A .crt file is a digital security certificate used to verify the identity of a website, server, or organization and to establish an encrypted connection. These files are a critical component of the Public Key Infrastructure (PKI) and are most commonly associated with SSL/TLS security on the internet. A .crt file contains the public key of the entity it identifies, along with metadata such as the certificate's expiration date, the name of the issuing Certificate Authority (CA), and the digital signature of the issuer. When a user visits a secure website (HTTPS), the web server sends its .crt file to the browser to prove its authenticity. The browser then checks if the certificate is signed by a trusted root authority. If valid, an encrypted session is established, protecting sensitive data from interception. While .crt files can be stored in binary DER format, they are most frequently found in PEM format, which is a Base64 encoded ASCII text format. These files are essential for securing web traffic, email communications, and virtual private networks.