  As the confidentiality and scalability of smart contracts have become a
crucial demand of blockchains, off-chain contract execution frameworks have
been promising. Some have recently expanded off-chain contracts to Multi-Party
Computation (MPC), which seek to transition the on-chain states by off-chain
MPC. The most general problem among these solutions is MPT, since its off-chain
MPC takes on- and off-chain inputs, delivers on- and off-chain outputs, and can
be publicly verified by the blockchain, thus capable of covering more
scenarios. However, existing Multi-Party Transaction (MPT) solutions lack at
least one of data availability, financial fairness, delivery fairness, and
delivery atomicity. These properties are crucially valued by communities, e.g.,
the Ethereum community, or users. Even worse, these solutions require high-cost
interactions between the blockchain and off-chain systems.
  This paper proposes a novel MPT-enabled off-chain contract execution
framework, DECLOAK. DECLOAK is the first to achieve data availability of MPT,
and our method can apply to other fields that seek to persist user data
on-chain. Moreover, DECLOAK solves all mentioned shortcomings with even lower
gas costs and weaker assumptions. Specifically, DECLOAK tolerates all but one
Byzantine party and TEE executors. Evaluating on 10 MPTs, DECLOAK reduces the
gas cost of the SOTA, Cloak, by 65.6%. Consequently, we are the first to not
only achieve such level secure MPT in practical assumption, but also
demonstrate that evaluating MPT in the comparable gas cost to normal Ethereum
transaction is possible. And the cost superiority of DECLOAK increases as the
number of MPT parties grows.
