By Aaron Gable · September 16, 2025

Let’s Encrypt has been proud to work with the IETF to maintain ACME as an open standard since we first developed the technology a decade ago. We’re happy to announce that IETF has published our latest addition to the ACME protocol, ACME Renewal Information (ARI), as RFC 9773. ARI helps keep the renewal process reliable during unexpected events affecting certificate validity.

Since the ACME protocol was first published as RFC 8555, the IETF ACME working group has remained active, defining various extensions to the original ACME protocol, initiated either by Let’s Encrypt or by colleagues from other organizations. For example, ACME WG documents have specified how to validate kinds of identifiers other than domain names, making it possible to use ACME to issue certificates for IP addresses, or even in PKIs other than the web PKI.

The publication of RFC 9773 is the culmination of a process that began in September 2021 with the first ARI draft. Along the way, numerous colleagues from Let’s Encrypt and elsewhere (thanked individually at the end of this post) contributed to the ARI specification and helped improve it.

Why implement ARI?

This is a good opportunity to remind our community about ARI and how implementing it can help users. If you’re an ACME client user, you may want to check the documentation for your client to see if it has implemented ARI yet. New functionality like this is a great reason to make sure you’re using up-to-date ACME client software. If you’re a client developer, questions about ARI implementation are welcome in the Community Forum’s Client Dev category.

Sometimes certificate authorities, including Let’s Encrypt, may perform mass revocations of an entire group or category of certificates. This most often happens when someone discovers that a certificate authority has made a mistake in how it validates or issues certificates, or has made a misstatement in how it describes its policies and procedures. In this case, the CA is required to revoke the affected certificates. This may happen through absolutely no fault of the subscribers. For example, in January 2022, we had to revoke approximately two million certificates due to a technical error in our validation processes.

When we have to revoke certificates, we want to make sure that the websites using those certificates don’t experience issues. That means those websites need to re-request issuance and install new certificates. Since CAs are sometimes required to revoke certificates on a 24 hour timeline or a 5 day timeline (depending on the nature of the incident), a process that relies on manual intervention from system administrators won’t reach most websites in time.

ARI allows a certificate authority to advise a client to perform an early renewal of a certificate that the client would have anticipated did not need to be renewed yet, broadly because the CA knows that an early renewal is helpful, or necessary, in particular circumstances. In the mass revocation scenario, this allows ARI-aware clients to avoid outages due to certificate invalidity, because they can replace their certificates even before the revocation occurs.

Of course, we and other certificate authorities work diligently to prevent mass revocation events. We’re encouraging ARI implementation as a form of emergency preparedness that can significantly mitigate the impact of this kind of problem, if and when it happens.

ARI also provides features to reduce the impact of load spikes where too many clients request certificates in a short period of time. Let’s Encrypt doesn’t need to use ARI for this today, because other improvements in popular clients’ renewal practices have already sufficiently smoothed out our load spikes. Even so, this will be a valuable ability for all ACME CAs to have available in the long term to better manage emergencies and disruptions.

On the server side, we added support for the ARI draft specification to our Boulder CA software in late 2021, so the Let’s Encrypt CA has supported ARI for some time. If you are implementing ARI in your own client, the Pebble ACME test-bed also supports ARI so you can test against that implementation.

Thanks

Thanks to all of the people who contributed to this process at the ACME WG and elsewhere, including:  Roland Shoemaker and Jacob Hoffman-Andrews for coming up with the initial idea of ARI and for helping me learn the IETF process; Samantha Frank, Matt Holt, Ilari Liusvaara, and Wouter Tinus for contributing client implementations; Freddy Zhang for contributing an independent server implementation; and Rob Stradling, Andrew Ayer, and J.C. Jones for providing meaningful feedback and suggestions that significantly improved this specification.

Finally, our congratulations also to Q Misell for the recent publication of RFC 9799, another ACME WG document that went through the standards process alongside ARI.