Generating Merkle Proofs

10 January 2023 (1y ago)

In the previous article "Building a Merkle Tree", we demonstrated how to construct a Merkle tree from scratch in Python. This article will focus on generating Merkle proofs, which are essential for verifying the inclusion of a specific data item in the tree.

What is a Merkle Proof?

A Merkle proof provides evidence that a particular data item is included in the Merkle tree. It consists of a series of hashes that trace a path from the data item to the root of the tree. By recomputing the hashes along this path, one can verify the integrity and inclusion of the data item.

Building the Merkle Tree

First, let's recall our MerkleTree class from the previous article, which includes the necessary methods to build the tree.

class MerkleTree:
    def __init__(self, data):
        self.leaves = data
        self.root = None
        self.tree = []

        self.hash_function = lambda x: hashlib.sha256(json.dumps(x, sort_keys=True).encode()).hexdigest()
        

    def build(self):
        if len(self.leaves) == 1:
            self.root = self.leaves[0]
            self.tree.append([self.root])
            return

        if len(self.leaves) % 2 != 0:
            self.leaves.append(self.leaves[-1])

        self.leaves = [
            self.hash_function(leaf)
            for leaf in self.leaves
        ]

        self.tree.append(self.leaves)

        while len(self.leaves) > 1:
            if len(self.leaves) % 2 != 0:
                self.leaves.append(self.leaves[-1])

            self.leaves = [
                self.hash_function(self.leaves[i] + self.leaves[i + 1])
                for i in range(0, len(self.leaves), 2)
            ]

            self.tree.append(self.leaves)

        self.root = self.leaves[0]

    def get_root(self):
        if self.root is None:
            self.build()
        return self.root

Generating a Merkle Proof

To generate a Merkle proof, we need to find the path from the target data item to the root of the tree, collecting the necessary hashes along the way. This would involve collecting the sibling hashes at each level of the tree and also keeping track of whether the target is the left or right child of the parent node.

class MerkleTree:
    ...
    
    def generate_proof(self, target):
        if self.root is None:
            self.build()
        
        target_hash = self.hash_function(target)
        proof = []

        idx = self.tree[0].index(target_hash)

        for level in range(len(self.tree) - 1):
            sibling_idx = idx + 1 if idx % 2 == 0 else idx - 1
            sibling_hash = self.tree[level][sibling_idx]
            proof.append((sibling_hash, idx % 2 == 1))
            idx //= 2

        return proof

Verifying a Merkle Proof

To verify the Merkle proof, we need to recompute the hashes from the data item to the root using the provided proof. This is done by building the hash path from the target to the root based on the proof provided and comparing the final hash with the root hash.

def verify_proof(self, proof, target, root):
    computed_hash = self.hash_function(target)
    for proof_hash, is_left in proof:
        if is_left:
            computed_hash = self.hash_function(proof_hash + computed_hash)
        else:
            computed_hash = self.hash_function(computed_hash + proof_hash)
    return computed_hash == root

Testing the Code

Let's test our implementation with a sample data set.

data = ['Alice', 'Bob', 'Charlie', 'Dave', 'Eve']
tree = MerkleTree(data)
root = tree.get_root()
print("Merkle Root:", root)

target = 'Alice'
proof = tree.generate_proof(target)
print("Merkle Proof for Bob:", proof)

is_valid = tree.verify_proof(proof, target, root)
print("Is the proof valid?", is_valid)

# Output: True

Conclusion

In this article, we demonstrated how to generate and verify Merkle proofs, which are crucial for ensuring data integrity in a Merkle tree. In the next article, we will explore practical applications of Merkle proofs in blockchain technology and distributed systems.

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