Nethereum.Merkle 6.1.0

Nethereum.Merkle

Comprehensive Merkle tree implementations for Ethereum smart contract verification, airdrops, whitelisting, and large-scale state management.

Overview

Nethereum.Merkle provides production-ready Merkle tree data structures optimized for blockchain use cases:

• Standard Merkle Trees: For airdrops, whitelisting, and contract verification
• OpenZeppelin Compatible: Interoperable with OpenZeppelin's JavaScript library and Solidity contracts
• Incremental Trees: Efficient updates without rebuilding the entire tree
• Sparse Merkle Trees: Handle millions of records with database-backed storage
• Proof Generation & Verification: Create and verify cryptographic proofs on-chain and off-chain

Use Merkle trees to efficiently verify membership in large datasets, enable token airdrops, implement whitelists, or manage scalable state commitments.

Installation

dotnet add package Nethereum.Merkle

Dependencies

Nethereum Dependencies:

• Nethereum.ABI - ABI encoding for struct-based Merkle leaves
• Nethereum.Util - Keccak-256 hashing and byte array utilities

Key Concepts

What is a Merkle Tree?

A Merkle tree is a cryptographic data structure that allows efficient verification of large datasets:

1. Leaves: Data elements (hashed)
2. Branches: Hashes of pairs of child nodes
3. Root: Single hash representing the entire dataset

Key Property: You can prove an element exists in the dataset by providing a small "proof" (log₂(n) hashes) instead of the entire dataset.

Pairing Strategies

When combining hash pairs, Nethereum.Merkle supports:

• Sorted Pairing (PairingConcatType.Sorted): Hashes are ordered before concatenation (OpenZeppelin standard)
• Normal Pairing (PairingConcatType.Normal): Hashes concatenated in given order

Use Cases

1. Token Airdrops: Distribute tokens to thousands of addresses efficiently
2. Whitelisting: Verify user eligibility on-chain with minimal gas
3. State Commitments: Compress large state into a single hash
4. Fraud Proofs: Prove invalid state transitions in layer 2 solutions
5. NFT Metadata: Prove authenticity of off-chain metadata

Quick Start

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;
using System.Collections.Generic;

// Create a simple merkle tree with string addresses
var addresses = new List<string>
{
    "0x95222290DD7278Aa3Ddd389Cc1E1d165CC4BAfe5",
    "0xA61b1fB89Dd42fcDDD2D3fA19c2B715c426692c7",
    "0xfa6179E49EE57a06391F218965b35B632F930472"
};

var merkleTree = new MerkleTree<string>(
    new Sha3KeccackHashProvider(),
    new HexStringByteArrayConvertor()
);
merkleTree.BuildTree(addresses);

// Get the root hash for your smart contract
var rootHash = merkleTree.Root.Hash.ToHex(true);

// Generate proof for an address
var proof = merkleTree.GetProof(addresses[0]);

// Verify proof
var isValid = merkleTree.VerifyProof(proof, addresses[0]);

Usage Examples

Example 1: Simple Merkle Tree with Character Data

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;
using Nethereum.Hex.HexConvertors.Extensions;
using System.Linq;

// Create a list of characters
var elements = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="
    .ToCharArray()
    .ToList();

// Build the merkle tree
var merkleTree = new MerkleTree<char>(
    new Sha3KeccackHashProvider(),
    new ChartByteArrayConvertor()
);
merkleTree.BuildTree(elements);

// Get the root hash
var hexRoot = merkleTree.Root.Hash.ToHex(true);
// Result: "0xec0dffcb601ee38fa372bbf1d89ed16761db0a0b215480032b783f8c33230783"

// Generate proof for 'A'
var proofs = merkleTree.GetProof('A');

// Verify the proof
var isValid = merkleTree.VerifyProof(proofs, 'A');  // Returns true

Source: tests/Nethereum.Contracts.IntegrationTests/MerkleDrop/MerkleUnitTests.cs

Example 2: OpenZeppelin-Compatible Merkle Tree for Airdrops

using Nethereum.Merkle;
using Nethereum.ABI.FunctionEncoding.Attributes;
using System.Collections.Generic;
using System.Numerics;

