CURLOPT_SSLCERTTYPE man page
CURLOPT_SSLCERTTYPE — specify type of the client SSL certificate
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSLCERTTYPE, char *type);
Pass a pointer to a zero terminated string as parameter. The string should be the format of your certificate. Supported formats are "PEM" and "DER", except with Secure Transport. OpenSSL (versions 0.9.3 and later) and Secure Transport (on iOS 5 or later, or OS X 10.7 or later) also support "P12" for PKCS#12-encoded files.
The application does not have to keep the string around after setting this option.
All TLS based protocols: HTTPS, FTPS, IMAPS, POP3S, SMTPS etc.
If built TLS enabled. Added in 7.9.3
Returns CURLE_OK if TLS is supported, CURLE_UNKNOWN_OPTION if not, or CURLE_OUT_OF_MEMORY if there was insufficient heap space.
curl_easy_setopt(3), CURLOPT_PROXY_SSLCERTTYPE(3), CURLOPT_SSLCERT(3).