  In this letter we propose Meta-key, a data-sharing mechanism that enables
users share their encrypted data under a blockchain-based decentralized storage
architecture. All the data-encryption keys are encrypted by the owner's public
key and put onto the blockchain for safe and secure storage and easy
key-management. Encrypted data are stored in dedicated storage nodes and proxy
re-encryption mechanism is used to ensure secure data-sharing in the untrusted
environment. Security analysis of our model shows that the proxy re-encryption
adopted in our system is naturally free from collusion-attack due to the
specific architecture of Meta-key.