// Define your airdrop recipient struct (must match Solidity struct)
[Struct("AirdropRecipient")]
public class AirdropRecipient
{
    [Parameter("address", 1)]
    public string User { get; set; }

    [Parameter("uint256", "amount", 2)]
    public BigInteger Amount { get; set; }
}

// Create recipients list
var recipients = new List<AirdropRecipient>
{
    new AirdropRecipient
    {
        User = "0x95222290DD7278Aa3Ddd389Cc1E1d165CC4BAfe5",
        Amount = BigInteger.Parse("1000000000000000000")  // 1 token
    },
    new AirdropRecipient
    {
        User = "0xA61b1fB89Dd42fcDDD2D3fA19c2B715c426692c7",
        Amount = BigInteger.Parse("2000000000000000000")  // 2 tokens
    },
    new AirdropRecipient
    {
        User = "0xfa6179E49EE57a06391F218965b35B632F930472",
        Amount = BigInteger.Parse("500000000000000000")   // 0.5 tokens
    }
};

// Build OpenZeppelin-compatible merkle tree
var merkleTree = new OpenZeppelinStandardMerkleTree<AirdropRecipient>();
merkleTree.BuildTree(recipients);

// Deploy your smart contract with this root
var rootHash = merkleTree.Root.Hash.ToHex(true);

// User wants to claim - generate their proof
var userToClaim = recipients[0];
var proof = merkleTree.GetProof(userToClaim);

// User submits proof + their data to claim() function on-chain
// Contract verifies using OpenZeppelin's MerkleProof.sol

Source: tests/Nethereum.Contracts.IntegrationTests/MerkleDrop/OpenZeppelinMerkleUnitTests.cs

Example 3: Whitelist with Single Parameter

using Nethereum.Merkle;
using Nethereum.ABI.FunctionEncoding.Attributes;
using System.Collections.Generic;

[Struct("WhitelistEntry")]
public class WhitelistEntry
{
    [Parameter("address", 1)]
    public string User { get; set; }
}

// Create whitelist
var whitelist = new List<WhitelistEntry>
{
    new WhitelistEntry { User = "0x95222290DD7278Aa3Ddd389Cc1E1d165CC4BAfe5" },
    new WhitelistEntry { User = "0xA61b1fB89Dd42fcDDD2D3fA19c2B715c426692c7" },
    new WhitelistEntry { User = "0xfa6179E49EE57a06391F218965b35B632F930472" },
    new WhitelistEntry { User = "0x1f9090aaE28b8a3dCeaDf281B0F12828e676c326" }
};

var merkleTree = new OpenZeppelinStandardMerkleTree<WhitelistEntry>();
merkleTree.BuildTree(whitelist);

// Store root hash in your smart contract constructor
var rootHash = merkleTree.Root.Hash.ToHex(true);

// User proves they're whitelisted
var userEntry = whitelist[0];
var proof = merkleTree.GetProof(userEntry);
var isWhitelisted = merkleTree.VerifyProof(proof, userEntry);  // true

Source: tests/Nethereum.Contracts.IntegrationTests/MerkleDrop/OpenZeppelinMerkleUnitTests.cs

Example 4: Lean Incremental Merkle Tree (Efficient Updates)

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;
using System.Numerics;

// Create an incremental tree (efficient for frequent updates)
var tree = new LeanIncrementalMerkleTree<BigInteger>(
    new Sha3KeccackHashProvider(),
    new BigIntegerByteArrayConvertor(),
    PairingConcatType.Normal
);

// Insert leaves one at a time (tree updates incrementally)
tree.InsertLeaf(BigInteger.Parse("100"));
tree.InsertLeaf(BigInteger.Parse("200"));
tree.InsertLeaf(BigInteger.Parse("300"));

// Get current root
var root = tree.Root.ToHex(true);

// Insert many leaves at once
var newValues = new[] {
    BigInteger.Parse("400"),
    BigInteger.Parse("500")
};
tree.InsertMany(newValues);

// Update existing leaf
tree.Update(0, BigInteger.Parse("150"));  // Change first leaf from 100 to 150

// Check if value exists
var hasValue = tree.Has(BigInteger.Parse("200"));  // true
var index = tree.IndexOf(BigInteger.Parse("200"));  // 1

