Lesson 02 · Hashing a leaf - Build a Merkle Tree

Hashing a leaf

Data is never hashed raw in a Merkle tree - a leaf is hashed with a one-byte prefix that marks it as a leaf. Today you add that leaf prefix and hash your first leaf.

The goal

Hash a piece of data as a leaf by prefixing it with the leaf marker 0x00.

Start here - the target

TO DO

Scenario: A leaf hash uses the 0x00 prefix

GivenHashLeaf prepends the leaf-marker byte 0x00 to the data, then runs hashBytes on the result

WhenHashLeaf is called on the bytes of "alice"

Thenit returns 0x00063049

AndHashLeaf of "bob" returns 0x6bfe63ee

Background

Every value in a Merkle tree is either a leaf (a piece of your actual data) or an internal node (a hash of two child hashes). If you hashed both the same way, an attacker could later present an internal node as though it were a leaf, or the reverse. The defense is domain separation - give each kind its own one-byte prefix before hashing. Leaves get 0x00; internal nodes, in a couple of lessons, will get 0x01.

So HashLeaf does not hash the data directly. It prepends the 0x00 marker and hashes that. The extra byte is invisible to the caller but means a leaf hash lives in a different “namespace” than a node hash. Today you only need the leaf side; pin those two exact values and the rest of the tree grows from them.

Make it work

const leafPrefix = 0x00
func HashLeaf(data []byte) Hash {
  buf := append([]byte{leafPrefix}, data...)
  return hashBytes(buf)
}

CheckpointDONE

Data is hashed into leaf hashes carrying the 0x00 marker. Commit and stop here.