The .crt file extension is most commonly associated with X.509 digital certificates. These files are used extensively in public key infrastructure (PKI) to verify the identity of an individual, device, or service on a network, particularly for secure communication protocols like SSL/TLS (used for HTTPS). A CRT file typically contains the public key of the entity it certifies, along with information about the certificate issuer (Certificate Authority or CA), the validity period, and the subject's identity details. While often stored in PEM (Privacy-Enhanced Mail) format, which is ASCII-encoded text, they can also be in DER (Distinguished Encoding Rules) binary format. These certificates are crucial for establishing trust in web browsers, email clients, and various server applications (like web servers) to ensure that data exchanged is encrypted and that the communicating parties are who they claim to be. They are fundamental components of secure internet infrastructure.