.CRT = The extension CRT is used for certificates. The certificate can be DER encoded or PEM encoded. The extensions CER and CRT are almost synonymous. This situation is common in various unix/linux systems.

CER = Microsoft version of CRT certificate. You can use Microsoft tools to convert CRT files to CER files (CRT and CER must be the same encoding, DER or PEM). Files with the extension CER can be recognized by IE and used as commands to call Microsoft's cryptoAPI (specifically, rudll32.exe

Extension information

There are several certificate extensions in Windows , such as .cer and .crt. Generally speaking, .cer files are binary data, and .crt files contain ASCII data.

The cer file contains certificates encoded according to DER (Distinguished Encoding Rules) rules. Data, this is the encoding format specified in the x.690 standard.

X.509 is the most basic public key format standard, which stipulates the various information that the certificate needs to contain. Certificates are all in this format, which contain the public key, the publisher's digital signature, validity period, etc. It should be emphasized that it only does not contain the private key. The relevant formats are: DER, PEM, CER, CRT.

Reference: Baidu Encyclopedia - cer