]> 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.

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/p/ coap://www.example.com/.well-known/cmp/p// ]]>

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 ): 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: /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 Informative References

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