Automated Certificate Management Environment (ACME) Renewal Information (ARI) Extension
draft-ietf-acme-ari-08

ACME Working Group                                              A. Gable
Internet-Draft                          Internet Security Research Group
Intended status: Standards Track                        26 February 2025
Expires: 30 August 2025

Automated Certificate Management Environment (ACME) Renewal Information
                            (ARI) Extension
                         draft-ietf-acme-ari-08

Abstract

   This document specifies how an ACME server may provide suggestions to
   ACME clients as to when they should attempt to renew their
   certificates.  This allows servers to mitigate load spikes, and
   ensures clients do not make false assumptions about appropriate
   certificate renewal periods.

Current Implementations

   Draft note: this section will be removed by the editor before final
   publication.

   Let's Encrypt's Boulder (https://github.com/letsencrypt/boulder)
   software fully implements the server side of an earlier version of
   this draft, and that implementation is deployed in both the
   Production (https://acme-v02.api.letsencrypt.org/directory) and
   Staging (https://acme-staging-v02.api.letsencrypt.org/directory)
   environments.  Google Trust Services has done the same
   (https://security.googleblog.com/2023/05/google-trust-services-acme-
   api_0503894189.html).  Client implementations include Lego
   (https://github.com/go-acme/lego), eggsampler
   (https://github.com/eggsampler/acme), ACMEz
   (https://github.com/mholt/acmez), and win-acme (https://github.com/
   win-acme/win-acme).

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 30 August 2025.

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   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Extensions to the Directory Object  . . . . . . . . . . . . .   3
   4.  Getting Renewal Information . . . . . . . . . . . . . . . . .   4
     4.1.  The "renewalInfo" Resource  . . . . . . . . . . . . . . .   4
     4.2.  RenewalInfo Objects . . . . . . . . . . . . . . . . . . .   5
     4.3.  Schedule for checking the RenewalInfo resource  . . . . .   6
       4.3.1.  Server choice of Retry-After  . . . . . . . . . . . .   7
       4.3.2.  Client handling of Retry-After  . . . . . . . . . . .   7
       4.3.3.  Error handling  . . . . . . . . . . . . . . . . . . .   7
   5.  Extensions to the Order Object  . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  ACME Resource Type  . . . . . . . . . . . . . . . . . . .  10
     7.2.  ACME Renewal Info Object Fields . . . . . . . . . . . . .  11
     7.3.  ACME Order Object Fields  . . . . . . . . . . . . . . . .  11
     7.4.  ACME Error Types  . . . . . . . . . . . . . . . . . . . .  12
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Appendix A.  Example Certificate  . . . . . . . . . . . . . . . .  13
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  13

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1.  Introduction

   Most ACME [RFC8555] clients today choose when to attempt to renew a
   certificate in one of three ways.  They may be configured to renew at
   a specific interval (e.g., via cron), they may parse the issued
   certificate to determine its expiration date and renew a specific
   amount of time before then, or they may parse the issued certificate
   and renew when some percentage of its validity period has passed.
   The first two techniques create significant barriers against the
   issuing Certification Authority (CA) changing certificate lifetimes.
   All three techniques may lead to load clustering for the issuing CA
   due to the inability of the issuing CA to schedule renewal requests.

   Allowing issuing CAs to suggest a period in which clients should
   renew their certificates enables dynamic time-based load balancing.
   This allows a CA to better respond to exceptional circumstances.  For
   example, a CA could suggest that clients renew prior to a mass-
   revocation event to mitigate the impact of the revocation, or a CA
   could suggest that clients renew earlier than they normally would to
   reduce the size of an upcoming mass-renewal spike.

   This document specifies ACME Renewal Information (ARI), a mechanism
   by which ACME servers may provide suggested renewal windows to ACME
   clients, and by which ACME clients may inform ACME servers that they
   have successfully renewed and replaced a certificate.

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.

   Throughout this document, the word "renewal" and its variants are
   taken to encompass any combination of "Renewal", "Re-Key", and
   "Modification" as defined in [RFC3647].

   This document assumes that the certificates being issued by the ACME
   server are in compliance with [RFC5280], and in particular contain
   the Authority Key Identifier extension and the keyIdentifier field
   within that extension.

3.  Extensions to the Directory Object

   An ACME server which wishes to provide renewal information MUST
   include a new field, renewalInfo, in its directory object.

