Network Working Group                                            L. Liao
Internet-Draft                                                  NIO Inc.
Intended status: Standards Track                         1 November 2026
Expires: 5 May 2027


                       EST for C509 Certificates
                       draft-liao-ace-est-c509-04

Abstract

   This document defines Enrollment over Secure Transport (EST) protocol
   operations over HTTPS and secure CoAP for use with C509 certificates.
   The operations specified in this document support CA certificate
   distribution, C509 certificate enrollment, C509 certificate re-
   enrollment, and server-side key generation using C509 certificates.
   This document also defines operations for server capability
   discovery, Certificate Revocation List (CRL) distribution, and a
   challenge-response proof of possession for KEM-only keys.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-liao-ace-est-c509/.

   Discussion of this document takes place on the Authentication and
   Authorization for Constrained Environments Working Group mailing list
   (mailto:ace@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ace/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/ace/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ace-wg/xxx.

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/.






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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 5 May 2027.

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   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   7
   3.  Protocol Design . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Client Authentication . . . . . . . . . . . . . . . . . .   8
     3.2.  TLS Certificate Type Negotiation  . . . . . . . . . . . .   8
     3.3.  Discovery and URIs  . . . . . . . . . . . . . . . . . . .   8
     3.4.  EST URL Structure and Path Components . . . . . . . . . .   9
     3.5.  Channel Binding . . . . . . . . . . . . . . . . . . . . .   9
     3.6.  Message Binding . . . . . . . . . . . . . . . . . . . . .  10
     3.7.  Message Fragmentation . . . . . . . . . . . . . . . . . .  10
     3.8.  Delayed Responses . . . . . . . . . . . . . . . . . . . .  10
     3.9.  Response Cache  . . . . . . . . . . . . . . . . . . . . .  10
     3.10. CBOR Transfer . . . . . . . . . . . . . . . . . . . . . .  10
   4.  C509 Certification Request (C509 CSR) . . . . . . . . . . . .  12
     4.1.  empty-publickey Algorithm . . . . . . . . . . . . . . . .  13
     4.2.  CRAttribute C509ChangeSubjectName . . . . . . . . . . . .  13
     4.3.  Proof of Possession . . . . . . . . . . . . . . . . . . .  14
       4.3.1.  Proof of Possession for KEM Private Keys  . . . . . .  14
         4.3.1.1.  Challenge Issuance  . . . . . . . . . . . . . . .  14
         4.3.1.2.  PoP Nonce . . . . . . . . . . . . . . . . . . . .  15
         4.3.1.3.  MAC Computation by the EST Client . . . . . . . .  16
         4.3.1.4.  Verification by the EST Server  . . . . . . . . .  17
         4.3.1.5.  MAC Algorithms  . . . . . . . . . . . . . . . . .  17
   5.  EST Operations for Server Capability Discovery  . . . . . . .  18
     5.1.  caps  . . . . . . . . . . . . . . . . . . . . . . . . . .  18



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       5.1.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  18
       5.1.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  18
     5.2.  csrattrs / att  . . . . . . . . . . . . . . . . . . . . .  20
       5.2.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  20
       5.2.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  20
   6.  EST Operations for Distribution of CA Certificates  . . . . .  20
     6.1.  cacerts / crts  . . . . . . . . . . . . . . . . . . . . .  20
       6.1.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  21
       6.1.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  21
   7.  EST Operations for Distribution of C509 CRLs  . . . . . . . .  22
     7.1.  crli  . . . . . . . . . . . . . . . . . . . . . . . . . .  22
       7.1.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  22
       7.1.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  23
     7.2.  crl . . . . . . . . . . . . . . . . . . . . . . . . . . .  24
       7.2.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  24
       7.2.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  24
   8.  EST Operations for Certificate Enrollment . . . . . . . . . .  24
     8.1.  kemc  . . . . . . . . . . . . . . . . . . . . . . . . . .  25
       8.1.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  25
       8.1.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  25
     8.2.  simpleenroll / sen  . . . . . . . . . . . . . . . . . . .  25
       8.2.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  26
       8.2.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  26
     8.3.  simplereenroll / sren . . . . . . . . . . . . . . . . . .  26
       8.3.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  26
       8.3.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  27
     8.4.  serverkeygen / skc  . . . . . . . . . . . . . . . . . . .  27
       8.4.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  27
       8.4.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  28
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  28
     9.1.  Transport Security  . . . . . . . . . . . . . . . . . . .  28
     9.2.  Server Key Generation . . . . . . . . . . . . . . . . . .  29
     9.3.  C509 Certificate Validation . . . . . . . . . . . . . . .  29
     9.4.  C509 CRL Validation . . . . . . . . . . . . . . . . . . .  29
     9.5.  KEM Proof of Possession . . . . . . . . . . . . . . . . .  29
     9.6.  Capability Discovery  . . . . . . . . . . . . . . . . . .  30
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
     10.1.  C509 Public Key Algorithms Registry  . . . . . . . . . .  30
     10.2.  C509 Signature Algorithms Registry . . . . . . . . . . .  30
     10.3.  C509 CR Attributes Registry  . . . . . . . . . . . . . .  31
     10.4.  Media Types Registry . . . . . . . . . . . . . . . . . .  32
       10.4.1.  Media Type application/c509-kemchall+cbor  . . . . .  32
     10.5.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  33
     10.6.  Resource Type (rt=) Link Target Attribute Values
            Registry . . . . . . . . . . . . . . . . . . . . . . . .  33
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  34
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  34
     11.2.  Informative References . . . . . . . . . . . . . . . . .  36



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   Appendix A.  Message Flow Diagrams of EST over HTTPS
           Operations  . . . . . . . . . . . . . . . . . . . . . . .  36
     A.1.  caps  . . . . . . . . . . . . . . . . . . . . . . . . . .  36
     A.2.  cacerts for single CA certificate . . . . . . . . . . . .  37
     A.3.  cacerts for CA certificate set  . . . . . . . . . . . . .  37
     A.4.  cacerts for CA certificate chain  . . . . . . . . . . . .  37
     A.5.  crli  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.6.  crl . . . . . . . . . . . . . . . . . . . . . . . . . . .  38
     A.7.  kemc  . . . . . . . . . . . . . . . . . . . . . . . . . .  39
     A.8.  simpleenroll  . . . . . . . . . . . . . . . . . . . . . .  39
     A.9.  simplereenroll  . . . . . . . . . . . . . . . . . . . . .  40
     A.10. serverkeygen  . . . . . . . . . . . . . . . . . . . . . .  40
   Appendix B.  Message Flow Diagrams of EST over CoAP/DTLS
           Operations  . . . . . . . . . . . . . . . . . . . . . . .  41
     B.1.  crts for single CA certificate  . . . . . . . . . . . . .  41
     B.2.  crts for CA certificate set . . . . . . . . . . . . . . .  41
     B.3.  crts for CA certificate chain . . . . . . . . . . . . . .  42
     B.4.  crli  . . . . . . . . . . . . . . . . . . . . . . . . . .  42
     B.5.  crl . . . . . . . . . . . . . . . . . . . . . . . . . . .  43
     B.6.  kemc  . . . . . . . . . . . . . . . . . . . . . . . . . .  43
     B.7.  sen . . . . . . . . . . . . . . . . . . . . . . . . . . .  43
     B.8.  sren  . . . . . . . . . . . . . . . . . . . . . . . . . .  44
     B.9.  skc . . . . . . . . . . . . . . . . . . . . . . . . . . .  44
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  45
   Change log  . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
     Since draft-liao-ace-est-c509-03  . . . . . . . . . . . . . . .  45
     Since draft-liao-ace-est-c509-02  . . . . . . . . . . . . . . .  46
     Since draft-liao-ace-est-c509-01  . . . . . . . . . . . . . . .  47
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  47

1.  Introduction

   Enrollment over Secure Transport (EST) [RFC7030] defines HTTPS-based
   operations for X.509 [RFC5280] certificate enrollment and CA
   certificate distribution.  Payloads are DER-encoded and wrapped in
   CMS (Cryptographic Message Syntax, [RFC5652]) structures.  C509
   [I-D.ietf-cose-cbor-encoded-cert] defines a compact, CBOR-encoded
   alternative to DER X.509 certificates.  C509 certificates are
   substantially smaller.

   Although C509 was developed with constrained devices in mind, its
   benefits extend to unconstrained devices operating over low-bandwidth
   links and to large-scale deployments.  Smaller, CBOR-encoded
   certificates reduce bandwidth and storage requirements, accelerate
   TLS handshakes, and lower parsing and serialization overhead even on
   powerful endpoints; because C509 does not use ASN.1/DER,
   implementations can avoid complex ASN.1 parsing code, which reduces
   code size and complexity and lowers the attack surface for



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   certificate parsing libraries.  In complex systems (for example,
   connected cars) that contain diverse device classes—microcontrollers,
   sensor chips, and SoCs—using a common certificate format wherever
   practical simplifies integration and provisioning.  Using C509
   consistently across device classes simplifies provisioning,
   interoperability, and over-the-air updates, and can reduce overall
   operational costs and latency.

   This document defines EST operations that carry C509 objects in place
   of DER X.509 objects, following the same secure transport and URI
   path structure as [RFC7030] and [RFC9148].

   A key property of this design is that EST clients need only minimal
   CBOR processing:

   *  For non-KEM-only key types, the C509 CSR is typically pre-
      provisioned as an opaque binary blob by the device manufacturer or
      a provisioning tool; the EST client sends it verbatim as the POST
      body of simpleenroll / sen or simplereenroll / sren without
      interpreting its contents.

