I'm studying the SSL Client authentication in a server to send specific rest API. I'm using cURL to send the request and I'm following the instruction from a company. I can send the request and receive the answer but I want to understand more. In the request I have to specify a certificate (.cer), the key file (.pem) and a passphrase. The .pem file includes a private key. Can someone explain why I need to specify the private key? Is it my private key (client)? What is the process that the server uses to verify my identity? What is the relation between the certificate, the key file and the passphrase? Hope someone can help me to understand!