what if you were the owner of the iphone and square hardware and the business or person you were giving money to had a barcode/number/qr code that you would scan/enter which would go to a third party(probably square) service that verifies your account is in good standing and tells the receiver on their iphone/android then the transaction is completed square takes a small cut 1 cent per transaction for being the trusted middle man. No worry about using untrusted system or anything.