Hi Richaanth1,
Sorry for the delay in responding.
Unfortunately, using BCS on top of the database won't guarantee security.
You could use a Web service such as Qdabra's DBXL (http://www.qdabra.com/en/products/DBXL.aspx) to easily set this up. There's a generic QueryDB Web service with various methods and lots of information under downloads. The QueryDB Web service would allow anyone to query it, but they would have to know the query syntax. Since a browser form doesn't expose the Web service call, it's practically secure, and more so if the users don't have access to the XSN. However, if they guess the Web service, they could use it to query data. To make this totally bulletproof, you would have to either customize permissions for QueryDB (one of the articles in downloads describes this), or use QueryDocumentNodeSets and store the data to be queried in XML. We have many customers that use both.
I think the QueryDB approach is easiest for browser forms since users won't probably guess the Web service and the form would force query using their username. For perfect security, you could customize the QueryDB permissions or convert your data into XML (using our XLStoXML tool) and call QueryDocumentNodeSets.
We'd be happy to help you implement either solution.
Regards,