Tableau Server SSL requires one key file (.key), one certificate (.crt), and sometimes a certificate chain file (also .crt extension). You may not have the private key file you need.
You can view the encrypted file contents directly in a text editor, look for either
-----BEGIN RSA PRIVATE KEY-----
You can also use the openssl command available on many OSs to check the files.
openssl x509 -in certificate.crt -text -noout
openssl rsa -in privateKey.key -check