Future arms treaties will require verifying the number and types of nuclear warheads in storage. However, no nuclear power would permit verification tests that reveal secret warhead designs. Efforts to address this challenge have focused on crafting "information barriers": electronic sensors and software that digest a claimed warhead's secret specs but spit out only an unclassified "yes"—the real deal—or a "no." But these methods are prone to tampering. Now, there may be a solution to this conundrum. Researchers have devised a new approach to verifying a nuclear warhead's identity without laying eyes on it. Known as a zero-knowledge protocol, a name adopted from a technique used in cryptography, their proposal, outlined in this week's issue of Nature, is setting the arms control community abuzz.