# Blockchain Notes

## 期中考範圍 Chapter 1
### Security Objectives
* CIA triad
* Confidentiality:
* **Data Confidentiality**
* Shield data from all unauthorized parties
• Is achieved through data encryption
• Shield data from unauthorized parties who are differentiated on the basis
of access right
* **User Confidentiality**
* There is reason to expect that a user remains unidentifiable
• Ensure untraceability
• Ensure unlinkability
* Integrity
* Data integrity is concerned with ensuring that data are not tampered
* Availabitity
* Data and online services are of no use if they are not available when authorized users need them
* addtional two
* Authentication
* Authentication extends to both data authentication and user authentication
* **Data authentication**
• Verify that the origin of a piece of data is what it claims to be
* **User authentication**
• Verify that a user is whom they claim to be
* Accountability
* Thwart a user from falsely denying that they have sent or received a particular communication
• Similar to the concept of non-repudiation

### Properties of hash functions (6 points)
* Deterministic: Same input always get the smae output
* Variable-length input fixed-length output
* avalanche effect: 雪崩效應
* Pre-image resistance: Get only hash value is computationally infeasible to find what the input is
* Seconde pre-image resistance: give input and hash, there is computationally infeasible to find anoth different input that produces the same hash
* Collision resistance: computationally infeasible to find two different inputs that produce the same hash value

### Distributed Ledger Technology(DLT) 分散式帳本

* Define: DLT is a method of record-keeping whereby every person involved in the record-keeping possesses a copy of the ledger
* DLT is a method of record-keeping whereby every person involved in the record-keeping possesses a copy of the ledger

### Merkle Hash Tree
* is a complete binary tree
* using bottom-up construction, all leaf nodes are transactions
*

### Proof of Work
The proof-of-work (PoW) consensus algorithm in Bitcoin requires
miners to vary the 32-bit nonce and iteratively calculate the block
header hash until the resulting hash satisfies the difficulty target

### Digital Signature
* Digital signature achieves user accountability, data authentication and integrity protection
* Signing requires the signer’s message and private key
* Verifying requires the verifier to perform the arithmetic comparison using the signer’s public key, message, and digital signature

### RSA vs ECDSA(Elliptic Curve Crytography)
* Elliptic Curve Digital Signature Algorithm (ECDSA) is adopted in Bitcoin and widely used in the blockchain system
* RSA:
N=p*q
r=(p-1)(q-1), 選一個e<r 且 e與r互質
d = e相對於r的模逆元
n=plain text 轉換後的數字
c=plain text 加密後的秘文
(N, e) 公鑰
(N, d) 私鑰
加密: c = n^e mod N
解密: n = c^d mod N

### P2PKH vs P2SH
* P2PKH: Pay to Public Key Hash:
The act of specifying the receiver’s public key hash in a bitcoin transaction is
a mechanism called “Pay to Public Key Hash (P2PKH)

### Public key infrastructure(PKI): 公開金鑰基礎建設架構
* A certificate is a digital document that serves as proof that a particular public key belongs to a specific user (the subject)

### Privacy(以下兩種方法解決)

#### Coinjoin
**Define:** A trustless method that allows multiple bitcoin spenders to come together
and mix their input and output, so that observers cannot easily trace how
they are spending their bitcoin

#### Zero-Knowledge Proofs
**Define:** 不提供任何有關訊息的資料，但仍然可以說服對方該筆訊息是正確的。
Such proofs have three properties:
1. Completeness. If the prover is honest, the prover will eventually convince the verifier
2. Soundness. The prover can only convince the verifier if the statement is true
3. Zero-knowledge. The verifier learns nothing about the statement aside from the fact that it is true

## 期末考範圍(Chapter2, 3, 4)

### Chapter2 **Consensus for Blockchain and Distributed Ledger Technologies**

#### Consensus Algorithm
For consensus algorithms to achieve the objective to maintain a validblockchain ledger, it needs to be able to do the following
* If bad actors try to broadcast invalid transactions, they should be ignored by the good nodes and not included in the ledger
* When bad nodes try to mine a block with invalid or fraudulent transactions, the majority of the network should agree not mine on top of it

#### Network fault tolerance
The consensus algorithm is the set of rules that the nodes (or “generals”) have to follow
* A. All good nodes have to agree on the same plan of action (whether a transaction is valid)

a. All good nodes will follow the rules set by the consensus algorithm; check the ledger history to ensure that a transaction is valid before putting on the ledger. Bad nodes can do what they want such as work together to invalidate legitimate transactions or validate double-spent transactions (by invalidating the first transaction)

b. The consensus algorithm must guarantee A no matter what the bad nodes do
* B. As long as the number of bad nodes is small enough, the good nodes will not adopt an invalid transaction

* The algorithm can ensure correct operation only if fewer than one-third of the processes are faulty →
**n>=3m+1**(𝑛 is the number of generals in the system;𝑚 is the number of traitors)

* A malicious actor needs to control around 50% of the network to successfully launch a Byzantine attack on a PoW network

* In general, there are two fundamental stages to a BFT system:
1. **Agreement**
• Nodes agree on what should be done
• Each consensus algorithm has its way for the majority of the nodes to reach an
agreement
• Once the outcome is agreed upon, it moves on to the execution stage and puts the outcome on the ledger
2. **Execution**

