Pass a pointer to a zero terminated string as parameter. It will be used as the identifier for the crypto engine you want to use for your private key.
CURLE_OK - Engine found.
CURLE_SSL_ENGINE_NOTFOUND - Engine not found, or OpenSSL was not built with engine support.
CURLE_SSL_ENGINE_INITFAILED - Engine found but initialization failed.
CURLE_NOT_BUILT_IN - Option not built in, OpenSSL is not the SSL backend.
CURLE_UNKNOWN_OPTION - Option not recognized.
CURLE_OUT_OF_MEMORY - Insufficient heap space.
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