Let’s Encrypt is a free, automated, and open certificate authority brought to you by the nonprofit Internet Security Research Group (ISRG).
What is a certificate
A certificate or digital certificate is a unique, digitally signed document which authoritatively identifies the identity of an individual or organization. Using public key cryptography, its authenticity can be verified to ensure that the software or website you are using is legitimate. On the Internet, a certificate is signed by a trusted CA (certificate authority), and verified with the authority’s public key. The decrypted certificate contains a verified public key of the certificate holder (website operator), with which encrypted HTTPS communications can be established.
An operating system or web browser may alert the user when loading software or a website whose digital certificate is not verified by a trusted CA.
What is SSL (TLS)
Transport Layer Security (TLS), and its now-deprecated predecessor, Secure Sockets Layer (SSL), are cryptographic protocols designed to provide communications security over a computer network. Several versions of the protocols find widespread use in applications such as web browsing, email, instant messaging, and voice over IP (VoIP). Websites can use TLS to secure all communications between their servers and web browsers.
The TLS protocol aims primarily to provide privacy and data integrity between two or more communicating computer applications.:3 When secured by TLS, connections between a client (e.g., a web browser) and a server (e.g., wikipedia.org) should have one or more of the following properties:
- The connection is private (or secure) because symmetric cryptography is used to encrypt the data transmitted. The keys for this symmetric encryption are generated uniquely for each connection and are based on a shared secret that was negotiated at the start of the session (see § TLS handshake). The server and client negotiate the details of which encryption algorithm and cryptographic keys to use before the first byte of data is transmitted (see § Algorithm below). The negotiation of a shared secret is both secure (the negotiated secret is unavailable to eavesdroppers and cannot be obtained, even by an attacker who places themselves in the middle of the connection) and reliable (no attacker can modify the communications during the negotiation without being detected).
- The identity of the communicating parties can be authenticated using public-key cryptography. This authentication can be made optional, but is generally required for at least one of the parties (typically the server).
- The connection is reliable because each message transmitted includes a message integrity check using a message authentication code to prevent undetected loss or alteration of the data during transmission.
In addition to the properties above, careful configuration of TLS can provide additional privacy-related properties such as forward secrecy, ensuring that any future disclosure of encryption keys cannot be used to decrypt any TLS communications recorded in the past.
TLS supports many different methods for exchanging keys, encrypting data, and authenticating message integrity (see § Algorithm below). As a result, secure configuration of TLS involves many configurable parameters, and not all choices provide all of the privacy-related properties described in the list above (see the § Key exchange (authentication), § Cipher security, and § Data integrity tables).
Attempts have been made to subvert aspects of the communications security that TLS seeks to provide, and the protocol has been revised several times to address these security threats (see § Security). Developers of web browsers have also revised their products to defend against potential security weaknesses after these were discovered (see TLS/SSL support history of web browsers).
The TLS protocol comprises two layers: the TLS record and the TLS handshake protocols.
TLS is a proposed Internet Engineering Task Force (IETF) standard, first defined in 1999, and the current version is TLS 1.3 defined in RFC 8446 (August 2018). TLS builds on the earlier SSL specifications (1994, 1995, 1996) developed by Netscape Communications for adding the HTTPS protocol to their Navigator web browser.