Smart Contract (Ethereum)

Put code in blockchain

  • a special tx type
    • a program (a class object): “smart contract”
    • a special account
      • has address
      • can hold balance
      • can receive / send
    • initialization execution
    • have permanent storage space
  • calling a function in a smart contract
    • by send
  • a language-level virtual machine
    • avoid non-deterministic operation
      • external: trusted data sources
    • avoid computation and storage abuse: gas
    • isolation for safety

Application exampls

  • IoT
  • paycheck systems
  • insurance

Application: token systems

  • token systems examples
    • reward points
    • new currency (coins)
    • shares
  • a ledger
    • key variable: balances
    • key function: transfer
  • ERC20: a template to set up a token system quickly
    • totalSupply()
    • balanceOf(account)
    • transfer(recipient, amount)
    • transferFrom(sender, recipient, amount)
    • approve(spender, amount)
    • allowance(owner, spender)
  • You can add other functions / smart contracts to “trade” tokens
    • between the token and ETH
    • between tokens

Application: stabilize price

  • between token and eth
    • set up a smart contract that buy a token at a price in the future
  • between token/eth and USD
    • A and B both deposit in a smart contract 1000 eth, worth of X USD at the time
    • after a pre-defined time period, return X-worth of eth to A and the rest to B

The DAO attack

  • decentralized autonomous organization
  • a token system that allows deposit and withdraw
function getBalance(address user) constant returns(uint) {
  return userBalances[user];
}

function addToBalance() {
  userBalances[msg.sender] += msg.amount;
}

function withdrawBalance() {
  amountToWithdraw = userBalances[msg.sender];
  if (!(msg.sender.call.value(amountToWithdraw)())) { throw; }
  userBalances[msg.sender] = 0;
}
  • attack/ recursive withdraw
function () {
  // To be called by a vulnerable contract with a withdraw function.
  // This will double withdraw.

  vulnerableContract v;
  uint times;
  if (times == 0 && attackModeIsOn) {
    times = 1;
    v.withdraw();
   
   } else { times = 0; }
}
  • a possible fix
function withdrawBalance() {
  amountToWithdraw = userBalances[msg.sender];
  userBalances[msg.sender] = 0;
  if (amountToWithdraw > 0) {
    if (!(msg.sender.send(amountToWithdraw))) { throw; }
  }
}