###### tags: `blockchain`

# 為什麼區塊鏈交易難以被竄改

需有ECDSA簽章與blockchain運作流程基礎知識

https://ethereum.org/en/developers/tutorials/how-to-mint-an-nft/#sign-txn
STEP 8 sign

## 原先困擾是 為什麼交易在區塊鏈節點無法竄改交易

畢竟節點是有廣播交易的功能 為何不能更改交易value 傳輸錯誤金額?

**因為使用者是在前端進行相關交易簽名的**
下圖的signTransaction
在該動作便已產生正確的簽名資訊
區塊節點只能去驗證簽名產出的是否為該地址公鑰
除非該節點擁有你的私鑰 不然沒有辦法自行製造交易

下圖中前端就已完成簽名相關動作
簽名交易訊息(signTransaction)->發送簽名(sendSignedTransaction)
**所以節點是沒有簽名動作的** **只接收交易和簽名資訊**

要竄改只會在前端發生
例如一開始簽署的合約金額就在前端被更改等等

require("dotenv").config()
const API_URL = process.env.API_URL
const PUBLIC_KEY = process.env.PUBLIC_KEY
const PRIVATE_KEY = process.env.PRIVATE_KEY

const { createAlchemyWeb3 } = require("@alch/alchemy-web3")
const web3 = createAlchemyWeb3(API_URL)

const contract = require("../artifacts/contracts/MyNFT.sol/MyNFT.json")
const contractAddress = "0x81c587EB0fE773404c42c1d2666b5f557C470eED"
const nftContract = new web3.eth.Contract(contract.abi, contractAddress)

async function mintNFT(tokenURI) {
  const nonce = await web3.eth.getTransactionCount(PUBLIC_KEY, "latest") //get latest nonce

  //the transaction
  const tx = {
    from: PUBLIC_KEY,
    to: contractAddress,
    nonce: nonce,
    gas: 500000,
    data: nftContract.methods.mintNFT(PUBLIC_KEY, tokenURI).encodeABI(),
  }

  const signPromise = web3.eth.accounts.signTransaction(tx, PRIVATE_KEY)
  signPromise
    .then((signedTx) => {
      web3.eth.sendSignedTransaction(
        signedTx.rawTransaction,
        function (err, hash) {
          if (!err) {
            console.log(
              "The hash of your transaction is: ",
              hash,
              "\nCheck Alchemy's Mempool to view the status of your transaction!"
            )
          } else {
            console.log(
              "Something went wrong when submitting your transaction:",
              err
            )
          }
        }
      )
    })
    .catch((err) => {
      console.log(" Promise failed:", err)
    })
}