By default, OpenSSL files are created in PEM (Privacy Enhanced Mail) format. SSL files that are created in Windows operating environments are created in DER (Distinguished Encoding Rules) format.
Under Windows, you can import a file that is created in either PEM or DER format. However, a digital certificate that is created in DER format must be converted to PEM format before it can be included in a trust list under UNIX.
An example of converting a server digital certificate from PEM input format to DER output format follows :
OpenSSL> x509 -inform PEM -outform DER -in server.pem -out server.der
An example of converting a server digital certificate from DER input format to PEM output format follows:
OpenSSL> x509 -inform DER -outform PEM -in server.der -out server.pem