Let’s Encrypt: Free SSL certificate for all sites

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.

The key principles behind Let’s Encrypt are:

• Free: Anyone who owns a domain name can use Let’s Encrypt to obtain a trusted certificate at zero cost.
• Automatic: Software running on a web server can interact with Let’s Encrypt to painlessly obtain a certificate, securely configure it for use, and automatically take care of renewal.
• Secure: Let’s Encrypt will serve as a platform for advancing TLS security best practices, both on the CA side and by helping site operators properly secure their servers.
• Transparent: All certificates issued or revoked will be publicly recorded and available for anyone to inspect.
• Open: The automatic issuance and renewal protocol is published as an open standard that others can adopt.
• Cooperative: Much like the underlying Internet protocols themselves, Let’s Encrypt is a joint effort to benefit the community, beyond the control of any one organization.

How it works

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. First, the agent proves to the CA that the web server controls a domain. Then, the agent can request, renew, and revoke certificates for that domain.

Domain Validation for Let's Encrypt SSL Certificates

Let’s Encrypt identifies the server administrator by public key. The first time the agent software interacts with Let’s Encrypt, it generates a new key pair and proves to the Let’s Encrypt CA that the server controls one or more domains. This is similar to the traditional CA process of creating an account and adding domains to that account.

To kick off the process, the agent asks the Let’s Encrypt CA what it needs to do in order to prove that it controls example.com. The Let’s Encrypt CA will look at the domain name being requested and issue one or more sets of challenges. These are different ways that the agent can prove control of the domain. For example, the CA might give the agent a choice of either:

• Provisioning a DNS record under example.com, or
• Provisioning an HTTP resource under a well-known URI on http://example.com/

Along with the challenges, the Let’s Encrypt CA also provides a nonce that the agent must sign with its private key pair to prove that it controls the key pair.

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