|Founder||Electronic Frontier Foundation
University of Michigan
|Headquarters||San Francisco, California, U.S.|
|Services||X.509 certificate authority|
|Internet Security Research Group|
Let's Encrypt is a certificate authority that launched on April 12, 2016 that provides free X.509 certificates for Transport Layer Security (TLS) encryption via an automated process designed to eliminate the current complex process of manual creation, validation, signing, installation, and renewal of certificates for secure websites.
The project aims 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.
By being as transparent as possible, they hope to both protect their own trustworthiness and guard against attacks and manipulation attempts. For that purpose they regularly publish transparency reports, publicly log all ACME transactions (e.g. by using Certificate Transparency), and use open standards and free software as much as possible.
Let's Encrypt is a service provided by the Internet Security Research Group (ISRG), a public benefit organization. Major sponsors are the Electronic Frontier Foundation (EFF), the Mozilla Foundation, OVH, Akamai, and Cisco Systems. Other partners include the certificate authority IdenTrust, the University of Michigan (U-M), the Stanford Law School, the Linux Foundation as well as Stephen Kent from Raytheon/BBN Technologies and Alex Polvi from CoreOS.
Technical Advisory Board
- Rich Salz (Akamai Technologies)
- Joe Hildebrand (Cisco Systems)
- Jacob Hoffman-Andrews (Electronic Frontier Foundation)
- J. C. Jones (Mozilla Foundation)
- Russ Housley (Independent)
- Ryan Hurst (Google)
- Stephen Kent (Independent)
- Karen O'Donoghue (Internet Society)
In June 2015, Let's Encrypt generated an RSA root certificate with its private key stored on a hardware security module which is kept offline. The root certificate is used to sign two intermediate certificates which are cross-signed by the certificate authority IdenTrust. One of the intermediate certificates is used to sign issued certificates, while the other is kept offline as a backup in case of problems with the first intermediate certificate. Because the IdenTrust certificate is preinstalled in major web browsers, Let's Encrypt certificates can normally be validated and are accepted upon installation, out of the box, even while no browser vendors include the ISRG root certificate as a trust anchor.
The challenge–response protocol used to automate enrolling with this new certificate authority is called Automated Certificate Management Environment (ACME). It involves various requests to the web server on the domain that is covered by the certificate. Based on whether the resulting responses match the expectations, control of the enrollee over the domain is assured (domain validation). In order to do that, the ACME client software sets up a special TLS server on the server system that gets queried by the ACME certificate authority server with special requests using Server Name Indication (Domain Validation using Server Name Indication, DVSNI).
The validation processes are run multiple times over separate network paths. Checking DNS entries is provisioned to be done from multiple geographically diverse locations to make DNS spoofing attacks harder to do.
ACME interactions are based on exchanging JSON documents over HTTPS connections. A draft specification is available on GitHub, and a version has been submitted to the Internet Engineering Task Force (IETF) as a proposal for an Internet standard.
The certificate authority consists of a piece of software called Boulder, written in Go, that implements the server side of the ACME protocol. It is published as free software with source code under the terms of version 2 of the Mozilla Public License (MPL). It provides a RESTful API that can be accessed over a TLS-encrypted channel.
An Apache-licensed Python certificate management program called certbot (formerly letsencrypt) gets installed on the client side (the web server of an enrollee). This is used to order the certificate, to conduct the domain validation process, to install the certificate, to configure the HTTPS encryption in the HTTP server, and later to regularly renew the certificate. After installation and agreeing to the user license, executing a single command is enough to get a valid certificate installed. Additional options like OCSP stapling or HTTP Strict Transport Security (HSTS) can also be enabled. Automatic setup initially only works with Apache and nginx.
Let's Encrypt issues certificates valid for 90 days. Their reason is that these certificates "limit damage from key compromise and mis-issuance" and encourage automation. The official certbot client and most of the third-party clients allow automation of the certificate renewal.
Several third-party client implementations in several languages were created by the community.
History and schedule
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 standardisation. 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. The cross-signature from IdenTrust is planned to be available when Let's Encrypt opens for the public.
On September 14, 2015, Let's Encrypt issued its first certificate, which was for the domain helloworld
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 December 3, 2015, Let's Encrypt announced commencement of the public beta.
On March 8, 2016, Let's Encrypt issued its millionth certificate after seven months of existence.
On April 12, 2016, Let's Encrypt left Beta.
On April 21, 2016, 44 days after issuing its millionth certificate, Let's Encrypt issued its 2 millionth certificate. By June 3, 2016, Let's Encrypt issued over 4 million certificates. As of June 22, 2016, Let's Encrypt has issued over 5 million certificates, of which 3.8 million are unexpired and unrevoked. Their active certificates cover more than 7 million unique domains, in part due to support by large hosting companies. On September 9, 2016, they had issued over 10 million certificates, by November 27, 2016 they had issued over 20 million, and by December 2016, 24 million. On June 28, 2017, they announced that they've issued 100 million certificates. 
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- "Milestone: 100 Million Certificates Issued - Let's Encrypt - Free SSL/TLS Certificates". letsencrypt.org. Retrieved 2017-07-02.
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