This document provides a gentle introduction to the data structures and formats that define the certificates used in HTTPS. It should be accessible to anyone with a little bit of computer science experience and a bit of familiarity with certificates. An HTTPS certificate is a type of file, like any other file. Its contents follow a format defined by RFC 5280. The definitions are expressed in ASN.1, which is a language used to define file formats or (equivalently) data structures.| letsencrypt.org
Let’s Encrypt has improved how we manage Online Certificate Status Protocol (OCSP) responses by deploying Redis and generating responses on-demand rather than pre-generating them, making us more reliable than ever. About OCSP Responses OCSP is used to communicate the revocation status of TLS certificates. When an ACME agent signs a request to revoke a certificate, our Let’s Encrypt Certificate Authority (CA) verifies whether or not the request is authorized and if it is, we begin publishi...| letsencrypt.org
Let’s Encrypt helps to protect a huge portion of the Web by providing TLS certificates to more than 235 million websites. A database is at the heart of how Let’s Encrypt manages certificate issuance. If this database isn’t performing well enough, it can cause API errors and timeouts for our subscribers. Database performance is the single most critical factor in our ability to scale while meeting service level objectives. In late 2020, we upgraded our database servers and we’ve been ve...| letsencrypt.org
When a certificate is no longer safe to use, you should revoke it. This can happen for a few different reasons. For instance, you might accidentally share the private key on a public website; hackers might copy the private key off of your servers; or hackers might take temporary control over your servers or your DNS configuration, and use that to validate and issue a certificate for which they hold the private key.| letsencrypt.org
Since Let’s Encrypt started issuing certificates in 2015, people have repeatedly requested the ability to get certificates for IP addresses, an option that only a few certificate authorities have offered. Until now, they’ve had to look elsewhere, because we haven’t provided that feature. Today, we’ve issued our first certificate for an IP address, as we announced we would in January. As with other new certificate features on our engineering roadmap, we’ll now start gradually rolling...| letsencrypt.org
Since its inception, Let’s Encrypt has been sending expiration notification emails to subscribers that have provided an email address to us via the ACME API. This service ended on June 4, 2025. The decision to end the service is the result of the following factors: Over the past 10 years more and more of our subscribers have been able to put reliable automation into place for certificate renewal. Providing expiration notification emails means that we have to retain millions of email address...| letsencrypt.org
The Certificate Transparency ecosystem has been improving transparency for the web PKI since 2013. It helps make clear exactly what certificates each certificate authority has issued and makes sure errors or compromises of certificate authorities are detectable. Let’s Encrypt participates in CT both as a certificate issuer and as a log operator. For the past year, we’ve also been running an experiment to help validate a next-generation design for Certificate Transparency logs.| letsencrypt.org
A profile is a collection of characteristics that describe both the validation process required to get a certificate, and the final contents of that certificate. For the vast majority of Let’s Encrypt subscribers, you should never have to worry about this: we automatically select the best profile for you, and ensure that it complies with all of the requirements and best practices that govern the Web PKI. But some people might be interested in proactively selecting a specific profile, so thi...| letsencrypt.org
Let’s Encrypt protects a vast portion of the Web by providing TLS certificates to over 550 million websites—a figure that has grown by 42% in the last year alone. We currently issue over 340,000 certificates per hour. To manage this immense traffic and maintain responsiveness under high demand, our infrastructure relies on rate limiting. In 2015, we introduced our first rate limiting system, built on MariaDB. It evolved alongside our rapidly growing service but eventually revealed its lim...| letsencrypt.org
This year we will continue to pursue our commitment to improving the security of the Web PKI by introducing the option to get certificates with six-day lifetimes (“short-lived certificates”). We will also add support for IP addresses in addition to domain names. Our longer-lived certificates, which currently have a lifetime of 90 days, will continue to be available alongside our six-day offering. Subscribers will be able to opt in to short-lived certificates via a certificate profile mech...| letsencrypt.org
We are excited to announce a new extension to Let’s Encrypt’s implementation of the ACME protocol that we are calling “profile selection.” This new feature will allow site operators and ACME clients to opt in to the next evolution of Let’s Encrypt. As of today, the staging environment is advertising a new field in its directory resource: GET /directory HTTP/1.1 HTTP/1.1 200 OK Content-Type: application/json { ... "meta": { "profiles": { "classic": "The same profile you're accustomed...| letsencrypt.org
Earlier this year we announced our intent to provide certificate revocation information exclusively via Certificate Revocation Lists (CRLs), ending support for providing certificate revocation information via the Online Certificate Status Protocol (OCSP). Today we are providing a timeline for ending OCSP services: January 30, 2025 OCSP Must-Staple requests will fail, unless the requesting account has previously issued a certificate containing the OCSP Must Staple extension May 7, 2025 Prior t...| letsencrypt.org
