Palo Alto Networks

3000 Tannery Way Santa Clara CA 95054 United States of America msahni@paloaltonetworks.com

Palo Alto Networks

3000 Tannery Way Santa Clara CA 95054 United States of America stripathi@paloaltonetworks.com

sec ace This document specifies the use of the Constrained Application Protocol (CoAP) as a transfer mechanism for the Certificate Management Protocol (CMP). CMP defines the interaction between various PKI entities for the purpose of certificate creation and management. CoAP is an HTTP-like client-server protocol used by various constrained devices in the Internet of Things space.

Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at .

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Table of Contents

• .  Introduction
  • .  Requirements Language
• .  CoAP Transfer Mechanism for CMP
  • .  CoAP URI Format
  • .  Discovery of CMP RA/CA
  • .  CoAP Request Format
  • .  CoAP Block-Wise Transfer Mode
  • .  Multicast CoAP
  • .  Announcement PKIMessage
• .  Proxy Support
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction The Certificate Management Protocol (CMP) is used by the PKI entities for the generation and management of certificates. One of the requirements of CMP is to be independent of the transport protocol in use. CMP has mechanisms to take care of required transactions, error reporting, and protection of messages. The Constrained Application Protocol (CoAP) defined in , , and is a client-server protocol like HTTP. It is designed to be used by constrained devices over constrained networks. The recommended transport for CoAP is UDP; however, specifies the support of CoAP over TCP, TLS, and WebSockets. This document specifies the use of CoAP over UDP as a transport medium for CMP version 2, CMP version 3 (designated as CMP in this document), and the Lightweight CMP Profile. In general, this document follows the HTTP transfer for CMP specifications defined in and specifies the requirements for using CoAP as a transfer mechanism for CMP. This document also provides guidance on how to use a "CoAP-to-HTTP" proxy to ease adoption of a CoAP transfer mechanism by enabling the interconnection with existing PKI entities already providing CMP over HTTP.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

CoAP Transfer Mechanism for CMP A CMP transaction consists of exchanging PKIMessages between PKI end entities (EEs), registration authorities (RAs), and certification authorities (CAs). If the EEs are constrained devices, then they may prefer, as a CMP client, the use of CoAP instead of HTTP as the transfer mechanism. In general, the RAs and CAs are not constrained and can support both CoAP and HTTP client and server implementations. This section specifies how to use CoAP as the transfer mechanism for CMP.

CoAP URI Format The CoAP URI format is described in . The CoAP endpoints MUST support use of the path prefix "/.well-known/" as defined in and the registered name "cmp" to help with endpoint discovery and interoperability. Optional path segments MAY be added after the registered application name (i.e., after "/.well-known/cmp") to provide distinction. The path segment 'p' followed by an arbitraryLabel <name> could, for example, support the differentiation of specific CAs or certificate profiles. Further path segments, for example, as specified in Lightweight CMP Profile, could indicate PKI management operations using an operationLabel <operation>. A valid full CMP URI can look like this: coap://www.example.com/.well-known/cmp coap://www.example.com/.well-known/cmp/<operation> coap://www.example.com/.well-known/cmp/p/<profileLabel> coap://www.example.com/.well-known/cmp/p/<profileLabel>/<operation>

Discovery of CMP RA/CA The EEs can be configured with enough information to form the CMP server URI. The minimum information that can be configured is the scheme, i.e., "coap:" or "coaps:", and the authority portion of the URI, e.g., "example.com:5683". If the port number is not specified in the authority, then the default port numbers MUST be assumed for the "coap:" and "coaps:" scheme URIs. The default port for "coap:" scheme URIs is 5683 and the default port for "coaps:" scheme URIs is 5684 . Optionally, in the environments where a Local RA or CA is deployed, EEs can also use the CoAP service discovery mechanism to discover the URI of the Local RA or CA. The CoAP CMP endpoints supporting service discovery MUST also support resource discovery in the Constrained RESTful Environments (CoRE) Link Format, as described in . The link MUST include the 'ct' attribute defined in with the value of "application/pkixcmp", as defined in the "CoAP Content-Formats" IANA registry.

