I received a dreaded "cURL error: Unknown SSL protocol error in connection to secure.authorize.net:443" when setting up the Authorize.net payment gateway for use in a Drupal Ubercart store recently. Oh noes! Here's how I fixed it.
Find the module ubercart > payment > uc_authorizenet
On line 558 (your mileage may vary) I added:
curl_setopt($ch, CURLOPT_SSLVERSION, 3); // Force SSLv3 to fix Unknown SSL Protocol error
Goodbye sweet, sweet error. I barely knew ye.
Ami K., Director of Marketing & Business Development GHT Limited
Thanks John! Very helpful. Training went really smoothly and everything has been very simple for us to do. Thank you!