FunctionSelector

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In 2021, Poly Network’s bridge contracts were hacked, losing $611M due to a collision vulnerability—inspiring me to build a collision checker.

A web-based tool for calculating Ethereum function selectors and detecting selector collisions. This tool is essential for smart contract developers and security researchers working with Ethereum smart contracts.

What is FunctionSelector?

FunctionSelector is a simple, lightweight web application that helps developers:

• Calculate the 4-byte function selector for any Ethereum function signature
• Check for potential selector collisions between different function signatures
• Understand the importance of selector uniqueness in smart contract security

Features

• Function Selector Calculator: Convert any function signature to its corresponding 4-byte selector
• Collision Detection: Check if two different function signatures produce the same selector
• Copy to Clipboard: Easy one-click copying of calculated selectors
• Modern UI: Clean, responsive interface built with Tailwind CSS
• No Dependencies: Runs entirely in the browser using CDN resources

Usage

1. Open the application: Simply open Index.html in any modern web browser
2. Calculate a selector:
   • Enter a function signature (e.g., transfer(address,uint256))
   • Click "Calculate Selector"
   • Copy the resulting 4-byte selector
3. Check for collisions:
   • Enter two different function signatures
   • Click "Check Collision"
   • View whether they produce the same selector

How it Works

Function selectors are the first 4 bytes of the Keccak-256 hash of the function signature. For example:

• Function signature: transfer(address,uint256)
• Keccak-256 hash: 0xa9059cbb2ab09eb219583f4a59a5d0623ade346d962bcd4e46b11da047c9049b
• Function selector: 0xa9059cbb

Security Importance

Function selector collisions can lead to critical vulnerabilities in:

• Proxy patterns: Where the same selector might call different functions
• Fallback functions: Unexpected function calls due to selector conflicts
• Contract upgrades: Potential security issues during contract upgrades

This tool helps developers identify and avoid such collisions during the development phase.

Example Usage

// Function signature examples:
"transfer(address,uint256)" → 0xa9059cbb
"approve(address,uint256)" → 0x095ea7b3
"balanceOf(address)" → 0x70a08231

Built with ❤️ by Zaki

For smart contract security and development best practices.