   *  For KEM-only key types, the EST client needs to communicate with
      the component that holds the private key (for example, a secure
      element or a hardware security module) to get the public key
      (C509SubjectPublicKeyInfo) and, after receiving the KEM challenge
      object (C509KemChall) from the EST server, the certification
      request (C509CertificationRequest); in this case, the EST client
      treats the C509SubjectPublicKeyInfo, C509KemChall, and
      C509CertificationRequest as opaque binary blobs.

   *  For all key types, the C509 certificate (and private key in the
      operation serverkeygen / skc) returned in the response is stored
      directly in persistent memory without modification.  The EST
      client MUST validate received C509 certificates against a trust
      anchor (see Section 9.3); apart from this validation, no parsing
      of the response is required.  This property makes the EST client
      implementation lightweight.

   This document uses C509CertificationRequest as defined in
   [I-D.ietf-cose-cbor-encoded-cert] as the C509 Certificate Signing
   Request (C509 CSR) format.  An EST client uses a C509 CSR to request
   issuance of a C509 certificate from an EST server.

   The operations for EST over HTTPS used in this document (those marked
   new are new operations defined in this document) are listed in
   Figure 1, and those for EST over CoAP in Figure 2.





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   +==========+===+=============+==============+===================+
   | Opera-   | M | Description |  Request     | Response          |
   | tion     | / |             |  Media Type  | Media Type        |
   |          | O |             |              |                   |
   +==========+===+=============+==============+===================+
   | caps     | M | Capability  | (none)       | text/plain;       |
   | (new)    |   | discovery   |              | charset=utf-8     |
   +----------+---+-------------+--------------+-------------------+
   | cacerts  | M | CA          | (none)       | - application/    |
   |          |   | certificate |              |   cose-c509+cbor  |
   |          |   | retrieval   |              | - application/    |
   |          |   |             |              |   cose-c509+cbor, |
   |          |   |             |              |   usage=chain     |
   |          |   |             |              | - application/    |
   |          |   |             |              |   cose-c509-cert  |
   |          |   |             |              |   +cbor           |
   +----------+---+-------------+--------------+-------------------+
   | crli     | O | CRL         | (none)       | application/      |
   | (new)    |   | metadata    |              | c509-crlinfo+cbor |
   |          |   | retrieval   |              |                   |
   +----------+---+-------------+--------------+-------------------+
   | crl      | O | CRL         | (none)       | application/      |
   | (new)    |   | retrieval   |              | c509-crl+cbor     |
   +----------+---+-------------+--------------+-------------------+
   | csr      | O | CSR         | (none)       | application/      |
   | attrs    |   | attributes  |              | cose-c509-        |
   |          |   | retrieval   |              | crtemplate+cbor   |
   +----------+---+-------------+--------------+-------------------+
   | kemc     | O | KEM         | application/ | application/      |
   | (new)    |   | challenge   | cose-c509-   | c509-kemchall     |
   |          |   | issuance    | pubkey+cbor  | +cbor             |
   +----------+---+-------------+--------------+-------------------+
   | simple   | M | Certificate | application/ | application/      |
   | enroll   |   | enrollment  | cose-c509-   | cose-c509-        |
   |          |   |             | pkcs10+cbor  | cert+cbor         |
   +----------+---+-------------+--------------+-------------------+
   | simple   | M | Certificate | application/ | application/      |
   | reenroll |   | reenroll-   | cose-c509-   | cose-c509-        |
   |          |   | ment        | pkcs10+cbor  | cert+cbor         |
   +----------+---+-------------+--------------+-------------------+
   | server   | O | Server-side | application/ | application/      |
   | keygen   |   | key         | cose-c509-   | cose-c509-        |
   |          |   | generation  | pkcs10+cbor  | pem+cbor          |
   +----------+---+-------------+--------------+-------------------+

     Figure 1: Operations for EST over HTTPS Used in This Document (M/
                          O: MANDATORY / OPTIONAL)




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   +===============+================+=====+==========+==========+
   | EST over      | Corresponding  | M/O | Request  | Response |
   | CoAP/DTLS     | EST over HTTPS |     | Content- | Content- |
   | Operation     | Operation      |     | Format   | Format   |
   +===============+================+=====+==========+==========+
   | (Not Related) | caps           | N/A | N/A      | N/A      |
   +---------------+----------------+-----+----------+----------+
   | crts          | cacerts        | M   | (none)   | - TBD    |
   |               |                |     |          | - TBD    |
   |               |                |     |          | - TBD    |
   +---------------+----------------+-----+----------+----------+
   | crli (new)    | crli           | O   | (none)   | TBD      |
   +---------------+----------------+-----+----------+----------+
   | crl (new)     | crl            | O   | (none)   | TBD      |
   +---------------+----------------+-----+----------+----------+
   | attr          | csrattrs       | O   | (none)   | TBD      |
   +---------------+----------------+-----+----------+----------+
   | kemc (new)    | kemc           | O   | TBD      | TBD      |
   +---------------+----------------+-----+----------+----------+
   | sen           | simpleenroll   | M   | TBD      | TBD      |
   +---------------+----------------+-----+----------+----------+
   | sren          | simplereenroll | M   | TBD      | TBD      |
   +---------------+----------------+-----+----------+----------+
   | skc           | serverkeygen   | O   | TBD      | TBD      |
   +---------------+----------------+-----+----------+----------+

     Figure 2: Operations for EST over CoAP/DTLS Used in This Document
                        (M/O: MANDATORY / OPTIONAL)

2.  Conventions and Definitions

   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.

   The following terms are used in this document:

   EST client:  The entity that contacts the EST server to obtain
      certificates or CA information, as defined in [RFC7030],
      Section 1.

   EST server:  The entity that processes EST requests, typically acting
      as an RA between the EST client and the CA, as defined in
      [RFC7030], Section 1.

   CA:  Certification Authority.  The entity that issues C509



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      certificates.

   C509 CSR:  C509 Certification Request.  A CBOR-encoded certification
      request used by an EST client to request issuance of a C509
      certificate.

   PoP:  Proof of Possession.  Verification that the requester holds the
      private key corresponding to the public key in the C509 CSR.

3.  Protocol Design

3.1.  Client Authentication

   While [RFC7030] permits a number of the EST functions to be used
   without authentication, this document requires that the client MUST
   be authenticated for all functions, as in [RFC9148].  This document
   requires EST to support certificate-based client authentication only;
   neither HTTP Basic nor Digest authentication (as described in
   Section 3.2.3 of [RFC7030]) is supported, as in [RFC9148].

3.2.  TLS Certificate Type Negotiation

   When C509 certificates are used for TLS or DTLS authentication, the
   client and server negotiate the certificate type using the
   server_certificate_type and client_certificate_type TLS extensions as
   defined in [RFC7250].

3.3.  Discovery and URIs

   In EST over HTTPS, the capabilities of the EST server are retrieved
   by using the operation caps.

   In EST over CoAP/DTLS, the capabilities are retrieved by sending a
   GET method to the EST server (as in Section 4.1 of [RFC9148]).  As in
   [RFC9148], the ct attribute lists the Content-Formats of both the
   request and the response.  Linefeeds are included only for
   readability.














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   REQ: GET /.well-known/core?rt=ace.est*

   RES: 2.05 Content
   </.well-known/est/crts>;rt="ace.est.crts";ct="TBD TBD TBD",
   </.well-known/est/sen>;rt="ace.est.sen";ct="TBD TBD",
   </.well-known/est/sren>;rt="ace.est.sren";ct="TBD TBD",
   </.well-known/est/att>;rt="ace.est.att";ct=TBD,
   </.well-known/est/skc>;rt="ace.est.skc";ct="TBD TBD",
   </.well-known/est/crli>;rt="ace.est.crli";ct=TBD,
   </.well-known/est/crl>;rt="ace.est.crl";ct=TBD,
   </.well-known/est/kemc>;rt="ace.est.kemc";ct="TBD3 TBD7"

   (TODO: the TBD values will be replaced with the real values once the
   Content-Formats are assigned in [I-D.ietf-cose-cbor-encoded-cert] and
   [I-D.liao-cose-c509-revocation].)

3.4.  EST URL Structure and Path Components

   The operations in this document follow the same URI path structure
   defined in [RFC7030], Section 3.2.2 for HTTPS and the corresponding
   secure CoAP mapping defined in [RFC9148].  Retrieval operations caps,
   cacerts / crts, csrattrs / att, crli, and crl use the GET method.

   Method: GET
   Request target: /.well-known/est/<operation>
   Request target: /.well-known/est/<label>/<operation>

   Enrollment operations kemc, simpleenroll / sen, simplereenroll /
   sren, and serverkeygen / skc use HTTP POST or CoAP POST:

   Method: POST
   Request target: /.well-known/est/<operation>
   Request target: /.well-known/est/<label>/<operation>

   An EST server MUST return HTTP 405 / CoAP 4.05 (Method Not Allowed)
   if a client uses POST for a retrieval operation or GET for an
   enrollment operation.

   The optional <label> path segment follows [RFC7030], Section 3.2.2
   for HTTPS and the equivalent path structure in [RFC9148] for CoAP: an
   EST server MAY use an additional path segment before the operation
   name to distinguish services for multiple CAs and / or certificate
   profiles.

3.5.  Channel Binding

   Channel binding is OPTIONAL.  The channel binding mechanisms
   specified in [RFC7030] and [RFC9148] apply.



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3.6.  Message Binding

   The message binding mechanism specified in [RFC9148], Section 4.4
   applies for CoAP in this document.

3.7.  Message Fragmentation

   The message fragmentation mechanism specified in [RFC9148],
   Section 4.6 applies for CoAP in this document.

