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A .crt file contains an X.509 digital certificate — the cryptographic document that proves a server's identity for HTTPS connections. When your browser shows a padlock icon, it verified the server's .crt certificate against a chain of trusted certificate authorities.
The extension is a convention, not a format specification. A .crt file is usually PEM-encoded (Base64 text, readable in a text editor) but can sometimes be DER-encoded (binary). Check the file: if you can read it and see BEGIN CERTIFICATE, it's PEM. If it's binary gibberish, it's DER.
You'll encounter .crt files when setting up SSL/TLS on web servers (Apache, Nginx), configuring email encryption, or managing certificate infrastructure. Certificate authorities deliver certificates as .crt files. To inspect a certificate: `openssl x509 -in server.crt -text -noout` shows the issuer, validity period, and subject details.