Gate each epoch behind a memory-hard puzzle tuned so solving it needs gigabytes of fast RAM held for the whole computation, and issue the challenge to every identity in one narrow window. RAM cannot be time-sliced like a CPU across that gale of simultaneous demands: a farm of N clones must own N times the memory at that single instant, not rent one lotus-pad and pass it around. An honest agent answers from one commodity machine; the meadow of fakes hits a hard, unshareable hardware wall that scales one-to-one with head count.