent provides the registrar certificate to the pledge to be put into the "proximity-registrar-cert" leaf in the pledge voucher-request object. This enables the registrar to verify, that it is the target registrar for the request. The registrar certificate may be configured at the pledge-agent or may be fetched by the pledge-agent based on a prior TLS connection establishment with the domain registrar. The pledge-agent triggers the pledge, to generate a pledge voucher-request object (PleVouReq) . Triggering is done using HTTPS POST with the operation path value of "/.well-known/brski/triggervoucherrequest". The pledge-agent triggervoucherrequest Content-Type header is: application/json: /* to be defined */(different format used as response): defines a JSON document to provide the registrar certificate to the pledge. The pledge shall construct the voucher request object as defined [I-D.ietf-anima-bootstrapping-keyinfra] and sign the request using the pledges IDevID credential. The response is encoded as JSON-in-JWS (or JWS-signed-JSON, tbd). After the voucher request exchange the pledge will be triggered to generate an enrollment request object. As in BRSKI the enrollment request object is a PKCS#10 request, with an additional wrapping signature using the IDevID. As the initial enrollment aims to request a general certificate, no certificate attributes are provided to the pledge. /* Discussion: The pledge-agent may have been pre-configured with the CSR attributes, that it could provide to the pledge to request a specific certificate (for a communication endpoint on the pledge. This is expected to be done later, once the pledge has an LDevID and can be further configured. */ The enrollment request is generated as authenticated self-signed object, which assures proof of possession of the private key corresponding to the contained public key in the enrollment request as well as a proof of identity, based on the IDevID of the pledge. This is done as described for use case 1 Section 5.1. The pledge-agent enrollment-request Content-Type header is: application/json: to be defined (different format used as response): defines a JSON document. Optional CSR parameter may be provided to the pledge. The pledge shall construct the certification request as PKCS#10 object and sign the request using the pledge's IDevID Fries, et al. Expires July 11, 2021 [Page 27] Internet-Draft BRSKI-AE January 2021 credential. The response is encoded as PKCS#10-in-JSON/JWS (tbd). If the pledge does not have specific information about the content of the LDevID to be applied for (like device name, etc.) the domain registrar will provide this information to the issuing CA. /* to be discussed */ The domain registrar may either enhance the PKCS#10 request or generate a structure containing the attributes to be included by the CA and sends both (the original PKCS#10 request and the enhancements) to the domain CA. As enhancing the PKCS#10 request destroys the initial proof of possession of the corresponding private key, the CA would need to accept RA-verified requests. With the collected voucher request object and the enrollment request object, the pledge-agent starts the interaction with the domain registrar. If the domain registrar is in a different network, the pledge-agent closes the TLS session with the pledge (to be resumed for provisioning of voucher object and certificate. /* further description necessary at least for */ Consideration of TLS session resumption for the second run Authentication of pledge-agent to domain registrar Behavior of registrar when pledge LDevID not used in TLS Values to be taken from the IDevID into the PKCS#10 (like pledge serial number or subjectName, or certificate template) PKCS#10-in-JSON/JWS (tbd) handling by domain registrar to request a generic LDevID from the domain CA service. Order of requests may be different as in BRSKI Definition of objects and encoding, some existing encoding may change as for the voucher? Once the pledge-agent has collected the response objects from the domain registrar (voucher and certificate), it will re-start the interaction the pledge. For this the pledge-agent resumes the previous TLS connection with the pledge. The pledge-agent will provide the information via two distinct endpoints at the pledge: The voucher response will be provided with a HTTPS POST using the operation path value of "/.well-known/brski/supplyvoucherresponse". Fries, et al. Expires July 11, 2021 [Page 28] Internet-Draft BRSKI-AE January 2021 The pledge-agent voucher-response Content-Type header is: application/jose: /* to be discussed, as the current voucher is a "application/voucher-cms+json" object). */ The pledge will generate a voucher status object and provides it in the response. The response is encoded as JSON-in-JWS ("application/jose"), signed by the IDevID