tornado-nova/circuits/merkleTree.circom

include "../node_modules/circomlib/circuits/poseidon.circom";

// Computes MiMC([left, right])
template HashLeftRight() {
    signal input left;
    signal input right;
    signal output hash;

    component hasher = Poseidon(2);
    hasher.inputs[0] <== left;
    hasher.inputs[1] <== right;
    hash <== hasher.out;
}

// if s == 0 returns [in[0], in[1]]
// if s == 1 returns [in[1], in[0]]
template DualMux() {
    signal input in[2];
    signal input s;
    signal output out[2];

    s * (1 - s) === 0
    out[0] <== (in[1] - in[0])*s + in[0];
    out[1] <== (in[0] - in[1])*s + in[1];
}

// Verifies that merkle proof is correct for given merkle root and a leaf
// pathIndices input is an array of 0/1 selectors telling whether given pathElement is on the left or right side of merkle path
template MerkleTree(levels) {
    signal input leaf;
    signal input pathElements[levels];
    signal input pathIndices;

    signal output root;

    component selectors[levels];
    component hashers[levels];

    component indexBits = Num2Bits(levels);
    indexBits.in <== pathIndices;

    for (var i = 0; i < levels; i++) {
        selectors[i] = DualMux();
        selectors[i].in[0] <== i == 0 ? leaf : hashers[i - 1].hash;
        selectors[i].in[1] <== pathElements[i];
        selectors[i].s <== indexBits.out[i];

        hashers[i] = HashLeftRight();
        hashers[i].left <== selectors[i].out[0];
        hashers[i].right <== selectors[i].out[1];
    }

    root <== hashers[levels - 1].hash;
}