# Carlos Sanchez I Beginners Track Capture the Flag CTF Intro I ETHGlobal Prague 2025 - Channel: [ETHGlobal](https://streameth.org/ethglobal) - Date: 2025-10-09 - Duration: 26:19 - Topics: CTF, Capture the Flag, Ethereum security, Solidity, smart contract hacking, Optimism, Scaffold-Ethereum, NFT flags, SVG NFTs, smart contract challenges, tx.origin, msg.sender, contract interaction, Web3 security, DeFi security, hardhat, foundry, Etherscan, AI Ninja, contract debugging, local blockchain, cryptogames, ETHGlobal, Prague 2025, smart contract development, blockchain education, security challenges, contract deployment, Web3 tools - Watch: https://streameth.org/watch/yt-wf4IJqnlrxo - YouTube: https://www.youtube.com/watch?v=wf4IJqnlrxo ## Description Carlos Sanchez introduces the Capture the Flag (CTF) challenge at ETHGlobal Prague 2025, designed to help developers deepen their Solidity and security skills. Learn how to solve 12 progressively difficult challenges deployed on Optimism, earning SVG NFTs as flags. The workshop covers using Scaffold-Ethereum tools, interacting with contracts through multiple methods (UI, scripts, Etherscan), and demonstrates solving the first two challenges. Perfect for developers looking to practice smart contract security in a gamified environment. 00:00 Introduction to CTF concept and history 00:48 How CTF games work in blockchain context 01:19 Why breaking things leads to deeper understanding 01:49 Popular Ethereum CTF examples 02:30 Defcon CTF experience and results 02:59 Introducing ctfbillgill.com challenge platform 03:35 How flag NFTs work with SVG images 04:15 Scaffold-Ethereum extension setup 04:47 Website walkthrough and first challenge 05:21 Connecting wallet and checking flags 05:53 Installing scaffold-eth CTF extension 06:36 Running local blockchain instance 07:17 Deploying challenge contracts locally 07:52 Examining challenge 1 contract code 08:29 Understanding the flag minting mechanism 09:07 Debug contract UI demonstration 09:39 Solving challenge 1 through UI 10:17 Funding account and completing challenge 10:52 Alternative methods to interact with contracts 11:28 Using AI Ninja contract interface 12:07 Solving via script with private key 13:11 Testing solution on local network 13:57 Running script on Optimism mainnet 14:46 Capturing first flag on live network 15:43 Introducing challenge 2 requirements 16:18 Understanding msg.sender vs tx.origin 16:54 Solution approach using intermediate contract 17:50 Writing solution contract code 18:42 Deploying solution contract locally 19:38 Testing solution contract interaction 20:24 Deploying solution to Optimism 21:23 Switching frontend to Optimism network 22:08 Capturing second flag on mainnet 22:58 Recommended next steps for learners 23:33 Importance of hands-on practice 24:23 Estimated completion times for challenges 25:35 Demo of AI Ninja contract interface tool _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ๐Ÿ‡จ๐Ÿ‡ฟ *ETHGlobal Prague* This workshop is specifically for ETHGlobal Prague, a 3-day hackathon held May 30 - June 1 2025, bringing together the most skilled web3 developers, designers and product builders from all around the globe for a weekend-long adventure to advance the Ethereum ecosystem! Watch the full ETHGlobal Prague 2025 playlist here: https://youtube.com/playlist?list=PLXzKMXK2aHh5Vv-ZobDU2YMWekDFYvOqd&si=SoQ5O2TOAK8RkYf- _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ โœ… Follow Buidl Guidl Website: https://buidlguidl.com โœ… *Follow ETHGlobal* X: https://x.com/ETHGlobalโ€‹ Warpcast: https://warpcast.com/ethglobal Website: https://ethglobal.com YouTube: https://www.youtube.com/c/ETHGlobal _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _Are you interested in Ethereum development and entrepreneurship?_ ๐Ÿ‘‰ Sign up for the next ETHGlobal event: https://ethglobal.com/events ๐ŸŽ Get exclusive access and perks with ETHGlobal Plus! https://ethglobal.com/plus ๐Ÿ“ฃ Want us to throw an event in your city? Tell us where! https://ethglobal.com/city _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _