Be sure to visit the SSL. Certificates and Encodings At its core an X. 509 certificate usually refers to the IETF’s PKIX Certificate and Binary vs digital option Profile of the X.
X509 File Extensions The first thing we have to understand is what each type of file extension is. There is a lot of confusion about what DER, PEM, CRT, and CER are and many have incorrectly said that they are all interchangeable. While in certain cases some can be interchanged the best practice is to identify how your certificate is encoded and then label it correctly. The DER extension is used for binary DER encoded certificates.