Today we are announcing our intent to end Online Certificate Status Protocol (OCSP) support in favor of Certificate Revocation Lists (CRLs) as soon as possible. OCSP and CRLs are both mechanisms by which CAs can communicate certificate revocation information, but CRLs have significant advantages over OCSP. Let’s Encrypt has been providing an OCSP responder since our launch nearly ten years ago. We added support for CRLs in 2022. Websites and people who visit them will not be affected by thi...| letsencrypt.org
Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. It is a service provided by the Internet Security Research Group (ISRG). We give people the digital certificates they need in order to enable HTTPS (SSL/TLS) for websites, for free, in the most user-friendly way we can. We do this because we want to create a more secure and privacy-respecting Web. You can read about our most recent year in review by downloading our annual report.| letsencrypt.org
Please note that the Let's Encrypt Growth and Let's Encrypt Certificates Issued Per Day charts are undergoing updates and may not reflect the most recent data. Let's Encrypt Growth Percentage of Web Pages Loaded by Firefox Using HTTPS (14-day moving average, source: Firefox Telemetry) Let's Encrypt Certificates Issued Per Day| letsencrypt.org
Are you an organization looking to support our work? Becoming a sponsor may be a better fit for you. Learn more. We're making it possible for everyone to experience a secure and privacy-respecting Web. We make it easy to get certificates for HTTPS, because ease of use is critical for adoption. We provide certificates free of charge, because cost excludes people. Our certificates are available in every country in the world, because the secure Web is for everyone.| letsencrypt.org
This page describes all of the current and relevant historical Certification Authorities operated by Let’s Encrypt. Note that a CA is most correctly thought of as a key and a name: any given CA may be represented by multiple certificates which all contain the same Subject and Public Key Information. In such cases, we have provided the details of all certificates which represent the CA. Root CAs Our root key material is kept safely offline.| letsencrypt.org
We highly recommend testing against our staging environment before using our production environment. This will allow you to get things right before issuing trusted certificates and reduce the chance of your running up against rate limits. The ACME URL for our ACME v2 staging environment is: https://acme-staging-v02.api.letsencrypt.org/directory If you’re using Certbot, you can use our staging environment with the --test-cert or --dry-run flag. For other ACME clients, please read their instr...| letsencrypt.org
CAA is a type of DNS record that allows site owners to specify which Certificate Authorities (CAs) are allowed to issue certificates containing their domain names. It was first standardized in 2013, and the version we use today was standardized in 2019 by RFC 8659 and RFC 8657. By default, every public CA is allowed to issue certificates for any domain name in the public DNS, provided they validate control of that domain name.| letsencrypt.org
The objective of Let’s Encrypt and the ACME protocol is to make it possible to set up an HTTPS server and have it automatically obtain a browser-trusted certificate, without any human intervention. This is accomplished by running a certificate management agent on the web server. To understand how the technology works, let’s walk through the process of setting up https://example.com/ with a certificate management agent that supports Let’s Encrypt. There are two steps to this process.| letsencrypt.org
Last updated: Nov 12, 2024 | See all Documentation Let’s Encrypt uses the ACME protocol to verify that you control a given domain name and to issue you a certificate. To get a Let’s Encrypt certificate, you’ll need to choose a piece of ACME client software to use. The ACME clients below are offered by third parties. Let’s Encrypt does not control or review third party clients and cannot make any guarantees about their safety or reliability.| letsencrypt.org
Let’s Encrypt provides rate limits to ensure fair usage by as many people as possible. We believe these rate limits are high enough to work for most people by default. We’ve also designed them so that renewing a certificate almost never hits a rate limit, and so that large organizations can gradually increase the number of certificates they can issue without requiring intervention from Let’s Encrypt. If you’re actively developing or testing a Let’s Encrypt client, please utilize our...| letsencrypt.org
When you get a certificate from Let’s Encrypt, our servers validate that you control the domain names in that certificate using “challenges,” as defined by the ACME standard. Most of the time, this validation is handled automatically by your ACME client, but if you need to make some more complex configuration decisions, it’s useful to know more about them. If you’re unsure, go with your client’s defaults or with HTTP-01.| letsencrypt.org
Update Feb 05, 2024 It’s been two years, and the Android compatibility cross-sign mentioned below is close to expiring. See our recent blog post for a detailed explanation of the changes coming over the course of 2024. Update September 30, 2021 As planned, the DST Root CA X3 cross-sign has expired, and we’re now using our own ISRG Root X1 for trust on almost all devices. For more details about the plan, keep reading!| letsencrypt.org
Let’s Encrypt issues certificates through an automated API based on the ACME protocol. In order to interact with the Let’s Encrypt API and get a certificate, a piece of software called an “ACME client” is required. No part of the process for getting a certificate happens on this website, which is merely informational. The first question to answer for people who want to get started with Let’s Encrypt is: will my hosting provider get and manage certificates from Let’s Encrypt for me...| letsencrypt.org
