science

Let's Encrypt

Let's Encrypt is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Let's Encrypt rather than just read about it. In short: Let's Encrypt is a non-profit certificate authority run by the Internet Security Research Group (ISRG) that provides X.509 certificates for Transport Layer Security (TLS) encryption without charging fees. It is the world's largest certificate authority, used by more than 700 million websites, with the goal of creating a more secure and privacy-respecting web through the widespread adoption of HTTPS.

Key takeaways

Let's Encrypt belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Let's Encrypt to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Let's Encrypt from memory before moving on to harder problems.

Reference excerpt

Let's Encrypt is a non-profit certificate authority run by the Internet Security Research Group (ISRG) that provides X.509 certificates for Transport Layer Security (TLS) encryption without charging fees. It is the world's largest certificate authority, used by more than 700 million websites, with the goal of creating a more secure and privacy-respecting web through the widespread adoption of HTTPS. The Internet Security Research Group, the provider of the service, is a public benefit organization. Major sponsors of the ISRG include the Electronic Frontier Foundation (EFF), the Mozilla Foundation, OVHcloud, Cisco Systems, Facebook, Google Chrome, the Internet Society, AWS, nginx, and the Gates Foundation. Other partners include the certificate authority IdenTrust, the University of Michigan, and the Linux Foundation.

Overview

The mission of the organization is to create a more secure and privacy-respecting World Wide Web by promoting the widespread adoption of HTTPS. Let's Encrypt certificates are valid for 90 days by default, during which renewal can take place at any time. Optionally, certificates can be issued which are valid for 45 days (tlsserver profile) and 6 days (shortlived profile). This is handled by an automated process designed to overcome manual creation, validation, signing, installation, and renewal of certificates for secure websites. The project claims its goal is to make encrypted connections to World Wide Web servers ubiquitous. By eliminating payment, web server configuration, validation email management and certificate renewal tasks, it is meant to significantly lower the complexity of setting up and maintaining TLS encryption. On a Linux web server, execution of only two commands is sufficient to set up HTTPS encryption and acquire and install certificates. To that end, a software package was included into the official Debian and Ubuntu software repositories. Current initiatives of major browser developers such as Mozilla and Google to deprecate unencrypted HTTP are counting on the availability of Let's Encrypt. The project is acknowledged to have the potential to accomplish encrypted connections as the default case for the entire Web. The service only issues domain-validated certificates, since they can be fully automated. Organization Validation and Extended Validation Certificates both require human validation of any registrants, and are therefore not offered by Let's Encrypt. Support of ACME v2 and wildcard certificates was added in March 2018. The domain validation (DV) utilized by Let's Encrypt dates back to 2002 and was at first controversial when introduced by GeoTrust before becoming a widely accepted method for the issuance of SSL certificates. By being as transparent as possible, the organization hopes to both protect its own trustworthiness and guard against attacks and manipulation attempts. For that purpose it regularly publishes transparency reports, publicly logs all ACME transactions (e.g. by using Certificate Transparency), and uses open standards and free software as much as possible.

History The Let's Encrypt project was started in 2012 by two Mozilla employees, Josh Aas and Eric Rescorla, together with Peter Eckersley at the Electronic Frontier Foundation and J. Alex Halderman at the University of Michigan. Internet Security Research Group, the company behind Let's Encrypt, was incorporated in May 2013. Let's Encrypt was announced publicly on November 18, 2014. On January 28, 2015, the ACME protocol was officially submitted to the IETF for standardization. On April 9, 2015, the ISRG and the Linux Foundation declared their collaboration. The root and intermediate certificates were generated in the beginning of June. On June 16, 2015, the final launch schedule for the service was announced, with the first certificate expected to be issued sometime in the week of July 27, 2015, followed by a limited issuance period to test security and scalability. General availability of the service was originally planned to begin sometime in the week of September 14, 2015. On August 7, 2015, the launch schedule was amended to provide more time for ensuring system security and stability, with the first certificate to be issued in the week of September 7, 2015 followed by general availability in the week of November 16, 2015. On September 14, 2015, Let's Encrypt issued its first certificate, which was for the domain helloworld.letsencrypt.org. On the same day, ISRG submitted its root program applications to Mozilla, Microsoft, Google and Apple. On October 19, 2015, the intermediate certificates became cross-signed by IdenTrust, causing all certificates issued by Let's Encrypt to be trusted by all major browsers. On November 12, 2015, Let's Encrypt announced that general availability would be pushed back and that the first public beta would commence on December 3, 2015. The public beta ran from December 3, 2015 to April 12, 2016. It launched on April 12, 2016. On March 3, 2020, Let's Encrypt announced that it would have to revoke over 3 million certificates on March 4, due to a flaw in its Certificate Authority software. Through working with software vendors and contacting site operators, Let's Encrypt was able to get 1.7 million of the affected certificates renewed before the deadline. They ultimately decided not to revoke the remaining affected certificates, as the security risk was low and the certificates were to expire within the next 90 days. The mass-revocation event has significantly increased the global revocation rate. In March 2020, Let's Encrypt was awarded the Free Software Foundation's annual Award for Projects of Social Benefit. On February 27, 2020, Let's Encrypt announced having issued a billion certificates. In April 2022, Let's Encrypt was awarded the Levchin Prize for “fundamental improvements to the certificate ecosystem that provide free certificates for all”. As of 2025, Let's Encrypt serves more than 700 million websites worldwide, making it the largest certificate authority in the world. In January 2025, Let's Encrypt announced the retirement of its free email expiry notifications and recommended Red Sift Certificates Lite as its certificate monitoring service.

Technology

Chain of trust

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Let's Encrypt

Start with the simplest possible case. Write down what Let's Encrypt claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Let's Encrypt before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Let's Encrypt ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Let's Encrypt

In research: Let's Encrypt appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Let's Encrypt in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Let's Encrypt is common in secondary-school and first-year university syllabi. It links to neighbouring topics Certificate authorities, Internet properties established in 2014, Linux Foundation projects, so understanding it makes those chapters shorter.
In everyday life: Look for Let's Encrypt outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Let's Encrypt in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Let's Encrypt means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Let's Encrypt out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Let's Encrypt in simple terms?

Why does Let's Encrypt matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Let's Encrypt?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Let's Encrypt.