3.8.  Delayed Responses

   The mechanism for delayed responses specified in [RFC9148],
   Section 4.7 applies for CoAP in this document.  The retry-later
   mechanism for the operations simpleenroll and simplereenroll
   specified in [RFC7030], Section 4.2.3 applies for HTTPS in this
   document.

3.9.  Response Cache

   Successful responses whose content is not expected to change in the
   near future may include caching metadata:

   *  For EST over HTTPS, EST servers SHOULD include ETag, Last-
      Modified, Cache-Control, and Expires headers.

   *  For EST over secure CoAP, EST servers SHOULD include the options
      ETag and Max-Age.

3.10.  CBOR Transfer

   All POST request bodies and all response bodies in this document,
   except the caps response, are CBOR-encoded.  The CBOR encoding MUST
   be deterministic as specified in Sections 4.2.1 and 4.2.2 of
   [RFC8949].

   For HTTPS, following [RFC8951], the POST request bodies and the CBOR-
   based response bodies are base64-encoded as specified in Section 4 of
   [RFC4648]; the CBOR encoding is the input to the base64 encoding
   routine.  This format is used regardless of any Content-Transfer-
   Encoding header, and any value in such a header MUST be ignored.  As
   specified in [RFC8951], Section 3.1, white space including CR, LF,
   spaces (ASCII 32), and tabs (ASCII 9) SHOULD be tolerated by
   receivers; senders are not required to insert any kind of white
   space.






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   For CoAP, CBOR payloads are carried directly in binary form, without
   base64 encoding, in the message body using the relevant CoAP Content-
   Format.

   An EST server that serves both X.509 and C509 certificates under the
   same path distinguishes the requests by the media type (HTTP Content-
   Type) or Content-Format (CoAP) of the request.  If the media type or
   Content-Format of a POST request is not supported, the EST server
   MUST return HTTP 415 / CoAP 4.15 (Unsupported Media Type /
   Unsupported Content-Format).

   The media types used in this document are:







































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   +=====================+=======+================================+=================================+
   |Media Type           |CoAP   |Structure                       |Defined in                       |
   |                     |Content|                                |                                 |
   |                     |Format |                                |                                 |
   +=====================+=======+================================+=================================+
   |application/cose-    |TBD    |C509Certificate                 |[I-D.ietf-cose-cbor-encoded-cert]|
   |c509-cert+cbor       |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD    |COSE_C509                       |[I-D.ietf-cose-cbor-encoded-cert]|
   |c509+cbor            |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD    |COSE_C509                       |[I-D.ietf-cose-cbor-encoded-cert]|
   |c509+cbor;usage=chain|       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD    |C509CertificationRequest        |[I-D.ietf-cose-cbor-encoded-cert]|
   |c509-pkcs10+cbor     |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD3   |C509SubjectPublicKeyInfo        |[I-D.liao-cose-c509-additions]   |
   |c509-pubkey+cbor     |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/         |TBD7   |C509KemChall                    |This document                    |
   |c509-kemchall+cbor   |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD    |C509CertificationRequestTemplate|[I-D.ietf-cose-cbor-encoded-cert]|
   |c509-crtemplate+cbor |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/cose-    |TBD    |C509PEM (key + certificate)     |[I-D.ietf-cose-cbor-encoded-cert]|
   |c509-pem+cbor        |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/         |TBD    |C509CRL                         |[I-D.liao-cose-c509-revocation]  |
   |c509-crl+cbor        |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+
   |application/         |TBD    |C509CRLInfo                     |[I-D.liao-cose-c509-revocation]  |
   |c509-crlinfo+cbor    |       |                                |                                 |
   +---------------------+-------+--------------------------------+---------------------------------+

                 Table 1: Media Types Used in This Document

   The caps operation returns text/plain; charset=utf-8 with a list of
   capability keywords.

4.  C509 Certification Request (C509 CSR)

   A C509 CSR is a CBOR-encoded certification request used to request
   issuance of a C509 certificate.  It is the C509 analogue of the
   PKCS#10 CSR [RFC2986] used in standard EST operations.





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   This document uses C509CertificationRequest as defined in
   [I-D.ietf-cose-cbor-encoded-cert], Section 4.  An EST client sends a
   C509 CSR to request certificate issuance or re-enrollment.  The media
   type is application/cose-c509-pkcs10+cbor.

   For server-side key generation requests (Section 8.4), the EST client
   does not possess the private key and does not know the public key
   that the EST server will generate.  In this case, the
   subjectPublicKeyAlgorithm in TBSCertificationRequest MUST be set to
   the integer code for empty-publickey (see Section 4.1) and
   subjectPublicKey MUST be a zero-length byte string (h'').  Because no
   private/public key is available, no PoP signature can be computed/
   verified: the signatureAlgorithm MUST be set to the id-alg-unsigned
   integer code (see [I-D.ietf-cose-cbor-encoded-cert]) and
   signatureValue MUST be a zero-length byte string.  EST servers MUST
   accept C509 CSRs using empty-publickey and id-alg-unsigned for
   serverkeygen / skc requests and MUST NOT verify a PoP signature in
   this case.

4.1.  empty-publickey Algorithm

   This document defines a new C509 public key algorithm, empty-
   publickey, to be used exclusively in C509 CSRs for serverkeygen / skc
   (server key generation) requests to indicate that no public key is
   available in the CSR.

   The empty-publickey algorithm code (TBD1) MUST NOT appear in C509
   certificates.  It is only valid in the subjectPublicKeyAlgorithm
   field of TBSCertificationRequest when the corresponding
   subjectPublicKey is a zero-length byte string (h'').  EST servers
   MUST reject any C509 CSR using empty-publickey in a simpleenroll /
   sen or simplereenroll / sren request.

4.2.  CRAttribute C509ChangeSubjectName

   The ChangeSubjectName attribute for X.509 PKI is defined in
   [RFC6402].  The corresponding definition for C509 certification
   requests is a CBOR array consisting of a subject and a subjectAlt.
   At least one of subject and subjectAlt MUST NOT be null.

   C509ChangeSubjectName = [
     subject     C509Name / null,
     subjectAlt  SubjectAltName / null
   ]

   This CRAttribute MAY be included in a simplereenroll / sren request
   to change the Subject field and/or the SubjectAltName extension in
   the newly generated certificate.



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4.3.  Proof of Possession

   simpleenroll / sen and simplereenroll / sren MUST verify the PoP
   signature in the C509 CSR before issuing a certificate.  As an
   exception, if the subjectPublicKeyAlgorithm in the C509 CSR is a KEM
   algorithm, the EST server MUST instead verify the PoP MAC as
   specified in Section 4.3.1.  The serverkeygen / skc operation does
   not require PoP verification because the EST server generates the key
   pair itself.

4.3.1.  Proof of Possession for KEM Private Keys

   Some public-key algorithms are KEM-only (key-encapsulation
   mechanisms) and do not provide a signature operation suitable for the
   traditional PoP signature carried in a C509CertificationRequest.  For
   CSRs whose subjectPublicKeyAlgorithm is a KEM algorithm, an EST
   server MUST obtain explicit proof that the requester holds the
   corresponding KEM private key.

   This document specifies an interactive KEM challenge-response PoP
   mechanism based on the unilateral key confirmation specified in
   Section 4.4 of [NIST.SP.800-227], in which the EST client (the
   decapsulator) is the key-confirmation provider and the EST server
   (the encapsulator) is the key-confirmation recipient.  Together with
   the client authentication required by this document (Section 3.1),
   this provides PoP as described in Section 4.5 of [NIST.SP.800-227].

4.3.1.1.  Challenge Issuance

   Upon receipt of a KEM public key (C509SubjectPublicKeyInfo) in the
   kemc operation (Section 8.1), the EST server runs the encapsulation
   algorithm of the KEM with this public key, which produces a shared
   secret key K and a ciphertext c, and returns a CBOR-encoded KEM
   challenge object (C509KemChall) with media type application/
   c509-kemchall+cbor.

   C509KemChall = [
     keyId         : bstr .size 32,
     serverId      : bstr .size 32,
     encapsulation : bstr,
     ? lifetime    : uint
   ]

   The media type of C509KemChall is application/c509-kemchall+cbor (see
   Section 10.4.1); the corresponding CoAP Content-Format is defined in
   Section 10.5.  The "magic number" is TBD6, using the reserved CBOR
   tag 55799 and Content-Format TBD7, as described in [RFC9277],
   Section 2.2.



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   The fields are:

   *  keyId is the SHA-256 hash of the deterministically encoded (see
      Section 3.10) C509SubjectPublicKeyInfo received in the kemc
      request.

   *  serverId is a 32-byte identifier of the EST server, chosen by the
      EST server.  It is RECOMMENDED to use the SHA-256 hash of the
      certificate that the EST server uses for TLS or DTLS server
      authentication.

   *  encapsulation is the ciphertext c.

   *  lifetime, if present, is the number of seconds, counted from the
      sending of the response, after which the EST server considers the
      challenge expired.  A relative value is used so that the EST
      client does not need a synchronized clock.  If lifetime is absent,
      the lifetime is determined by the EST server policy and is not
      communicated to the EST client.  An EST client SHOULD submit the
      enrollment request before the lifetime has elapsed; if the
      challenge has expired, the EST client needs to request a new
      challenge.

   The KEM algorithm is the one identified by the
   subjectPublicKeyAlgorithm of the C509SubjectPublicKeyInfo received in
   the kemc request.

