DER encoded RSA private key
From Just Solve the File Format Problem
DER encoded RSA private key is a private key format that stores the same information as PEM encoded RSA private key, but encoded in DER format instead of PEM format.
Examples
To view the contents of a file:
openssl rsa -noout -text -inform DER -in example-der.key
To convert to PEM encoded RSA private key format:
openssl rsa -inform DER -in example-der.key -out example-pem.key
