Digital advertising has a serious problem: fraud. Widespread fraudulent practices are inflating measures of the traffic that digital ads attract, leading some advertisers to pay higher rates than they should. Can blockchain fix it? A handful of blockchain startups believe so. They argue that a shared digital ledger to authenticate and safeguard advertising data, a transparent exchange for trading digital ads, and a special ad-focused browser that pays its users in tokens could each help fix that multibillion-dollar problem.