Internet DRAFT - draft-acme-ari
draft-acme-ari
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
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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.
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Please review these documents carefully, as they describe your rights
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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.
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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
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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
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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.
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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.
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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.
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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:
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+=================+============+===============+
| 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,
<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>.
[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,
<https://www.rfc-editor.org/info/rfc6960>.
[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>.
8. Informative References
[boulder] Internet Security Research Group, "Boulder", 2022,
<https://github.com/letsencrypt/boulder>.
[lestaging]
Internet Security Research Group, "Let's Encrypt Staging
Environment", 2022,
<https://acme-staging-v02.api.letsencrypt.org/directory>.
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Appendix A. Example Certificates
A.1. Example End-Entity Certificate
-----BEGIN CERTIFICATE-----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-----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-----
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Acknowledgments
TODO acknowledge.
Author's Address
A. Gable
Internet Security Research Group
Email: aaron@letsencrypt.org
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