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]

Contents

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]

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

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. [15]

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. [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]

See also

References

  1. 1 2 Steven J. Vaughan-Nichols (9 April 2015). "Securing the web once and for all: The Let's Encrypt Project". ZDNet.
  2. 1 2 sh. "ietf-wg-acme/acme-spec". GitHub . Retrieved 2017-04-05.
  3. ↑ Chris Brook (18 November 2014). "EFF, Others Plan to Make Encrypting the Web Easier in 2015". ThreatPost.
  4. ↑ Barnes, R.; Hoffman-Andrews, J.; McCarney, D.; Kasten, J. (2019-03-12). Automatic Certificate Management Environment (ACME). IETF. doi: 10.17487/RFC8555 . RFC 8555 . Retrieved 2019-03-13.
  5. ↑ "Automated Certificate Management Environment (acme)". IETF Datatracker. Retrieved 2019-03-12.
  6. ↑ "Certbot". EFF . Retrieved 2016-08-14.
  7. ↑ "certbot/certbot". GitHub . Retrieved 2016-06-02.
  8. ↑ "Announcing Certbot: EFF's Client for Let's Encrypt". LWN. 2016-05-13. Retrieved 2016-06-02.
  9. ↑ "letsencrypt/boulder". GitHub . Retrieved 2015-06-22.
  10. ↑ "ACME Client Implementations - Let's Encrypt - Free SSL/TLS Certificates". letsencrypt.org. 20 February 2025.
  11. ↑ Warren, Brad (7 March 2024). "Should Caddy and Traefik Replace Certbot?". Electronic Frontier Foundation. Retrieved 16 September 2025.
  12. ↑ "mod_md - Apache HTTP Server Version 2.4". httpd.apache.org. Archived from the original on 2025-09-25. Retrieved 2025-10-07.
  13. ↑ "NGINX Introduces Native Support for ACME Protocol – NGINX Community Blog". 12 August 2025. Retrieved 14 September 2025.
  14. ↑ "Native ACME Support Comes to NGINX". Let's Encrypt . 11 September 2025. Retrieved 14 September 2025.
  15. ↑ "End of Life Plan for ACMEv1 - API Announcements". Let's Encrypt Community Support. 2021-05-05. Retrieved 2021-06-12.
  16. ↑ "ACME v2 API Endpoint Coming January 2018 - Let's Encrypt - Free SSL/TLS Certificates". letsencrypt.org. 14 June 2017.
  17. ↑ "Staging endpoint for ACME v2". Let's Encrypt Community Support. January 5, 2018.
  18. ↑ "Challenge Types - Let's Encrypt Documentation". Let's Encrypt . 2020-12-08. Retrieved 2021-05-12.
  19. ↑ Barnes, R.; Hoffman-Andrews, J.; McCarney, D.; Kasten, J. (2019-03-12). Automatic Certificate Management Environment (ACME). IETF. doi: 10.17487/RFC8555 . RFC 8555 . Retrieved 2021-05-12. The values "tls-sni-01" and "tls-sni-02" are reserved because they were used in pre-RFC versions of this specification to denote validation methods that were removed because they were found not to be secure in some cases.
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