An exploratory study of smart contracts in the Ethereum blockchain platform

Abstract

Ethereum is a blockchain platform that supports smart contracts. Smart contracts are pieces of code that perform general-purpose computations. For instance, smart contracts have been used to implement crowdfunding initiatives that raised a total of US$6.2 billion from January to June of 2018. In this paper, we conduct an exploratory study of smart contracts. Differently from prior studies that focused on particular aspects of a subset of smart contracts, our goal is to have a broader understanding of all contracts that are currently deployed in Ethereum. In particular, we elucidate how frequently used the contracts are (activity level), what they do (category), and how complex they are (source code complexity). To conduct this study, we mined and cross-linked data from four sources: Ethereum dataset on the Google BigQuery platform, Etherscan, State of the DApps, and CoinMarketCap. Our study period runs from July 2015 (inception of Ethereum) until September 2018. With regards to activity level, we notice that it is concentrated on a very small subset of the contracts. More specifically, only 0.05% of the smart contracts are the target of 80% of the transactions that are sent to contracts. New solutions to cope with Ethereum’s limited scalability should take such an activity imbalance into consideration. With regards to categories, we highlight that the new and widely advertised rich programming model of smart contracts is currently being used to develop very simple applications that tend to be token-centric (e.g., ICOs, Crowdsales, etc). Finally, with regards to code complexity, we observe that the source code of high-activity verified contracts is small, with at most 211 instructions in 80% of the cases. These contracts also commonly include at least two subcontracts and libraries in their source code. The comment ratio of these contracts is also significantly higher than that of GitHub top-starred projects written in Java, C++, and C#. Hence, the source code of high-activity verified smart contracts exhibit particular complexity characteristics compared to other popular programming languages. Further studies are necessary to uncover the actual reasons behind such differences. Finally, based on our findings, we propose an open research agenda to drive and foster future studies in the area.

Appendix: The Top-10 Most Active Contracts on Ethereum

The top two contracts are part of decentralized currency exchange ÐApps. The webpage of IDEX is shown in Fig. 20. On the left-hand side, we can see the list of cryptocurrencies that are supported by IDEX. On the right-hand side, it shows the price chart and volume of exchanges for the cryptocurrency that we selected (AURA). Right below the price chart, we can see options for buying and selling aura (using Ether). We highlight that both etherdelta_2 and IDEX_1 contracts do not define a token of their own (check column “Own Token"). Instead, they operate on cryptocurrency tokens created by other contracts.

Fig. 20

A screenshot of the IDEX currency exchange ÐApp

The third contract was an ICO for the EOS token. It was a crowdfunding initiative for the EOSIO project, whose goal is to build a new blockchain platform that can process millions of transactions per second. As of August 2018, the EOS token is one of the most valuable cryptocurrencies with a market capitalization of US$5 billion.⁴⁴ The EOSIO blockchain has been released as open source software and stable versions are already available. A convenient quickstart guide providing a Docker image is available at the EOSIO Developer portal.⁴⁵

The fourth contract is part of a game ÐApp called CryptoKitties. This contract deals with core aspects of the game and has a higher number of lines of code compared to the others in our list. CryptoKitties is considered the first game to achieve widespread success on the Ethereum platform. As described in their website, “CryptoKitties is a game centered around breedable, collectible, and oh-so-adorable creatures we call CryptoKitties! Each cat is one-of-a-kind and 100% owned by you; it cannot be replicated, taken away, or destroyed”. The game is clearly in the realm of digital collectibles, allowing people to buy, sell, and trade CryptoKitties (similarly to traditional collectibles like trading cards). While the vast majority of cryptokitties sell for less than US$100, a few rare kitties sell for far more money (Galea 2017). For instance, one of the rarest kitties is the very first one created by the developers. The token representing this kitty was sold on December 2nd 2017 for US$113,082.15.⁴⁶ Figure 21 shows kitties for sale and Fig. 22 shows the profile of the second kitty (id #1044853).

Fig. 21

The CryptoKitties game. The image shows rare Generation 0 kitties (i.e., those created by the developers of the game) for sale. Below each kitty is its unique identifier. After a kitty breeds with another kitty, it will be temporarily unable to breed again. The “Fast” tag below these kitties indicates that this recovery time is short for them (1 minute)

Fig. 22

The profile of the kitty with ID 1044853 (check Fig. 21). At the top of the page, we can see the kitty’s name and its owner. The owner defines the start and end prices (similarly to an auction). Bio is a simple biography of the kitty. Catttributes are the attributes of the kitty, which indicate its rarity and also influence the profile of its children once it breads with another kitty

The fifth contract is called TronToken. It defines and manages the Tronix (TRX) token. This token is the cryptocurrency that was sold in the ICO to bootstrap the Tron project, which advertises itself as “one of the largest blockchain-based operating systems in the world”. The project raised US$70 million in the ICO (all tokens were sold). Ultimately, TRON is a domain-specific blockchain platform. Tron aims to be a content distribution platform for the digital entertainment industry, in which creators have the power to freely publish, store, and own their content, interacting directly with consumers. The selling point of Tron is making entertainment content easier to sell and cheaper to consume by removing the man-in-the-middle. The Tron project is already operational and people use the token contract to operate on the tokens (e.g., transfer tokens between accounts). The Tron blockchain can be explored by means of the TronScan website,⁴⁷ which operates analogously to the Etherscan.io website (Ethereum blockchain explorer).

The sixth contract is not verified, so its name is not available. However, searching for its address on Etherscan revealed that such a contract is part of the Poloniex Exchange ÐApp.

The seventh and eighth contracts are part of the Bittrex ÐApp, which is yet another cryptocurrency exchange. Interestingly, the Controller contract is used internally by the Bittrex company, as it manages the creation of wallets and other managerial tasks.

The ninth contract is an attempt to have a Bitcoin-like token in Ethereum. The key difference compared to regular Ethereum tokens is that it is mintable. Instead of issuing a supply of coins via an ICO or similar mechanism, the contract offers a mine() function that delivers 1 BTCM per call. And that’s the only way to generate coins. The contract allows only 50 calls to mine() per 10 minutes (across the whole Ethereum platform, not per client). The maximum supply allowed by the contract is capped at 21,000,000 BTCM (same as Bitcoin). This supply is projected to be achieved (minted) in 132 years.

Finally, the last contract OMGToken is the token contract for the OMG token, which was sold in an ICO to crowdfund the OmiseGO ÐApp. OmigoGo is yet another cryptocurrency exchange. The advisors of OmiseGo include Vitalin Buterin and Gavin Wood, who are the co-founders of Ethereum. The OmiseGo (OMG) token was the first Ethereum cryptocurrency to surpass a market capitalization of US$1 billion (later on, other coins achieved similar status) (Russell 2017).