21 - Bringing Down the Complexity: Fast Composable Protocols for Card Games Without Secret State

While many cryptographic protocols for card games have

been proposed, all of them focus on card games where players have some

state that must be kept secret from each other, e.g closed cards and

bluffs in Poker. This scenario poses many interesting technical challenges,

which are addressed with cryptographic tools that introduce significant

computational and communication overheads (e.g. zero-knowledge proofs).

In this paper, we consider the case of games that do not require any secret

state to be maintained (e.g. Blackjack and Baccarat). Basically, in these

games, cards are chosen at random and then publicly advertised, allowing

for players to publicly announce their actions (before or after cards

are known). We show that protocols for such games can be built from

very lightweight primitives such as digital signatures and canonical random

oracle commitments, yielding constructions that far outperform all

known card game protocols in terms of communication, computational

and round complexities. Moreover, in constructing highly efficient protocols,

we introduce a new technique based on verifiable random functions

for extending coin tossing, which is at the core of our constructions. Besides

ensuring that the games are played correctly, our protocols support

financial rewards and penalties enforcement, guaranteeing that winners

receive their rewards and that cheaters get financially penalized. In order

to do so, we build on blockchain-based techniques that leverage the

power of stateful smart contracts to ensure fair protocol execution.