   The EST server MUST retain the challenge state, consisting of at
   least keyId, serverId, c, K, the identity of the authenticated EST
   client, and the expiry time of the challenge, until the challenge is
   used or has expired.  A new challenge replaces any outstanding
   challenge for the same keyId and EST client.

4.3.1.2.  PoP Nonce

   Because the KEM key pair to be certified is a static key pair, the
   key-confirmation provider (the EST client) contributes a nonce as its
   ephemeral data, as required by Section 4.4.1 of [NIST.SP.800-227].
   The nonce is carried in the C509 CSR in the CR attribute
   C509KemPopNonce:

   C509KemPopNonce = bstr .size 32









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   The EST client MUST generate a fresh random nonce for each C509 CSR.
   The 256-bit length satisfies the requirement of Appendix C.2 of
   [NIST.SP.800-227] for all security strengths up to 256 bits.  The
   C509KemPopNonce attribute MUST be present in a C509 CSR whose
   subjectPublicKeyAlgorithm is a KEM algorithm, and MUST NOT be present
   otherwise.

4.3.1.3.  MAC Computation by the EST Client

   The EST client decapsulates encapsulation with its KEM private key to
   obtain the shared secret key K.  The key-confirmation key KC_Key is K
   in its entirety (Section 4.4.2 of [NIST.SP.800-227]); no data key is
   derived.

   The EST client constructs the TBSCertificationRequest, which includes
   the C509KemPopNonce attribute, and then constructs MAC_Data as
   specified in Section 4.4.1 of [NIST.SP.800-227]:

   MAC_Data = KC_Step_Label || ID_P || ID_R || Eph_P || Eph_R
              || Extra_P || Extra_R

   where:

   *  KC_Step_Label is the 6-byte ASCII string "KC_1_D", indicating that
      the decapsulator provides the first key-confirmation message;

   *  ID_P is keyId (32 bytes), identifying the KEM key pair of the EST
      client;

   *  ID_R is serverId (32 bytes) from the C509KemChall;

   *  Eph_P is the value of the C509KemPopNonce attribute (32 bytes);

   *  Eph_R is the ciphertext encapsulation, whose length is fixed by
      the KEM algorithm;

   *  Extra_P is the CBOR-encoded TBSCertificationRequest; and

   *  Extra_R is empty.

   All fields except Extra_P have a fixed length, so MAC_Data can be
   parsed unambiguously.









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   The EST client computes the MAC tag over MAC_Data with KC_Key and
   places it in the C509CertificationRequest: the signatureAlgorithm
   MUST be set to the integer code of the MAC algorithm (e.g., TBD4 for
   HMAC-SHA256, see Section 10.2) and the signatureValue MUST contain
   the MAC tag.  The EST client then destroys K and submits a
   simpleenroll / sen or simplereenroll / sren request as usual.

4.3.1.4.  Verification by the EST Server

   The EST server:

   1.  constructs a C509SubjectPublicKeyInfo from the
       subjectPublicKeyAlgorithm and subjectPublicKey fields of the
       TBSCertificationRequest, encodes it deterministically, and
       computes keyId from this encoding as described in
       Section 4.3.1.1;

   2.  retrieves the challenge state for this keyId and verifies that
       the challenge has not expired and that the request is sent by the
       same authenticated EST client to which the challenge was issued;

   3.  verifies that the C509KemPopNonce attribute is present and is 32
       bytes long;

   4.  constructs MAC_Data as specified in Section 4.3.1.3, using
       serverId and c from the challenge state; and

   5.  verifies the MAC tag in signatureValue over MAC_Data, using the
       MAC algorithm indicated by signatureAlgorithm and K from the
       challenge state.

   Each challenge MUST be used at most once: the EST server MUST delete
   the challenge state, including K, after the first enrollment request
   that refers to it, whether or not the verification succeeds.  If any
   step fails, the EST server MUST reject the request.  Otherwise, the
   EST server proceeds with certificate issuance as for signature-based
   PoP.

4.3.1.5.  MAC Algorithms

   The MAC algorithm MUST be one of the approved MAC algorithms for key
   confirmation listed in Appendix C.1 of [NIST.SP.800-227], and the
   length of KC_Key MUST be permitted for that algorithm.  EST servers
   MUST support HMAC-SHA256 (with integer value TBD4).  EST clients
   SHOULD use HMAC-SHA256 and MAY use another approved MAC algorithm,
   for example if required by the security strength of the KEM.  If the
   EST server does not support the MAC algorithm indicated by
   signatureAlgorithm, it MUST reject the request with HTTP 400 / CoAP



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   4.00 (Bad Request).  EST servers MUST enforce challenge expiry and
   limit the number of outstanding challenges per EST client to mitigate
   denial-of-service risks.

5.  EST Operations for Server Capability Discovery

5.1.  caps

   This operation applies only to EST over HTTPS, not to EST over CoAP/
   DTLS.

   The caps operation allows an EST client to discover which
   C509-specific operations the EST server supports before invoking
   them.  EST clients and EST servers MUST support caps.

5.1.1.  Request

   The EST client sends a GET request for the server's C509 capability
   list.  No request body is sent.

   Method: GET
   HTTP request target: /.well-known/est/<label>/caps

5.1.2.  Response

   On success, the EST server returns an HTTP 200 response with:

   *  Media type: text/plain; charset=utf-8

   *  Body: A plain-text list of capability keywords, one keyword per
      line.  The EST server MUST terminate each line with <CR><LF>.  The
      EST client MUST be able to parse lines terminated by <CR><LF>,
      <CR>, or <LF>.  Keywords are unquoted and case-insensitive.

   The following keywords are defined.  An EST server MUST include a
   keyword in the caps response if and only if it supports the
   corresponding operation.














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   +================+==================================================+
   | Keyword        | Description                                      |
   +================+==================================================+
   | cacerts        | EST server supports cacerts (Section 6.1)        |
   +----------------+--------------------------------------------------+
   | crli           | EST server supports crli (Section 7.1)           |
   +----------------+--------------------------------------------------+
   | crl            | EST server supports crl (Section 7.2)            |
   +----------------+--------------------------------------------------+
   | csrattrs       | EST server supports csrattrs (Section 5.2)       |
   +----------------+--------------------------------------------------+
   | kemc           | EST server supports kemc (Section 8.1)           |
   +----------------+--------------------------------------------------+
   | simpleenroll   | EST server supports simpleenroll (Section 8.2)   |
   +----------------+--------------------------------------------------+
   | simplereenroll | EST server supports simplereenroll               |
   |                | (Section 8.3)                                    |
   +----------------+--------------------------------------------------+
   | serverkeygen   | EST server supports serverkeygen (Section 8.4)   |
   +----------------+--------------------------------------------------+

              Table 2: caps Keywords Defined in This Document

   If an EST client receives an error response to a caps request, it
   cannot determine which OPTIONAL operations the EST server supports.
   In this case, the EST client MAY still attempt the MANDATORY
   operations cacerts, simpleenroll, and simplereenroll, and SHOULD NOT
   attempt the OPTIONAL operations.  An EST client MUST be prepared to
   receive an error response for any operation.

   Successful caps responses MAY include caching metadata as specified
   in Section 3.9.

   Example response:

   cacerts
   crli
   crl
   csrattrs
   kemc
   simpleenroll
   simplereenroll
   serverkeygen








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5.2.  csrattrs / att

   The csrattrs / att operation returns a
   C509CertificationRequestTemplate that an EST client MAY use to
   construct a C509 CSR.

5.2.1.  Request

   The EST client sends a GET request for the CSR template.  No request
   body is sent.

   Method: GET
   HTTP Request target: /.well-known/est/<label>/csrattrs
   CoAP Request target: /.well-known/est/<label>/att

5.2.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/cose-c509-crtemplate+cbor (HTTP) /
      Content-Format TBD (CoAP)

   *  Body: A C509CertificationRequestTemplate as defined in
      [I-D.ietf-cose-cbor-encoded-cert].

   A response code of HTTP 404 / CoAP 4.04 indicates that CSR attributes
   are not available.

   Successful csrattrs / att responses MAY include caching metadata as
   specified in Section 3.9.

6.  EST Operations for Distribution of CA Certificates

6.1.  cacerts / crts

   Depending on the Accept option, the cacerts / crts operation returns
   different forms of the CA certificates (TODO: the TBD will be
   replaced with the real value once the Content-Formats are assigned in
   [I-D.ietf-cose-cbor-encoded-cert] and
   [I-D.liao-cose-c509-revocation]):

   *  Accept: application/cose-c509-cert+cbor (HTTP) / Accept=TBD
      (CoAP): C509Certificate (the issuing CA certificate)

   *  Accept: application/cose-c509+cbor; usage=chain (HTTP) /
      Accept=TBD (CoAP): COSE_C509 (certificate chain)




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   *  No Accept or Accept: application/cose-c509+cbor (HTTP) /
      Accept=TBD (CoAP): COSE_C509 (certificate set)

6.1.1.  Request

   The EST client sends a GET request for the CA certificate(s).  No
   request body is sent.  The Accept option specifies the expected media
   type / Content-Format of the response.  If an Accept option is not
   included in the request, the client is not expressing any preference
   and the server SHOULD return the CA certificate set with media type
   application/cose-c509+cbor (HTTP) / Content-Format TBD (CoAP).

   Method: GET

   HTTP request target: /.well-known/est/<label>/cacerts
                        (Accept: <accepted media type>)
   CoAP request target: /.well-known/est/<label>/crts
                        (Accept=<accepted Content-Format>)

6.1.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  For Request with Accept: application/cose-c509-cert+cbor (HTTP) /
      Accept=TBD (CoAP):

      -  Media type: application/cose-c509-cert+cbor (HTTP) / Content-
         Format TBD (CoAP)

      -  Body: A C509Certificate representing the issuing CA
         certificate, i.e., the CA certificate whose key is used to
         issue certificates to the EST client.  This form cannot carry
         the "Root CA Key Update" certificates defined in [RFC9810],
         Section 4.4; an EST client that needs them uses one of the
         other forms.

