what type of security you need in tableau. Are you need browser security then go with ssl in server.
If you want to set some users access in same workbook then go with user level security.
Sankar's answer relates to Tableau Server which is Tableau's distribution, consumption, security managing software.
Be aware there are distinct products - Desktop and Server.
The start of this thread has a good summary of your options, and why Tableau does it this way -
Check Russell Christopher's response in particular.
As ever there are workarounds, but it depends on what you are trying to secure, why and also how you consume/ distribute.