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Smart Contracts @ Ethereum By Vineet Kumar Singh Innovation Lead, Citi Innovation Lab HyperLedger Corda

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Smart Contracts @ Ethereum By Vineet Kumar Singh Innovation Lead, Citi Innovation Lab HyperLedger Corda

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Contracts don't make anything possible that was previously impossible, but rather, they allow you to solve common problems in a way that minimizes trust. Minimal trust often makes things more convenient by allowing human judgements to be taken out of the loop, thus allowing complete automation*. *https://en.bitcoin.it/wiki/Contract

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Decentralized provable execution of logic. A smart contract’s state is stored on the public blockchain. A smart contract program is executed by a network of miners who reach consensus on the outcome of the execution, and update the contract’s state on the blockchain accordingly. Users can send money or data to a contract; or receive money or data from a contract

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Block i Block i+1 Block i+2 Block i+3 Block i+4 Mined Block Code Storage Miners Contracts Users Turing Compatible Smart Contracts Compatible Blockchains Data Money Blockchain

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Contract Development Solidity, Serpent, LLL Contract Interaction WEB3 Javascript Module

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Contract Development Key Concepts

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Solidity Contracts ● State Variables ● Functions ● Function Modifiers ● Events ● Structs ● Enum Types Contracts in Solidity are similar to Classes in Object Oriented Languages.

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State Variables contract SimpleStorage { uint storedData; // State variable // ... } ● external ● public ● internal ● private

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Functions contract SimpleAuction { function bid() { // Function // ... } }

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Function Modifiers contract Purchase { address public seller; modifier onlySeller() { // Modifier if (msg.sender != seller) throw; _ } function abort() onlySeller { // Modifier usage // ... } }

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Events contract SimpleAuction { event HighestBidIncreased(address bidder, uint amount); // Event function bid() { // ... HighestBidIncreased(msg.sender, msg.value); // Triggering event } }

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Structs contract Ballot { struct Voter { // Struct uint weight; bool voted; address delegate; uint vote; } }

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Enums contract Purchase { enum State { Created, Locked, Inactive } // Enum }

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Walkthrough: Escrow Contract Selling goods over the internet without any trusted third party

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Bob Alice 1. Buyer is scammer ??

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Bob Alice Buyer is scammer ??

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Alice Buyer is scammer ?? ??

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Bob Alice 2. Seller is scammer ??

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Bob Alice 2. Seller is scammer ??

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Bob 2. Seller is scammer ?? ??

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Bob Alice Use an Escrow third Party Eve

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Bob Alice Use an Escrow Third Party Eve

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Bob Alice Use an Escrow Third Party ?? ??

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Bob Alice Two Party Escrow original idea Oleg Andreev

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Bob Alice Two Party Escrow Seller puts 2x amount

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Bob Alice Two Party Escrow Buyer puts amount + deposit

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Bob Alice Two Party Escrow Buyer puts amount + deposit

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Bob Alice Two Party Escrow Buyer Confirms: Received Buyer gets deposit, Seller gets deposit + sale

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Bob Alice Two Party Escrow Buyer unhappy with goods Received Seller refunds or money in escrow until they resolve

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Code

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Web3 API’s

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Directory Structure

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Checking for the RPC Connection to Local Node

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Compiling the Solidity Contract

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Deploying Contract To Blockchain

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Secure Contract Principles... ● Error in encoding state machine. ● Failing to use cryptography. ● Misaligned Incentives. Ethereum Specific Mistakes ● Call stack bug ● Blockhash bug ● Not checking return value of sender.send();

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Thank You