   *  For Request with Accept: application/cose-c509+cbor; usage=chain
      (HTTP) / Accept=TBD (CoAP):

      -  Media type: application/cose-c509+cbor; usage=chain (HTTP) /
         Content-Format TBD (CoAP)

      -  Body: A COSE_C509 representing an ordered certificate chain.
         The first element is the issuing CA certificate; subsequent
         elements are intermediate and root CA certificates in chain
         order.  If a Root CA key update applies, the EST server SHOULD
         include the four "Root CA Key Update" certificates defined in



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         [RFC9810], Section 4.4 in the response chain.  If present,
         these certificates SHALL take the place of the root CA
         certificate at the end of the chain, in the order NewWithNew,
         NewWithOld, OldWithOld, and OldWithNew; that is, the chain is:
         issuing CA certificate, intermediate CA certificates (if any),
         NewWithNew, NewWithOld, OldWithOld, OldWithNew.  In this case,
         the last intermediate CA certificate (or the issuing CA
         certificate if there is no intermediate CA) is issued under the
         new root CA key.

   *  For Request without Accept option or with Accept: application/
      cose-c509+cbor (HTTP) / Accept=TBD (CoAP):

      -  Media type: application/cose-c509+cbor (HTTP) / Content-Format
         TBD (CoAP)

      -  Body: A COSE_C509 representing a certificate set.  The first
         element is the issuing CA certificate; no order is defined for
         the subsequent elements.  If a Root CA key update applies, the
         EST server SHOULD include the four "Root CA Key Update"
         certificates defined in [RFC9810], Section 4.4 in the
         certificate set.  No order is defined for these certificates.

   If the server cannot return the CA certificates in the form requested
   by the Accept option, it MUST return HTTP 406 / CoAP 4.06 (Not
   Acceptable).

   Successful cacerts / crts responses MAY include caching metadata as
   specified in Section 3.9.

7.  EST Operations for Distribution of C509 CRLs

   The crli and crl operations provide C509 CRL access.  The C509 CRL
   format is defined in [I-D.liao-cose-c509-revocation].

7.1.  crli

   The crli operation returns metadata about the current C509 CRL for
   the target CA as a C509CRLInfo object without the full revocation
   list.  This enables an EST client to check whether its locally cached
   CRL is still current before requesting the full crl.  C509CRLInfo is
   defined in [I-D.liao-cose-c509-revocation].

7.1.1.  Request

   The EST client sends a GET request for CRL metadata.  No request body
   is sent.




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   Method: GET
   Request target: /.well-known/est/<label>/crli
                   [?crlnumber=<n>][&crldp=<dp>]

   The optional crlnumber query parameter carries the decimal
   representation of the CRL number the client is interested in.  When
   present, the EST server MUST return the C509CRLInfo for the CRL with
   that exact crlNumber.  If no CRL with the requested crlnumber is
   available, the EST server MUST return HTTP 404 / CoAP 4.04 (Not
   Found).  When crlnumber is absent, the server returns the C509CRLInfo
   for the most recent CRL.

   The optional crldp query parameter carries a distribution point name
   extracted from the CRL Distribution Points extension of the
   certificate whose revocation status is to be checked (see
   [I-D.ietf-cose-cbor-encoded-cert]).  The value is the URI of the
   distribution point, percent-encoded as specified in Section 2.1 of
   [RFC3986].  When present, the EST server MUST return the C509CRLInfo
   for the CRL associated with that distribution point.  When both
   crlnumber and crldp are present, the server MUST return the
   C509CRLInfo matching both criteria.

   For CoAP, each query parameter is carried in its own Uri-Query
   option, for example crlnumber=5 and crldp=<percent-encoded URI>.

7.1.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/c509-crlinfo+cbor (HTTP) / Content-Format
      TBD (CoAP)

   *  Body: A C509CRLInfo as defined in [I-D.liao-cose-c509-revocation].

   C509CRLInfo carries all CRL fields from C509CRLInfoData — including
   crlType, signatureAlgorithm, authoritySubject,
   authorityKeyIdentifier, crlNumber, thisUpdate, nextUpdate,
   baseCrlNumber, and crlExtensions — without the revokedCertsList.  An
   EST client can use these fields to compare crlNumber, nextUpdate, or
   perform a freshness check against its local cache before deciding
   whether to download the full C509CRL via crl.

   If no matching CRL is available, the EST server MUST return HTTP 404
   / CoAP 4.04 (Not Found).

   Successful crli responses MAY include caching metadata as specified
   in Section 3.9.



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7.2.  crl

   The crl operation returns the C509 CRL for the target CA.

7.2.1.  Request

   The EST client sends a GET request for the C509 CRL.  No request body
   is sent.

   Method: GET
   Request target: /.well-known/est/<label>/crl
                   [?crlnumber=<n>][&crldp=<dp>]

   The optional crlnumber query parameter carries the decimal
   representation of the CRL number.  When present, the EST server MUST
   return the full C509CRL with that exact crlNumber.  When crlnumber is
   absent, the server returns the most recent CRL.

   The optional crldp query parameter is encoded as specified for crli
   (Section 7.1.1).  When present, the EST server MUST return the full
   C509CRL for the CRL associated with that distribution point.  When
   both crlnumber and crldp are present, the server MUST return the
   C509CRL matching both criteria.

   For CoAP, the query parameters are carried in Uri-Query options as
   specified for crli (Section 7.1.1).

7.2.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/c509-crl+cbor (HTTP) / Content-Format TBD
      (CoAP)

   *  Body: A C509CRL as defined in [I-D.liao-cose-c509-revocation].

   If no matching CRL is available, the EST server MUST return HTTP 404
   / CoAP 4.04 (Not Found).

   Successful crl responses MAY include caching metadata as specified in
   Section 3.9.

8.  EST Operations for Certificate Enrollment







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8.1.  kemc

   The kemc operation requests a KEM-based Proof-of-Possession challenge
   for a submitted C509 public key whose subjectPublicKeyAlgorithm is a
   KEM algorithm.

8.1.1.  Request

   An authenticated EST client sends a POST request containing a
   C509SubjectPublicKeyInfo ([I-D.liao-cose-c509-additions]) to request
   a KEM challenge.

   Method: POST
   Request target: /.well-known/est/<label>/kemc
   Media type: application/cose-c509-pubkey+cbor (HTTP) /
               Content-Format TBD (CoAP)
   Body: C509SubjectPublicKeyInfo

   If the request does not contain a KEM public key, the EST server MUST
   return HTTP 400 / CoAP 4.00 (Bad Request).  If the server supports
   KEM PoP for the submitted algorithm, it issues a challenge;
   otherwise, it MUST return HTTP 501 / CoAP 5.01 (Not Implemented).

8.1.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/c509-kemchall+cbor (HTTP) / Content-Format
      TBD7 (CoAP)

   *  Body: A C509KemChall (Section 4.3.1).

   A client that receives a C509KemChall decapsulates the shared secret
   key, computes the PoP MAC as specified in Section 4.3.1.3, and then
   proceeds with a normal enrollment request (simpleenroll / sen or
   simplereenroll / sren) carrying the MAC in the C509 CSR.  The EST
   server verifies the MAC as specified in Section 4.3.1.4 and issues
   the certificate on success.

8.2.  simpleenroll / sen

   The simpleenroll / sen operation requests issuance of a new C509
   certificate from the EST server.







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8.2.1.  Request

   An authenticated EST client sends a POST request containing a C509
   CSR (Section 4).

   Method: POST
   HTTP Request target: /.well-known/est/<label>/simpleenroll
   CoAP Request target: /.well-known/est/<label>/sen
   Media type: application/cose-c509-pkcs10+cbor (HTTP) /
               Content-Format TBD (CoAP)
   Body: C509CertificationRequest

   The C509CertificationRequest MUST include a valid PoP signature.  The
   EST server MUST verify the PoP signature against the public key in
   the C509 CSR before issuing a certificate, as required by [RFC7030],
   Section 3.4.  As an exception, if the public key in the C509 CSR is a
   KEM public key, the C509CertificationRequest MUST instead include a
   valid PoP MAC, and the EST server MUST verify it as specified in
   Section 4.3.1.

8.2.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/cose-c509-cert+cbor (HTTP) / Content-
      Format TBD (CoAP)

   *  Body: A C509Certificate issued for the subject in the C509 CSR, as
      defined in [I-D.ietf-cose-cbor-encoded-cert].

8.3.  simplereenroll / sren

   The simplereenroll / sren operation renews or rekeys an existing C509
   certificate.

8.3.1.  Request

   An authenticated EST client sends a POST request containing a C509
   CSR for re-enrollment.  The request Subject field and SubjectAltName
   extension MUST be identical to the corresponding fields in the
   certificate being renewed or rekeyed.

   The C509ChangeSubjectName attribute defined in Section 4.2 MAY be
   included in the CSR to request that these fields be changed in the
   new certificate.





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   Method: POST
   HTTP Request target: /.well-known/est/<label>/simplereenroll
   CoAP Request target: /.well-known/est/<label>/sren
   Media type: application/cose-c509-pkcs10+cbor (HTTP) /
               Content-Format TBD (CoAP)
   Body: C509CertificationRequest

   Re-enrollment processing follows [RFC7030], Section 4.2.2.  The EST
   server MUST verify the PoP signature in the C509 CSR or, if the
   public key in the C509 CSR is a KEM public key, the PoP MAC as
   specified in Section 4.3.1.

