> The only purpose of the exchange rate is to be a source of legal random numbers, since a die or deck of cards would be a source of illegal random numbers.

Incidentally, Illegal lotteries in the US historically used similar sources of numbers. In their case it was apparently more of a way to guarantee that the drawing wasn't fixed than to present it as legal. [1]

[1] https://en.wikipedia.org/wiki/Numbers_game#Gameplay