This page does not describe the possible keys for the input array.
Here are the SSL option keys from the cURL source code as of 2008-07-24:
CERT: String that holds file name of the SSL certificate to use
CERTTYPE: String that holds file type of the SSL certificate to use
KEY: String that holds file name of the SSL certificate to use
KEYTYPE: String that holds file type of the SSL certificate to use
PASSWD: String that holds the SSL or SSH private key password.
ENGINE: String that holds the SSL crypto engine.
ENGINE_DEFAULT: flag to set engine as default.