Braga Blockchain - Ethereum Smart Contracts programming
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The document provides an introduction to Ethereum smart contracts, highlighting key concepts such as blockchain technology, transaction processes, and the Ethereum virtual machine. It explains the structure and function of smart contracts, including programming languages like Solidity, and touches on potential challenges such as immutability and security issues. Additionally, it discusses examples and resources for further exploration of Ethereum development.
![SOLIDITY
IMPLEMENTING YOUR OWN COIN
// from previous page ...
function mint(address receiver, uint amount) {
if (msg.sender != minter) return;
balances[receiver] += amount;
}
function send(address receiver, uint amount) {
if (balances[msg.sender] < amount) return;
balances[msg.sender] -= amount;
balances[receiver] += amount;
Sent(msg.sender, receiver, amount);
}
}
emanuel / br-blockchain-2](https://image.slidesharecdn.com/bragablockchainsmartcontractstalk-171208181119/85/Braga-Blockchain-Ethereum-Smart-Contracts-programming-28-320.jpg)
![ETHEREUM CHALLENGES
EXAMPLE OF A RE-ENTRANCY ATTACK PROBLEM
This can be exploited by an external contract
msg.sender calling withdraw again (line 4) while
balance is still non zero
mapping (address => uint) private balances;
// msg.sender is a user withdrawing funds
function withdraw() public {
uint amount = balances[msg.sender];
if (!(msg.sender.call.value(amount)()))
{ revert; }
balances[msg.sender] = 0;
}
emanuel / br-blockchain-2](https://image.slidesharecdn.com/bragablockchainsmartcontractstalk-171208181119/85/Braga-Blockchain-Ethereum-Smart-Contracts-programming-34-320.jpg)
Braga Blockchain - Ethereum Smart Contracts programming
- 1. ETHEREUM SMART CONTRACTS INTRO emanuel / br-blockchain-2
- 2. EMANUEL MOTA - @EMOTA7 Founder of Yari Labs [email protected] @yarilabs emanuel / br-blockchain-2
- 3. ABOUT THE TALK Intro Blockchain Overview Bitcoin vs Ethereum Smart Contracts Solidity Questions emanuel / br-blockchain-2
- 4. INTRO Quick questions about the audience emanuel / br-blockchain-2
- 5. BLOCKCHAIN OVERVIEW emanuel / br-blockchain-2
- 6. BLOCKCHAIN OVERVIEW A blockchain is a globally shared, transactional database. everyone can read entries in the database changes can only happen via transactions accepted by all others emanuel / br-blockchain-2
- 7. BLOCKCHAIN OVERVIEW transactions are always cryptographically signed transactions are bundled in blocks and chained together emanuel / br-blockchain-2
- 8. emanuel / br-blockchain-2
- 9. BITCOIN Cryptocurrency (Bitcoin) Bitcoin Blockchain One shared distributed ledger Support for a "Script" Limited smart contracts capabilities emanuel / br-blockchain-2
- 10. ETHEREUM Cryptocurrency (Ether) Bitcoin like distributed ledger Ethereum Virtual Machine (EVM) Turing complete emanuel / br-blockchain-2
- 11. ETHEREUM Ethereum Blockchain Contracts (code) Storage Logs Events emanuel / br-blockchain-2
- 12. ETHEREUM Two kinds of accounts External Accounts (Wallets controlled by humans) Contract Accounts (controlled by code) every account has a balance emanuel / br-blockchain-2
- 13. ETHEREUM Code execution costs GAS Transaction is a message sent from one account to another and can have a data payload emanuel / br-blockchain-2
- 14. WORLD COMPUTER ? emanuel / br-blockchain-2
- 15. SMART CONTRACTS emanuel / br-blockchain-2
- 16. SMART CONTRACTS "A smart contract is a computer program that directly controls digital assets and which is run in such an environment that it can be trusted to faithfully execute." (Vitalik Buterin) emanuel / br-blockchain-2
- 17. emanuel / br-blockchain-2
- 18. SMART CONTRACTS emanuel / br-blockchain-2
- 19. SOURCE FOR SMART CONTRACTS SECTION: HTTPS://SOLIDITY.READTHEDOCS.IO/ emanuel / br-blockchain-2
- 20. SMART CONTRACTS Contract = code (i.e. functions) and data (its state) that resides at a speci c address on the Ethereum blockchain EVM is the runtime for Smart Contracts on Ethereum Accounts have a persistent memory area which is called storage: key-value store that maps 256-bit words to 256-bit words emanuel / br-blockchain-2
- 21. SMART CONTRACTS Contracts can neither read nor write to any storage apart from its own Contracts also have access to memory - linearly and can be addressed at byte level Memory is expanded by a 256bits when reading or writing & expansion must be paid in gas emanuel / br-blockchain-2
