Internet-Draft | est-renew | October 2025 |
Shekh-Yusef, et al. | Expires 6 April 2026 | [Page] |
This document describes an extension to RFC7030, Enrollment over Secure Transport to give an indication to a end-entity device when it should start attempting to renew its certificates.¶
Prior art is that client decides, with a typical recommmendation to start when the remaining lifetime of the certificate is at the 50% point. As typical certificate lifetimes are reduced from years to fractions of a year, the 50% may be far too early, and this document provides a way to give alternate advice.¶
This note is to be removed before publishing as an RFC.¶
Status information for this document may be found at https://datatracker.ietf.org/doc/draft-yusef-lamps-rfc7030-renewal-recommendation/.¶
Discussion of this document takes place on the lamps Working Group mailing list (mailto:spasm@ietf.org), which is archived at https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at https://www.ietf.org/mailman/listinfo/spasm/.¶
Source for this draft and an issue tracker can be found at https://github.com/mcr/rfc7030-renewal-recommendation.¶
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[RFC9773], Section 1 explains why certificate lifetimes and renewal times need more deterministic control in the ACME [RFC8555] ecosystem. Similar arguments apply to the [RFC7030] ecosystem.¶
This document extends [RFC7030] to add support for renewal information, by adding a new entry to the HTTP URIs for Control list defined in [RFC7030], Section 3.2.2¶
This mechanism enables EST servers to provide suggested detailed renewal operations to EST clients.¶
The /renewal-info URI is added, as an OPTIONAL operation, to the table in figure 5 in section 3.2.2 of [RFC7030].¶
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.¶
To retrieve the renewal information, the EST client uses the following HTTP request-line:¶
GET /.well-known/est/renewal-info/<certificate-id>;¶
The request includes 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, the period character ".", 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.¶
Thus, the full request URL is constructed as follows (split onto multiple lines for readability), where the "||" operator indicates string concatenation:¶
url = /.well-known/est/renewal-info || '/' || base64url(AKI keyIdentifier) || '.' || base64url(Serial)¶
The structure the EST RenewalInfo object 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.¶
For example:¶
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" } }¶
[RFC7030] mistakenly declared that all content would be base64 encoded. [RFC8951] clarifies that the content is to be base64 encoded, whether or not there is a Content-Transfer-Encoding header present. It further clarifies that future extensions (such as this document) will not use base64 encoding. The response detailed above is not base64 encoded.¶
Clients MUST attempt renewal at a time of their choosing based on the suggested renewal window, obtained in the previous step.¶
The following algorithm is RECOMMENDED for choosing a renewal time:¶
Select a uniform 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 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.¶
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.).¶
The advice in [RFC9773], Section 4.3 applies:¶
Clients SHOULD fetch a certificate's RenewalInfo immediately after issuance.¶
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).¶
A very short certificate lifetime renewal time will cause clients to communicate with the EST Registrar more frequently.¶
EST connections for renewals typically make use of mutually authenticated TLS. When the client certificate being an IDevID, or the last issued certificate, often an LDevID, there is potential to disclose identities during this connection when using TLS 1.2.¶
TLS 1.3 does not suffer from this problem, and it's use is RECOMMENDED as per [I-D.ietf-uta-require-tls13]¶
Not sure what yet.¶
Might need a header allocation¶