How can I convert a certificate file from .crt to .cer?

A valid CA certificate can be imported to the SonicWall security appliance. You can use a certificate signed and verified by a third party CA. SonicWall will support only to import PKCS#7 (.p7b), PEM (.pem) or DER (.der or .cer) encoded file. This article illustrates the steps to convert the certificate file format from .crt to .cer.
 

• .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 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.