Digital cryptocurrencies—including bitcoin and litecoin, along with dozens of others—have struggled to win mainstream acceptance in the U.S. Interest in this so-called “Internet money” is not going away, however, which is why regulators are developing rules that that they hope can avert a repeat of last year’s Mt. Gox meltdown, when the world’s largest bitcoin exchange unexpectedly shut down after losing hundreds of thousands of bitcoins in a cyber attack.