Blockchain has already started disrupting or improving several industries with innovative solutions impacting our lives in more ways than we could have ever imagined. One such application of blockchain is a cryptocurrency. In this workshop, we create a simple cryptocurrency based on smart contracts, and talk about a few best practises on writing secure smart contracts to get you started with developing decentralized applications.

1. t
Introduction to
Blockchain &
Smart Contracts
April 30, 2018Sandip Pandey
1
Create your own Cryptocurrency Token

2. - Blockchain Enthusiast
- Blockchain Research Engineer
Blockchain Labs, TU Delft
- EIT Digital Alumni (2016)
https://github.com/xoriole
https://www.linkedin.com/in/sandippandey/
About Me
2

3. 1500+ more

4. Blockchain is a Solution looking for a Problem!

6. Source: Bitcoin: A Peer-to-Peer Electronic Cash System, S. Nakamoto

9. → SHOW US SOME PROOF
Proof of work
Proof of burn
Proof of activity
Proof of stake
Proof of capacity
Proof of elapsed time Proof of ...

10. - First digital decentralized cryptocurrency
- Founder: Satoshi Nakamoto
- Proof of work consensus
- Pseudonymous
Eg. 1CF6yUN61c4zmE2wTibhPhh53sJ5s4VLsa

11. Bitcoin Scripts are limited in capability & Restrictive
Input Script
Output Script

12. A decentralized platform that runs smart contracts
Smart Contract
A collection of code (functions) and data (state) that resides at a specific
address on the blockchain
- Digital Token
- Trustless Crowdsale
- Democratic Autonomous Organizations
- so on …
One step further towards a Programmable Economy

14. Trade
Finance
Supply
Chain &
Logistics
Health &
Medicine
Self
Sovereign
Identity
...Mortgages
Energy &
Smart Grid
Securities &
Insurance

15. ❏ Multiple interacting entities
❏ Data represents some form of value or asset
❏ Data is shared among the entities
❏ All updates on data need to be logged
❏ Entities need to trust the shared state of data & all updates made on them
❏ CONSISTENCY, VALIDITY and IMMUTABILITY of the data is important
❏ Incremental data dependency
❏ High operational costs (intermediary)

16. Immutability isn’t always a good thing
- Data written on blockchain cannot be changed or deleted
- No correction of mistakes (Hard Fork?)
- Legal issues: EU - Right to be forgotten
- Think before writing privacy sensitive information on blockchain
Performance Issues
- Transaction delay & Confirmation time
- Transaction throughput (tx/second)

18. User Account
- Address
- Balance
Smart Contract
- Address
- Balance
- Code
- State
Source: http://www.gjermundbjaanes.com/img/posts/smart_contract_infographic.png

19. - Transfer of Value
- Smart Contract creation
- Call contract

20. - Transfer of Value
- Smart Contract creation
- Call contract
{
‘to’: ‘0x990FcbF55D6eD9493c62e11da388A4D89d857200’,
‘value’: 0.500
‘data’: ‘0x’
}

21. - Transfer of Value
- Smart Contract creation
- Call contract
{
‘to’: ‘’,
‘value’: 0.0
‘data’: ‘0x60806040526040805190810160405280601a….’
}

22. - Transfer of Value
- Smart Contract creation
- Call contract
{
‘to’: ‘0x990FcbF55D6eD9493c62e11da388A4D89d857200’, // contract
‘value’: 0.0
‘data’: ‘0xa9059cbb000000000000000000000000990fcbf55d...c8’
}

23. Source: http://www.gjermundbjaanes.com/img/posts/calling_a_smart_contract_infographic.png

25. ❏ Ethereum wallet
❏ Some ethers
❏ An IDE to write and deploy smart contract

26. ❏ Many options - Ethereum wallet (official)/Mist, Metamask, MyEtherWallet, ...
❏ We use Metamask chrome plugin ( https://metamask.io )

27. Create an account with a
good password
Copy the seed words and
keep it safe.
Your wallet account home
screen
Copy your address.
For eg. 0xC594...277

28. ❏ Ether Network Chains
❏ Main network (ETH)
❏ Test networks
❏ Ropsten
❏ Kovan
❏ Rinkeby << We’ll use this one
❏ Others
❏ Expanse (EXP)
❏ Ubiq (UBQ)
❏ ...

29. ❏ Rinkeby Faucet
https://faucet.rinkeby.io/

30. ❏ Options
❏ Ethereum Wallet/Mist
❏ Remix IDE
https://remix.ethereum.org
❏ ...

32. Some information about token
- Name,
- Symbol,
- Number of decimals,
- total supply

33. Some information about token
- Name,
- Symbol,
- Number of decimals,
- total supply
Some information of account and account balance
- Which address has how many tokens?