// Generate proof
var proof = tree.GenerateProof(1);  // Proof for index 1

// Verify proof
var isValid = tree.VerifyProof(proof, BigInteger.Parse("200"), tree.Root);

// Export tree (for storage or sharing)
var exported = tree.Export();  // JSON string

// Import tree later
var imported = LeanIncrementalMerkleTree<BigInteger>.Import(
    new Sha3KeccackHashProvider(),
    new BigIntegerByteArrayConvertor(),
    exported,
    s => BigInteger.Parse(s)  // Leaf mapper
);

Source: src/Nethereum.Merkle/LeanIncrementalMerkleTree.cs

Example 5: Sparse Merkle Tree for Large Datasets

using Nethereum.Merkle.Sparse;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;
using System.Collections.Generic;
using System.Threading.Tasks;

// Create sparse merkle tree with in-memory storage
// (use DatabaseSparseMerkleTreeStorage for millions of records)
var storage = new InMemorySparseMerkleTreeStorage<string>();

var sparseTree = new SparseMerkleTree<string>(
    depth: 256,  // Tree depth (bits for key space)
    hashProvider: new Sha3KeccackHashProvider(),
    byteArrayConvertor: new HexStringByteArrayConvertor(),
    storage: storage
);

// Set leaves by key (async for database support)
await sparseTree.SetLeafAsync("key1", "value1");
await sparseTree.SetLeafAsync("key2", "value2");
await sparseTree.SetLeafAsync("key3", "value3");

// Batch update for performance (critical for processing blocks)
var updates = new Dictionary<string, string>
{
    ["key4"] = "value4",
    ["key5"] = "value5",
    ["key6"] = "value6"
};
await sparseTree.SetLeavesAsync(updates);

// Get root hash (cached for performance)
var root = await sparseTree.GetRootHashAsync();

// Get individual leaf
var value = await sparseTree.GetLeafAsync("key1");

// Get leaf count
var count = await sparseTree.GetLeafCountAsync();  // 6

// Clear all data
await sparseTree.ClearAsync();

Source: src/Nethereum.Merkle/Sparse/SparseMerkleTree.cs

Example 6: Using MerkleDropMerkleTree for Token Airdrops

using Nethereum.Merkle;
using System.Collections.Generic;
using System.Numerics;

// MerkleDropItem is pre-defined for airdrop scenarios
var airdropItems = new List<MerkleDropItem>
{
    new MerkleDropItem
    {
        Index = 0,
        Account = "0x95222290DD7278Aa3Ddd389Cc1E1d165CC4BAfe5",
        Amount = BigInteger.Parse("1000000000000000000")
    },
    new MerkleDropItem
    {
        Index = 1,
        Account = "0xA61b1fB89Dd42fcDDD2D3fA19c2B715c426692c7",
        Amount = BigInteger.Parse("2500000000000000000")
    },
    new MerkleDropItem
    {
        Index = 2,
        Account = "0xfa6179E49EE57a06391F218965b35B632F930472",
        Amount = BigInteger.Parse("750000000000000000")
    }
};

// Build airdrop merkle tree
var merkleDropTree = new MerkleDropMerkleTree();
merkleDropTree.BuildTree(airdropItems);

// Get root for smart contract
var root = merkleDropTree.Root.Hash.ToHex(true);

// Generate proof for a specific recipient
var proof = merkleDropTree.GetProof(airdropItems[0]);

// Recipient calls claim(proof, index, account, amount) on contract

Source: src/Nethereum.Merkle/MerkleDropMerkleTree.cs

Example 7: Custom Merkle Tree with Custom Data Types

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;
using System.Collections.Generic;
using System.Text;

// Create custom byte array convertor for your data type
public class MyDataConvertor : IByteArrayConvertor<MyCustomData>
{
    public byte[] ConvertToByteArray(MyCustomData value)
    {
        // Serialize your data type to bytes
        var json = JsonConvert.SerializeObject(value);
        return Encoding.UTF8.GetBytes(json);
    }
}

public class MyCustomData
{
    public string Name { get; set; }
    public int Value { get; set; }
}