8.3.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/cose-c509-cert+cbor (HTTP) / Content-
      Format TBD (CoAP)

   *  Body: A renewed C509Certificate, as defined in
      [I-D.ietf-cose-cbor-encoded-cert].

8.4.  serverkeygen / skc

   The serverkeygen / skc operation requests server-side key generation
   and returns the generated private key and the issued C509
   certificate.

   As discussed in [RFC9148], Section 9, transporting private keys
   generated by the EST server is inherently risky.  The use of server-
   generated private keys increases the risk of digital identity theft.
   Therefore, implementations SHOULD NOT use EST functions that rely on
   server-generated private keys.

8.4.1.  Request

   An authenticated EST client sends a POST request containing a C509
   CSR.  The subjectPublicKeyAlgorithm in the C509 CSR MUST be set to
   empty-publickey (Section 4.1) and subjectPublicKey MUST be a zero-
   length byte string (h''), because the key pair is generated by the
   EST server.  The signatureAlgorithm MUST be the id-alg-unsigned
   integer code (see [I-D.ietf-cose-cbor-encoded-cert]) and
   signatureValue MUST be a zero-length byte string.  EST servers MUST
   accept C509 CSRs that use empty-publickey and id-alg-unsigned for
   serverkeygen / skc and MUST NOT verify a PoP signature in this case.





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   Method: POST
   HTTP Request target: /.well-known/est/<label>/serverkeygen
   CoAP Request target: /.well-known/est/<label>/skc
   Media type: application/cose-c509-pkcs10+cbor (HTTP) /
               Content-Format TBD (CoAP)
   Body: C509CertificationRequest

8.4.2.  Response

   On success, the EST server returns an HTTP 200 / CoAP 2.05 response
   with:

   *  Media type: application/cose-c509-pem+cbor (HTTP) / Content-Format
      TBD (CoAP)

   *  Body: A C509PEM.

   The C509CertData field in the C509PEM MUST contain only the issued
   C509 certificate for the generated key pair.

   The EST server SHOULD delete the private key from its storage as soon
   as the response has been transmitted successfully, unless the
   deployment policy requires retention for key escrow or disaster
   recovery (see Section 9).  The private key is protected only by the
   TLS or DTLS channel; no additional encryption is applied.

9.  Security Considerations

   The security requirements of [RFC7030] apply in full to all
   operations defined in this document.

9.1.  Transport Security

   All operations defined in this document MUST be carried out over
   HTTPS (HTTP over TLS) or secure CoAP (CoAP over DTLS) as required by
   [RFC7030], Section 3 and [RFC9148].  Implementations MUST NOT fall
   back to plain HTTP or unsecured CoAP.

   EST clients and servers SHOULD use C509 certificates
   [I-D.ietf-cose-cbor-encoded-cert] for TLS or DTLS authentication when
   both peers support C509.  This enables end-to-end C509 usage,
   including the handshake itself, and reduces size and parsing overhead
   consistently.  EST servers SHOULD continue to accept X.509
   certificates [RFC5280] for TLS or DTLS client authentication for
   interoperability with clients that do not yet support C509.

   All operations MUST require client authentication (see Section 3.1).




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9.2.  Server Key Generation

   The serverkeygen / skc operation delivers a generated private key to
   the EST client over TLS or DTLS (see Section 8.4).  EST servers
   SHOULD delete the private key after successful transmission.  EST
   clients MUST store the key material securely immediately upon
   receipt.

   Because the C509 CSR in a serverkeygen / skc request carries no proof
   of possession, the EST server MUST verify that the authenticated EST
   client is authorized to request a certificate for the subject and
   subject alternative names in the C509 CSR.

9.3.  C509 Certificate Validation

   EST clients MUST validate received C509 certificates against an
   independently configured trust anchor according to
   [I-D.ietf-cose-cbor-encoded-cert].

9.4.  C509 CRL Validation

   A C509 CRL received via the crl operation is protected by the secure
   transport only while in transit.  Before using a C509 CRL, an EST
   client MUST verify its signature and validate the CRL issuer as
   specified in [I-D.liao-cose-c509-revocation].  An EST client uses a
   C509CRLInfo received via the crli operation only to decide whether to
   retrieve the full C509CRL; it MUST NOT use a C509CRLInfo in place of
   a validated C509CRL for revocation decisions.

9.5.  KEM Proof of Possession

   The KEM PoP mechanism (Section 4.3.1) follows the key confirmation
   specified in Section 4.4 of [NIST.SP.800-227].  The MAC tag can only
   be computed by a party that can decapsulate the ciphertext, i.e.,
   that holds the KEM private key.  MAC_Data binds the tag to the KEM
   public key (keyId), the EST server (serverId), the ciphertext, a
   fresh client nonce, and the complete TBSCertificationRequest, so that
   a tag cannot be reused for a different CSR, a different challenge, or
   a different EST server.  Binding the challenge state to the
   authenticated EST client prevents another client from using the
   challenge, and the single-use requirement prevents replay.

   The shared secret key K is used only as the key-confirmation key.
   Both the EST client and the EST server MUST destroy K after use, as
   required by Section 4.4.2 of [NIST.SP.800-227].






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9.6.  Capability Discovery

   An attacker that could modify a caps response could make an EST
   client believe that an operation is not supported, or that an
   operation is supported when it is not.  This is prevented by the
   requirement that all operations are carried out over TLS or DTLS with
   an authenticated EST server (see Section 9.1).

10.  IANA Considerations

10.1.  C509 Public Key Algorithms Registry

   IANA is requested to register the following entry in the "C509 Public
   Key Algorithms" registry under the registry group "CBOR Encoded X.509
   (C509)" defined in [I-D.ietf-cose-cbor-encoded-cert]:

    +=============+==================================================+
    | Field       | Value                                            |
    +=============+==================================================+
    | Value       | TBD1                                             |
    +-------------+--------------------------------------------------+
    | Name        | empty-publickey                                  |
    +-------------+--------------------------------------------------+
    | Identifiers | N/A                                              |
    +-------------+--------------------------------------------------+
    | OID         | N/A                                              |
    +-------------+--------------------------------------------------+
    | Parameters  | N/A                                              |
    +-------------+--------------------------------------------------+
    | DER         | N/A                                              |
    +-------------+--------------------------------------------------+
    | Comments    | Exclusively for use in subjectPublicKeyAlgorithm |
    |             | of a TBSCertificationRequest for server-side key |
    |             | generation (serverkeygen / skc).  MUST NOT       |
    |             | appear in C509 certificates.                     |
    +-------------+--------------------------------------------------+
    | Reference   | This document                                    |
    +-------------+--------------------------------------------------+

                  Table 3: empty-publickey Registration

10.2.  C509 Signature Algorithms Registry

   IANA is requested to register the following entry in the "C509
   Signature Algorithms" registry under the registry group "CBOR Encoded
   X.509 (C509)" defined in [I-D.ietf-cose-cbor-encoded-cert]:





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    +=============+==================================================+
    | Field       | Value                                            |
    +=============+==================================================+
    | Value       | TBD4                                             |
    +-------------+--------------------------------------------------+
    | Name        | hmacWithSHA256                                   |
    +-------------+--------------------------------------------------+
    | Identifiers | id-hmacWithSHA256                                |
    +-------------+--------------------------------------------------+
    | OID         | 1.2.840.113549.2.9                               |
    +-------------+--------------------------------------------------+
    | Parameters  | N/A                                              |
    +-------------+--------------------------------------------------+
    | DER         | 06 08 2A 86 48 86 F7 0D 02 09                    |
    +-------------+--------------------------------------------------+
    | Comments    | HMAC over SHA256.  Exclusively for use in the    |
    |             | signatureAlgorithm of a C509CertificationRequest |
    |             | for the KEM proof of possession.  MUST NOT       |
    |             | appear in C509 certificates.                     |
    +-------------+--------------------------------------------------+
    | Reference   | This document                                    |
    +-------------+--------------------------------------------------+

                    Table 4: HMAC-SHA256 Registration

10.3.  C509 CR Attributes Registry

   IANA is requested to register the following entries in the "C509 CR
   Attributes" registry under the registry group "CBOR Encoded X.509
   (C509)" defined in [I-D.ietf-cose-cbor-encoded-cert]:





















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   +-------+-----------------------------------------------------------+
   | Value | CR Attribute                                              |
   +=======+===========================================================+
   |  TBD5 | Name:            CMC Change Subject Name                  |
   |       | Identifiers:     id-cmc-changeSubjectName                 |
   |       | OID:             1.3.6.1.5.5.7.7.36                       |
   |       | DER:             06 08 2B 06 01 05 05 07 07 24            |
   |       | Comments:        RFC 6402                                 |
   |       | attributeValue:  C509ChangeSubjectName                    |
   +-------+-----------------------------------------------------------+
   |  TBD8 | Name:            KEM PoP Nonce                            |
   |       | Identifiers:     N/A                                      |
   |       | OID:             N/A                                      |
   |       | DER:             N/A                                      |
   |       | Comments:        Nonce for the KEM proof of possession    |
   |       |                  in C509 CSRs (RFC XXXX)                  |
   |       | attributeValue:  C509KemPopNonce                          |
   +-------+-----------------------------------------------------------+

                 Figure 3: C509 CR Attribute Registrations

10.4.  Media Types Registry

   IANA is requested to register the following media type in the "Media
   Types" registry.

