This paper presents a protocol for establishing a trust-less exchange system for digital assets on top of turing-complete blockchain technologies such as Ethereum. By combining smart contracts and digital signatures, we show how rules about asset trades can be enforced at low latency. These rules allow the exchange to be centralized but avoid placing trust in a third-party. This idea combines the advantages of centralized exchanges such as speed and price with the benefits of trust-less smart contracts giving unprecedented transaction security and settlement speed.