of the pledge (LDevID not available yet). The enrollment response will be provided with a HTTPS POST using the operation path value of "/.well-known/brski/supplyenrollresponse". The pledge-agent enroll-response Content-Type header is: application/pkcs7-mime: to be defined (contains LDevID + certificate chain). /* to be discussed */: the enrollment response object may also be an application/jose object with a signature of the domain registrar. This may be used either to transport additional data which is bound to the LDevID or it may be considered for enrollment status to ensure that in an error case the registrar providing the certificate can be identified. The pledge will generate an enrollment status object as confirmation, showing it can apply the certificate and possesses the corresponding private key The response ist encoded as JSON-in-JWS. The signature is created using the LDevID of the pledge. 5.2.4. Protocol flow The following protocol description assumes an already established TLS channel as described in Section 22.214.171.124 and focuses on the exchange of signature wrapped objects using endpoints defined for the pledge in Section 126.96.36.199 +--------+ +-------+ +-----------+ +--------+ +---------+ | Pledge | | Pledge| | Domain | | Domain | | Vendor | | | | Agent | | Registrar | | CA | | Service | | | | | | (JRC) | | | | (MASA) | +--------+ +-------+ +-----------+ +--------+ +---------+ | | | | Internet | | opt: configure | | | | - registrar-certificate | | | | - optional CSR attributes | | | | | | | | |<TLS Establishment>| | | | Fries, et al. Expires July 11, 2021 [Page 29] Internet-Draft BRSKI-AE January 2021 | | | | | [example: trigger voucher and certification request generation ] | | | | | | |<-trigger PleVouReq| | | | | (registrar-cert) | | | | |- Voucher Request->| | | | | | | | | |<--trigger ER------| | | | | | | | | |----Cert Request-->| | | | | |<---- TLS --->| | | | | | | | | | | | | | |--- VouReq -->| | | | | [accept device?] | | | | [contact vendor] | | | | |----- Voucher Request ------>| | | |----- Pledge ID ------------>| | | |----- Domain ID ------------>| | | |----- optional: nonce ------>| | | | [extract DomainID] | | | [update audit log] | | |<--------- Voucher ---------| | |<-- Voucher --| | | | | |<----- device audit log ----| | | | | | [optional retrieve CA certs] | | | | |- CACertReq ->| | | | | |- CACertReq -->| | | | |<-CACertResp --| | | |< CACertResp -| | | | | | | | [certification request handling pledge-agent and infrastructure] | | |-- CertReq -->| | | | | |-- CertReq --->| | | | |<--CertResp----| | | |<-- CertResp -| | | | | | | | | | | | | [push voucher and certificate to pledge, optionally push CA certs] | | | | | | | < TLS Resumption >| | | | | | | | | |<--supply Voucher--| | | | |- Voucher Status-->| | | | | | | | | |<---supply Cert----| | | | |-- Enroll Status-->| | | | Fries, et al. Expires July 11, 2021 [Page 30] Internet-Draft BRSKI-AE January 2021 | | | | | | [voucher status telemetry ] | | | |VoucherStatus>| | | | |[verify audit log and voucher]| | | | | | | | | [enroll Status] | | | |-- CertConf ->| | | | | |-- CertConf -->| | | | | | | Figure 4: Request handling of the pledge using a pledge-agent As shown in Figure 4 the pledge-agent collects the voucher request and enrollment request objects from a pledge. As the pledge-agent is intended to work between the pledge and the domain registrar, a collection of requests from multiple pledges is possible, allowing a bulk bootstrapping of multiple pledges using the connection between the pledge-agent and the domain registrar. The information exchange between the pledge-agent and the domain registrar resembles the exchanges between the pledge and the domain registrar from BRSKI in the PULL case. [RFC Editor: please delete] /* to be discussed: Description on how the registrar makes the decision if he is connected with pledge directly (as in BRSKI PULL) or with a pledge-agent (PUSH). This may result in a case statement (client-side authentication in TLS, user authentication above TLS, etc.) for the TLS connection establishment in the original BRSKI document in section 5.1 */ Once the pledge-agent has finished the exchanges with the domain registrar to get the voucher object and the enrollment object, it can close the TLS connection to the domain registrar and provide the objects to the pledge(s). The content of the response objects is defined through the voucher [RFC8366] and the certificate [RFC5280]. 