CURLINFO_SSL_ENGINES man page
CURLINFO_SSL_ENGINES — get an slist of OpenSSL crypto-engines
CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_SSL_ENGINES,
struct curl_slist **engine_list);
Pass the address of a 'struct curl_slist *' to receive a linked-list of OpenSSL crypto-engines supported. Note that engines are normally implemented in separate dynamic libraries. Hence not all the returned engines may be available at run-time. NOTE: you must call curl_slist_free_all(3) on the list pointer once you're done with it, as libcurl will not free the data for you.
All TLS based ones.
Added in 7.12.3. Available in OpenSSL builds with "engine" support.
Returns CURLE_OK if the option is supported, and CURLE_UNKNOWN_OPTION if not.