CMP over CoAP: Constrained Application Protocol (CoAP) Transfer for the Certificate Management Protocol
How small IoT devices can talk to a certificate authority using CoAP instead of HTTP.
Sahni, M., Ed. and S. Tripathi, Ed., "Constrained Application Protocol (CoAP) Transfer for the Certificate Management Protocol", RFC 9482, DOI 10.17487/RFC9482, November 2023, <https://www.rfc-editor.org/info/rfc9482>.
@misc{rfc9482,
series = {Request for Comments},
number = 9482,
howpublished = {RFC 9482},
publisher = {RFC Editor},
doi = {10.17487/RFC9482},
url = {https://www.rfc-editor.org/info/rfc9482},
author = {M. Sahni and S. Tripathi},
title = {{Constrained Application Protocol (CoAP) Transfer for the Certificate Management Protocol}},
pagetotal = 9,
year = 2023,
month = nov,
} RFC text
Contents
Internet Engineering Task Force (IETF) M. Sahni, Ed.
Request for Comments: 9482 S. Tripathi, Ed.
Category: Standards Track Palo Alto Networks
ISSN: 2070-1721 November 2023
Constrained Application Protocol (CoAP) Transfer for the Certificate
Management Protocol
Abstract
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
https://www.rfc-editor.org/info/rfc9482.
Copyright Notice
Copyright (c) 2023 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 Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.
Table of Contents
1. Introduction
1.1. Requirements Language
2. CoAP Transfer Mechanism for CMP
2.1. CoAP URI Format
2.2. Discovery of CMP RA/CA
2.3. CoAP Request Format
2.4. CoAP Block-Wise Transfer Mode
2.5. Multicast CoAP
2.6. Announcement PKIMessage
3. Proxy Support
4. Security Considerations
5. IANA Considerations
6. References
6.1. Normative References
6.2. Informative References
Acknowledgements
Authors' Addresses
1. Introduction
The Certificate Management Protocol (CMP) [RFC4210] 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 [RFC7252],
[RFC7959], and [RFC8323] 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,
[RFC8323] 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 [RFC4210], CMP version 3 [RFC9480]
(designated as CMP in this document), and the Lightweight CMP Profile
[RFC9483]. In general, this document follows the HTTP transfer for
CMP specifications defined in [RFC6712] 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.
1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. CoAP Transfer Mechanism for CMP
A CMP transaction consists of exchanging PKIMessages [RFC4210]
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.
2.1. CoAP URI Format
The CoAP URI format is described in Section 6 of [RFC7252]. The CoAP
endpoints MUST support use of the path prefix "/.well-known/" as
defined in [RFC8615] 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
[RFC9483], 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>
2.2. 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 [RFC7252].
Optionally, in the environments where a Local RA or CA is deployed,
EEs can also use the CoAP service discovery mechanism [RFC7252] 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 [RFC6690]. The link MUST include the 'ct' attribute defined in
Section 7.2.1 of [RFC7252] with the value of "application/pkixcmp",
as defined in the "CoAP Content-Formats" IANA registry.
2.3. 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 [RFC7252]. 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 [RFC7252] to the client, or it
MAY send a separate response [RFC7252] 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.
2.4. 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
[RFC7959] 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 [RFC9112] 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.
2.5. Multicast CoAP
CMP PKIMessages sent over CoAP MUST NOT use a Multicast destination
address.
2.6. 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 Section 5.1.2 of [RFC4210]):
[15] CA Key Update Announcement
[16] Certificate Announcement
[17] Revocation Announcement
[18] CRL Announcement
An EE MAY use the CoAP Observe Option [RFC7641] 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
[RFC7641] in the response to the client. Upon receiving a Success
2.xx response without the Observe Option [RFC7641], 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 Section 6.5 of
[RFC4210]. If supported, EEs MAY also use "support messages" defined
in Section 4.3 of Lightweight CMP Profile [RFC9483] 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.
3. 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 Section 10 of [RFC7252]. 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
Section 2.2. 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 [RFC8446] in case of the "coaps://"
scheme for routing CoAP requests.
