A proposal to add a currency token to an anonymous email system. Each phone acts as its own email server running peer-to-peer between two phones. A central registry maintains identity and IP addresses so phones can communicate.
So. The biggest problem is that email is not a guaranteed delivery protocol. In 2014, this system was losing about 2% of its emails. No sane bank will sign up to lose 2% of its money. After a few days, I defined the overall design as a guaranteed delivery system between mobile devices and started researching.
A second problem is scalability. The client wanted to scale to 100 million devices. So guaranteed delivery and high scalability. Eventually, I looked at IoT solutions...
Other constraints - how well does the IoT toolkit match the existing environment? Cost, effort and applicability are concerns. I recommended the RTI toolkit as the best option with an annual licensing cost of $7500 per developer. At that point, this project died. There's initial sequence diagrams and even coding of the Bank Manager, mock tokens, unit tests.