Cryptocurrencies like Bitcoin and Ethereum are targets of choice for online criminals, who often exploit weaknesses in major trading platforms or individual users’ digital “wallets” to make major scores. A recent $1.5-billion heist of Ethereum from the Bybit platform — attributed by the FBI to North Korean hackers — is believed to be the largest yet in an ever-longer litany of thefts. How common is crypto theft? Cryptocurrencies are based on blockchain technology, which publicly records transactions between people holding and exchanging them. That has not kept a lid on theft, with an estimated $2.2 billion worth of the assets...