Certificate Renewal Recommendations for Enrollment over Secure Transport
draft-ietf-lamps-est-renewal-info-00

LAMPS Working Group                                       R. Shekh-Yusef
Internet-Draft                                                     Ciena
Intended status: Standards Track                           M. Richardson
Expires: 16 August 2026                         Sandelman Software Works
                                                            M. Ounsworth
                                                         Entrust Limited
                                                        12 February 2026

Certificate Renewal Recommendations for Enrollment over Secure Transport
                  draft-ietf-lamps-est-renewal-info-00

Abstract

   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.

About This Document

   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-ietf-lamps-est-renewal-info/.

   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/lamps-wg/lamps-est-renewal-info.

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/.

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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 16 August 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Protocol Details  . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Renewal Information Request . . . . . . . . . . . . . . .   3
     3.2.  Renewal Information Response  . . . . . . . . . . . . . .   4
       3.2.1.  Base64 Not Used . . . . . . . . . . . . . . . . . . .   4
   4.  Renewal Operations  . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Fetching Schedule . . . . . . . . . . . . . . . . . . . .   5
   5.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   5
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   9.  Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   6
     10.2.  Informative References . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

   [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.

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   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].

2.  Terminology

   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.  Protocol Details

3.1.  Renewal Information Request

   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)

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3.2.  Renewal Information Response

   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"
         }
       }

3.2.1.  Base64 Not Used

   [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.

4.  Renewal Operations

   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:

   1.  Select a uniform 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.

   4.  Otherwise, if the selected time is before the next time that the
       client would wake up normally, attempt renewal immediately.

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   5.  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.).

4.1.  Fetching Schedule

   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).

5.  Privacy Considerations

   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]

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6.  Security Considerations

   Not sure what yet.

7.  IANA Considerations

   Might need a header allocation

8.  Acknowledgements

   Many bits of text are taken from [RFC9773].

9.  Changelog

10.  References

10.1.  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>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [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>.

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   [RFC8951]  Richardson, M., Werner, T., and W. Pan, "Clarification of
              Enrollment over Secure Transport (EST): Transfer Encodings
              and ASN.1", RFC 8951, DOI 10.17487/RFC8951, November 2020,
              <https://www.rfc-editor.org/info/rfc8951>.

10.2.  Informative References

   [I-D.ietf-uta-require-tls13]
              Salz, R. and N. Aviram, "New Protocols Using TLS Must
              Require TLS 1.3", Work in Progress, Internet-Draft, draft-
              ietf-uta-require-tls13-12, 14 April 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-uta-
              require-tls13-12>.

   [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>.

   [RFC9773]  Gable, A., "ACME Renewal Information (ARI) Extension",
              RFC 9773, DOI 10.17487/RFC9773, June 2025,
              <https://www.rfc-editor.org/info/rfc9773>.

Authors' Addresses

   Rifaat Shekh-Yusef
   Ciena
   Email: rifaat.s.ietf@gmail.com

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

   Mike Ounsworth
   Entrust Limited
   Email: mike.ounsworth@entrust.com

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