What is DER Format?
DER files are digital certificates in binary format, instead of the instead of the ASCII PEM format.
DER files may end with .der or .cer, so to differentiate between DER.cer and PEM.cer files, you may need to use a text editor to read the file. A DER file should not have any BEGIN/END statements and will show garbled binary content.
Both digital certificates and private keys can be encoded in DER format.
DER is often used with Java platforms.