在 PEM、DER、PKCS#7 與 PFX 格式間轉換
# Certificate: PEM to DER
openssl x509 -in cert.pem -outform DER -out cert.der
# Certificate: DER to PEM
openssl x509 -in cert.der -inform DER -out cert.pem
# Private Key: PEM to DER
openssl pkcs8 -in key.pem -outform DER -out key.der -nocrypt