Blockchains have become a buzzword, and many blockchain proponents believe a smart contract is a panacea for redefining the digital economy. But the community has a misconception that any kind of contract could be implemented as a blockchain smart contract. There is no doubt that Turing-complete scripting languages in blockchain techniques, such as Ethereum, can be used to draft many important smart contracts, but the digital economy is much more than Turing-complete smart contracts. Many protocols/contracts in our daily lives could not be implemented using Turing-complete smart contracts. As an example, we have formulated an Obama-Trump contract and show such a contract cannot be implemented using blockchain smart-contract techniques.
As the Internet increasingly becomes part of our daily lives, it will be convenient to have a digital payment system or design digital currency for society. It is generally easy to design an electronic cash system using public key infrastructure (PKI) systems. But PKI-based electronic cash is also easy to trace. Theoretically, banknotes could be traced using sequence numbers, though there is no convenient infrastructure to trace banknote sequence numbers back to users. Banknotes thus maintain sufficient anonymity.
Indeed, not all contracts are executable, but some contracts can be reformulated to be executable. For instance, if they ask Trump to put in custody (an entity that Trump and Obama both trust) his tax return and, once done, if Obama releases his birth certificate then the smart contract releases Trumps tax return from custody. This is definitely executable and Trump will get in such agreement if and only if he is willing to release his tax return.
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