Automatic Certificate Management Environment

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Automatic Certificate Management Environment - Wikipedia

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The Automatic Certificate Management Environment (ACME) protocol is a communications protocol for automating interactions between certificate authorities and their users' servers, allowing the automated deployment of public key infrastructure at very low cost.^[1]^[2] It was designed by the Internet Security Research Group (ISRG) for their Let's Encrypt service.^[1]

The protocol, based on passing JSON-formatted messages over HTTPS,^[2]^[3] has been published as an Internet Standard in RFC 8555^[4] by its own chartered IETF working group.^[5]

Client implementations

The ISRG provides free and open-source reference implementations for ACME: certbot is a Python-based implementation of server certificate management software using the ACME protocol,^[6]^[7]^[8] and boulder is a certificate authority implementation, written in Go.^[9]

Since 2015 a large variety of client options have appeared for all operating systems.^[10]

API versions

API version 1

API v1 specification was published on April 12, 2016. It supports issuing certificates for fully-qualified domain names, such as example.com or cluster.example.com, but not wildcards like *.example.com. Let's Encrypt turned off API v1 support on 1 June 2021.^[11]

API version 2

API v2 was released March 13, 2018 after being pushed back several times. ACME v2 is not backwards compatible with v1. Version 2 supports wildcard domains, such as *.example.com, allowing for many subdomains to have trusted TLS, e.g. https://cluster01.example.com, https://cluster02.example.com, https://example.com, on private networks under a single domain using a single shared "wildcard" certificate.^[12] A major new requirement in v2 is that requests for wildcard certificates require the modification of a Domain Name Service TXT record, verifying control over the domain.

Changes to ACME v2 protocol since v1 include:^[13]

The authorization/issuance flow has changed
JWS request authorization has changed
The "resource" field of JWS request bodies is replaced by a new JWS header: "url"
Directory endpoint/resource renaming
URI → URL renaming in challenge resources
Account creation and ToS agreement are combined into one step. Previously, these were two steps.
A new challenge type was implemented, TLS-ALPN-01. Two earlier challenge types, TLS-SNI-01 and TLS-SNI-02, were removed because of security issues.^[14]^[15]