CURLOPT_SSLENGINE man page
CURLOPT_SSLENGINE — set SSL engine identifier
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSLENGINE, char *id);
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.
The application does not have to keep the string around after setting this option.
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
Only if the SSL backend is OpenSSL built with engine support.
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.
curl_easy_setopt(3), CURLOPT_SSLENGINE_DEFAULT(3), CURLOPT_SSLKEYTYPE(3).