4. 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 [RFC7252].
* The CMP does not provide confidentiality of the CMP payloads. If
confidentiality is desired, CoAP over DTLS [RFC9147] SHOULD be
used to provide confidentiality for the CMP payloads; although, it
cannot conceal that the CMP is used within the DTLS layer.
* Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
securing CoAP. DTLS [RFC9147] 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 [RFC7641], 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 Section 2.6 to obtain time critical
changes, such as Certificate Revocation List (CRL) [RFC5280]
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.
5. IANA Considerations
IANA has registered "application/pkixcmp" (ID 259) in the "CoAP
Content-Formats" registry <https://www.iana.org/assignments/core-
parameters> to transfer CMP transactions over CoAP.
Type name: application
Subtype name: pkixcmp
Reference: RFC 9482 [RFC4210]
IANA has also registered a new path segment "ann" in the "CMP Well-
Known URI Path Segments" registry <https://www.iana.org/assignments/
cmp> 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 <https://www.iana.org/assignments/
well-known-uris>.
IANA has also added this document as a reference for the "p" entry in
the "CMP Well-Known URI Path Segments" registry
<https://www.iana.org/assignments/cmp/>.
6. References
6.1. Normative References
[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>.
[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>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6712] Kause, T. and M. Peylo, "Internet X.509 Public Key
Infrastructure -- HTTP Transfer for the Certificate
Management Protocol (CMP)", RFC 6712,
DOI 10.17487/RFC6712, September 2012,
<https://www.rfc-editor.org/info/rfc6712>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[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>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/info/rfc9112>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
[RFC9480] Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
Management Protocol (CMP) Updates", RFC 9480,
DOI 10.17487/RFC9480, November 2023,
<https://www.rfc-editor.org/info/rfc9480>.
[RFC9483] Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", RFC 9483,
DOI 10.17487/RFC9483, November 2023,
<https://www.rfc-editor.org/info/rfc9483>.
6.2. Informative References
[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>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[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>.
Acknowledgements
The authors would like to thank Hendrik Brockhaus, David von Oheimb,
and Andreas Kretschmer for their guidance in writing the content of
this document and providing valuable feedback.
Authors' Addresses
Mohit Sahni (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: msahni@paloaltonetworks.com
Saurabh Tripathi (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: stripathi@paloaltonetworks.com
AI ELI5
On this page
§1Why CMP Needs a CoAP Transfer
The Certificate Management Protocol (CMP, RFC 4210) is how PKI entities create and manage certificates. One of its requirements is to be independent of the transport protocol in use.
The Constrained Application Protocol (CoAP, RFC 7252) is a client-server protocol like HTTP, designed for constrained devices on constrained networks. Its recommended transport is UDP. This document specifies CoAP over UDP as a transport for CMP version 2, CMP version 3 (RFC 9480) and the Lightweight CMP Profile (RFC 9483), and follows the HTTP transfer rules of RFC 6712 in general.
End entities (EEs) may be constrained devices, while registration authorities (RAs) and certification authorities (CAs) are usually not and can support both CoAP and HTTP.
Getting an ID card from the office normally means a lot of paperwork. Tiny gadgets cannot carry that, so this gives them a lighter way to ask.
§2.1URI Format and Discovery
CoAP endpoints MUST support the path prefix /.well-known/ with the registered name cmp. Optional path segments may follow. The segment p plus a label can pick a specific CA or certificate profile, and a further segment can name a PKI operation, as in the Lightweight CMP Profile.
A device can be configured with just the scheme (coap: or coaps:) and the authority. If no port is given, the defaults are 5683 for coap: and 5684 for coaps:.
Where a Local RA or CA is deployed, devices can optionally use CoAP service discovery. Endpoints that support it MUST also support resource discovery in CoRE Link Format, and the link MUST include the ct attribute with the value application/pkixcmp.
Every certificate office gets a standard street address, so a gadget knows where to knock. It can also shout out and ask who is nearby.
