Automatic Certificate Management Environment

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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

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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]

Web servers like Caddy, Traefik Proxy,[11] Nginx (starting in August, 2025), and Apache HTTP Server[12] (2.4.30 and later) have built in support for automatically acquiring a TLS certificate using the ACME protocol.[13][14]

API versions

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API version 1

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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.[15]

API version 2

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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.[16] 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:[17]

  • 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.[18][19]

See also

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References

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  15. ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
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  19. ^ Lua error in Module:Citation/CS1/Configuration at line 2172: attempt to index field '?' (a nil value).
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