10.4.1.  Media Type application/c509-kemchall+cbor

   When the application/c509-kemchall+cbor media type is used, the
   payload is a C509KemChall structure.

   Type name: application

   Subtype name: c509-kemchall+cbor

   Required parameters: N/A

   Optional parameters: N/A

   Encoding considerations: binary

   Security considerations: See Section 9 of RFC XXXX.

   Interoperability considerations: N/A

   Published specification: RFC XXXX





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   Applications that use this media type: Applications that employ C509
   KEM challenges.

   Fragment identifier considerations: N/A

   Additional information:

   *  Deprecated alias names for this type: N/A

   *  Magic number(s): TBD6 (derived from the CoAP Content-Format TBD7
      as described in [RFC9277], Section 2.2)

   *  File extension(s): .c509

   *  Macintosh file type code(s): N/A

   Person & email address to contact for further information:
   iesg@ietf.org

   Intended usage: COMMON

   Restrictions on usage: N/A

   Author: ACE WG

   Change controller: IETF

10.5.  CoAP Content-Formats Registry

   IANA is requested to add the entry for application/c509-kemchall+cbor
   to the "CoAP Content-Formats" registry in the registry group
   "Constrained RESTful Environments (CoRE) Parameters".

   +----------------------+---------+-----------+-------+------------+
   | Content              | Content | Media     | ID    | Reference  |
   | Format               | Coding  | Type      |       |            |
   +======================+=========+===========+=======+============+
   | application/         | -       | RFC XXXX  | TBD7  | RFC XXXX   |
   | c509-kemchall+cbor   |         |           |       |            |
   +----------------------+---------+-----------+-------+------------+

                     Figure 4: CoAP Content-Format IDs

10.6.  Resource Type (rt=) Link Target Attribute Values Registry

   IANA is requested to add the following entries to the "Resource Type
   (rt=) Link Target Attribute Values" registry in the registry group
   "Constrained RESTful Environments (CoRE) Parameters":



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        +==============+=============================+===========+
        | Value        | Description                 | Reference |
        +==============+=============================+===========+
        | ace.est.crli | C509 CRL metadata retrieval | RFC XXXX  |
        +--------------+-----------------------------+-----------+
        | ace.est.crl  | C509 CRL retrieval          | RFC XXXX  |
        +--------------+-----------------------------+-----------+
        | ace.est.kemc | KEM challenge issuance      | RFC XXXX  |
        +--------------+-----------------------------+-----------+

                   Table 5: Resource Type Registrations

   RFC Editor: Please replace "RFC XXXX" with the RFC number of this
   document and remove this note.

11.  References

11.1.  Normative References

   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J.,
              Furuhed, M., and L. Liao, "CBOR Encoded X.509 Certificates
              (C509 Certificates)", Work in Progress, Internet-Draft,
              draft-ietf-cose-cbor-encoded-cert-21, 24 September 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-21>.

   [I-D.liao-cose-c509-additions]
              Liao, L., "Additions to C509 Structures", Work in
              Progress, Internet-Draft, draft-liao-cose-c509-additions-
              00, 17 September 2026,
              <https://datatracker.ietf.org/doc/html/draft-liao-cose-
              c509-additions-00>.

   [I-D.liao-cose-c509-revocation]
              Liao, L., Tiloca, M., Höglund, J., Höglund, R., and S.
              Raza, "CBOR Encoded Certificate Revocation Management",
              Work in Progress, Internet-Draft, draft-liao-cose-c509-
              revocation-00, 6 July 2026,
              <https://datatracker.ietf.org/doc/html/draft-liao-cose-
              c509-revocation-00>.

   [NIST.SP.800-227]
              Alagic, G., Barker, E., Chen, L., Moody, D., Roginsky, A.,
              Silberg, H., and N. Waller, "Recommendation for Key-
              Encapsulation Mechanisms", NIST Special Publication
              800-227, September 2025,
              <https://doi.org/10.6028/NIST.SP.800-227>.



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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/rfc/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/rfc/rfc4648>.

   [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/rfc/rfc5280>.

   [RFC6402]  Schaad, J., "Certificate Management over CMS (CMC)
              Updates", RFC 6402, DOI 10.17487/RFC6402, November 2011,
              <https://www.rfc-editor.org/rfc/rfc6402>.

   [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/rfc/rfc7030>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/rfc/rfc7250>.

   [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/rfc/rfc8174>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

   [RFC8951]  Richardson, M., Werner, T., and W. Pan, "Clarification of
              Enrollment over Secure Transport (EST): Transfer Encodings
              and ASN.1", RFC 8951, DOI 10.17487/RFC8951, November 2020,
              <https://www.rfc-editor.org/rfc/rfc8951>.



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   [RFC9148]  van der Stok, P., Kampanakis, P., Richardson, M., and S.
              Raza, "EST-coaps: Enrollment over Secure Transport with
              the Secure Constrained Application Protocol", RFC 9148,
              DOI 10.17487/RFC9148, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9148>.

   [RFC9277]  Richardson, M. and C. Bormann, "On Stable Storage for
              Items in Concise Binary Object Representation (CBOR)",
              RFC 9277, DOI 10.17487/RFC9277, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9277>.

   [RFC9810]  Brockhaus, H., von Oheimb, D., Ounsworth, M., and J. Gray,
              "Internet X.509 Public Key Infrastructure -- Certificate
              Management Protocol (CMP)", RFC 9810,
              DOI 10.17487/RFC9810, July 2025,
              <https://www.rfc-editor.org/rfc/rfc9810>.

11.2.  Informative References

   [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/rfc/rfc2986>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/rfc/rfc5652>.

Appendix A.  Message Flow Diagrams of EST over HTTPS Operations

A.1.  caps

   EST Client                                   EST Server
     |                                               |
     | Method: GET                                   |
     | Request target: /.well-known/est/<label>/caps |
     |---------------------------------------------->|
     |                                               |
     | Status: 200 OK                                |
     | Media type: text/plain;charset=utf-8          |
     |                                               |
     | <CR-LF separated list of supported            |
     |  operations>                                  |
     |<----------------------------------------------|
     |                                               |

          Figure 5: Message flow of EST over HTTPS operation caps




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A.2.  cacerts for single CA certificate

   EST Client                                      EST Server
     |                                                  |
     | Method: GET                                      |
     | Request target: /.well-known/est/<label>/cacerts |
     | Accept: application/cose-c509-cert+cbor          |
     |------------------------------------------------->|
     |                                                  |
     | Status: 200 OK                                   |
     | Media type: application/cose-c509-cert+cbor      |
     |                                                  |
     | <Base64-encoded C509Certificate>                 |
     |<-------------------------------------------------|
     |                                                  |

       Figure 6: Message flow of EST over HTTPS operation cacerts for
                           single CA certificate

A.3.  cacerts for CA certificate set

   EST Client                                      EST Server
     |                                                  |
     | Method: GET                                      |
     | Request target: /.well-known/est/<label>/cacerts |
     |------------------------------------------------->|
     |                                                  |
     | Status: 200 OK                                   |
     | Media type: application/cose-c509+cbor           |
     |                                                  |
     | <Base64-encoded COSE_C509>                       |
     |<-------------------------------------------------|
     |                                                  |

     Figure 7: Message flow of EST over HTTPS operation cacerts for CA
                              certificate set

A.4.  cacerts for CA certificate chain













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   EST Client                                      EST Server
     |                                                  |
     | Method: GET                                      |
     | Request target: /.well-known/est/<label>/cacerts |
     | Accept: application/cose-c509+cbor;              |
     |         usage=chain                              |
     |------------------------------------------------->|
     |                                                  |
     | Status: 200 OK                                   |
     | Media type: application/cose-c509+cbor;          |
     |             usage=chain                          |
     |                                                  |
     | <Base64-encoded COSE_C509>                       |
     |<-------------------------------------------------|
     |                                                  |

          Figure 8: Message flow of EST over HTTPS cacerts for CA
                             certificate chain

A.5.  crli

   EST Client                                     EST Server
     |                                                |
     | Method: GET                                    |
     | Request target: /.well-known/est/<label>/crli  |
     |   [?crlnumber=<n>][&crldp=<dp>]                |
     |----------------------------------------------->|
     |                                                |
     | Status: 200 OK                                 |
     | Media type: application/c509-crlinfo+cbor      |
     |                                                |
     | <Base64-encoded C509CRLInfo>                   |
     |<-----------------------------------------------|
     |                                                |

          Figure 9: Message flow of EST over HTTPS operation crli

A.6.  crl













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   EST Client                                    EST Server
     |                                                |
     | Method: GET                                    |
     | Request target: /.well-known/est/<label>/crl   |
     |   [?crlnumber=<n>][&crldp=<dp>]                |
     |----------------------------------------------->|
     |                                                |
     |  Status: 200 OK                                |
     |  Media type: application/c509-crl+cbor         |
     |                                                |
     |  <Base64-encoded C509CRL>                      |
     |<-----------------------------------------------|
     |                                                |

          Figure 10: Message flow of EST over HTTPS operation crl

A.7.  kemc

   EST Client                                  EST Server
     |                                                |
     | [TLS client certificate]                       |
     |                                                |
     | Method: POST                                   |
     | Request target: /.well-known/est/<label>/kemc  |
     | Media type: application/cose-c509-pubkey+cbor  |
     |                                                |
     | <Base64-encoded C509SubjectPublicKeyInfo>      |
     |----------------------------------------------->|
     |                                                |
     |                                                |
     | Status: 200 OK                                 |
     | Media type: application/c509-kemchall+cbor     |
     |                                                |
     | <Base64-encoded C509KemChall>                  |
     |<-----------------------------------------------|
     |                                                |

