  Blockchain has been applied to data sharing to ensure the integrity of data
and chain of custody. Sharing big data such as large biomedical data files is a
challenge to blockchain systems since the ledger is not designed to maintain
big files, access control is an issue, and users may be dishonest. We call big
data such as big files stored outside of a ledger that includes the blockchain
and world state at a blockchain node as "off-state" and propose an off-state
sharing protocol for a blockchain system to share big data between pairs of
nodes. In our protocol, only encrypted files are transferred. The cryptographic
key is stored in the world state in a secure way and can be accessed only by
authorized parties. A receiver has to request the corresponding cryptographic
key from the sender to decrypt such encrypted files. All requests are run
through transactions to establish reliable chain of custody. We design and
implement a prototypical blockchain off-state sharing system, BOSS, with
Hyperledger Fabric. Extensive experiments were performed to validate the
feasibility and performance of BOSS.
