ACME Working Group A. Gable Internet-Draft Internet Security Research Group Intended status: Standards Track 17 August 2022 Expires: 18 February 2023 Automated Certificate Management Environment (ACME) Renewal Information (ARI) Extension draft-acme-ari-00 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 Staging environment (available at [lestaging], source at [boulder]) implements this draft specification. 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/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on 18 February 2023. Copyright Notice Copyright (c) 2022 IETF Trust and the persons identified as the document authors. All rights reserved. Gable Expires 18 February 2023 [Page 1] Internet-Draft ACME ARI August 2022 This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/ license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3 3. Extensions to the ACME Protocol: The "directory" Resource . . 3 4. Extensions to the ACME Protocol: The "renewalInfo" Resource . . . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Getting Renewal Information . . . . . . . . . . . . . . . 4 4.2. Updating Renewal Information . . . . . . . . . . . . . . 6 5. Security Considerations . . . . . . . . . . . . . . . . . . . 7 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 6.1. New Registries . . . . . . . . . . . . . . . . . . . . . 8 6.2. ACME Resource Type . . . . . . . . . . . . . . . . . . . 8 6.3. ACME Renewal Info Object Fields . . . . . . . . . . . . . 8 7. Normative References . . . . . . . . . . . . . . . . . . . . 9 8. Informative References . . . . . . . . . . . . . . . . . . . 9 Appendix A. Example Certificates . . . . . . . . . . . . . . . . 10 A.1. Example End-Entity Certificate . . . . . . . . . . . . . 10 Example CA Certificate . . . . . . . . . . . . . . . . . . . . 10 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 11 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 11 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. Gable Expires 18 February 2023 [Page 2] Internet-Draft ACME ARI August 2022 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. +=============+==============+ | Field | URL in Value | +=============+==============+ | renewalInfo | Renewal info | +-------------+--------------+ Table 1 Gable Expires 18 February 2023 [Page 3] Internet-Draft ACME ARI August 2022 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 Gable Expires 18 February 2023 [Page 4] Internet-Draft ACME ARI August 2022 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: 1. Select a uniforn random time within the suggested window. 2. If the selected time is in the past, attempt renewal immediately. 3. Otherwise, if the client can schedule itself to attempt renewal at exactly the selected time, do so. Gable Expires 18 February 2023 [Page 5] Internet-Draft ACME ARI August 2022 4. Otherwise, if the selected time is before the next time that the client would wake up normally, attempt renewal immediately. 5. 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. Gable Expires 18 February 2023 [Page 6] Internet-Draft ACME ARI August 2022 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. Gable Expires 18 February 2023 [Page 7] Internet-Draft ACME ARI August 2022 6. IANA Considerations Draft note: The following changes to IANA registries have not yet been made. 6.1. New Registries Within the "Automated Certificate Management Environment (ACME) Protocol" registry, IANA has created the new "ACME Renewal Info Object Fields" registry (Section 6.4). 6.2. ACME Resource Type Within the "Automated Certificate Management Environment (ACME) Protocol" registry, the following entry has been added to the "ACME Resource Types" registry. +=============+=====================+===============+ | Field Name | Resource Type | Reference | +=============+=====================+===============+ | renewalInfo | Renewal Info object | This document | +-------------+---------------------+---------------+ Table 2 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: Gable Expires 18 February 2023 [Page 8] Internet-Draft ACME ARI August 2022 +=================+============+===============+ | Field Name | Field type | Reference | +=================+============+===============+ | suggestedWindow | object | This document | +-----------------+------------+---------------+ Table 3 7. 