Automated Certificate Management Environment (ACME) Renewal Information (ARI) Extension

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Table of Contents

1. Introduction

Most ACME 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 CA changing certificate lifetimes. All three techniques lead to load clustering for the issuing CA.¶

Being able to indicate to the client a period in which the issuing CA suggests renewal would allow both dynamic changes to the certificate validity period and proactive smearing of load. This document specifies a mechanism by which ACME servers may provide suggested renewal windows to ACME clients.¶

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.¶

3. Extensions to the ACME Protocol: The "order" Resource

An ACME server which wishes to provide renewal information MUST include a new field, "renewalInfo", in finalized Order objects.¶

renewalInfo (optional, string): A URL for renewal information for the certificate that has been issued in response to this order.¶

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

{
  "status": "valid",
  "expires": "2021-01-20T14:09:07.99Z",

  "identifiers": [
    { "type": "dns", "value": "www.example.org" },
    { "type": "dns", "value": "example.org" }
  ],

  "notBefore": "2021-01-01T00:00:00Z",
  "notAfter": "2021-01-08T00:00:00Z",

  "authorizations": [
    "https://example.com/acme/authz/PAniVnsZcis",
    "https://example.com/acme/authz/r4HqLzrSrpI"
  ],

  "finalize": "https://example.com/acme/order/TOlocE8rfgo/finalize",
  "certificate": "https://example.com/acme/cert/mAt3xBGaobw",
  "renewalInfo": "https://example.com/acme/renewal/eXoM9UwLgbL"
}

Conforming clients SHOULD store the "renewalInfo" URL locally so that they can poll it at any time during the lifetime of the certificate.¶

4. Extensions to the ACME Protocol: The "renewalInfo" Resource

We define a new resource type, the "renewalInfo" resource, as part of the ACME protocol.¶

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.¶

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

{
  "suggestedWindow": {
    "start": "2021-01-03T00:00:00Z",
    "end": "2021-01-07T00:00:00Z"
  }
}

Conforming servers MUST provide the renewalInfo resource via POST-as-GET; they SHOULD provide it via unauthenticated GET as well. Conforming clients SHOULD use unauthenticated GET to request renewalInfo resources.¶

The server SHOULD include a Retry-After header indicating the polling interval that the ACME server recommends. Conforming clients SHOULD query the "renewalInfo" URL again after the Retry-After period has passed, as the server may provide a different suggestedWindow.¶

Conforming clients SHOULD select a random time within the suggested window to attempt to renew the certificate. If the selected time is in the past, the client SHOULD attempt renewal immediately.¶

5. Security Considerations

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

This document specifies that renewalInfo resources should be exposed via unauthenticated GET requests, a departure from RFC8555's requirement that clients must 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 load from clients which do not respect the Retry-After header.¶

6. IANA Considerations

Draft note: The following changes to IANA registries have not yet been made.¶

Acknowledgments

TODO acknowledge.¶

Author's Address