That's the best I'll be able to do for assurances.
Not enough to me on a security/trust point of view.
Who knows how secure your encryption is and what your app will do with the key.
Only an audit of the code could remove this uncertainty.
Sorry to say, but I have no reason to trust you, as you should have none to trust me.
RE: Introducing Steem Exchanger, A Desktop App To Manage Your STEEM Account and For AutoTrading Tokens at Steem-Engine.com