To convert a certificate from PEM to DER:

openssl x509 -in input.crt -inform PEM -out output.crt -outform DER

To convert a certificate from DER to PEM:

openssl x509 -in input.crt -inform DER -out output.crt -outform PEM

To convert a key from PEM to DER:

openssl rsa -in input.key -inform PEM -out output.key -outform DER

To convert a key from DER to PEM:

openssl rsa -in input.key -inform DER -out output.key -outform PEM