# Cumulus: Blockchain-Enabled Privacy Preserving Data Audit in Cloud

## TL;DR

This paper introduces Cumulus, a blockchain-based system for privacy-preserving data audits in cloud storage that handles malicious and colluding parties, using smart contracts and state channels for practical implementation.

## Contribution

It presents a novel blockchain-enabled protocol for secure data auditing that accounts for adversarial behavior and collusion, improving upon existing PoR schemes.

## Key findings

- Prototype implementation on Ethereum demonstrates practical performance.
- The protocol effectively handles malicious and colluding parties.
- Blockchain smart contracts facilitate dispute resolution and payments.

## Abstract

Data owners upload large files to cloud storage servers, but malicious servers may potentially tamper data. To check integrity of remote data, Proof of Retrievability (PoR) schemes were introduced. Existing PoR protocols assume that data owners and third-party auditors are honest and audit only the potentially malicious cloud server to check integrity of stored data. In this paper we consider a system where any party may attempt to cheat others and consider collusion cases. We design a protocol that is secure under such adversarial assumptions and use blockchain smart contracts to act as mediator in case of dispute and payment settlement. We use state channels to reduce blockchain interactions in order to build a practical audit solution. We implement and evaluate a prototype using Ethereum as the blockchain platform and show that our scheme has comparable performance.