5.3. Domain registrar support of different enrollment options Well-known URIs for different endpoints on the domain registrar are already defined as part of the base BRSKI specification. In addition, alternative enrollment endpoints may be supported at the domain registrar. The pledge / pledge-agent will recognize if its supported enrollment option is supported by the domain registrar by sending a request to its preferred enrollment endpoint. The following provides an illustrative example for a domain registrar supporting different options for EST as well as CMP to be used in BRSKI-AE. The listing contains the supported endpoints for the Fries, et al. Expires July 11, 2021 [Page 31] Internet-Draft BRSKI-AE January 2021 bootstrapping, to which the pledge may connect. This includes the voucher handling as well as the enrollment endpoints. </brski/voucherrequest>,ct=voucher-cms+json </brski/voucher_status>,ct=json </brski/enrollstatus>,ct=json </est/cacerts>;ct=pkcs7-mime </est/simpleenroll>;ct=pkcs7-mime </est/simplereenroll>;ct=pkcs7-mime </est/fullcmc>;ct=pkcs7-mime </est/serverkeygen>;ct= pkcs7-mime </est/csrattrs>;ct=pkcs7-mime </cmp/initialization>;ct=pkixcmp </cmp/certification>;ct=pkixcmp </cmp/keyupdate>;ct=pkixcmp </cmp/p10>;ct=pkixcmp </cmp/getCAcert>;ct=pkixcmp </cmp/getCSRparam>;ct=pkixcmp [RFC Editor: please delete] /* Open Issues: o Clarify, if /.well-known discovery can be performed as discussed in the design team (usage of GET /.well-known/brski to collect information about enrollment specific endpoint support, to be specified in a separate draft). Also, is a discovery option necessary at all, as the pledge will most likely implement only one enrollment option? It can be helpful in the pledge-agent use case, when the pledge-agent has no information about the supported enrollment options (less likely). o In addition to the current content types, we may specify that the response provide information about different content types as multiple values. This would allow to further adopt the encoding of the objects exchanges (ASN.1, JSON, CBOR, ...). -> dependent on the utilized protocol. */ 6. Example for signature-wrapping using existing enrollment protocols This sections map the requirements to support proof of possession and proof of identity to selected existing enrollment protocols. Note that that the work in the ACE WG described in [I-D.selander-ace-coap-est-oscore] may be considered here as well, as it also addresses the encapsulation of EST in a way to make it Fries, et al. Expires July 11, 2021 [Page 32] Internet-Draft BRSKI-AE January 2021 independent from the underlying TLS using OSCORE resulting in an authenticated self-contained object. 6.1. EST Handling When using EST [RFC7030], the following constraints should be considered: o Proof of possession is provided by using the specified PKCS#10 structure in the request. o Proof of identity is achieved by signing the certification request object, which is only supported when Full PKI Request (the /fullcmc endpoint) is used. This contains sufficient information for the RA 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 would be calculated using the IDevID credential of the pledge. o [RFC Editor: please delete] /* TBD: in this case the binding to the underlying TLS connection is not be necessary. */ o When the RA is not available, as per [RFC7030] Section 4.2.3, a 202 return code should be returned by the Registrar. The pledge in this case would retry a simpleenroll with a PKCS#10 request. Note that if the TLS connection is teared down for the waiting time, the PKCS#10 request would need to be rebuilt if it contains the unique identifier (tls_unique) from the underlying TLS connection for the binding. o [RFC Editor: please delete] /* TBD: clarification of retry for fullcmc is necessary as not specified in the context of EST */ 6.2. Lightweight CMP Handling Instead of using CMP [RFC4210], this specification refers to the lightweight CMP profile [I-D.ietf-lamps-lightweight-cmp-profile], as it restricts the full featured CMP to the functionality needed here. For this, the following constrains should be observed: o For proof of possession, the defined approach in Lightweight CMP section 5.1.1 (based on CRMF) and 5.1.5 based on PCKS#10 should be supported. o Proof of identity can be provided by using the signatures to protect the certificate request message as outlined in section 4.2. Fries, et al. Expires July 11, 2021 [Page 33] Internet-Draft BRSKI-AE January 2021 o When the RA/CA is not available, a waiting indication should be returned in the PKIStatus by the Registrar. The pledge in this case would retry using the PollReqContent with a request identifier certReqId provided in the initial CertRequest message as specified in section 6.1.4 with delayed enrollment. 