Bitcoin is a decentralized cryptocurrency providing an open, self-regulating alternative to classical currencies. Bitcoin uses a peer-to-peer network where users submit transactions without intermediaries. Bitcoin miners collect transactions, solve computational puzzles (proof-of-work) to reach consensus, and add the transactions to a distributed public ledger known as the blockchain. The original Bitcoin paper argues that transaction processing is secure and irreversible as long as the largest colluding group of miners represents less than 50% of total computing capacity and at least about one hour has elapsed. Critically this transaction latency limits Bitcoin's suitability for real-time transactions, furthermore recent work has exposed vulnerabilities to transaction reversibility, double-spending, and strategic mining attacks. This paper introduces ByzCoin, a novel protocol that leverages scalable collective signing to commit Bitcoin transactions irreversibly within seconds.