// Create merkle tree with custom type
var items = new List<MyCustomData>
{
    new MyCustomData { Name = "Alice", Value = 100 },
    new MyCustomData { Name = "Bob", Value = 200 },
    new MyCustomData { Name = "Charlie", Value = 150 }
};

var merkleTree = new MerkleTree<MyCustomData>(
    new Sha3KeccackHashProvider(),
    new MyDataConvertor(),
    PairingConcatType.Sorted  // OpenZeppelin compatible
);

merkleTree.BuildTree(items);

// Get root
var root = merkleTree.Root.Hash.ToHex(true);

// Generate and verify proof
var proof = merkleTree.GetProof(items[0]);
var isValid = merkleTree.VerifyProof(proof, items[0]);

Source: src/Nethereum.Merkle/MerkleTree.cs

Example 8: Dynamically Adding Leaves to Existing Tree

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Util.ByteArrayConvertors;

// Start with initial set
var addresses = new List<string>
{
    "0x95222290DD7278Aa3Ddd389Cc1E1d165CC4BAfe5",
    "0xA61b1fB89Dd42fcDDD2D3fA19c2B715c426692c7"
};

var merkleTree = new MerkleTree<string>(
    new Sha3KeccackHashProvider(),
    new HexStringByteArrayConvertor()
);
merkleTree.BuildTree(addresses);

var initialRoot = merkleTree.Root.Hash.ToHex(true);

// Add single leaf (tree rebuilds)
merkleTree.InsertLeaf("0xfa6179E49EE57a06391F218965b35B632F930472");

var newRoot = merkleTree.Root.Hash.ToHex(true);
// Root has changed!

// Add multiple leaves at once
var newAddresses = new[]
{
    "0x1f9090aaE28b8a3dCeaDf281B0F12828e676c326",
    "0x742d35Cc6634C0532925a3b844Bc9e7595f0bEb"
};
merkleTree.InsertLeaves(newAddresses);

// Tree now has 5 leaves total
var finalRoot = merkleTree.Root.Hash.ToHex(true);

Source: src/Nethereum.Merkle/MerkleTree.cs

Example 9: Static Proof Verification (Without Building Tree)

using Nethereum.Merkle;
using Nethereum.Util.HashProviders;
using Nethereum.Hex.HexConvertors.Extensions;

// You have a proof from somewhere (API, database, etc.)
var proof = new List<byte[]>
{
    "0x1f675bff07515f5df96737194ea945c36c41e7b4fcef307b7cd4d0e602a69111".HexToByteArray(),
    "0xe62e1dfc08d58fd144947903447473a090c958fe34e2425d578237fcdf1ab5a4".HexToByteArray(),
    "0x1907ce7877ec74782a26c166b562bfbdd4c8d8833f98ad82ae9dc8e98db20093".HexToByteArray()
};

var rootHash = "0xec0dffcb601ee38fa372bbf1d89ed16761db0a0b215480032b783f8c33230783".HexToByteArray();
var itemHash = "0x3f4a1640bcca71e45d053d67ab9891fe44608f4db37cc45e5523588c76c79539".HexToByteArray();

// Verify without building the tree (static method)
var hashProvider = new Sha3KeccackHashProvider();
var isValid = MerkleTree<object>.VerifyProof(
    proof,
    rootHash,
    itemHash,
    hashProvider,
    PairingConcatType.Sorted
);

// Returns true if proof is valid

Source: src/Nethereum.Merkle/MerkleTree.cs

API Reference

MerkleTree<T>

Generic Merkle tree implementation with pluggable hashing and serialization.

Constructor:

MerkleTree(
    IHashProvider hashProvider,
    IByteArrayConvertor<T> byteArrayConvertor,
    PairingConcatType pairingConcatType = PairingConcatType.Sorted
)

Key Properties:

• MerkleTreeNode Root: The root node of the tree
• List<MerkleTreeNode> Leaves: All leaf nodes
• List<List<MerkleTreeNode>> Layers: All layers of the tree (for proof generation)

Key Methods:

• void BuildTree(List<T> items): Build tree from items
• void InsertLeaf(T item): Add single item and rebuild
• void InsertLeaves(IEnumerable<T> items): Add multiple items and rebuild
• List<byte[]> GetProof(T item): Generate proof for an item
• List<byte[]> GetProof(int index): Generate proof by leaf index
• List<byte[]> GetProof(byte[] hashLeaf): Generate proof by leaf hash
• bool VerifyProof(IEnumerable<byte[]> proof, T item): Verify proof for item
• bool VerifyProof(IEnumerable<byte[]> proof, byte[] itemHash): Verify proof for hash
• static bool VerifyProof(proof, rootHash, itemHash, hashProvider, pairingType): Static verification

OpenZeppelinStandardMerkleTree<T>

Merkle tree compatible with OpenZeppelin's JavaScript library and Solidity contracts.

var tree = new OpenZeppelinStandardMerkleTree<TAbiStruct>();

• Inherits from AbiStructSha3KeccackMerkleTree<T>
• Uses sorted pairing (OpenZeppelin standard)
• Works with ABI-annotated structs ([Struct] and [Parameter] attributes)

Requirements:

• Type T must have [Struct] attribute
• Properties must have [Parameter] attributes with correct types and order

AbiStructMerkleTree<T>

Merkle tree for ABI-encoded structs.

var tree = new AbiStructMerkleTree<MyStruct>();

• Uses AbiStructEncoderPackedByteConvertor<T> for encoding
• Uses Keccak-256 (Sha3) hashing
• Sorted pairing by default

MerkleDropMerkleTree

Specialized tree for token airdrops using MerkleDropItem.

var tree = new MerkleDropMerkleTree();

MerkleDropItem Properties:

• BigInteger Index: Sequential index
• string Account: Recipient address
• BigInteger Amount: Token amount

LeanIncrementalMerkleTree<T>

Efficient incremental tree optimized for frequent updates.

Constructor:

LeanIncrementalMerkleTree(
    IHashProvider hashProvider,
    IByteArrayConvertor<T> byteArrayConvertor,
    PairingConcatType pairingType = PairingConcatType.Normal
)

Key Properties:

• byte[] Root: Current root hash (auto-updated)
• List<T> Leaves: Current leaves
• int Size: Number of leaves
• int Depth: Tree depth

Key Methods:

• void InsertLeaf(T leaf): Add single leaf (incremental update)
• void InsertMany(IEnumerable<T> leaves): Batch insert
• void Update(int index, T newLeaf): Update existing leaf
• void UpdateMany(int[] indices, T[] newLeaves): Batch update
• bool Has(T leaf): Check if leaf exists
• int IndexOf(T leaf): Find leaf index
• MerkleProof GenerateProof(int leafIndex): Generate proof
• bool VerifyProof(MerkleProof proof, T leaf, byte[] root): Verify proof
• string Export(Func<byte[], string> formatter = null): Export to JSON
• static Import(hashProvider, convertor, json, leafMapper, ...): Import from JSON

SparseMerkleTree<T>

High-performance sparse tree for millions of records with pluggable storage.

Constructor:

SparseMerkleTree(
    int depth,  // 1-256
    IHashProvider hashProvider,
    IByteArrayConvertor<T> byteArrayConvertor,
    ISparseMerkleTreeStorage<T> storage
)

Key Properties:

• int Depth: Tree depth (key space size)
• string EmptyLeafHash: Hash of empty leaf

Key Methods:

• async Task SetLeafAsync(string key, T value): Set leaf value
• async Task<T> GetLeafAsync(string key): Get leaf value
• async Task<string> GetRootHashAsync(): Get cached root hash
• async Task SetLeavesAsync(Dictionary<string, T> updates): Batch update (optimized)
• async Task<long> GetLeafCountAsync(): Count non-empty leaves
• async Task ClearAsync(): Clear all data

Synchronous Overloads:

• void SetLeaf(string key, T value)
• T GetLeaf(string key)
• string GetRootHash()

Storage Implementations:

• InMemorySparseMerkleTreeStorage<T>: For testing/small datasets
• DatabaseSparseMerkleTreeStorage<T>: For production/large datasets (requires ISparseMerkleRepository)

MerkleProof

Container for merkle proof data.

Properties:

• List<byte[]> ProofNodes: Hashes needed to verify membership

MerkleTreeNode

Node in the merkle tree.