34. Some information about token
- Name,
- Symbol,
- Number of decimals,
- total supply
Some information of account and account balance
- Which address has how many tokens?
Movement of tokens
- Transfer of tokens from one account to another

35. pragma solidity ^0.4.12;
contract MyFirstToken {
string public name = "MyFirstToken";
string public symbol = "MFT";
uint8 public decimals = 2;
uint256 public totalSupply;
// This creates an array with all balances
mapping (address => uint256) public balanceOf;
/**Constructor. Initializes contract with some initial supply tokens.*/
function MyFirstToken( uint256 initialSupply) public {
totalSupply = initialSupply * 10 ** uint256(decimals);
balanceOf[msg.sender] = totalSupply;
}
// transfer function in next slide
}
[ http://bit.ly/my-first-token ]

36. pragma solidity ^0.4.12;
contract MyFirstToken {
/** Transfer tokens.*/
function transfer(address _to, uint _value) public {
// Prevent transfer to 0x0 address.
require(_to != 0x0);
// Check if the sender has enough
require(balanceOf[msg.sender] >= _value);
// Check for overflows
require(balanceOf[_to] + _value >= balanceOf[_to]);
// Save this for an assertion in the future
uint previousBalances = balanceOf[msg.sender] + balanceOf[_to];
// Subtract from the sender
balanceOf[msg.sender] -= _value;
// Add the same to the recipient
balanceOf[_to] += _value;
//Make sure math is right. Assertions should never fail
assert(balanceOf[msg.sender] + balanceOf[_to] == previousBalances);
}
}
[ http://bit.ly/my-first-token ]

37. Requirements
- Basic Token +
- Foundation creates the token
- Token should be transferable
- Only alumni can send/receive tokens
- Foundation can register alumni in blockchain
http://bit.ly/alumni-credits
[ http://bit.ly/alumni-credits ]

38. pragma solidity ^0.4.12;
contract AlumniCredits {
string public name = "AlumniCredits";
string public symbol = "EITDAC";
uint8 public decimals = 2;
uint256 public totalSupply;
// Foundation address
address public foundation;
// This creates an array with all balances
mapping (address => uint256) public balanceOf;
/**Constructor. Initializes contract with some initial supply tokens.*/
function AlumniCredits( uint256 initialSupply) public {
foundation = msg.sender; // contract creator is the foundation
totalSupply = initialSupply * 10 ** uint256(decimals);
balanceOf[foundation] = totalSupply;
registered[foundation] = true;
}
}
[ http://bit.ly/alumni-credits ]

39. pragma solidity ^0.4.12;
contract AlumniCredits {
// All registered/valid alumni addresses approved by the foundation
mapping (address => bool) registeredAlumni;
/** Transfer tokens.*/
function transfer(address _to, uint _value) public {
//Make sure receiver is also alumni
require(registeredAlumni[_alumni] == true);
// Rest is same as basic token…
}
// Register an alumni address.Can be done by foundation only.
function registerAlumni(address _address) public onlyFoundation {
registeredAlumni[_address] = true;
}
//Check if an address belongs to a registered alumni
function isRegistered(address _address) public view returns (bool registered){
return registeredAlumni[_address] == true;
}
}
[ http://bit.ly/alumni-credits ]

41. // Abstract contract for the full ERC 20 Token standard
// https://github.com/ethereum/EIPs/issues/20
pragma solidity ^0.4.8;
contract Token {
string public name;
string public symbol;
uint8 public decimals = 18;
uint256 public totalSupply;
function balanceOf(address _owner) constant returns (uint256 balance);
function transfer(address _to, uint256 _value) returns (bool success);
function transferFrom(address _from, address _to, uint256 _value) returns (bool success);
function approve(address _spender, uint256 _value) returns (bool success);
function allowance(address _owner, address _spender) constant returns (uint256 remaining);
event Transfer(address indexed _from, address indexed _to, uint256 _value);
event Approval(address indexed _owner, address indexed _spender, uint256 _value);
}

42. - Determined at the time contract is created
- Derived from creator address and “nonce”
- A Contract cannot be re-deployed at the same contract address

43. - Determined at the time contract is created
- Derived from creator address and “nonce”
- A Contract cannot be re-deployed at the same contract address
BUG IN THE CONTRACT
HOW TO FIX THEM?

44. 1. Separate Data and Logic
Eg. prepare separate contracts for data storage and application logic
2. Maintain a versioning of contracts (sort of)
3. Delegator/Proxy Contract
Delegator delegates all the calls to latest implementation of the contract

45. Identity & Access Control
- Who has what level of control on the contract functions and data?
- Access & privilege control
Data Privacy
- Data encryption, Encryption Algorithms,
- Digital Signatures etc.

46. Future is decentralized!