2. Ethereum virtual machine

Decompiling bytecode

Several projects have created tools to attempt making bytecode more readable. For example, you can try decompiling a contract on mainnet using or Unfortunately, some parts of the original contract source, such as functions names or event names, are always lost due to optimization done by the compiler. Nonetheless, most function names can still be brute forced by comparing function signatures to large datasets containing popular function and event names (see

Contract calls usually require an “ABI” (Application Binary Interface), which is a piece of data documenting all functions and events, including their needed input and output. When calling a function on a contract, the function signature is determined by hashing the name of the function including its inputs (using keccak256), and truncating everything but the first 4 bytes.

Example Solidity contract, along with its ABI

As you can see in the image above, our function HelloWorld() resolves to the signature hash 0x7fffb7bd. If we would like to call this function, our transaction data needs to start with 0x7fffb7bd. Arguments which need to be passed to a function (none in this case) can be added in 32-byte pieces called words after the signature hash in a transaction’s input data.

If an argument contains over 32 bytes (256 bits) of data, like an array or string, the argument is split into multiple words which are added to the input data after all other arguments have been included. Moreover, the total size of all words gets included as another word, before all array words. At the location where the argument would have been included, the start position of the array words (including the size word) is added instead.


Ethereum provides a decentralized ecosystem for developers to build awesome products using Solidity & the EVM. Although interacting with the EVM through smart contracts might be a lot more expensive than running programs on traditional servers, there are lots of use cases in which decentralization is favored over costs.

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