At a bustling money changer in northwestern Syria, a 46-year-old farmer gripped a plastic card like a lifeline. She had never heard of cryptocurrency, but the card held $500 of it to help restart her farm after nearly 14 years of civil war. Where had such technology come from, she asked. The answer surprised her: Afghanistan. Blockchain-based cash transfers are not the kind of innovation that many people would expect from a country better known for its repressive Taliban leadership, which views the internet with suspicion. But in a nation that has largely turned its back on the world, an Afghan start-up is building tools that it hopes will transform how humanitarian aid is delivered in countries shattered by conflict. "We've lived through these challenges ourselves, so we know how to develop an approach that works," said Zakia Hussaini, 26, a programmer at the start-up, HesabPay, which designed the technology driving Ms. Almahmoud's card. An early proponent of the platform was the United Nations High Commissioner for Refugees. The agency uses it to support more than 86,000 families in Afghanistan in one of the biggest public blockchain aid initiatives in the world. Mercy Corps, which donated the funds to Ms. Almahmoud, worked with HesabPay to expand its reach to include Syria, and programs for Sudan and Haiti are in development. Today, the platform has more than 650,000 wallets in Afghanistan, of which about 50,000 are in regular use.