Hi Sandeep,
I was not visiting site for quiet some time due to project schedules.
Here is a trick to make Dashboard secured.
Keep an object covering the whole canvas/page of dashboard with required transparency %. On that keep a UserName and/or password field to validate. Once validated make the object covering the data invisible.
This is a successful method which has been in practice already. Revert in case you need more clarification.
Thanks
BaaRaa.