It is not technically difficult
The amount and the person to be paid must be displayed on the card or you do not know how much you are paying or who you are paying it to. The buttons needed to authenticate must be on the card or you do not know who is logging them. The software must be open source so it can be checked. The software must be stored in ROM that can be read by external devices so you can tell it matches the source code.
The hard part is collecting a bigger bribe for the banks than the people who want insecure banking.