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                      +=============+==============+
                      | Field       | URL in Value |
                      +=============+==============+
                      | renewalInfo | Renewal info |
                      +-------------+--------------+

                                 Table 1

   HTTP/1.1 200 OK
   Content-Type: application/json

   {
     "newNonce": "https://acme.example.com/new-nonce",
     "newAccount": "https://acme.example.com/new-account",
     "newOrder": "https://acme.example.com/new-order",
     "newAuthz": "https://acme.example.com/new-authz",
     "revokeCert": "https://acme.example.com/revoke-cert",
     "keyChange": "https://acme.example.com/key-change",
     "renewalInfo": "https://acme.example.com/renewal-info",
     "meta": {
       "termsOfService": "https://example.com/acme/terms",
       "website": "https://example.com/acme/docs",
       "caaIdentities": ["example.com"],
       "externalAccountRequired": false
     }
   }

4.  Getting Renewal Information

4.1.  The "renewalInfo" Resource

   The "renewalInfo" resource is a new resource type introduced to the
   ACME protocol.  This new resource allows clients to query the server
   for suggestions on when they should renew certificates.

   To request the suggested renewal information for a certificate, the
   client sends an unauthenticated GET request to a path under the
   server's renewalInfo URL.

   The path component is a unique identifier for the certificate in
   question.  The unique identifier is constructed by concatenating the
   base64url-encoding [RFC4648] of the keyIdentifier field of the
   certificate's Authority Key Identifier (AKI) [RFC5280] extension, a
   literal period, and the base64url-encoding of the DER-encoded Serial
   Number field (without the tag and length bytes).  All trailing "="
   characters MUST be stripped from both parts of the unique identifier.

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   Thus the full request URL is constructed as follows (split onto
   multiple lines for readability), where the "||" operator indicates
   string concatenation and the renewalInfo URL is taken from the
   Directory object:

   url = renewalInfo || '/' ||
         base64url(AKI keyIdentifier) || '.' || base64url(Serial)

   For example, to request renewal information for the end-entity
   certificate given in Appendix A, the client would make the request as
   follows:

   1.  The keyIdentifier field of the certificate's AKI extension has
       the hexadecimal bytes
       69:88:5B:6B:87:46:40:41:E1:B3:7B:84:7B:A0:AE:2C:DE:01:C8:D4 as
       its ASN.1 Octet String value.  The base64url encoding of those
       bytes is aYhba4dGQEHhs3uEe6CuLN4ByNQ=.
   2.  The certificate's Serial Number field has the hexadecimal bytes
       00:87:65:43:21 as its DER encoding (note the leading zero byte to
       ensure the serial number remains positive despite the leading 1
       bit in 0x87).  The base64url encoding of those bytes is AIdlQyE=.
   3.  Stripping the trailing padding characters and concatenating with
       the separator, the unique identifier is therefore
       aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE, and the client makes the
       request:

   GET /renewal-info/aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE HTTP/1.1
   Host: acme.example.com
   Accept: application/json

4.2.  RenewalInfo Objects

   The structure of an ACME renewalInfo resource is as follows:

   suggestedWindow (object, required): A JSON object with two keys,
   "start" and "end", whose values are timestamps, encoded in the format
   specified in [RFC3339], which bound the window of time in which the
   CA recommends renewing the certificate.

   explanationURL (string, optional): A URL pointing to a page which may
   explain why the suggested renewal window has its current value.  For
   example, it may be a page explaining the CA's dynamic load-balancing
   strategy, or a page documenting which certificates are affected by a
   mass revocation event.  Clients SHOULD provide this URL to their
   operator, if present.

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   HTTP/1.1 200 OK
   Content-Type: application/json
   Retry-After: 21600

   {
     "suggestedWindow": {
       "start": "2025-01-02T04:00:00Z",
       "end": "2025-01-03T04:00:00Z"
     },
     "explanationURL": "https://acme.example.com/docs/ari"
   }

   Clients MUST attempt renewal at a time of their choosing based on the
   suggested renewal window.  The following algorithm is RECOMMENDED for
   choosing a renewal time:

   1.  Query the renewalInfo resource to get a suggested renewal window.
   2.  Select a uniform random time within the suggested window.
   3.  If the selected time is in the past, attempt renewal immediately.
   4.  Otherwise, if the client can schedule itself to attempt renewal
       at exactly the selected time, do so.
   5.  Otherwise, if the selected time is before the next time that the
       client would wake up normally, attempt renewal immediately.
   6.  Otherwise, sleep until the time indicated by the Retry-After
       header and return to Step 1.

