You're storing bank related sensitive personal data. Don't hack your own solution. Use an existing, proven solution. Most likely you will also be running into all kinds of security and privacy laws, regulations and liabilities when dealing with such data. Find someone who knows these regulations and who can help you and advise you.
Don't try to do this yourself. "Anyone can build a security system they they themselves cannot break." - I think that's a Bruce Schneider quote. Heed it.
Edit to react on comment:
Even when dealing with private finance software you're probably dealing with bank account numbers, social security numbers, etcetera. So you are probably still running into various kinds of regulations.
Systems like OpenID and Oracle SSO only cover authentication. Regulations also dictate minimum security measures on how you should store data in your database, how you should treat backups, how you should deal with people (e.g. developers) accessing the database, etcetera, etcetera. If you don't follow these and something goes wrong, you're liable.
I really urge you to seek help from someone knowledgeable in the field. Explain them what you want to do, what you want to store, etcetera. They can tell you what (if any) regulations apply. Only then can you start looking at how you are going to implement this and what off-the-shelf components you can use.