7. IANA Considerations This document requires the following IANA actions: IANA is requested to enhance the Registry entitled: "BRSKI well- known URIs" with the following: URI document description triggervoucherrequest [THISRFC] create voucher request triggerenrollrequest [THISRFC] create enrollment request supplyvoucherresponse [THISRFC] supply voucher response supplyenrollresponse [THISRFC] supply enrollment response supplyCACerts [THISRFC] supply CA certs [RFC Editor: please delete] /* to be done: IANA consideration to be included for the defined namespaces in Section 5.1.5 and Section 5.3 . */ 8. Privacy Considerations [RFC Editor: please delete] /* to be done: clarification necessary */ 9. Security Considerations 9.1. Exhaustion attack on pledge Exhaustion attack on pledge based on DoS attack (connection establishment, etc.) 9.2. PSK usage in TLS establishment TLS is used to provide a proximity information to the pledge. As the devices in scope may not feature an input or output interface for local interaction, a PSK is use to establish the connection between the pledge and the pledge-agent. Certificate based authentication of the pledge cannot be used, as the device does not have the appropriate information contained in the IDevID. The PSK is build using a KDF, which uses the serial number of the device, potential further device related information and a randomizer value. This information is stored within the pledge and is also part of product information (also an QR code attached to the device). If a potential attacker is able to physically access the device, he may read this Fries, et al. Expires July 11, 2021 [Page 34] Internet-Draft BRSKI-AE January 2021 information and is to connect to the pledge. Without physical proximity to the device, to capture the QR code information, the attacker may guess the device' serial number but will not be able to construct the PSK as the randomizer value is not known. 9.3. Misuse of acquired voucher and enrollment responses Pledge-agent that uses acquired voucher and enrollment response for domain 1 in domain 2: can be detected in Voucher Request processing on domain registrar side. Requires domain registrar to verify the proximity-registrar-cert leaf in the voucher request against his own as well as the association of the pledge to his domain based on the serial number contained in the voucher. Misbinding of pledge by a faked domain registrar is countered as described in BRSKI security considerations (section 11.4). 10. Acknowledgments We would like to thank the various reviewers for their input, in particular Brian E. Carpenter, Michael Richardson, Giorgio Romanenghi, Oskar Camenzind, for their input and discussion on use cases and call flows. 11. References 11.1. Normative References [I-D.ietf-anima-bootstrapping-keyinfra] Pritikin, M., Richardson, M., Eckert, T., Behringer, M., and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping- keyinfra-45 (work in progress), November 2020. [I-D.ietf-tls-external-psk-importer] Benjamin, D. and C. Wood, "Importing External PSKs for TLS", draft-ietf-tls-external-psk-importer-06 (work in progress), December 2020. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <https://www.rfc-editor.org/info/rfc2119>. [RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed., "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013, <https://www.rfc-editor.org/info/rfc7030>. Fries, et al. Expires July 11, 2021 [Page 35] Internet-Draft BRSKI-AE January 2021 [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>. 11.2. Informative References [I-D.ietf-lamps-cmp-updates] Brockhaus, H., "CMP Updates", draft-ietf-lamps-cmp- updates-06 (work in progress), November 2020. [I-D.ietf-lamps-lightweight-cmp-profile] Brockhaus, H., Fries, S., and D. Oheimb, "Lightweight CMP Profile", draft-ietf-lamps-lightweight-cmp-profile-04 (work in progress), November 2020. [I-D.ietf-tls-external-psk-guidance] Housley, R., Hoyland, J., Sethi, M., and C. Wood, "Guidance for External PSK Usage in TLS", draft-ietf-tls- external-psk-guidance-01 (work in progress), November 2020. [I-D.selander-ace-coap-est-oscore] Selander, G., Raza, S., Furuhed, M., Vucinic, M., and T. Claeys, "Protecting EST Payloads with OSCORE", draft- selander-ace-coap-est-oscore-04 (work in progress), November 2020. [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. Fries, et al. Expires July 11, 2021 [Page 36] Internet-Draft BRSKI-AE January 2021 [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>. [RFC4210] Adams, C., Farrell, S., Kause, T., and T. Mononen, "Internet X.509 Public Key Infrastructure Certificate Management Protocol (CMP)", RFC 4210, DOI 10.17487/RFC4210, September 2005, <https://www.rfc-editor.org/info/rfc4210>. [RFC4211] Schaad, J., "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", RFC 4211, DOI 10.17487/RFC4211, September 2005, <https://www.rfc-editor.org/info/rfc4211>. [RFC5272] Schaad, J. and M. Myers, "Certificate Management over CMS (CMC)", RFC 5272, DOI 10.17487/RFC5272, June 2008, <https://www.rfc-editor.org/info/rfc5272>. [RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S., Housley, R., and W. Polk, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008, <https://www.rfc-editor.org/info/rfc5280>. [RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70, RFC 