- 22. SMART CONTRACTS Contracts can call other contracts (they can even call themselves) 1024 max call stack depth call (new contexts) vs delegatecall (same context, e.g. msg.sender + msg.value) Events Contracts can purge themselves from the blockchain (OPCODE selfdestruct) emanuel / br-blockchain-2
- 23. SMART CONTRACTS Contracts are de ned through a transaction sent to 0x000.....000 (zero acc) Data of the transaction is the compiled contract emanuel / br-blockchain-2
- 24. SMART CONTRACTS PROGRAMMING LANGUAGES Available Programming languages Solidity (Javascript/Java like) Serpent (Python inspired) LLL (Lisp inspired) others in the making ... emanuel / br-blockchain-2
- 25. SOLIDITY PROGRAMMING LANGUAGE emanuel / br-blockchain-2
- 26. SOLIDITY SIMPLE STORAGE DEMO Anyone can call set anytime overwriting the value. The history would be preserved on the Blockchain. pragma solidity ^0.4.0; contract SimpleStorage { uint storedData; function set(uint x) { storedData = x; } function get() constant returns (uint) { return storedData; } } emanuel / br-blockchain-2
- 27. SOLIDITY IMPLEMENTING YOUR OWN COIN pragma solidity ^0.4.0; contract Coin { // keyword "public" exposes vars to the outside. address public minter; mapping (address => uint) public balances; // Events allow light clients to react on changes. event Sent(address from, address to, uint amount); // The constructor is run only on creation. function Coin() { minter = msg.sender; } // continues on next page ... emanuel / br-blockchain-2
- 28. SOLIDITY IMPLEMENTING YOUR OWN COIN // from previous page ... function mint(address receiver, uint amount) { if (msg.sender != minter) return; balances[receiver] += amount; } function send(address receiver, uint amount) { if (balances[msg.sender] < amount) return; balances[msg.sender] -= amount; balances[receiver] += amount; Sent(msg.sender, receiver, amount); } } emanuel / br-blockchain-2
- 29. SOLIDITY STOP/REMOVE A CONTRACT FROM BLOCKCHAIN: SELFDESTRUCT ! pragma solidity ^0.4.0; contract owned { function owned() { owner = msg.sender; } address owner; } // a contract can be derived from another contract contract mortal is owned { function kill() { if (msg.sender == owner) selfdestruct(owner); } } //... } emanuel / br-blockchain-2
- 30. ETHEREUM CHALLENGES emanuel / br-blockchain-2
- 31. ETHEREUM CHALLENGES Transaction takes a couple of minutes to be mined Storage and execution is expensive Ecosystem is young emanuel / br-blockchain-2
- 32. ETHEREUM CHALLENGES Deal with immutability Test contracts fully before deployment Revert payments you don’t expect Avoid unrecoverable states emanuel / br-blockchain-2
- 33. THE DAO HACK emanuel / br-blockchain-2
- 34. ETHEREUM CHALLENGES EXAMPLE OF A RE-ENTRANCY ATTACK PROBLEM This can be exploited by an external contract msg.sender calling withdraw again (line 4) while balance is still non zero mapping (address => uint) private balances; // msg.sender is a user withdrawing funds function withdraw() public { uint amount = balances[msg.sender]; if (!(msg.sender.call.value(amount)())) { revert; } balances[msg.sender] = 0; } emanuel / br-blockchain-2
- 35. INTERESTING TECHNOLOGY OFF-CHAIN emanuel / br-blockchain-2
- 36. INTERESTING TECHNOLOGY OFF-CHAIN Oracles (ex: ) Whisper - comunication protocol for DApps ( ) P2P Data Storage Swarm - distributed storage with incentives ( ) IPFS - Interplanetary Filesystem ( ) http://www.oraclize.it https://github.com/ethereum/wiki/wiki/Whisper https://github.com/ethersphere/swarm https://ipfs.io emanuel / br-blockchain-2
- 37. SOME USEFULL LINKS emanuel / br-blockchain-2
- 38. SOME USEFULL LINKS Online compiler Another online compiler Online tools (block explorer, tx submit) https://ethereum.github.io/browser-solidity/ https://etherchain.org/solc https://testnet.etherscan.io https://etherscan.io emanuel / br-blockchain-2
- 39. SOME USEFULL LINKS (contract repository w/ source code) Infura.io (so you don’t have to run your own node/s) Truf e . Embark Open Zeppelin https://etherchain.org/ http://ether.fund/contracts/ https://github.com/ConsenSys/truf e https://github.com/iurimatias/embark- framework https://openzeppelin.org/ emanuel / br-blockchain-2
- 40. QUESTIONS ? @yarilabs emanuel / br-blockchain-2
- 41. EMANUEL MOTA @YARILABS [email protected] twitter: @emota7 github: emanuel HTTP://YARILABS.COM emanuel / br-blockchain-2