   In all cases, renewal attempts are subject to the client's existing
   error backoff and retry intervals.

   In particular, cron-based clients may find they need to increase
   their run frequency to check ARI more frequently.  Those clients will
   need to store information about failures so that increasing their run
   frequency doesn't lead to retrying failures without proper backoff.
   Typical information stored should include: number of failures for a
   given order (defined by the set of names on the order), and time of
   the most recent failure.

   A RenewalInfo object in which the end timestamp equals or precedes
   the start timestamp is invalid.  Servers MUST NOT serve such a
   response, and clients MUST treat one as though they failed to receive
   any response from the server (e.g., retry at an appropriate interval,
   renew on a fallback schedule, etc.).

4.3.  Schedule for checking the RenewalInfo resource

   Clients SHOULD fetch a certificate's RenewalInfo immediately after
   issuance.

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   During the lifetime of a certificate, the renewal information needs
   to be fetched frequently enough that clients learn about changes in
   the suggested window quickly, but without overwhelming the server.
   This protocol uses the Retry-After header [RFC9110] to indicate to
   clients how often to retry.  Note that in other HTTP applications,
   Retry-After often indicates the minimum time to wait before retrying
   a request.  In this protocol, it indicates the desired (i.e. both
   requested minimum and maximum) amount of time to wait.

   Clients MUST NOT check a certificate's RenewalInfo after the
   certificate has expired.  Clients MUST NOT check a certificate's
   RenewalInfo after they consider the certificate to be replaced (for
   instance, after a new certificate for the same identifiers has been
   received and configured).

4.3.1.  Server choice of Retry-After

   Servers set the Retry-After header based on their requirements on how
   quickly to perform a revocation.  For instance, a server that needs
   to revoke certificates within 24 hours of notification of a problem
   might choose to reserve twelve hours for investigation, six hours for
   clients to fetch RenewalInfo, and six hours for clients to perform a
   renewal.  Setting a small value for Retry-After means that clients
   can respond more quickly, but also incurs more load on the server.
   Servers should estimate their expected load based on the number of
   clients, keeping in mind that third parties may also monitor
   RenewalInfo endpoints.

4.3.2.  Client handling of Retry-After

   After an initial fetch of a certificate's RenewalInfo, clients MUST
   fetch it again as soon as possible after the time indicated in the
   Retry-After header (backoff on errors takes priority, though).
   Clients MUST set reasonable limits on their checking interval.  For
   example, values under one minute could be treated as if they were one
   minute, and values over one day could be treated as if they were one
   day.

4.3.3.  Error handling

   Temporary errors include, for instance:

   *  Connection timeout
   *  Request timeout
   *  5xx HTTP errors

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   On receiving a temporary error, clients SHOULD do exponential backoff
   with a capped number of tries.  If all tries are exhausted, clients
   MUST treat the request as a long-term error.

   Long term errors include, for instance:

   *  Retry-After is invalid or not present
   *  RenewalInfo object is invalid
   *  DNS lookup failure
   *  Connection refused
   *  Non-5xx HTTP error

   On receiving a long term error, clients MUST perform the next
   RenewalInfo fetch as soon as possible after six hours have passed (or
   some other locally configured default).

5.  Extensions to the Order Object

   In order to convey information regarding which certificate requests
   represent renewals of previous certificates, a new field is added to
   the Order object:

   replaces (string, optional): A string uniquely identifying a
   previously-issued certificate which this order is intended to
   replace.  This unique identifier is constructed in the same way as
   the path component for GET requests described above.

   Clients SHOULD include this field in New Order requests if there is a
   clear predecessor certificate, as is the case for most certificate
   renewals.  Clients SHOULD NOT include this field if the ACME Server
   has not indicated that it supports this protocol by advertising the
   renewalInfo resource in its Directory.

