Certificate file types

Two types of encoding used in certificate

1. DER(Distinguished Encoding Rule) - DER encoded certificate.
2. PEM - Base64 encoded certificate - starts with BEGIN CERTIFICATE and ends with END CERTIFICATE.

Above can also used and extensions.

Common extensions
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1. CRT - Most common among *nix systems
2. CER - alternate form of .CRT (Microsoft Convention)

The only time CRT and CER can safely be interchanged is when the encoding type can be identical. (ie PEM encoded CRT = PEM encoded CER)