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Apr 2017

Simple Decentralized Storage
  • David Vorick
  • Luke Champine

The authors introduce Sia, a platform for decentralized storage. Sia enables the formation of storage contracts between peers. Contracts are agreements between a storage provider and their client, defining what data will be stored and at what price. They require the storage provider to prove, at regular intervals, that they are still storing their client’s data.

Contracts are stored in a blockchain, making them publicly auditable. In this respect, Sia can be viewed as a Bitcoin derivative that includes support for such contracts. Sia will initially be implemented as an altcoin, and later financially connected to Bitcoin via a two-way peg.


Sia is a decentralized cloud storage platform that intends to compete with existing storage solutions, at both the P2P and enterprise level. Instead of renting storage from a centralized provider, peers on Sia rent storage from each other. Sia itself stores only the storage contracts formed between parties, defining the terms of their arrangement. A blockchain, similar to Bitcoin, is used for this purpose.

By forming a contract, a storage provider (also known as a host) agrees to store a client’s data, and to periodically submit proof of their continued storage until the contract expires. The host is compensated for every proof they submit, and penalized for missing a proof. Since these proofs are publicly verifiable (and are publicly available in the blockchain), network consensus can be used to automatically enforce storage contracts. Importantly, this means that clients do not need to personally verify storage proofs; they can simply upload their file and let the network do the rest.

We acknowledge that storing data on a single untrusted host guarantees little in the way of availability, bandwidth, or general quality of service. Instead, we recommend storing data redundantly across multiple hosts. In particular, the use of erasure codes can enable high availability without excessive redundancy.

Sia will initially be implemented as a blockchain-based altcoin. Future support for a two-way peg with Bitcoin is planned, as discussed in “Enabling Blockchain Innovations with Pegged Sidechains”. The Sia protocol largely resembles Bitcoin except for the changes noted below.


Sia is a variant on the Bitcoin protocol that enables decentralized file storage via cryptographic contracts. These contracts can be used to enforce storage agreements between clients and hosts. After agreeing to store a file, a host must regularly submit storage proofs to the network. The host will automatically be compensated for storing the file regardless of the behavior of the client.

Importantly, contracts do not require hosts to transfer files back to their client when requested. Instead, an out-of-band ecosystem must be created to reward hosts for uploading. Clients and hosts must also find a way to coordinate; one mechanism would be the arbitrary data field in the blockchain. Various precautions have been enumerated which mitigate Sybil attacks and the unreliability of hosts.

Siafunds are used as a mechanism of generating revenue for Nebulous Inc., the company responsible for the release and maintenance of Sia. By using Siafunds instead of premining, Nebulous more directly correlates revenue to actual use of the network, and is largely unaffected by market games that malicious entities may play with the network currency. Miners may also derive a part of their block subsidy from siafunds, with similar benefits. Long term, we hope to add support for two-way-pegs with various currencies, which would enable consumers to insulate themselves from the instability of a single currency.

We believe Sia will provide a fertile platform for decentralized cloud storage in trustless environments.

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