An analysis of the Oracles used in Ethereum's blockchain

Smart contracts are programs whose business logic runs in a decentralized architecture named blockchain where each of the executing nodes trusts and agrees with the execution outcomes. Blockchain is an isolated execution environment, so smart contracts cannot access external data by themselves. The blockchain Oracle, or simply Oracle, is the software that allows smart contracts to interact with the outside world. In the past, a wide variety of studies have examined the oracles from various perspectives such as their implementation characteristics and their reliability. Unlike some other studies, this paper analyzes the use of Oracles through a static analysis of smart contracts. First, we analyze the source code of a corpus of over 40K smart contracts to find the most blockchain Oracles used by the Ethereum blockchain. On the basis of previous works, we examine the features of the most used blockchain Oracles in Ethereum. Lastly, we discussed our findings and the possible reasons why some blockchain oracles are more widely used than others. The purpose of this study is to shed light on how and which oracles are actually used by Ethereum smart contracts, as an alternative to other very interesting studies that analysed the blockchain oracle characteristics.