The problem is smartphones don't have much processing power, and what they do have uses batteries. So either they are trivial for servers to generate en masse or prohibitive for smartphones.
There's also botnets, whose resources hijackers are happy to exploit.
Sure but certainly one could still make it costly enough to prohibit mass sending but not too costly to drain a cell battery under casual use. Or am I underestimating the level of complexity needed for a viable proof of work?
Something lightweight enough to be feasible on a cell phone is likely enough to be trivially computable in bulk by a decent bit of hardware. Think about all those obsolete bitcoin miners out there.
It sounds like proof-of-work alone is insufficient. The bit* type systems sound promising but that seems to preclude mobile. Interesting. What a funny problem we have built for ourselves.
Interesting that SMS appears to have simply legislated the problem from existence (or so says the article), I suppose if anyone can connect to a telephony network for cheap the problem would persist there too.
There's also botnets, whose resources hijackers are happy to exploit.