CoAP Request Format The CMP PKIMessages MUST be DER encoded and sent as the body of the CoAP POST request. A CMP client MUST send each CoAP request marked as a Confirmable message . If the CoAP request is successful, then the CMP RA or CA MUST return a Success 2.xx response code; otherwise, the CMP RA or CA MUST return an appropriate Client Error 4.xx or Server Error 5.xx response code. A CMP RA or CA may choose to send a piggybacked response to the client, or it MAY send a separate response in case it takes some time for the RA or CA to process the CMP transaction. When transferring CMP PKIMessage over CoAP, the content-format "application/pkixcmp" MUST be used.

CoAP Block-Wise Transfer Mode A CMP PKIMessage consists of a header, body, protection, and extraCerts structure, which may contain many optional and potentially large fields. Thus, a CMP message can be much larger than the Maximum Transmission Unit (MTU) of the outgoing interface of the device. The EEs and RAs or CAs MUST use the block-wise transfer mode to transfer such large messages instead of relying on IP fragmentation. If a CoAP-to-HTTP proxy is in the path between EEs and an RA or EEs and a CA and if the server supports, then it MUST use the chunked transfer encoding to send data over the HTTP transport. The proxy MUST try to reduce the number of packets sent by using an optimal chunk length for the HTTP transport.

Multicast CoAP CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination address.

Announcement PKIMessage A CMP server may publish announcements that can be triggered by an event or periodically for the other PKI entities. Here is the list of CMP announcement messages prefixed by their respective ASN.1 identifier (see ): [15] CA Key Update Announcement [16] Certificate Announcement [17] Revocation Announcement [18] CRL Announcement An EE MAY use the CoAP Observe Option to register itself to get any announcement messages from the RA or CA. The EE can send a GET request to the server's URI suffixed by "/ann". For example, a path to register for announcement messages may look like this: coap://www.example.com/.well-known/cmp/ann coap://www.example.com/.well-known/cmp/p/<profileLabel>/ann If the server supports CMP announcement messages, then it MUST send an appropriate Success 2.xx response code; otherwise, it MUST send an appropriate Client Error 4.xx or Server Error 5.xx response code. If for some reason the server cannot add the client to its list of observers for the announcements, it can omit the Observe Option in the response to the client. Upon receiving a Success 2.xx response without the Observe Option , after some time, a client MAY try to register again for announcements from the CMP server. Since a server can remove the EE from the list of observers for announcement messages, an EE SHOULD periodically reregister itself for announcement messages. Alternatively, an EE MAY periodically poll for the current status of the CA via the "PKI Information Request" message; see . If supported, EEs MAY also use "support messages" defined in Lightweight CMP Profile to get information about the CA status. These mechanisms will help constrained devices that are acting as EEs to conserve resources by eliminating the need to create an endpoint for receiving notifications from the RA or CA. It will also simplify the implementation of a CoAP-to-HTTP proxy.

Proxy Support This section provides guidance on using a CoAP-to-HTTP proxy between EEs and RAs or CAs in order to avoid changes to the existing PKI implementation. Since the CMP payload is the same over CoAP and HTTP transfer mechanisms, a CoAP-to-HTTP cross-protocol proxy can be implemented based on . The CoAP-to-HTTP proxy can either be located closer to the EEs or closer to the RA or CA. The proxy MAY support service discovery and resource discovery, as described in . The CoAP-to-HTTP proxy MUST function as a reverse proxy, only permitting connections to a limited set of preconfigured servers. It is out of scope of this document to specify how a reverse proxy can route CoAP client requests to one of the configured servers. Some recommended mechanisms are as follows:

• Use the Uri-Path option to identify a server.
• Use separate hostnames for each of the configured servers and then use the Uri-Host option for routing the CoAP requests.
• Use separate hostnames for each of the configured servers and then use Server Name Indication in case of the "coaps://" scheme for routing CoAP requests.

