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.
Double-click on the *.crt file to open it into the certificate display.
Select the Details tab, then select the Copy to file option.
Choose next on the Certificate Wizard.
Select Base-64 encoded X.509 (.CER) in the File format window, then Next.
Select Browse (to locate a destination) and type in the filename.
Choose Next, then the certificate file with the format .cer will be saved in the selected destination.