PartialMerkleTree - corda

corda / net.corda.core.crypto / PartialMerkleTree

# PartialMerkleTree

`class PartialMerkleTree`

Building and verification of Partial Merkle Tree. Partial Merkle Tree is a minimal tree needed to check that a given set of leaves belongs to a full Merkle Tree.

Example of Merkle tree with 5 leaves.

                 h15
              /       \
             h14       h55
            /  \      /  \
          h12  h34   h50 h00
         / \   / \   / \  / \
        l1 l2 l3 l4 l5 0 0  0

l* denote hashes of leaves, h* - hashes of nodes below. 0 denotes zero hash, we use it to pad not full binary trees, so the number of leaves is always a power of 2.

Example of Partial tree based on the tree above.

               ___
            /       \
            _        _
          /  \      /  \
        h12   _     _   h00
             / \   / \
            I3 l4 I5 0

We want to check l3 and l5 - now turned into IncudedLeaf (I3 and I5 above). To verify that these two leaves belong to the tree with a hash root h15 we need to provide a Merkle branch (or partial tree). In our case we need hashes: h12, l4, 0 and h00. Verification is done by hashing the partial tree to obtain the root and checking it against the obtained h15 hash. Additionally we store included hashes used in calculation and compare them to leaves hashes we got (there can be a difference in obtained leaves ordering - that's why it's a set comparison not hashing leaves into a tree). If both equalities hold, we can assume that l3 and l5 belong to the transaction with root h15.

### Types

#### PartialTree | The structure is a little different than that of Merkle Tree. Partial Tree might not be a full binary tree. Leaves represent either original Merkle tree leaves or cut subtree node with stored hash. We differentiate between the leaves that are included in a filtered transaction and leaves that just keep hashes needed for calculation. Reason for this approach: during verification it's easier to extract hashes used as a base for this tree. `sealed class PartialTree`

### Constructors

#### <init> | Building and verification of Partial Merkle Tree. Partial Merkle Tree is a minimal tree needed to check that a given set of leaves belongs to a full Merkle Tree. `PartialMerkleTree(root: PartialTree)`

### Properties

#### root | `val root: PartialTree`

### Functions

#### leafIndex | Method to return the index of the input leaf in the partial Merkle tree structure. `fun leafIndex(leaf: SecureHash): Int`
#### verify | Function to verify a PartialMerkleTree against an input Merkle root and a list of leaves. The tree should only contain the leaves defined in hashesToCheck. `fun verify(merkleRootHash: SecureHash, hashesToCheck: List<SecureHash>): Boolean`

### Companion Object Functions

#### build | `fun build(merkleRoot: MerkleTree, includeHashes: List<SecureHash>): PartialMerkleTree`
#### rootAndUsedHashes | Recursive calculation of root of this partial tree. Modifies usedHashes to later check for inclusion with hashes provided. `fun rootAndUsedHashes(node: PartialTree, usedHashes: MutableList<SecureHash>): SecureHash`