Security Considerations

• If PKIProtection is used, the PKIHeader and PKIBody of the CMP are cryptographically protected against malicious modifications. As such, UDP can be used without compromising the security of the CMP. Security considerations for CoAP are defined in .
• The CMP does not provide confidentiality of the CMP payloads. If confidentiality is desired, CoAP over DTLS SHOULD be used to provide confidentiality for the CMP payloads; although, it cannot conceal that the CMP is used within the DTLS layer.
• defines how to use DTLS for securing CoAP. DTLS associations SHOULD be kept alive and reused where possible to amortize on the additional overhead of DTLS on constrained devices.
• An EE might not witness all of the announcement messages when using the CoAP Observe Option , since the Observe Option is a "best-effort" approach and the server might lose its state for subscribers to its announcement messages. The EEs may use an alternate method described in to obtain time critical changes, such as Certificate Revocation List (CRL) updates.
• Implementations SHOULD use the available datagram size and avoid sending small datagrams containing partial CMP PKIMessage data in order to reduce memory usage for packet buffering.
• A CoAP-to-HTTP proxy can also protect the PKI entities by handling UDP and CoAP messages. The proxy can mitigate attacks, like denial-of-service attacks, replay attacks, and resource-exhaustion attacks, by enforcing basic checks, like validating that the ASN.1 syntax is compliant to CMP messages and validating the PKIMessage protection before sending them to PKI entities.
• Since the proxy may have access to the CMP-level metadata and control over the flow of CMP messages, proper role-based access control should be in place. The proxy can be deployed at the edge of the "end entities" network or in front of an RA and CA to protect them. However, the proxy may itself be vulnerable to resource-exhaustion attacks as it's required to buffer the CMP messages received over CoAP transport before sending it to the HTTP endpoint. This can be mitigated by using short timers for discarding the buffered messages and rate limiting clients based on the resource usage.

IANA Considerations IANA has registered "application/pkixcmp" (ID 259) in the "CoAP Content-Formats" registry to transfer CMP transactions over CoAP.

Type name:

application

Subtype name:

pkixcmp

Reference:

RFC 9482

IANA has also registered a new path segment "ann" in the "CMP Well-Known URI Path Segments" registry for the EEs to register themselves for the announcement messages.

Path Segment:

ann

Description:

The path to send a GET request with the CoAP Observe Option to register for CMP announcement messages.

Reference:

RFC 9482

IANA has added this document as a reference for the "cmp" entry in the "Well-Known URIs" registry. IANA has also added this document as a reference for the "p" entry in the "CMP Well-Known URI Path Segments" registry.

