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Bitcoin Internals: Verifying Merkle Roots using Merkle Proofs in JavaScript
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const fetchLatestBlock = () => | |
fetch(`https://blockchain.info/q/latesthash?cors=true`) | |
.then(r => r.text()); | |
const fetchMerkleRootAndTransactions = block => | |
fetch(`https://blockchain.info/rawblock/${block}?cors=true`) | |
.then(r => r.json()) | |
.then(d => [d.mrkl_root, d.tx.map(t => t.hash)]); | |
const random = arr => | |
arr[Math.floor(Math.random() * arr.length)]; | |
const toBytes = hex => | |
hex.match(/../g).reduce((acc, hex) => [...acc, parseInt(hex, 16)], []); | |
const toHex = bytes => | |
bytes.reduce((acc, bytes) => acc + bytes.toString(16).padStart(2, '0'), ''); | |
const toPairs = arr => | |
Array.from(Array(Math.ceil(arr.length / 2)), (_, i) => arr.slice(i * 2, i * 2 + 2)); | |
const hashPair = (a, b = a) => { | |
const bytes = toBytes(`${b}${a}`).reverse(); | |
const hashed = sha256.array(sha256.array(bytes)); | |
return toHex(hashed.reverse()); | |
}; | |
const merkleProof = (txs, tx, proof = []) => { | |
if (txs.length === 1) { | |
return proof; | |
} | |
const tree = []; | |
toPairs(txs).forEach(pair => { | |
const hash = hashPair(...pair); | |
if (pair.includes(tx)) { | |
const idx = pair[0] === tx | 0; | |
proof.push([idx, pair[idx]]); | |
tx = hash; | |
} | |
tree.push(hash); | |
}); | |
return merkleProof(tree, tx, proof); | |
}; | |
const merkleProofRoot = (proof, tx) => | |
proof.reduce((root, [idx, tx]) => idx ? hashPair(root, tx) : hashPair(tx, root), tx); | |
fetchLatestBlock() | |
.then(fetchMerkleRootAndTransactions) | |
.then(([root, txs]) => { | |
const tx = random(txs); | |
const proof = merkleProof(txs, tx); | |
const isValid = merkleProofRoot(proof, tx) === root; | |
console.log(isValid); | |
}); |
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