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The .cer extension is Windows' preferred name for certificate files. Functionally identical to .crt, the distinction is largely a platform convention: Linux and web servers tend to use .crt, Windows tends to use .cer. The file contains the same X.509 certificate data either way.
On Windows, .cer files are DER-encoded by default (binary format). On Linux, they're more often PEM-encoded (Base64 text). Windows double-clicks open the Certificate dialog automatically, showing issuer, subject, validity, and the certificate chain. This native integration makes .cer the natural choice for Windows certificate management.
Converting between encodings: `openssl x509 -in cert.cer -inform der -out cert.pem -outform pem` converts DER to PEM. The reverse: `openssl x509 -in cert.pem -outform der -out cert.cer`. The cryptographic data is identical — only the encoding differs.