How to Call Ethereum Smart Contracts

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Introduction

In CTF smart contract challenges, one major hurdle is understanding how to interact with contracts—especially reverse engineering contracts without source code. This guide compiles practical methods for calling Ethereum smart contracts using tools like Remix IDE and MetaMask.

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Method 1: Deploying Source Code for Interaction

Scenario

You have access to the contract address and source code.

Example: CallMe Challenge

Contract Code:

pragma solidity ^0.4.21;
contract CallMeChallenge {
  bool public isComplete = false;
  function callme() public {
    isComplete = true;
  }
}

Steps:

  1. Compile the code in Remix.
  2. Set Environment to Injected Web3 (ensure MetaMask is logged into Ropsten).
  3. Deploy using the At Address field (paste the contract address).
  4. Interact with the callme() function via the UI.
Note: Modifying function names in the source code breaks compatibility.

Method 2: Using Web3.js

Option A: Browser Console (with MetaMask)

Example: Nickname Challenge

  1. Get ABI from Remix after compiling the source.

  2. Execute this in the browser console:

    var abiDefinition = [{"constant":false,"inputs":[{"name":"nickname","type":"bytes32"}],"name":"setNickname","outputs":[],"payable":false,"stateMutability":"nonpayable","type":"function"},{"constant":true,"inputs":[{"name":"","type":"address"}],"name":"nicknameOf","outputs":[{"name":"","type":"bytes32"}],"payable":false,"stateMutability":"view","type":"function"}];
var CaptureTheEther = web3.eth.contract(abiDefinition);
var cte = CaptureTheEther.at("0x71c46Ed333C35e4E6c62D32dc7C8F00D125b4fee");
cte.setNickname("your nickname", console.log);

Option B: Node.js Script

Dependencies:

npm install web3@^0.20.1 ethereumjs-tx@^1.3.7

Script:

let Web3 = require("web3");
let Tx = require("ethereumjs-tx");
let web3 = new Web3(new Web3.providers.HttpProvider("https://ropsten.infura.io/v3/YOUR_API_KEY"));
let rawTransaction = {
  "to": "0xCONTRACT_ADDRESS",
  "data": "0xa3c8e393" // Keccak-256 hash of `callme()`
};
let privKey = new Buffer.from("YOUR_PRIVATE_KEY", "hex");
let tx = new Tx(rawTransaction);
tx.sign(privKey);
web3.eth.sendRawTransaction('0x' + tx.serialize().toString('hex'), (err, hash) => {
  console.log(err || hash);
});

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Method 3: Using Web3.py

Installation:

pip install web3

Script:

from web3 import Web3, HTTPProvider

config = {
  "abi": [{"constant": False, "inputs": [], "name": "callme", "outputs": [], "type": "function"}],
  "address": "0xCONTRACT_ADDRESS"
}

web3 = Web3(HTTPProvider("https://ropsten.infura.io/YOUR_API_KEY"))
contract = web3.eth.contract(address=config['address'], abi=config['abi'])

txn = contract.functions.callme().buildTransaction({
  'nonce': web3.eth.getTransactionCount('YOUR_ADDRESS'),
  'gas': 7600000,
  'gasPrice': web3.eth.gasPrice
})
signed_txn = web3.eth.account.signTransaction(txn, private_key="YOUR_PRIVATE_KEY")
print(web3.eth.sendRawTransaction(signed_txn.rawTransaction).hex())

FAQs

Q1: Can I interact with a contract without ABI?

A: Yes, but only for public functions using their function signatures (hashed names).

Q2: Why does MetaMask show "Insufficient Funds"?

A: Ensure your account has enough ETH for gas fees on the Ropsten testnet.

Q3: How do I find a contract’s ABI?

A: Use Remix after compiling or Etherscan’s "Contract" tab for verified contracts.

Key Takeaways

Always verify transactions on Etherscan!