To track your https pages correctly, your server hosting Piwik should work when called from https. You will need to buy a SSL certificate for your Piwik server.
Many Piwik users successfully use Piwik to report analytics of Ecommerce and SSL secure shops.
The Piwik tag detects the protocol of the page being tracked, and then automatically calls the normal http or secure https Piwik server.
To check whether your Piwik server works over HTTPS, go to:
https://url/to/piwik/piwik.php?rec=1&idsite=1 – if it returns a blank page it is working. If
https://url/to/piwik/piwik.php?rec=1&idsite=1 returns an error, please contact your sysadmin to ask him to enable SSL on the Piwik server.
On a related note, you can also force the Piwik login form to use secure SSL connections.