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   POST /new-order HTTP/1.1
   Host: acme.example.com
   Content-Type: application/jose+json

   {
     "protected": base64url({
       "alg": "ES256",
       "kid": "https://acme.example.com/acct/evOfKhNU60wg",
       "nonce": "5XJ1L3lEkMG7tR6pA00clA",
       "url": "https://acme.example.com/new-order"
     }),
     "payload": base64url({
       "identifiers": [
         { "type": "dns", "value": "acme.example.com" }
       ],
       "replaces": "aYhba4dGQEHhs3uEe6CuLN4ByNQ.AIdlQyE"
     }),
     "signature": "H6ZXtGjTZyUnPeKn...wEA4TklBdh3e454g"
   }

   Servers SHOULD check that the identified certificate and the New
   Order request correspond to the same ACME Account, that they share at
   least one identifier, and that the identified certificate has not
   already been marked as replaced by a different Order that is not
   "invalid".  Correspondence checks beyond this (such as requiring
   exact identifier matching) are left up to Server policy.  If any of
   these checks fail, the Server SHOULD reject the new-order request.
   If the Server rejects the request because the identified certificate
   has already been marked as replaced, it MUST return an HTTP 409
   (Conflict) with a problem document of type "alreadyReplaced" (see
   Section 7.4).

   If the Server accepts a new-order request with a "replaces" field, it
   MUST reflect that field in the response and in subsequent requests
   for the corresponding Order object.

   This replacement information may serve many purposes, including but
   not limited to:

   *  granting New Order requests which arrive during the suggested
      renewal window of their identified predecessor certificate higher
      priority or allow them to bypass rate limits, if the Server's
      policy uses such;
   *  tracking the replacement of certificates which have been affected
      by a compliance incident, so that they can be revoked immediately
      after they are replaced; and
   *  tying together certificates issued under the same contract with an
      entity identified by External Account Binding.

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6.  Security Considerations

   The extensions to the ACME protocol described in this document builds
   upon the Security Considerations and threat model defined in
   [RFC8555], Section 10.1.

   This document specifies that renewalInfo resources are exposed and
   accessed via unauthenticated GET requests, a departure from RFC8555's
   requirement that clients send POST-as-GET requests to fetch resources
   from the server.  This is because the information contained in
   renewalInfo resources is not considered confidential, and because
   allowing renewalInfo to be easily cached is advantageous to shed the
   load from clients which do not respect the Retry-After header.  As
   always, servers should take measures to ensure that unauthenticated
   requests for renewal information cannot result in denial-of-service
   attacks.  These measures might include ensuring that a cache does not
   include superfluous request headers or query parameters in its cache
   key, instituting IP-based rate limits, or other general best-practice
   measures.

   Note that this protocol could exhibit undesired behavior in the
   presence of significant clock skew between the ACME client and
   server.  For example, if a server places the suggested renewal window
   wholly in the past to encourage a client to renew immediately, a
   client with a sufficiently slow clock might nonetheless see the
   window as being in the future.  Similarly, a server which wishes to
   schedule renewals very precisely may have difficulty doing so if some
   clients have skewed clocks (or do no implement ARI at all).  Server
   operators should take this concern into account when setting
   suggested renewal windows.  However, many other protocols (including
   TLS handshakes themselves) fall apart with sufficient clock skew, so
   this is not unique to this protocol.

7.  IANA Considerations

7.1.  ACME Resource Type

   IANA will add the following entry to the "ACME Resource Types"
   registry within the "Automated Certificate Management Environment
   (ACME) Protocol" registry group at https://www.iana.org/assignments/
   acme (https://www.iana.org/assignments/acme):

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           +=============+=====================+===============+
           | Field Name  | Resource Type       | Reference     |
           +=============+=====================+===============+
           | renewalInfo | Renewal Info object | This document |
           +-------------+---------------------+---------------+

                                  Table 2

7.2.  ACME Renewal Info Object Fields

   IANA will add the following new registry to the "Automated
   Certificate Management Environment (ACME) Protocol" registry group at
   https://www.iana.org/assignments/acme
   (https://www.iana.org/assignments/acme):

   Registry Name: ACME Renewal Info Object Fields

   Registration Procedure: Specification Required.  The designated
   expert should ensure that any new fields added to this registry carry
   useful and unique information that does not better belong elsewhere
   in the ACME protocol.