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>§2.3Sending CMP Messages in CoAP Requests
CMP PKIMessages MUST be DER encoded and sent as the body of a CoAP POST request. The client MUST mark each request as a Confirmable message, and the content-format application/pkixcmp MUST be used.
On success the RA or CA MUST return a 2.xx response. Otherwise it MUST return an appropriate 4.xx (client error) or 5.xx (server error) code. The server may send a piggybacked response, or a separate response if processing takes some time.
CMP messages over CoAP MUST NOT use a multicast destination address.
The gadget puts its request in an envelope marked 'please confirm you got this'. The office answers with a code that says whether it worked.
§2.4Block-Wise Transfer for Large Messages
A PKIMessage has a header, body, protection and extraCerts, some of which can be large, so a message can be much bigger than the MTU of the device's outgoing interface. EEs, RAs and CAs MUST use the block-wise transfer mode (RFC 7959) for such messages instead of relying on IP fragmentation.
If a CoAP-to-HTTP proxy sits in the path and the server supports it, the proxy MUST use chunked transfer encoding (RFC 9112) on the HTTP side and try to reduce the number of packets by choosing an optimal chunk length.
If a parcel is too big for the mail slot, you send it in several small pieces instead of squashing it through.
§2.6Announcements and the Proxy
A CMP server may publish announcements: CA Key Update (15), Certificate (16), Revocation (17) and CRL (18). An EE MAY use the CoAP Observe Option (RFC 7641) to register by sending a GET to the server URI suffixed with /ann. If the server cannot add the client as an observer, it can omit the Observe Option in its response, and the client MAY try again later. An EE SHOULD periodically reregister. As an alternative, an EE MAY poll with the PKI Information Request message or use support messages from RFC 9483.
Because the CMP payload is the same over CoAP and HTTP, a CoAP-to-HTTP cross-protocol proxy can be built on Section 10 of RFC 7252. It can sit near the EEs or near the RA or CA, and it MUST act as a reverse proxy that only permits connections to a limited set of preconfigured servers. Routing is out of scope, but the RFC suggests using the Uri-Path option, separate hostnames with the Uri-Host option, or Server Name Indication for coaps://.
A gadget can sign up for news from the certificate office, or just ask now and then. A helper at the door can translate between the gadget's light language and the office's usual one.
- [15]CA Key Update Announcement — Announces a CA key update.
- [16]Certificate Announcement — Announces a certificate.
- [17]Revocation Announcement — Announces a revocation.
- [18]CRL Announcement — Announces a CRL.
§4Security Considerations
If PKIProtection is used, the PKIHeader and PKIBody are cryptographically protected against modification, so UDP can be used without weakening CMP. CMP itself does not give confidentiality. If it is wanted, CoAP over DTLS (RFC 9147) SHOULD be used, and DTLS associations SHOULD be kept alive and reused to amortise overhead on constrained devices.
Observe is best-effort, so an EE might miss announcements. For time-critical changes such as CRL updates it may use the polling alternative. Implementations SHOULD use the available datagram size and avoid small datagrams carrying partial PKIMessage data.
A CoAP-to-HTTP proxy can help protect PKI entities by checking ASN.1 syntax and PKIMessage protection, which can mitigate denial-of-service, replay and resource-exhaustion attacks. It may itself be exposed to resource exhaustion because it buffers messages. The RFC suggests short timers for buffered messages and rate limiting clients, and says proper role-based access control should be in place.
The messages carry their own lock, but nobody hides what is inside unless you add a secret tunnel. The helper at the door can also turn away troublemakers.
§5IANA Registrations
IANA registered the content-format application/pkixcmp (ID 259) in the CoAP Content-Formats registry. It also registered the new path segment ann in the CMP Well-Known URI Path Segments registry, for registering for announcement messages.
This document was also added as a reference for the cmp entry in the Well-Known URIs registry and for the p entry in the CMP Well-Known URI Path Segments registry.
The official list of names was updated so that everyone uses the same label for these messages.