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Cert types

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.  Correctly labeled certificates will be much easier to manipulat

Encodings (also used as extensions)

  • .DER = The DER extension is used for binary DER encoded certificates. These files may also bear the CER or the CRT extension.   Proper English usage would be “I have a DER encoded certificate” not “I have a DER certificate”.
  • .PEM = The PEM extension is used for different types of X.509v3 files which contain ASCII (Base64) armored data prefixed with a “—– BEGIN …” line.

Common Extensions

  • .CRT = The CRT extension is used for certificates. The certificates may be encoded as binary DER or as ASCII PEM. The CER and CRT extensions are nearly synonymous.  Most common among *nix systems
  • CER = alternate form of .crt (Microsoft Convention) You can use MS to convert .crt to .cer (.both DER encoded .cer, or base64[PEM] encoded .cer)  The .cer file extension is also recognized by IE as a command to run a MS cryptoAPI command (specifically rundll32.exe cryptext.dll,CryptExtOpenCER) which displays a dialogue for importing and/or viewing certificate contents.
  • .KEY = The KEY extension is used both for public and private PKCS#8 keys. The keys may be encoded as binary DER or as ASCII PEM.

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)