SSL/TLS X.509 certificates are digital files that are used for Secure Sockets Layer (SSL) or Transport Layer Security (TLS). An SSL/TLS certificate is one of the most popular types of X.509 certificates, or a type of public key certificate which uses the X.509 standard. X.509 certificates contain a public key and the identity of a hostname, organization, or individual.
The SSL/TLS certificate fulfills two functions as machine identities: Authentication and Data Encryption.
An SSL/TLS certificate is most reliable when issued by a trusted Certificate Authority (CA). The CA has to follow very strict rules and policies about who may or may not receive an SSL Certificate. So, when you have a valid SSL Certificate from a trusted CA, there is a higher degree of trust. When a certificate authority (CA) signs them or another entity validates them, the owner of that certificate can leverage the public key to establish secure connections with another party or validate documents someone digitally signed using the corresponding private key.
Some of X.509 SSL/TLS certificates are self-signed. And these certificates will not be trusted for public-facing applications. Because of this, they are mainly used to encrypt and authenticate data within an organization’s network.
SSL/TLS certificates are X.509 certificates with Extended Key Usage: Server Authentication (22.214.171.124.126.96.36.199.1). The "Extended Key Usage" extension lists the "roles" for the entity that uses the certificate. In other words, an entity must use SSL/TLS certificates only for server authentication and nothing else. Otherwise, that entity risks violating the issuing CA's policies.
There are also other common types of X.509 certificates, like Client Authentication (188.8.131.52.184.108.40.206.2) and Code Signing (220.127.116.11.18.104.22.168.3). These files form the basis of encryption and authentication schemes.
As SSL/TLS certificates enable encryption, they are integral to Hyper Text Transfer Protocol Secure(HTTPS), a protocol which encrypts all communication exchanged between a website and your browser.
A standard web site without SSL/TLS security displays “HTTP” at the beginning of the web site address in the browser address bar. This stands for “Hypertext Transfer Protocol,” and is the conventional way to transmit information over the Internet. On the other hand, a web site that is secured with an SSL Certificate will have “HTTPS” before the address. This stands for “Hypertext Transfer Protocol Secure”.
Every browser has a slightly different way of displaying secure connections. But for all of them, you can check that a website you're visiting is using HTTPS by looking for "HTTPS" in the address bar.
Some browsers may also feature a padlock symbol next to the website's address. If you click on that symbol, your web browser should display the name of the organization that owns the SSL/TLS certificate. That symbol turns green when your web browser detects an Extended Validation (EV) SSL certificate. If the information does not match, or the certificate has expired, the browser displays an error message or warning. In addition, many browsers now flag all sites using HTTP as insecure.
If the certificate has expired, the web browser will display an error message or warning. These alerts could lead a visitor to navigate away from a website. To prevent this from happening, organizations that own websites and use HTTPS need to manage their certificates and make sure the ones they want to keep don't expire.
Read more about how to check SSL/TLS certificates.