Pass 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 is used as an identifier passed to the engine. You have to set the crypto engine with CURLOPT_SSLENGINE. "DER" format key file currently does not work because of a bug in OpenSSL.
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