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the Internet X.509 Public Key Infrastructure (PKI) Certificate Management Protocol (CMP). Protocol messages are defined for X.509v3 certificate creation and management. CMP provides on-line interactions between PKI components, including an exchange between a Certification Authority (CA) and a client system. [STANDARDS-TRACK] This specification defines Web Linking using a link format for use by constrained web servers to describe hosted resources, their attributes, and other relationships between links. Based on the HTTP Link Header field defined in RFC 5988, the Constrained RESTful Environments (CoRE) Link Format is carried as a payload and is assigned an Internet media type. "RESTful" refers to the Representational State Transfer (REST) architecture. A well-known URI is defined as a default entry point for requesting the links hosted by a server. [STANDARDS-TRACK] This document describes how to layer the Certificate Management Protocol (CMP) over HTTP. It is the "CMPtrans" document referenced in RFC 4210; therefore, this document updates the reference given therein. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP) is a specialized web transfer protocol for use with constrained nodes and constrained (e.g., low-power, lossy) networks. The nodes often have 8-bit microcontrollers with small amounts of ROM and RAM, while constrained networks such as IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs) often have high packet error rates and a typical throughput of 10s of kbit/s. The protocol is designed for machine- to-machine (M2M) applications such as smart energy and building automation. CoAP provides a request/response interaction model between application endpoints, supports built-in discovery of services and resources, and includes key concepts of the Web such as URIs and Internet media types. CoAP is designed to easily interface with HTTP for integration with the Web while meeting specialized requirements such as multicast support, very low overhead, and simplicity for constrained environments. The Constrained Application Protocol (CoAP) is a RESTful application protocol for constrained nodes and networks. The state of a resource on a CoAP server can change over time. This document specifies a simple protocol extension for CoAP that enables CoAP clients to "observe" resources, i.e., to retrieve a representation of a resource and keep this representation updated by the server over a period of time. The protocol follows a best-effort approach for sending new representations to clients and provides eventual consistency between the state observed by each client and the actual resource state at the server. The Constrained Application Protocol (CoAP) is a RESTful transfer protocol for constrained nodes and networks. Basic CoAP messages work well for small payloads from sensors and actuators; however, applications will need to transfer larger payloads occasionally -- for instance, for firmware updates. In contrast to HTTP, where TCP does the grunt work of segmenting and resequencing, CoAP is based on datagram transports such as UDP or Datagram Transport Layer Security (DTLS). These transports only offer fragmentation, which is even more problematic in constrained nodes and networks, limiting the maximum size of resource representations that can practically be transferred. Instead of relying on IP fragmentation, this specification extends basic CoAP with a pair of "Block" options for transferring multiple blocks of information from a resource representation in multiple request-response pairs. In many important cases, the Block options enable a server to be truly stateless: the server can handle each block transfer separately, with no need for a connection setup or other server-side memory of previous block transfers. Essentially, the Block options provide a minimal way to transfer larger representations in a block-wise fashion. A CoAP implementation that does not support these options generally is limited in the size of the representations that can be exchanged, so there is an expectation that the Block options will be widely used in CoAP implementations. Therefore, this specification updates RFC 7252. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This memo defines a path prefix for "well-known locations", "/.well-known/", in selected Uniform Resource Identifier (URI) schemes. In doing so, it obsoletes RFC 5785 and updates the URI schemes defined in RFC 7230 to reserve that space. It also updates RFC 7595 to track URI schemes that support well-known URIs in their registry. The Hypertext Transfer Protocol (HTTP) is a stateless application-level protocol for distributed, collaborative, hypertext information systems. This document specifies the HTTP/1.1 message syntax, message parsing, connection management, and related security concerns. This document obsoletes portions of RFC 7230. This document specifies version 1.3 of the Datagram Transport Layer Security (DTLS) protocol. DTLS 1.3 allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. The DTLS 1.3 protocol is based on the Transport Layer Security (TLS) 1.3 protocol and provides equivalent security guarantees with the exception of order protection / non-replayability. Datagram semantics of the underlying transport are preserved by the DTLS protocol. This document obsoletes RFC 6347. Informative References This memo profiles the X.509 v3 certificate and X.509 v2 certificate revocation list (CRL) for use in the Internet. An overview of this approach and model is provided as an introduction. The X.509 v3 certificate format is described in detail, with additional information regarding the format and semantics of Internet name forms. Standard certificate extensions are described and two Internet-specific extensions are defined. A set of required certificate extensions is specified. The X.509 v2 CRL format is described in detail along with standard and Internet-specific extensions. An algorithm for X.509 certification path validation is described. An ASN.1 module and examples are provided in the appendices. [STANDARDS-TRACK] The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations.

Acknowledgements The authors would like to thank , , and for their guidance in writing the content of this document and providing valuable feedback.

Authors' Addresses Palo Alto Networks

3000 Tannery Way Santa Clara CA 95054 United States of America msahni@paloaltonetworks.com

Palo Alto Networks

3000 Tannery Way Santa Clara CA 95054 United States of America stripathi@paloaltonetworks.com