You don’t need to hold encypted keys to do this at all and doing so only adds risks. The user password can be a private key generation seed. That is how a lot o

You don’t need to hold encypted keys to do this at all and doing so only adds risks. The user password can be a private key generation seed. That is how a lot of web wallets work already. The closest thing to what you describe is shapeshift.io or evercoin.com Limitation of this type of system is you can’t have a true order book and transaction fees are higher since you are on chain.