P15 - Accounts

Account-Based Ledger

Comparing Bitcoin and Ethereum:

• Bitcoin uses a transaction-based ledger, where account balances are inferred through UTXO.
• Ethereum uses an account-based ledger, similar to a banking system, where accounts directly store balance states.

Differences in transfer mechanisms:

• Bitcoin: When transferring from account B to C, the source of coins must be specified, and every UTXO must be entirely spent at once, so any remaining coins should be transferred back to another self-owned account.
• Ethereum: The source of coins during a transfer is the balance state of the account, as the account includes the balance state, so it does not need to be entirely spent at once.

         Transaction-based Ledger                    Account-Based Ledger             
 ┌────────────────────────────────────────┐     ┌───────────────────────────────────┐ 
 │      ┌──────────────────┐              │     │                                   │ 
 │      │                  │              │     │                                   │ 
 │┌─────▼───────┐         ┌┼─────────────┐│     │┌─────────────┐    ┌─────────────┐ │ 
 ││A───►B(10BTC)│         │B─┬──►C(3BTC) ││     ││A───►B(10ETH)│    │B───►C(3ETH) │ │ 
 ││             │         │  │           ││     ││             │    │             │ │ 
 ││             │         │  └──►B'(7BTC)││     ││             │    │             │ │ 
 │└─────────────┘         └──────────────┘│     │└─────────────┘    └─────────────┘ │ 
 └────────────────────────────────────────┘     └───────────────────────────────────┘               

Advantages of the account-based model: No need to consider the source of coins, inherently preventing double-spending attacks.

Replay Attack

Although the account-based ledger avoids double-spending attack, there is still a risk of replay attack.

Replay Attack: After transferring from account B to C, C maliciously broadcasts B’s transaction later, resulting in B being debited twice.

How to prevent Replay Attack?

1. Add a nonce state to the account as a transaction counter, recording the number of transactions posted by the account, the nonce becomes part of the transaction and is signed during transfers.
2. Transactions with a nonce less than or equal to the account’s current nonce are rejected.

┌─────────────────┐   
│ B -> C(10ETH)   │   
│                 │   
│   nonce = 21    │   
│                 │   
│    signed by A  │   
└─────────────────┘

Smart Contract Accounts

Two types of accounts exist in the Ethereum system:

1. Externally Owned Accounts: Controlled by user’s public-private key pairs, these accounts have a balance and a nonce.
2. Smart Contract Accounts: In addition to balance and nonce, these accounts include code and storage.
   • When a contract is created, it returns an address that can be used to make calls to the contract. During these calls, the coderemains unchanged.
   • Contract Accounts can’t initiate a transaction on their own, all transactions must be initiated by external accounts.
   • Contract Accounts also have a nonce, which records the number of times the contract has been invoked.
   • A contract can call another contract.

Why Ethereum Use Account-Based Ledger?

Ethereum is designed to support smart contracts, which require stable account addresses for execution.

Therefore, the account-based ledger is more suitable than the transaction-based ledger.

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