  Multi-party business processes rely on the collaboration of various players
in a decentralized setting. Blockchain technology can facilitate the automation
of these processes, even in cases where trust among participants is limited.
Transactions are stored in a ledger, a replica of which is retained by every
node of the blockchain network. The operations saved thereby are thus publicly
accessible. While this enhances transparency, reliability, and persistence, it
hinders the utilization of public blockchains for process automation as it
violates typical confidentiality requirements in corporate settings. In this
paper, we propose MARTSIA: A Multi-Authority Approach to Transaction Systems
for Interoperating Applications. MARTSIA enables precise control over process
data at the level of message parts. Based on Multi-Authority Attribute-Based
Encryption (MA-ABE), MARTSIA realizes a number of desirable properties,
including confidentiality, transparency, and auditability. We implemented our
approach in proof-of-concept prototypes, with which we conduct a case study in
the area of supply chain management. Also, we show the integration of MARTSIA
with a state-of-the-art blockchain-based process execution engine to secure the
data flow.
