A man who used to protect U.S. presidents is now guarding something new - Bitcoin. George Sax, a former special agent and senior official of the U.S. Secret Service, has joined British cryptocurrency company Blockchain Ltd. as global head of security. Blockchain, named after the software that underpins Bitcoin, manages more than 26 million digital wallets across 140 countries that permit users to buy, sell, and hold digital assets. Sax was in charge of the Secret Service\u2019s Office of Investigations and also managed security for the 2016 U.S. presidential campaign, the company said. He\u2019s joining Blockchain as anxiety mounts that cryptocurrency exchanges remain vulnerable to cyber attacks. In January, Coincheck Inc. said hackers stole more than $500 million of tokens from the Japanese digital-asset trading company. Last month, a South Korean crypto exchange called Coinrail announced that it was the victim of a \u201ccyber intrusion.\u201d \u201cWe are one of the top targets in the world for security threats by virtue of our place in the crypto ecosystem,\u201d said Peter Smith, Blockchain\u2019s chief executive officer and co-founder. \u201cWe have to secure our infrastructure, data centers, and our operations. It\u2019s a big job, and George will help us build a next generation of security.\u201d Even as Wall Street and regulators weigh whether Bitcoin and its ilk constitute a legitimate asset class, investors say guaranteeing the custody of crypto-assets is their top concern. Blockchain, which was founded in 2011 and has raised $70 million in equity capital, recently unveiled a platform designed for institutional investors. Antony Jenkins, the former CEO of Barclays Plc, is a director on Blockchain\u2019s board. Arthur Levitt, the onetime chairman of the Securities and Exchange Commission and a director at Bloomberg LP, is a board adviser. \u201cI\u2019m looking forward to applying my experience at the U.S. Secret Service and the U.S. Treasury to the company\u2019s already robust security program and enable millions across the globe access to this new financial system with confidence,\u201d Sax said in a statement.