5652, DOI 10.17487/RFC5652, September 2009, <https://www.rfc-editor.org/info/rfc5652>. [RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand Key Derivation Function (HKDF)", RFC 5869, DOI 10.17487/RFC5869, May 2010, <https://www.rfc-editor.org/info/rfc5869>. [RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018, <https://www.rfc-editor.org/info/rfc8446>. Fries, et al. Expires July 11, 2021 [Page 37] Internet-Draft BRSKI-AE January 2021 [RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019, <https://www.rfc-editor.org/info/rfc8615>. [RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol", RFC 8894, DOI 10.17487/RFC8894, September 2020, <https://www.rfc-editor.org/info/rfc8894>. Appendix A. History of changes [RFC Editor: please delete] From IETF draft 00 -> IETF 01: o Update of scope in Section 3.1 to include in which the pledge acts as a server. This is one main motivation for use case 2. o Rework of use case 2 in Section 5.2 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. o First description of exchanged object types (needs more work) o Clarification in discovery options for enrollment endpoints at the domain registrar based on well-known endpoints in Section 5.3 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. o Updated references. o Included Thomas Werner as additional author for the document. From individual version 03 -> IETF draft 00: o Inclusion of discovery options of enrollment endpoints at the domain registrar based on well-known endpoints in Section 5.3 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. o Missing details provided for the description and call flow in pledge-agent use case Section 5.2, e.g. to accommodate distribution of CA certificates. o Updated CMP example in Section 6 to use lightweight CMP instead of CMP, as the draft already provides the necessary /.well-known endpoints. Fries, et al. Expires July 11, 2021 [Page 38] Internet-Draft BRSKI-AE January 2021 o Requirements discussion moved to separate section in Section 4. Shortened description of proof of identity binding and mapping to existing protocols. o Removal of copied call flows for voucher exchange and registrar discovery flow from [I-D.ietf-anima-bootstrapping-keyinfra] in Section 5.1 to avoid doubling or text or inconsistencies. o 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: o 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. o Simplification of the architecture approach for the initial use case having an offsite PKI. o 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. o Update of provided examples of the addressing approach used in BRSKI to allow for support of multiple enrollment protocols in Section 5.1.5. From individual version 01 -> 02: o Update of introduction text to clearly relate to the usage of IDevID and LDevID. o Definition of the addressing approach used in BRSKI to allow for support of multiple enrollment protocols in Section 5.1.5. 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. Fries, et al. Expires July 11, 2021 [Page 39] Internet-Draft BRSKI-AE January 2021 o Update of description of architecture elements and changes to BRSKI in Section 5. o Enhanced consideration of existing enrollment protocols in the context of mapping the requirements to existing solutions in Section 4 and in Section 6. From individual version 00 -> 01: o Update of examples, specifically for building automation as well as two new application use cases in Section 3.2. o Deletion of asynchronous interaction with MASA to not complicate the use case. Note that the voucher exchange can already be handled in an asynchronous manner and is therefore not considered further. This resulted in removal of the alternative path the MASA in Figure 1 and the associated description in Section 5. o Enhancement of description of architecture elements and changes to BRSKI in Section 5. o Consideration of existing enrollment protocols in the context of mapping the requirements to existing solutions in Section 4. o New section starting Section 6 with the mapping to existing enrollment protocols by collecting boundary conditions. Authors' Addresses Steffen Fries Siemens AG Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: firstname.lastname@example.org URI: https://www.siemens.com/ Hendrik Brockhaus Siemens AG Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: email@example.com URI: https://www.siemens.com/ Fries, et al. Expires July 11, 2021 [Page 40] Internet-Draft BRSKI-AE January 2021 Eliot Lear Cisco Systems Richtistrasse 7 Wallisellen CH-8304 Switzerland Phone: +41 44 878 9200 Email: firstname.lastname@example.org Thomas Werner Siemens AG Otto-Hahn-Ring 6 Munich, Bavaria 81739 Germany Email: email@example.com URI: https://www.siemens.com/ Fries, et al. Expires July 11, 2021 [Page 41]