feat: P15 ETH Accounts & P21 Proof of Stake (POS) & P23 The DAO Incident

README.md

@@ -32,7 +32,7 @@ This course focuses on the fundamental concepts and implementation principles of
 
 ### Ethereum
 14. **[Overview](./ETH/Overview.md)**
-15. **Accounts**
+15. **[Accounts](./src/ETH/Accounts.md)**
 16. **State Trees**
 17. **Transaction and Receipt Trees**
 18. **GHOST Protocol**

src/ETH/Accounts.md

@@ -0,0 +1,59 @@
+# 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.
+
+```txt
+         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.
+
+```txt
+┌─────────────────┐   
+│ 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 `code`remains 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.

src/Introduction.md

@@ -32,7 +32,7 @@ This course focuses on the fundamental concepts and implementation principles of
 
 ### Ethereum
 14. **Overview**
-15. **Accounts**
+15. **[Accounts](./ETH/Accounts.md)**
 16. **State Trees**
 17. **Transaction and Receipt Trees**
 18. **GHOST Protocol**