Smart Contracts and Ethereum Winter School on Cryptocurrency Loi - - PowerPoint PPT Presentation
Smart Contracts and Ethereum Winter School on Cryptocurrency Loi Luu and Blockchain Technologies National University of Singapore Shanghai, Jan. 15-17 2017 Some slides are courtesy of Vitalik Buterin 1 Agenda Smart contracts and
Some slides are courtesy of Vitalik Buterin
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– Complex access policies depending on amount, withdrawal limits, etc – Dead man’s switch, “digital will”
– Prediction markets – Insurance – Micro-payments for computational services (file storage, bandwidth, computation, etc)
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**********
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value storage)
– Send ETH to other accounts – Read/write storage – Call (ie. start execution in) other contracts
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Private Storage Can be invoked by
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Ethereum VM Bytecode Stack Language Lower-Level Language Serpent Solidity
Functional, macros, looks like scheme
Types, invariants, looks like Javascript Looks like python Looks like Forth. Defined in Yellowpaper
Slide is courtesy of Andrew Miller
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60606040526040516102503 80380610250833981016040 528........ PUSH 60 PUSH 40 MSTORE PUSH 0 CALLDATALOAD ..... What you write What other see on the blockchain What people get from the disassembler
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– nonce: previous nonce + 1 – to: empty – value: value sent to the new contract – data: contains the code of the contract – gasprice (amount of ether per unit gas) – startgas (maximum gas consumable) – v, r, s (ECDSA signature values)
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– nonce: previous nonce + 1 – to: contract address – value: value sent to the new contract – data: data supposed to be read by the contract – gasprice (amount of ether per unit gas) – startgas (maximum gas consumable) – v, r, s (ECDSA signature values)
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Address Balance (BTC) 0x123456… 10 0x1a2b3f… 1 0xab123d… 1.1
Address Object 0x123456… X 0x1a2b3f… Y 0xab123d… Z
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Tx-n Tx-1
Miners
Tx-2
Block
A set of TXs Previous block New State Root Receipt Root Nonce
SHA3(Block) < D Broadcast Block
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Verify transactions & execute all code to update the state
This is a new block! I’m a leader This is a new block! This is a new block! This is a new block! This is a new block! This is a new block!
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uint i = 1; while (i++ > 0) { donothing(); }
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– If startgas is less than needed
happened
– Total gas spent by all transactions in a block < Gas Limit
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Bitcoin Network Ethereum Network BTCRelay
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Bitcoin Ethereum The verified Bitcoin transaction is relayed to the smart contract A Bitcoin transaction is submitted, BTCRelay verifies TX based on the block header Relayers constantly submit Bitcoin block headers
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ETH-BTC Swap contract 50 ETH for anyone who sends 1 BTC to my address BTCRelay I sent 1 Bitcoin to Alice address, here is the proof P Check proof P Bitcoin Network Send 1 BTC to Alice address Send 50 ETH
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Question: can we build a decentralized exchange between cryptocurrencies using all the relays?
– Miners go to mining pools for stable and frequent rewards – Decentralized platforms are secured by centralized entities
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Bitcoin Network Block Block Shares Block
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Pool operator
– Maintain a share-chain within the pool (just like the blockchain) – Pay miners in proportional to their contributions
– Broadcast to all miners – Check if the coinbase tx is correct and extend the share-chain
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Bitcoin Network Shares Block P2Pool
Bitcoin Network Shares Block
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P2Pool
– Reduce number of messages (txs) to the contract significantly
– Randomly verify only one share per submission – Probability of cheating being detected is proportional to the amount of cheating
SmartPool Submit Sample &Chec k
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Get 1.5 Reward with 2/3 probability Get 0 Reward with 1/3 probability Probabilistic verification passed detected Expected reward = 1
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Reward = 1
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E E E E E E
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Shard 1 Shard 2 Shard 3
– Scaling by using off-chain transactions – Can update the state multiple times – Only settlement transactions are on- chain
– Cannot create state channel for all applications – Still early research, more work needed
Blockchain TX1 TX2 X’s Initial State X’s Final State TX3 TX4 Many states i Alice Bob
Contract X
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PuzzleSolver() SetPuzzle reward=100 PuzzleSolver Contract SubmitSolution(solution) if isCorrect(solution): Send(reward) UpdateReward(newReward) reward=newReward Owner can update the reward anytime Anyone can submit a solution to claim the reward
Balance: 100
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Random TXs
PuzzleSolver() SetDifficulty reward=100 PuzzleSolver Contract SubmitSolution(solution) if isCorrect(solution): Send(reward) UpdateReward(newReward) reward=newReward Miners
Other TXs Solution for Puzzle
Random TXs SubmitSolution Other TXs
+100 Balance: 100 Balance: 0
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PuzzleSolver() SetDifficulty reward=100 PuzzleSolver Contract SubmitSolution(solution) if isCorrect(solution): Send(reward) UpdateReward(newReward) reward=newReward Miners
Other TXs Solution for Puzzle Update Reward to $0!
Block
UpdateReward = 0 SubmitSolution Other TXs
+0 Balance:100 Balance: 0
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Other TXs Solution for Puzzle Update Reward to $0! 70
Other TXs Solution for Puzzle Update Reward to $0! 71
– Most expensive vulnerability to date
... // Burn DAO Tokens if (balances[msg.sender] == 0) throw; withdrawRewardFor(msg.sender); totalSupply -= balances[msg.sender]; balances[msg.sender] = 0; paidOut[msg.sender] = 0; return true;
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withdrawRewardFor(msg.sender) splitDAO(proposal, address)
function() {}
rewardAccount.payOut(_account, reward) balances[msg.sender] = 0;
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withdrawRewardFor(msg.sender) splitDAO(proposal, address)
splitDAO()
rewardAccount.payOut(_account, reward)
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EXPLORER
CORE ANALYSIS
Z3 Bit-Vector Solver
VALIDATOR
ByteCode Ethereum State CFG BUILDER Visualizer 6060604052123 123123528.....
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F T F T T F F T T T T F
3 2 1
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Is there any value of x?
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5411 3056 340 83 1385 135 186 52 1000 2000 3000 4000 5000 6000
Callstack TOD Reentrancy Timestamp
Flagged Buggy Contracts
Total Unique F T F T T F F T 80
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