          Figure 11: Message flow of EST over HTTPS operation kemc

A.8.  simpleenroll











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   EST Client                                  EST Server
     |                                                |
     | [TLS client certificate]                       |
     |                                                |
     | Method: POST                                   |
     | Request target: /.well-known/est/<label>       |
     |                   /simpleenroll                |
     | Media type: application/cose-c509-pkcs10+cbor  |
     |                                                |
     | <Base64-encoded C509CertificationRequest>      |
     |----------------------------------------------->| Verify PoP,
     |                                                | Issue C509
     |                                                | cert
     | Status: 200 OK                                 |
     | Media type: application/cose-c509-cert+cbor    |
     |                                                |
     | <Base64-encoded C509Certificate>               |
     |<-----------------------------------------------|
     |                                                |

      Figure 12: Message flow of EST over HTTPS operation simpleenroll

A.9.  simplereenroll

   EST Client                                  EST Server
     |                                                |
     | [TLS client certificate]                       |
     |                                                |
     | Method: POST                                   |
     | Request target: /.well-known/est/<label>       |
     |                   /simplereenroll              |
     | Media type: application/cose-c509-pkcs10+cbor  |
     |                                                |
     | <Base64-encoded C509CertificationRequest>      |
     |----------------------------------------------->| Verify PoP,
     |                                                | Issue C509
     |                                                | cert
     | Status: 200 OK                                 |
     | Media type: application/cose-c509-cert+cbor    |
     |                                                |
     | <Base64-encoded C509Certificate>               |
     |<-----------------------------------------------|
     |                                                |

     Figure 13: Message flow of EST over HTTPS operation simplereenroll

A.10.  serverkeygen




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   EST Client                                   EST Server
     |                                                |
     | [TLS client certificate]                       |
     |                                                |
     | Method: POST                                   |
     | Request target: /.well-known/est/<label>       |
     |                   /serverkeygen                |
     | Media type: application/cose-c509-pkcs10+cbor  |
     |                                                |
     | <Base64-encoded C509 CSR (no public key)>      |
     |----------------------------------------------->|
     |                                                | Generate
     |                                                | keypair,
     |                                                | Issue C509
     | Status: 200 OK                                 | cert, Delete
     | Media type: application/cose-c509-pem+cbor     | key from server
     |                                                |
     | <Base64-encoded C509PEM>                       |
     |<-----------------------------------------------|
     | (key no longer on EST server)                  |
     |                                                |

      Figure 14: Message flow of EST over HTTPS operation serverkeygen

Appendix B.  Message Flow Diagrams of EST over CoAP/DTLS Operations

B.1.  crts for single CA certificate

   EST Client                                   EST Server
     |                                               |
     | GET example.com/.well-known/est/<label>/crts  |
     | (Accept: TBD)                                 |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded C509Certificate   |
     |   in binary format }                          |
     |<----------------------------------------------|
     |                                               |

      Figure 15: Message flow of EST over CoAP/DTLS operation crts for
                           single CA certificate

B.2.  crts for CA certificate set







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   EST Client                                   EST Server
     |                                               |
     | GET example.com/.well-known/est/<label>/crts  |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded COSE_C509         |
     |   (certificate set) in binary format }        |
     |<----------------------------------------------|
     |                                               |

      Figure 16: Message flow of EST over CoAP/DTLS operation crts for
                             CA certificate set

B.3.  crts for CA certificate chain

   EST Client                                   EST Server
     |                                               |
     | GET example.com/.well-known/est/<label>/crts  |
     | (Accept: TBD)                                 |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded COSE_C509         |
     |   (certificate chain) in binary format }      |
     |<----------------------------------------------|
     |                                               |

      Figure 17: Message flow of EST over CoAP/DTLS operation crts for
                            CA certificate chain

B.4.  crli

   EST Client                                   EST Server
     |                                               |
     | GET example.com/.well-known/est/<label>/crli  |
     |   [?crlnumber=<n>][&crldp=<dp>]               |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded C509CRLInfo       |
     |   in binary format }                          |
     |<----------------------------------------------|
     |                                               |

        Figure 18: Message flow of EST over CoAP/DTLS operation crli





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B.5.  crl

   EST Client                                   EST Server
     |                                               |
     | GET example.com/.well-known/est/<label>/crl   |
     |   [?crlnumber=<n>][&crldp=<dp>]               |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded C509CRL           |
     |   in binary format }                          |
     |<----------------------------------------------|
     |                                               |

        Figure 19: Message flow of EST over CoAP/DTLS operation crl

B.6.  kemc

   EST Client                                    EST Server
     |                                                |
     | POST example.com/.well-known/est/<label>/kemc  |
     | (Content-Format: TBD)                          |
     | { payload with CBOR-encoded                    |
     |   C509SubjectPublicKeyInfo in binary format }  |
     |----------------------------------------------->|
     |                                                |
     | 2.05 Content (Content-Format: TBD)             |
     | { payload with CBOR-encoded C509KemChall       |
     |   in binary format }                           |
     |<-----------------------------------------------|
     |                                                |

        Figure 20: Message flow of EST over CoAP/DTLS operation kemc

B.7.  sen
















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   EST Client                                   EST Server
     |                                               |
     | POST example.com/.well-known/est/<label>/sen  |
     | (Content-Format: TBD)                         |
     | { payload with CBOR-encoded                   |
     |  C509CertificationRequest in binary format }  |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded C509Certificate   |
     |   in binary format }                          |
     |<----------------------------------------------|
     |                                               |

        Figure 21: Message flow of EST over CoAP/DTLS operation sen

B.8.  sren

   EST Client                                   EST Server
     |                                               |
     | POST example.com/.well-known/est/<label>/sren |
     | (Content-Format: TBD)                         |
     | { payload with CBOR-encoded                   |
     |  C509CertificationRequest in binary format }  |
     |---------------------------------------------->|
     |                                               |
     | 2.05 Content (Content-Format: TBD)            |
     | { payload with CBOR-encoded C509Certificate   |
     |   in binary format }                          |
     |<----------------------------------------------|
     |                                               |

        Figure 22: Message flow of EST over CoAP/DTLS operation sren

B.9.  skc
















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   EST Client                                    EST Server
     |                                                |
     | POST example.com/.well-known/est/<label>/skc   |
     | (Content-Format: TBD)                          |
     | { payload with CBOR-encoded                    |
     |   C509CertificationRequest in binary format }  |
     |----------------------------------------------->|
     |                                                |
     | 2.05 Content (Content-Format: TBD)             |
     | { payload with CBOR-encoded C509PEM            |
     |   in binary format }                           |
     |<-----------------------------------------------|
     |                                                |

        Figure 23: Message flow of EST over CoAP/DTLS operation skc

Acknowledgements

   The author thanks the members of the ACE and COSE working groups for
   reviewing and commenting on intermediate versions of this draft.

Change log

      *RFC Editor's Note:* Please remove this section prior to
      publication of a final version of this document.

Since draft-liao-ace-est-c509-03

   *  Use C509SubjectPublicKeyInfo defined in
      [I-D.liao-cose-c509-additions] instead of C509PublicKey and the
      media type application/c509-pubkey+cbor previously defined in this
      document.

   *  Specify the order of the "Root CA Key Update" certificates in the
      CA certificate chain, and state that no order is defined for them
      in the CA certificate set.

   *  Allow a PoP MAC instead of a PoP signature for KEM public keys,
      and specify how the MAC is carried in the C509 CSR and how the
      server locates the challenge state.

   *  Align the KEM PoP with the key confirmation of [NIST.SP.800-227]:
      define MAC_Data, add the serverId field to C509KemChall, add the
      CR attribute C509KemPopNonce for the client nonce, and require
      single-use challenges bound to the authenticated EST client,
      remove the encapAlg field from C509KemChall, add the optional
      lifetime field to C509KemChall, and make HMAC-SHA256 mandatory to
      implement for EST servers.



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   *  Require (MUST instead of SHOULD) empty-publickey and id-alg-
      unsigned in serverkeygen / skc requests.

   *  Follow [RFC8951] for the base64 encoding of HTTPS payloads; CoAP
      payloads are binary.

   *  Require EST clients to validate received C509 certificates against
      a trust anchor.

   *  Specify HTTP 406 / CoAP 4.06 and HTTP 415 / CoAP 4.15 error
      responses.

   *  Add security considerations for server key generation, C509 CRL
      validation, and capability discovery.

   *  Add IANA registrations for the resource types ace.est.crli,
      ace.est.crl, and ace.est.kemc.

   *  Move RFC 5652 to the informative references.

   *  Use /.well-known/est/ as the path prefix for EST over CoAP, and
      <label> in the message flow diagrams.

   *  Specify the crldp query parameter as a percent-encoded
      distribution point URI taken from the certificate, and its
      encoding as CoAP Uri-Query options.

   *  Relax the client behavior after an error response to caps.

   *  Specify that the single-certificate form of cacerts / crts returns
      the issuing CA certificate.

   *  Editorial changes.

Since draft-liao-ace-est-c509-02

   *  Use both long name and short name of operations.

   *  Add message flows for the cacerts / crts operation to retrieve
      single CA certificate, and CA certificate chain.

   *  Add message flows for EST over CoAP/DTLS.

   *  Editorial changes.







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Since draft-liao-ace-est-c509-01

   *  Add EST of C509 certificate over secure CoAP.

   *  Use short operation names defined in [RFC9148] to replace the long
      names defined in [RFC7030].

Author's Address

   Lijun Liao
   NIO Inc.
   Email: lijun.liao@nio.io







































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