Cryptography & Block Structure

Blockchain uses cryptographic hashing to link blocks together and ensure data integrity. Learn what a hash is and how blocks are structured.

30 min•By Priygop Team•Last updated: Feb 2026

Cryptographic Hashing

A hash function takes any input and produces a fixed-length output (the hash). The same input always produces the same hash. Even a tiny change in input produces a completely different hash. It's impossible to reverse a hash to get the original input. Blockchain uses SHA-256 (Bitcoin) and Keccak-256 (Ethereum). Each block stores the hash of the previous block — this is what creates the 'chain'.

What's Inside a Block?

Index — Block number in the chain (0 = genesis block)
Timestamp — When the block was created
Transactions — List of all transactions in this block
Previous Hash — Hash of the block before it (creates the chain)
Nonce — Number used in mining (Proof of Work)
Hash — This block's own cryptographic fingerprint
Merkle Root — A single hash representing all transactions in the block

Block Structure Example

Example

// A simplified blockchain block structure

const genesisBlock = {
    index: 0,
    timestamp: "2009-01-03T18:15:05Z",
    transactions: ["Satoshi → Satoshi: 50 BTC (coinbase reward)"],
    previousHash: "0000000000000000000000000000000000000000000000000000000000000000",
    nonce: 2083236893,
    hash: "000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f",
};

const block1 = {
    index: 1,
    timestamp: "2009-01-12T03:30:25Z",
    transactions: [
        "Satoshi → Hal Finney: 10 BTC",  // First real Bitcoin transaction!
    ],
    previousHash: genesisBlock.hash,  // Links to genesis block
    nonce: 1639830024,
    hash: "00000000d1145790a8694403d4063f323d499e655c83426834d4ce2f8dd4a2ee",
};

console.log("Genesis Block:", genesisBlock.hash.substring(0, 20) + "...");
console.log("Block 1 links to:", block1.previousHash.substring(0, 20) + "...");
console.log("Chain is valid:", block1.previousHash === genesisBlock.hash);
console.log("\nIf we change Genesis Block data, its hash changes,");
console.log("breaking the link to Block 1 — tampering is detectable!");

Try It Yourself: Block Explorer

Try It Yourself: Block ExplorerHTML⚠ 1 error

⚠ Syntax Issues (1)

Line 30: Malformed tag: "< str.length; i++) { h ^= str.charCodeAt(i); h = (h * 0x01000193) >"

💡 Tags must use valid HTML names and be properly formed. Example: <div>, <p class="text">

HTML Editor

✕ 1 errorTab = 2 spaces

<!DOCTYPE html>
<html>
<head>
  <style>
    body { font-family: Arial, sans-serif; padding: 20px; background: #0f0f1a; color: #e0e0e0; }
    h2 { color: #a78bfa; }
    .block { background: #1a1a2e; border: 2px solid #a78bfa; border-radius: 10px; padding: 16px; margin: 12px 0; }
    .field { margin: 6px 0; font-size: 13px; }
    .label { color: #a78bfa; font-weight: bold; min-width: 130px; display: inline-block; }
    .hash { color: #34d399; font-family: monospace; font-size: 12px; word-break: break-all; }
    .tampered { border-color: #ef4444 !important; }
    .valid { border-color: #34d399 !important; }
    button { background: #7c3aed; color: white; border: none; padding: 9px 18px; border-radius: 6px; cursor: pointer; margin: 4px; }
    button.danger { background: #dc2626; }
    button:hover { opacity: 0.85; }
    #status { padding: 10px; border-radius: 6px; margin: 10px 0; font-weight: bold; }
  </style>
</head>
<body>
  <h2>🔍 Block Structure Explorer</h2>
  <div id="status"></div>
  <div id="blocks"></div>
  <button onclick="verifyChain()">✅ Verify Chain</button>
  <button class="danger" onclick="tamperBlock()">⚠️ Tamper with Block 1</button>
  <button onclick="resetChain()">🔄 Reset</button>

  <script>
    function simpleHash(str) {
      let h = 0x811c9dc5;
      for (let i = 0; i < str.length; i++) {
        h ^= str.charCodeAt(i);
        h = (h * 0x01000193) >>> 0;
      }
      return "0x" + h.toString(16).padStart(8, "0").toUpperCase();
    }

    let chain = [];

    function makeBlock(index, data, prevHash) {
      const b = { index, timestamp: new Date().toLocaleTimeString(), data, previousHash: prevHash };
      b.hash = simpleHash(JSON.stringify(b));
      return b;
    }

    function initChain() {
      chain = [];
      const g = makeBlock(0, "Genesis Block — First block ever", "0x00000000");
      const b1 = makeBlock(1, "Alice → Bob: 2.5 BTC", g.hash);
      const b2 = makeBlock(2, "Bob → Carol: 1.0 ETH", b1.hash);
      chain = [g, b1, b2];
    }

