CURLOPT_SSLKEYTYPE (3) - Linux Man Pages
CURLOPT_SSLKEYTYPE: set type of the private key file
NAMECURLOPT_SSLKEYTYPE - set type of the private key file
DESCRIPTIONPass a pointer to a zero terminated string as parameter. The string should be the format of your private key. Supported formats are "PEM", "DER" and "ENG".
The format "ENG" enables you to load the private key from a crypto engine. In this case CURLOPT_SSLKEY(3) is used as an identifier passed to the engine. You have to set the crypto engine with CURLOPT_SSLENGINE(3). "DER" format key file currently does not work because of a bug in OpenSSL.