We propose several decentralized ceremonies for constructing a powers-of-tau structured reference string (SRS). Our protocols make use of a blockchain platform to run in a permissionless manner, where anyone can contribute randomness in exchange for paying the requisite transaction fees. The resulting SRS is secure as long as any single party participates honestly. We introduce several protocols optimized for different sized powers-of-tau setups and using an on-chain or off-chain data availability model to store the resulting string. We implement our most efficient protocol on top of Ethereum, demonstrating practical concrete performance.