True internet privacy could finally become possible thanks to a new tool that can let you prove you’re over 18 without revealing your date of birth, or prove you have enough money in the bank for a financial transaction without revealing your balance. That limits the risk of a privacy breach or identity theft. The tool is an emerging cryptographic protocol called a zero-­knowledge proof. Though researchers have worked on it for decades, interest has exploded in the past year, thanks in part to the growing obsession with cryptocurrencies, most of which aren’t private. Much of the credit for a practical zero-knowledge proof goes to Zcash, a digital currency that launched in late 2016. Zcash’s developers used a method called a zk-SNARK (for “zero-knowledge succinct non-interactive argument of knowledge”) to give users the power to transact anonymously. That’s not normally possible in Bitcoin and most other public blockchain systems, in which transactions are visible to everyone.