#### Consensus for Trustless Blockchains
* Proof of Work(PoW)
* competitive
* **Steps**
* Receive new broadcasted transactions and check them against the ledger to ensure that they are legitimate (sender has signed the transaction and sender address has enough balance)
* Discard the transaction if it is invalid
* Package legitimate transactions into the current block
* Attempt to mine the block
* If successful, broadcast the mined block to the network. Start again from Step 1 and mine on top of the block you just created
* If not successful (next block is broadcasted before yours), check that the new block broadcasted is valid, go back to Step 1 and mine on top of the new block
* Proof of Stake(PoS)
* competitive
* Delegated Proof of Stake(DPoS)
* competitive
* Proof of Weight
* Proof of Believability (PoB)
* Proof of Space (PoSpace)
* Proof of Importance(PoI)
* Proof of Burn(PoB)
* random selection process
* The more you burn, the higher the chance of being selected to mine the next block
* This method also does not address the fairness issue; those with money to “burn” have higher chances of mining

#### Consensus for Trusted Blockchains
* Proof of Authority (PoA)
* Validation is done by approved accounts called **validators**
* 選出 Validator 三特點
* It must be a verified identity that links the node to **real world identity**
* It must be difficult to become a validator
* There must be complete **uniformity** in the checks and procedures for establishing an authority
* Usually used in private and permissioned blockchains

* Proof of Reputation(PoR)
* a potential validator needs to pass verification and prove their reputation
* Existing validators may vote on the eligibility of the new entrant or a weighted matrix may be used to decide
* Usually used in private and permissioned blockchains

* Proof of Elapsed Time (PoET)
* Practical Byzantine Fault Tolerance (PBFT)
* One of the first solutions proposed for the Byzantine Generals Problem
* There is a **leader node** in a PBFT system, and all other nodes are backup nodes
* Every node will communicate with each other to come to agreement on the state of the system using **majority voting**
* The number of malicious nodes cannot exceed **one-third** of the system
* Is usually only used in permissioned networks as there is no incentive mechanism to prevent untrusted malicious nodes from joining the network and making up the majority

#### Hybrid Blockchain Networks
* Delayed Proof of Work (dPoW)
* Directed Acyclic Graphs (DAGs)
* DAGs are generalized forms of blockchain and can be applied to **both trusted and trustless** networks
* The DAG structure allows transactions to be **added in a parallel fashion**, making it highly scalable
* Although deemed be the next-generation blockchain structure, one of the main issues of DAGs is that smart contracts are usually implemented via oracles and not be directly deployed on chain
* Tangle(DAG consensus)
* Hashgraph

### Chapter3 **Smart Contracts**

#### Smart Contracts
Def:
* Be proposed as a means to embed contractual clauses into digital assets
* For smart contracts to be useful for digital assets, **transparency** and **trust** are needed between contractual parties
* Smart contracts are **stateful decentralized applications** that are stored in the Ethereum blockchain for later execution by the EVM

#### Decentralized APP

#### ERC-20(Ethereum request for comment, 20 stands for unique ID)
* The Token rules or standard
* ERC-20 tokens follow a list of rules so that they can be shared, exchanged for other tokens or transferred to a crypto-wallet; these are defined in the ERC-20 Interface
* The ERC-20 standard consists of three optional rules and six mandatory rules
* Mandatory rules:
* totalSupply
* balanceOf
* Transfer
* transferFrom
* approve
* allowance
* Optional rules:
* Token Name
* Symbol
* Decimal (up to 18)

### Chapter4 **元宇宙 & NFT**

#### 元宇宙的7層構成要素
* 第一層 體驗層
元宇宙既不是三維的，也不是二維的，而是對現實空間、距離及物體的「非物質化」，現實生活中無法實現的體驗在元宇宙中都會變得觸手可及。
* 第二層 發現層
聚焦於將人們吸引到元宇宙的方式
* 主動發現機制: 使用者自發找尋，元宇宙的重要價值體現於在共有體驗基礎上的玩家之間的雙向互動
* 被動輸入機制: 使用者並無確切需求、沒有發起選擇時推薦給使用者
* 第三層 創作者經濟層
未來在元宇宙中，借助更多樣化的設計工具，創作者可以完成更多樣的創作，透過元宇宙中的資產市場進行交易，從而獲得收益。
* 第四層 空間計算層
軟體演算法能把人或物轉化為數位地圖，建立一個可量化、可操縱的數位世界，這為混合現實、虛擬計算提供了解決方案，而消除現實世界和虛擬世界之間的障礙。
* 第五層 去中心化層
* 元宇宙是個去中心化的世界，每個人都可以在這個世界裡自由、無限地創作內容
* 誰在元宇宙中創造的價值多，誰就會獲得更多的回報
* 對於自己創作的內容，創作者擁有獨立的資料主權
* 區塊鏈技術將為元宇宙的去中心化運作提供支援
* 第六層 人機互動層
微型裝置與人類軀體的結合將更加緊密，Augmented Reality(AR)/Virtual reality(VR)。
* 第七層 基礎設施層
括支援人們進入元宇宙的設備，以及將現實世界和虛擬世界連接到網路並提供內容的技術，例如更小的硬體設備、更快且低延遲的網路、持久耐用的電池。

#### 元宇宙四大核心屬性

* 同步和擬真
現實世界中產生的一切活動都將同步到虛擬世界中，同時，使用者在虛擬世界中進行互動也能得到真實的回饋資訊。
* 開源和建立
技術、資料的開源，借助各種標準和協定將程式碼進行封裝和模組化，使用者可依據自己的需求進行建立，豐富虛擬世界，不斷擴展元宇宙邊界。
* 持續性
從時間角度來講的，若把元宇宙比作一個遊戲，那麼它就是一個「永不結束的遊戲」，既不隨著某個營業公司的消亡而消亡，也不會因為某個國家的消亡而消亡。
* 循環經濟系統
使用者可以在元宇宙中工作賺錢，使用虛擬貨幣來消費，也可以將其兌換為現實世界中的貨幣，經濟系統是驅動元宇宙不斷發展的引擎。