    function renderBlocks() {
      document.getElementById("blocks").innerHTML = chain.map(b => `
        <div class="block" id="block-${b.index}">
          <div class="field"><span class="label">Block #${b.index}</span></div>
          <div class="field"><span class="label">Data:</span> ${b.data}</div>
          <div class="field"><span class="label">Prev Hash:</span> <span class="hash">${b.previousHash}</span></div>
          <div class="field"><span class="label">Hash:</span> <span class="hash">${b.hash}</span></div>
        </div>
      `).join("");
    }

    function verifyChain() {
      let valid = true;
      chain.forEach((b, i) => {
        const el = document.getElementById("block-" + b.index);
        if (i > 0 && b.previousHash !== chain[i-1].hash) {
          el.className = "block tampered";
          valid = false;
        } else {
          el.className = "block valid";
        }
      });
      const s = document.getElementById("status");
      s.style.background = valid ? "#064e3b" : "#7f1d1d";
      s.textContent = valid ? "✅ Chain is VALID — all hashes match!" : "❌ Chain is INVALID — tampering detected!";
    }

    function tamperBlock() {
      chain[1].data = "Alice → Bob: 999 BTC (TAMPERED!)";
      renderBlocks();
      document.getElementById("status").style.background = "#78350f";
      document.getElementById("status").textContent = "⚠️ Block 1 data changed! Click Verify Chain to see the effect.";
    }

    function resetChain() {
      initChain();
      renderBlocks();
      document.getElementById("status").textContent = "";
      document.getElementById("status").style.background = "transparent";
    }

    initChain();
    renderBlocks();
  </script>
</body>
</html>

<!DOCTYPE html>
<html>
<head>
  <style>
    body { font-family: Arial, sans-serif; padding: 20px; background: #0f0f1a; color: #e0e0e0; }
    h2 { color: #a78bfa; }
    .block { background: #1a1a2e; border: 2px solid #a78bfa; border-radius: 10px; padding: 16px; margin: 12px 0; }
    .field { margin: 6px 0; font-size: 13px; }
    .label { color: #a78bfa; font-weight: bold; min-width: 130px; display: inline-block; }
    .hash { color: #34d399; font-family: monospace; font-size: 12px; word-break: break-all; }
    .tampered { border-color: #ef4444 !important; }
    .valid { border-color: #34d399 !important; }
    button { background: #7c3aed; color: white; border: none; padding: 9px 18px; border-radius: 6px; cursor: pointer; margin: 4px; }
    button.danger { background: #dc2626; }
    button:hover { opacity: 0.85; }
    #status { padding: 10px; border-radius: 6px; margin: 10px 0; font-weight: bold; }
  </style>
</head>
<body>
  <h2>🔍 Block Structure Explorer</h2>
  <div id="status"></div>
  <div id="blocks"></div>
  <button onclick="verifyChain()">✅ Verify Chain</button>
  <button class="danger" onclick="tamperBlock()">⚠️ Tamper with Block 1</button>
  <button onclick="resetChain()">🔄 Reset</button>

let chain = [];

function makeBlock(index, data, prevHash) {
      const b = { index, timestamp: new Date().toLocaleTimeString(), data, previousHash: prevHash };
      b.hash = simpleHash(JSON.stringify(b));
      return b;
    }

function initChain() {
      chain = [];
      const g = makeBlock(0, "Genesis Block — First block ever", "0x00000000");
      const b1 = makeBlock(1, "Alice → Bob: 2.5 BTC", g.hash);
      const b2 = makeBlock(2, "Bob → Carol: 1.0 ETH", b1.hash);
      chain = [g, b1, b2];
    }

function renderBlocks() {
      document.getElementById("blocks").innerHTML = chain.map(b => `
        <div class="block" id="block-${b.index}">
          <div class="field"><span class="label">Block #${b.index}</span></div>
          <div class="field"><span class="label">Data:</span> ${b.data}</div>
          <div class="field"><span class="label">Prev Hash:</span> <span class="hash">${b.previousHash}</span></div>
          <div class="field"><span class="label">Hash:</span> <span class="hash">${b.hash}</span></div>
        </div>
      `).join("");
    }

function tamperBlock() {
      chain[1].data = "Alice → Bob: 999 BTC (TAMPERED!)";
      renderBlocks();
      document.getElementById("status").style.background = "#78350f";
      document.getElementById("status").textContent = "⚠️ Block 1 data changed! Click Verify Chain to see the effect.";
    }

function resetChain() {
      initChain();
      renderBlocks();
      document.getElementById("status").textContent = "";
      document.getElementById("status").style.background = "transparent";
    }

initChain();
    renderBlocks();
  </script>
</body>
</html>

Result

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Edit the code on the left, then click Run

HTML|98 lines|3737 chars|1 error, 0 warnings

UTF-8

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