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, . [RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet: Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002, . [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, . [RFC6960] Santesson, S., Myers, M., Ankney, R., Malpani, A., Galperin, S., and C. Adams, "X.509 Internet Public Key Infrastructure Online Certificate Status Protocol - OCSP", RFC 6960, DOI 10.17487/RFC6960, June 2013, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8555] Barnes, R., Hoffman-Andrews, J., McCarney, D., and J. Kasten, "Automatic Certificate Management Environment (ACME)", RFC 8555, DOI 10.17487/RFC8555, March 2019, . 8. Informative References [boulder] Internet Security Research Group, "Boulder", 2022, . [lestaging] Internet Security Research Group, "Let's Encrypt Staging Environment", 2022, . Gable Expires 18 February 2023 [Page 9] Internet-Draft ACME ARI August 2022 Appendix A. Example Certificates A.1. Example End-Entity Certificate -----BEGIN CERTIFICATE----- MIIDMDCCAhigAwIBAgIIPqNFaGVEHxwwDQYJKoZIhvcNAQELBQAwIDEeMBwGA1UE AxMVbWluaWNhIHJvb3QgY2EgM2ExMzU2MB4XDTIyMDMxNzE3NTEwOVoXDTI0MDQx NjE3NTEwOVowFjEUMBIGA1UEAxMLZXhhbXBsZS5jb20wggEiMA0GCSqGSIb3DQEB AQUAA4IBDwAwggEKAoIBAQCgm9K/c+il2Pf0f8qhgxn9SKqXq88cOm9ov9AVRbPA OWAAewqX2yUAwI4LZBGEgzGzTATkiXfoJ3cN3k39cH6tBbb3iSPuEn7OZpIk9D+e 3Q9/hX+N/jlWkaTB/FNA+7aE5IVWhmdczYilXa10V9r+RcvACJt0gsipBZVJ4jfJ HnWJJGRZzzxqG/xkQmpXxZO7nOPFc8SxYKWdfcgp+rjR2ogYhSz7BfKoVakGPbpX vZOuT9z4kkHra/WjwlkQhtHoTXdAxH3qC2UjMzO57Tx+otj0CxAv9O7CTJXISywB vEVcmTSZkHS3eZtvvIwPx7I30ITRkYk/tLl1MbyB3SiZAgMBAAGjeDB2MA4GA1Ud DwEB/wQEAwIFoDAdBgNVHSUEFjAUBggrBgEFBQcDAQYIKwYBBQUHAwIwDAYDVR0T AQH/BAIwADAfBgNVHSMEGDAWgBQ4zzDRUaXHVKqlSTWkULGU4zGZpTAWBgNVHREE DzANggtleGFtcGxlLmNvbTANBgkqhkiG9w0BAQsFAAOCAQEAx0aYvmCk7JYGNEXe +hrOfKawkHYzWvA92cI/Oi6h+oSdHZ2UKzwFNf37cVKZ37FCrrv5pFP/xhhHvrNV EnOx4IaF7OrnaTu5miZiUWuvRQP7ZGmGNFYbLTEF6/dj+WqyYdVaWzxRqHFu1ptC TXysJCeyiGnR+KOOjOOQ9ZlO5JUK3OE4hagPLfaIpDDy6RXQt3ss0iNLuB1+IOtp 1URpvffLZQ8xPsEgOZyPWOcabTwJrtqBwily+lwPFn2mChUx846LwQfxtsXU/lJg HX2RteNJx7YYNeX3Uf960mgo5an6vE8QNAsIoNHYrGyEmXDhTRe9mCHyiW2S7fZq o9q12g== -----END CERTIFICATE----- Example CA Certificate -----BEGIN CERTIFICATE----- MIIDSzCCAjOgAwIBAgIIOhNWtJ7Igr0wDQYJKoZIhvcNAQELBQAwIDEeMBwGA1UE AxMVbWluaWNhIHJvb3QgY2EgM2ExMzU2MCAXDTIyMDMxNzE3NTEwOVoYDzIxMjIw MzE3MTc1MTA5WjAgMR4wHAYDVQQDExVtaW5pY2Egcm9vdCBjYSAzYTEzNTYwggEi MA0GCSqGSIb3DQEBAQUAA4IBDwAwggEKAoIBAQDc3P6cxcCZ7FQOQrYuigReSa8T IOPNKmlmX9OrTkPwjThiMNEETYKO1ea99yXPK36LUHC6OLmZ9jVQW2Ny1qwQCOy6 TrquhnwKgtkBMDAZBLySSEXYdKL3r0jA4sflW130/OLwhstU/yv0J8+pj7eSVOR3 zJBnYd1AqnXHRSwQm299KXgqema7uwsa8cgjrXsBzAhrwrvYlVhpWFSv3lQRDFQg c5Z/ZDV9i26qiaJsCCmdisJZWN7N2luUgxdRqzZ4Cr2Xoilg3T+hkb2y/d6ttsPA kaSA+pq3q6Qa7/qfGdT5WuUkcHpvKNRWqnwT9rCYlmG00r3hGgc42D/z1VvfAgMB AAGjgYYwgYMwDgYDVR0PAQH/BAQDAgKEMB0GA1UdJQQWMBQGCCsGAQUFBwMBBggr BgEFBQcDAjASBgNVHRMBAf8ECDAGAQH/AgEAMB0GA1UdDgQWBBQ4zzDRUaXHVKql STWkULGU4zGZpTAfBgNVHSMEGDAWgBQ4zzDRUaXHVKqlSTWkULGU4zGZpTANBgkq hkiG9w0BAQsFAAOCAQEArbDHhEjGedjb/YjU80aFTPWOMRjgyfQaPPgyxwX6Dsid 1i2H1x4ud4ntz3sTZZxdQIrOqtlIWTWVCjpStwGxaC+38SdreiTTwy/nikXGa/6W ZyQRppR3agh/pl5LHVO6GsJz3YHa7wQhEhj3xsRwa9VrRXgHbLGbPOFVRTHPjaPg Gtsv2PN3f67DsPHF47ASqyOIRpLZPQmZIw6D3isJwfl+8CzvlB1veO0Q3uh08IJc fspYQXvFBzYa64uKxNAJMi4Pby8cf4r36Wnb7cL4ho3fOHgAltxdW8jgibRzqZpQ QKyxn2jX7kxeUDt0hFDJE8lOrhP73m66eBNzxe//FQ== -----END CERTIFICATE----- Gable Expires 18 February 2023 [Page 10] Internet-Draft ACME ARI August 2022 Acknowledgments TODO acknowledge. Author's Address A. Gable Internet Security Research Group Email: aaron@letsencrypt.org Gable Expires 18 February 2023 [Page 11]