   Template:

   *  Field name: The string to be used as a field name in the JSON
      object
   *  Field type: The type of value to be provided, e.g., string,
      boolean, array of string
   *  Reference: Where this field is defined

   Initial contents:

             +=================+============+===============+
             | Field Name      | Field type | Reference     |
             +=================+============+===============+
             | suggestedWindow | object     | This document |
             +-----------------+------------+---------------+
             | explanationURL  | string     | This document |
             +-----------------+------------+---------------+

                                 Table 3

7.3.  ACME Order Object Fields

   IANA will add the following entry to the "ACME Order Object Fields"
   registry within the "Automated Certificate Management Environment
   (ACME) Protocol" registry group at https://www.iana.org/assignments/
   acme (https://www.iana.org/assignments/acme):

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        +============+============+==============+===============+
        | Field Name | Field Type | Configurable | Reference     |
        +============+============+==============+===============+
        | replaces   | string     | true         | This document |
        +------------+------------+--------------+---------------+

                                 Table 4

7.4.  ACME Error Types

   IANA will add the following entry to the "ACME Error Types" registry
   within the "Automated Certificate Management Environment (ACME)
   Protocol" registry group at https://www.iana.org/assignments/acme
   (https://www.iana.org/assignments/acme):

    +=================+===================================+===========+
    | Type            | Description                       | Reference |
    +=================+===================================+===========+
    | alreadyReplaced | The request specified a           | This      |
    |                 | predecessor certificate which has | document  |
    |                 | already been marked as replaced   |           |
    +-----------------+-----------------------------------+-----------+

                                  Table 5

8.  References

8.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3339]  Klyne, G. and C. Newman, "Date and Time on the Internet:
              Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
              <https://www.rfc-editor.org/info/rfc3339>.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
              <https://www.rfc-editor.org/info/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/info/rfc5280>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8555]  Barnes, R., Hoffman-Andrews, J., McCarney, D., and J.
              Kasten, "Automatic Certificate Management Environment
              (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019,
              <https://www.rfc-editor.org/info/rfc8555>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/info/rfc9110>.

8.2.  Informative References

   [RFC3647]  Chokhani, S., Ford, W., Sabett, R., Merrill, C., and S.
              Wu, "Internet X.509 Public Key Infrastructure Certificate
              Policy and Certification Practices Framework", RFC 3647,
              DOI 10.17487/RFC3647, November 2003,
              <https://www.rfc-editor.org/info/rfc3647>.

Appendix A.  Example Certificate

   -----BEGIN CERTIFICATE-----
   MIIBQzCB66ADAgECAgUAh2VDITAKBggqhkjOPQQDAjAVMRMwEQYDVQQDEwpFeGFt
   cGxlIENBMCIYDzAwMDEwMTAxMDAwMDAwWhgPMDAwMTAxMDEwMDAwMDBaMBYxFDAS
   BgNVBAMTC2V4YW1wbGUuY29tMFkwEwYHKoZIzj0CAQYIKoZIzj0DAQcDQgAEeBZu
   7cbpAYNXZLbbh8rNIzuOoqOOtmxA1v7cRm//AwyMwWxyHz4zfwmBhcSrf47NUAFf
   qzLQ2PPQxdTXREYEnKMjMCEwHwYDVR0jBBgwFoAUaYhba4dGQEHhs3uEe6CuLN4B
   yNQwCgYIKoZIzj0EAwIDRwAwRAIge09+S5TZAlw5tgtiVvuERV6cT4mfutXIlwTb
   +FYN/8oCIClDsqBklhB9KAelFiYt9+6FDj3z4KGVelYM5MdsO3pK
   -----END CERTIFICATE-----

Acknowledgments

   My thanks to Roland Shoemaker and Jacob Hoffman-Andrews for coming up
   with the initial idea of ARI and for helping me learn the IETF
   process.  Thanks also to Samantha Frank, Matt Holt, Ilari Liusvaara,
   and Wouter Tinus for contributing client implementations, and to
   Freddy Zhang for contributing an independent server implementation.
   Finally, thanks to Rob Stradling, Andrew Ayer, and J.C.  Jones for
   providing meaningful feedback and suggestions which significantly
   improved this specification.

Author's Address

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Internet-Draft                  ACME ARI                   February 2025

   A. Gable
   Internet Security Research Group
   Email: aaron@letsencrypt.org

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