Convert Let’s Encrypt SSL certificates from pem to crt/key

openssl rsa -in <path_to_privkey.pem> -pubout > key.pub

openssl rsa -outform der -in privkey.pem -out privkey.key 

openssl x509 -outform der -in fullchain.pem -out fullchain.crt 

openssl x509 -outform der -in cert.pem -out cert.crt

openssl x509 -in cert.pem -outform pem -outform der -out cert.cer