To convert a certificate from PEM to DER:
<code>
openssl x509 -in input.crt -inform PEM -out output.crt -outform DER
</code>

To convert a certificate from DER to PEM:
<code>
openssl x509 -in input.crt -inform DER -out output.crt -outform PEM
</code>

To convert a key from PEM to DER:
<code>
openssl rsa -in input.key -inform PEM -out output.key -outform DER
</code>

To convert a key from DER to PEM:
<code>
openssl rsa -in input.key -inform DER -out output.key -outform PEM
</code>