How to convert .crt to .pem

If you'll be converting .crt to .pem, you may use the ff commands

openssl x509 -in server.crt -out server.der -outform DER
openssl x509 -in server.der -inform DER -out server.pem -outform PEM

Verify first if your key is in PEM format. Mostly, they begin in --BEGIN and can read with a text editor. If you'll be converting .key to .pem, just replace openssl x509 with openssl rsa. Normally, server.key is your private key and server.crt is your signed or returned certificate.