Polkadot Smart Contracts

About Me

• Kian Paimani aka. @kianenigma
• Engineering Lead @ Parity Technologies
• Polkadot Core Fellow
• blog.kianenigma.com (<-- slides)

Blockchain Models

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Blockchain Models

• Database 🤮
• A legacy of Bitcoing, and the (limited) UTXO model

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Blockchain Models

• State Machine ♾️
• Legacy of Ethereum, and the YP.

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Blockchain Models

• State Machine ♾️
  • State, represented as a list of key -> values
  • State Transition Function

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Blockchain Models

• Computer 💻
• Legacy of Ethereum (the "World Computer"), Polkadot JAM's Graypaper.

Note:

Database is utter BS
State Machine is accurate, but a bit to math-y
Computer is accurate, and one that most people relate to, we will use this one

The Blockchain Computer

• ­Computer has code and memory.
• ­Users can interact with the code (trigger a transfer 💸)
• ­memory stores valuable information (how much money I own 🤑)
• ­The main novelty of a computer is that it will execute code correctly, and therefore you can TRUST the memory. 🪄

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The Blockchain Computer

• First generation blockchains had a fixed code.
• What is the code / memory of the Bitcoin computer?
  • Transfer of BTC
  • Is it extensible?

Note:

First blockchains were computers with a (almost) fixed code. Bitcoin's code is ONLY how to do transfer of BTC, and therefore the State of the Bitcoin computer was only user balances. Nothing more.

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The Blockchain Computer

• Smart Contracts are the first step into making blockchain computers programmable.

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Blockchain Computer

• Ethereum is a programmable blockchain computer
• What is the code of the Ethereum computer?
  • Transfer of ETH
  • Upload a contract
  • Call into an existing contract, which might alter memory

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Blockchain Computer

• Execution of these contracts is as secure as the execution of the main blockchain's code. 🪄
• If you can trust Ethereum and its ability to correctly transfer ETH, you should also trust that it can correctly execute whatever is the code of any smart contract.

What Is A Smart Contract

A way to add programs with custom code and memory to the blockchain computer, making it extensible, with the very same security guarantees.

Anatomy Of Smart Contracts

A Smart Contract has:

• code
• state/memory
• EVM has an opinion to see contracts exactly like user accounts
  • address (at which it can be invoked)
  • balance: money owned by the contract

Note:
this is based on Ethereum/Solidity/EVM contracts
Bringing it all together

How to Write a Contract

• Ethereum Contracts are written in a language Solidity
  • Part of the Ethereum code is to execute Solidity
• This happens in a virtual machine known as EVM
• Solc: Solidity -> EVM Bytecode

Note:

EVM: Ethereum Virtual Machine

How to Execute a Contract

If Ethereum wants to execute a totally untrusted code, what challenges does it have?

• ­Deterministic cost (gas)
  • ­⏰ Metering!
  • ­⛽️ gas_limit!
• ­Deterministic output (EVM)
  • ­🫠 We I cannot make an HTTP request in a contract!

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Extending Memory

What about the fact that a contract can add more data to its own memory, which in turn bloats the overall blockchain memory?

• State cost needs to be taken into account
  • Part of gas
  • Deposit
  • State Rent

Lifecycle

Case Studies

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Case Study

Simple Escrow Contract

• Buyer deposits funds
• Both buyer and seller approve
  • Transfer funds to seller
• Timeout
  • Return funds to buyer

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Case Study

Web3 Roulette

• Deposit Phase
  • Anyone can deposit money into the contract
• Roulette Phase
  • One participants get nothing back, their money split among everyone else

Advance Topics

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Composability ~ Shared Environment

• VMs on a hardware
  • Ethereum => Hardware
  • Contracts => VMs
• Synchronous and Asynchronous

Note:

Contracts on Ethereum can always call one another

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Re-Entrancy Attacks

contract EtherStore {
    mapping(address => uint256) public balances;

    function deposit() {
        balances[msg.sender] += msg.value;
    }

    function withdraw() {
        uint256 bal = balances[msg.sender];
        require(bal > 0);

        (bool sent,) = msg.sender.call{value: bal}("");
        require(sent, "Failed to send Ether");

        balances[msg.sender] = 0;
    }
}

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Re-Entrancy Attacks

contract Attacker {
	// Our interface to EthStore contract
    EtherStore public etherStore;

    receive() external payable {
        etherStore.withdraw();
    }
}

• Checks-Effects-Interactions

Note:

Cyfrin Code Glossary: Re-entrancy Hack in Solidity

Smart Contracts in Polkadot

Polkadot

• Computer who's code allows other Blockchains to be executed and validated.
• Polkadot always assumed one of these external blockchains (Parachains) would deliver smart contract capability.

Past

• A lot of Polkadot-based blockchains use the above tech stack

Note:

• Writing smart contracts in Rust: Ink!, executed in a WASM.
• Writing in Solidity, executed in EVM pallet-evm

Both of these initiated in Parity, but are pursued as community projects now.

Now

• A new, much better virtual machine based on RiskV: PolkaVM

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Now

• Outcome: Polkadot will have fast Solidity-compatible Smart Contracts in H2 2025
• Demo

Open Question

• Smart Contract vs. AppChain Model

Outro

• Smart Contracts are the most basic form of building an application on top of a secure blockchain computer 🪄.
• Battle tested approach that has been used for almost a decade now

Questions