Physalia stops secret leaks by sharing data across servers
Physalia: Redistribution-Resistant Content Protection for Decentralized Storage
Cryptography and Security
Summary
Decentralized storage systems protect data by encrypting it and sharing keys, but if the key leaks, anyone can access the data. The authors designed Physalia, which splits the data itself into secret pieces stored on multiple servers, so someone who wants to leak the data must gather all pieces entirely. They also created a method that protects these pieces from being tampered with or corrupted by untrusted servers. Physalia was tested on an existing decentralized system to prove it works with real access rules.
What this means in practice
- •For cloud storage providers: Improve content protection by storing encrypted data shares across multiple servers to prevent easy piracy even if individual keys leak.
- •For blockchain platform teams: Implement robust decentralized storage with on-chain access policies that resist unauthorized redistribution of content.
- •For media streaming services: Enable more secure content delivery systems that reduce piracy risk by requiring reconstructing full data shares rather than leaked keys alone.$Commercial implications: Supports new anti-piracy products for digital media distributors seeking stronger decentralized protection.
Authors
Giacomo Giuliari, Karl Wüst
Abstract
In decentralized storage systems, access control is often implemented by encrypting the data before upload and sharing the decryption key with authorized parties. A leaked key, however, makes the data publicly accessible, which lowers the barrier to content piracy below that of traditional systems, where piracy requires redistributing the full data. We present Physalia, an end-to-end access-control system for decentralized storage that secret-shares the data itself, instead of just the key, across multiple servers. Leaking the data then requires transmitting it in full: We formalize this intuition and introduce the redistribution bandwidth an adversary must pay to leak protected content and show that Physalia raises it to the size of the data. Sharing across untrusted servers requires robustness against corrupted shares. We develop a robustness transform that turns any computational secret sharing scheme into a robust one and which is of independent interest. In contrast to existing schemes that rely on error correction, it uses signatures with ephemeral keys and adds only constant-size metadata per share. We show that this transform is secure and evaluate Physalia end-to-end on the Walrus decentralized storage system with an on-chain access policy.