ACE M. Sahni, Ed.
Internet-Draft S. Tripathi, Ed.
Intended status: Standards Track Palo Alto Networks
Expires: 16 October 2023 14 April 2023
CoAP Transfer for the Certificate Management Protocol
draft-ietf-ace-cmpv2-coap-transport-09
Abstract
This document specifies the use of 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 IoT space.
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
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and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 16 October 2023.
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/
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Please review these documents carefully, as they describe your rights
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provided without warranty as described in the Revised BSD License.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. CoAP Transfer Mechanism for CMP . . . . . . . . . . . . . . . 3
2.1. CoAP URI Format . . . . . . . . . . . . . . . . . . . . . 3
2.2. Discovery of CMP RA/CA . . . . . . . . . . . . . . . . . 4
2.3. CoAP Request Format . . . . . . . . . . . . . . . . . . . 4
2.4. CoAP Block-Wise Transfer Mode . . . . . . . . . . . . . . 4
2.5. Multicast CoAP . . . . . . . . . . . . . . . . . . . . . 5
2.6. Announcement PKIMessage . . . . . . . . . . . . . . . . . 5
3. Proxy Support . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Security Considerations . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 10
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 Certificate Management Protocol 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 the CMP version 2 [RFC4210] , CMP version 3
[I-D.ietf-lamps-cmp-updates] designated as CMP in this document and
Lightweight CMP Profile [I-D.ietf-lamps-lightweight-cmp-profile] .
This document, in general, follows the HTTP transfer for CMP
specifications defined in [RFC6712] and specifies the requirements
for using CoAP as a transfer mechanism for the CMP.
This document also provides guidance on how to use a "CoAP-to-HTTP"
proxy to ease adoption of CoAP transfer mechanism by enabling the
interconnection with existing PKI entities already providing CMP over
HTTP.
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1.1. Terminology
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. The RAs and CAs, in general, 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 the Certificate Management Protocol.
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, e.g., as specified in the Lightweight CMP Profile [I-
D.ietf-lamps-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/<name>
coap://www.example.com/.well-known/cmp/p/<name>/<operation>
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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 ports numbers MUST be assumed for the
"coap:" and the "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 Registration Authority
(LRA) or a Local 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 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 requests marked
as a Confirmable message [RFC7252]. If the CoAP request is
successful then the server MUST return a Success 2.xx response code
otherwise server 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 CA or RA
to process the CMP transaction.
When transferring CMP PKIMesssage over CoAP the content-format
"application/pkixcmp" MUST be used.
2.4. CoAP Block-Wise Transfer Mode
A CMP PKIMesssage consists of a header, body, protection, and
extraCerts structures 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.
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If a CoAP-to-HTTP proxy is in the path between EEs and CA or EEs and
RA then, if the server supports, 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 event triggered
or periodic, for the other PKI entities. Here is the list of CMP
announcement messages prefixed by their respective ASN.1 identifier
(section 5.1.2 [RFC4210])
[15] CA Key Update Announcement
[16] Certificate Announcement
[17] Revocation Announcement
[18] CRL Announcement
An EE MAY use 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 Announcements messages, then it MUST send
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 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. A client on receiving a 2.xx success
response without the Observe option [RFC7641] MAY try after some time
to register again for announcements from the CMP server. Since
server can remove the EE from the list of observers for announcement
messages, an EE SHOULD periodically re-register itself for
announcement messages.
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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
[I-D.ietf-lamps-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 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 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 pre-configured servers. It is out of scope of
this document on 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 "coaps://" scheme
for routing CoAP requests.
4. Security Considerations
* If PKIProtection is used, the PKIHeader and PKIBody of the CMP
protocol are cryptographically protected against malicious
modifications. As such, UDP can be used without compromising the
security of the CMP protocol. Security Considerations for CoAP
are defined in [RFC7252].
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* The CMP protocol does not provide confidentiality of the CMP
payloads. If confidentiality is desired, CoAP over DTLS [RFC9147]
MAY be used to provide confidentiality for the CMP payloads,
although it cannot conceal that the CMP protocol is used within
the DTLS layer.
* Section 9.1 of [RFC7252] defines how to use DTLS [RFC9147] for
securing the CoAP. DTLS [RFC9147] associations SHOULD be kept
alive and re-used 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 CRL updates.
* Implementations SHOULD use the available datagram size and avoid
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. 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 therefore proper role based
access control should be in place. Proxy can be deployed at the
edge of the "End Entities" network or in front of an RA and CA to
protect them. The proxy however 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
This document adds a new entry to the CoAP Content-Formats IANA
Registry (https://www.iana.org/assignments/core-parameters/core-
parameters.xhtml#content-formats) for the code of content-type
"application/pkixcmp", for transferring CMP transactions over CoAP,
from the identifier range 256-9999 reserved for IETF specifications.
Type name: application
Subtype name: pkixcmp
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Encoding: Content may contain arbitrary octet values. The octet
values are the ASN.1 DER encoding of a PKI message, as defined in the
[RFC4210] specifications.
Reference: This document and [RFC4210]
This document also adds a new path segment "ann" to the CMP Well-
Known URI Path Segments (https://www.iana.org/assignments/cmp/
cmp.xhtml#cmp-well-known-uri) IANA registry for the EEs to register
themselves for the announcement messages.
Path Segment: ann
Description: The path to send a GET request with CoAP Observer Option
to register for CMP announcement messages.
Reference: This document.
This document references the cmp, in the Well-Known URIs
(https://www.iana.org/assignments/well-known-uris/well-known-
uris.xhtml) IANA registry. Please add a reference of this document
to the Well-Known URIs (https://www.iana.org/assignments/well-known-
uris/well-known-uris.xhtml) IANA registry for that entry.
This document also refers the path segment "p" in the CMP Well-Known
URI Path Segments (https://www.iana.org/assignments/cmp/
cmp.xhtml#cmp-well-known-uri) IANA registry. Please add a reference
of this document to the CMP Well-Known URI Path Segments
(https://www.iana.org/assignments/cmp/cmp.xhtml#cmp-well-known-uri)
for that path segment.
[Note RFC Editor]: This document should be published together or
after the CMP version 3 [I-D.ietf-lamps-cmp-updates] as it references
IANA entries created by that Internet draft.
6. Acknowledgments
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.
7. References
7.1. Normative References
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[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>.
[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>.
[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>.
[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>.
[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>.
[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>.
[I-D.ietf-lamps-cmp-updates]
Brockhaus, H., von Oheimb, D., and J. Gray, "Certificate
Management Protocol (CMP) Updates", Work in Progress,
Internet-Draft, draft-ietf-lamps-cmp-updates-23, 29 June
2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
lamps-cmp-updates-23>.
[I-D.ietf-lamps-lightweight-cmp-profile]
Brockhaus, H., von Oheimb, D., and S. Fries, "Lightweight
Certificate Management Protocol (CMP) Profile", Work in
Progress, Internet-Draft, draft-ietf-lamps-lightweight-
cmp-profile-13, 8 July 2022,
<https://datatracker.ietf.org/doc/html/draft-ietf-lamps-
lightweight-cmp-profile-13>.
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[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
<https://www.rfc-editor.org/info/rfc8615>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[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>.
[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>.
[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>.
7.2. 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>.
[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>.
[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>.
Authors' Addresses
Mohit Sahni (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: msahni@paloaltonetworks.com
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Saurabh Tripathi (editor)
Palo Alto Networks
3000 Tannery Way
Santa Clara, CA 95054
United States of America
Email: stripathi@paloaltonetworks.com
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