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

Internet-Draft	ACME ARI	July 2022
Gable	Expires 27 January 2023	[Page]

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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 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 for dynamic smearing of load, enabling a CA to 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 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 "directory" Resource

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

Table 1
Field	URL in Value
renewalInfo	Renewal info

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

{
  "newNonce": "https://example.com/acme/new-nonce",
  "newAccount": "https://example.com/acme/new-account",
  "newOrder": "https://example.com/acme/new-order",
  "newAuthz": "https://example.com/acme/new-authz",
  "revokeCert": "https://example.com/acme/revoke-cert",
  "keyChange": "https://example.com/acme/key-change",
  "renewalInfo": "https://example.com/acme/renewal-info",
  "meta": {
    "termsOfService": "https://example.com/acme/terms/2021-10-05",
    "website": "https://www.example.com/",
    "caaIdentities": ["example.com"],
    "externalAccountRequired": false
  }
}

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

The "renewalInfo" resource is a new resource type introduced to ACME protocol. This new resource both allows clients to query the server for suggestions on when they should renew certificates, and allows clients to inform the server when they have completed renewal (or otherwise replaced the certificate to their satisfaction).¶

4.1. Getting Renewal Information

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

The full request URL is computed by concatenating the renewalInfo URL from the server's directory with a forward slash and the base64url-encoded [RFC4648] bytes of a DER-encoded CertID ASN.1 sequence [RFC6960]. Trailing '=' characters MUST be stripped.¶

For example, to request renewal information for the end-entity certificate given in Appendix A.1, issued by the CA certificate given in Appendix A.2, using SHA256, the client would make the following request (the path has been split onto multiple lines for readability):¶

GET https://example.com/acme/renewal-info/
        MFswCwYJYIZIAWUDBAIBBCCeWLRusNLb--vmWOkxm34qDjTMWkc
        3utIhOMoMwKDqbgQg2iiKWySZrD-6c88HMZ6vhIHZPamChLlzGH
        eZ7pTS8jYCCD6jRWhlRB8c

The ACME Server MAY restrict the hash algorithms which it accepts (for example, only allowing SHA256 to limit the number of potential cache keys); if it receives a request whose embedded hashAlgorithm field contains an unacceptable OID, it SHOULD respond with HTTP status code 400 (Bad Request).¶

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 is what it is. 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. Conforming clients SHOULD provide this URL to their operator, if present.¶

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

{
  "suggestedWindow": {
    "start": "2021-01-03T00:00:00Z",
    "end": "2021-01-07T00:00:00Z"
  },
  "explanationURL": "https://example.com/docs/example-mass-reissuance-event"
}

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 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:¶

Select a uniforn random time within the suggested window.¶
If the selected time is in the past, attempt renewal immediately.¶
Otherwise, if the client can schedule itself to attempt renewal at exactly the selected time, do so.¶
Otherwise, if the selected time is before the next time that the client would wake up normally, attempt renewal immediately.¶
Otherwise, sleep until the next normal wake time, re-check ARI, 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.¶

If the client receives no response or a malformed response (e.g. an end timestamp which precedes the start timestamp), it SHOULD make its own determination of when to renew the certificate, and MAY retry the renewalInfo request with appropriate exponential backoff behavior.¶

4.2. Updating Renewal Information

To update the renewal status of a certificate, the client sends a POST request to the server's renewalInfo URL.¶

The body of the POST is a JWS object which is authenticated to an account as defined in [RFC8555], Section 6.2, and whose JSON payload has the following structure:¶

certID (required, string): The CertID of the certificate whose renewal information should be updated, in the base64url-encoded version of the DER format with trailing "=" stripped. Note: this is identical to the final path component constructed for GET requests above.¶

replaced (required, boolean): Whether or not the client considers the certificate to have been replaced. A certificate is considered replaced when its revocation would not disrupt any ongoing services, for instance because it has been renewed and the new certificate is in use, or because it is no longer in use. Clients SHOULD NOT send a request where this value is false.¶

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

{
  "protected": base64url({
    "alg": "ES256",
    "kid": "https://example.com/acme/acct/evOfKhNU60wg",
    "nonce": "JHb54aT_KTXBWQOzGYkt9A",
    "url": "https://example.com/acme/renewal-info"
  }),
  "payload": base64url({
    "certID": "MFswCwYJ...RWhlRB8c",
    "replaced": true
  }),
  "signature": "Q1bURgJoEslbD1c5...3pYdSMLio57mQNN4"
}

The server MUST verify that the request is signed by the account key of the Subscriber to which the certificate was originally issued. If the server accepts the request and the update succeeds, it responds with HTTP status code 200 (OK). If the update is rejected or fails, for example because the certificate has already been marked as replaced, the server returns an error.¶

The server might use this renewal update to inform a number of processes, such as: not sending renewal reminder notifications for certificates that have been marked as replaced; sending empty or error responses to subsequent requests for the certificate's renewal information; or confidently revoking certificates subject to a mass revocation without fear of disrupting the Subscriber's operations.¶

5. Security Considerations

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

This document specifies that renewalInfo resources MUST be exposed and accessed 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.¶

Table 2
Field Name	Resource Type	Reference
renewalInfo	Renewal Info object	This document

6.3. ACME Renewal Info Object Fields

The "ACME Renewal Info Object Fields" registry lists field names that are defined for use in ACME renewal info objects.¶

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:¶

Table 3
Field Name	Field type	Reference
suggestedWindow	object	This document

Appendix A. Example Certificates

A.1. Example End-Entity Certificate

Example CA Certificate

Acknowledgments

TODO acknowledge.¶

Author's Address

A. Gable

Internet Security Research Group

Email: aaron@letsencrypt.org

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

Abstract

Current Implementations