Properties:

• byte[] Hash: Node hash value

Methods:

• bool Matches(byte[] hash): Check if hash matches
• MerkleTreeNode Clone(): Create copy

Pairing Strategies

PairingConcatType Enum

• Sorted: Sort hashes before concatenating (OpenZeppelin standard)
• Normal: Concatenate in given order

IPairConcatStrategy

Interface for custom pairing strategies.

Implementations:

• SortedPairConcatStrategy: Lexicographic sorting
• PairConcatStrategy: Direct concatenation

Factory:

• PairingConcatFactory.GetPairConcatStrategy(PairingConcatType): Get strategy instance

Related Packages

Used By (Consumers)

• Nethereum.Contracts - Uses merkle trees for airdrop contract deployments
• Smart Contract Verification - On-chain proof verification
• Token Distribution - ERC-20/ERC-721 airdrops
• Whitelisting Systems - NFT mints, presales

Dependencies

• Nethereum.ABI - ABI encoding for struct-based leaves
• Nethereum.Util - Keccak-256 and Poseidon hashing, byte conversions

Related ZK Packages

• Nethereum.ZkProofsVerifier - Groth16 proof verification on BN128 (Circom/snarkjs)
• Nethereum.Merkle.Binary - EIP-7864 Binary Merkle Trie for stateless execution

Important Notes

Gas Optimization

On-Chain Verification:

• Proof verification costs approximately keccak256(32 bytes) * depth gas
• For tree depth 20 (1M leaves): ~20 keccak operations
• Much cheaper than storing/checking entire list on-chain

Best Practices:

• Use sorted pairing for OpenZeppelin compatibility
• Keep tree balanced (number of leaves close to power of 2)
• For very large trees, consider sparse merkle trees

OpenZeppelin Compatibility

To ensure compatibility with OpenZeppelin's MerkleProof.sol:

1. Use OpenZeppelinStandardMerkleTree<T>
2. Use sorted pairing (PairingConcatType.Sorted)
3. Struct fields must match Solidity struct exactly (order and types)
4. Use keccak256(abi.encodePacked(...)) encoding

Solidity Example:

function claim(bytes32[] calldata proof, address account, uint256 amount) external {
    bytes32 leaf = keccak256(abi.encodePacked(account, amount));
    require(MerkleProof.verify(proof, merkleRoot, leaf), "Invalid proof");
    // ... claim logic
}

Performance Considerations

Standard MerkleTree:

• Building: O(n log n)
• Proof generation: O(log n)
• Proof verification: O(log n)
• Inserting leaves: O(n log n) (rebuilds tree)

LeanIncrementalMerkleTree:

• Insert single leaf: O(n) (linear scan to rebuild)
• Better for frequent reads, occasional writes
• Export/import for persistence

SparseMerkleTree:

• Set leaf: O(log n) with path invalidation optimization
• Batch updates: O(m log n) for m leaves
• Root computation: O(1) when cached, O(log n) when dirty
• Optimized for millions of records with database storage

Recommendation:

• < 10K items: Use MerkleTree<T> or OpenZeppelinStandardMerkleTree<T>
• 10K-100K items: Use LeanIncrementalMerkleTree<T> for updates
• > 100K items: Use SparseMerkleTree<T> with database storage

Tree Depth Calculation

For n leaves:

• Depth = ⌈log₂(n)⌉
• Proof size = depth × 32 bytes

Examples:

• 100 leaves: depth 7, proof 224 bytes
• 1,000 leaves: depth 10, proof 320 bytes
• 1,000,000 leaves: depth 20, proof 640 bytes

Security Considerations

Second Preimage Attacks:

• Nethereum.Merkle mitigates by using different encoding for leaves vs branches
• Leaves are hashed once, branches hash the concatenation of children

Collision Resistance:

• Keccak-256 provides 128-bit collision resistance
• Sufficient for all practical Ethereum use cases

Proof Validation:

• Always verify proofs on-chain before taking action
• Store merkle root on-chain (in contract storage or as constant)
• Never trust client-provided roots

Common Pitfalls

1. Forgetting to Rebuild: After InsertLeaf(), the tree is rebuilt automatically
2. Wrong Pairing Type: Use Sorted for OpenZeppelin compatibility
3. ABI Encoding Mismatch: Ensure C# struct matches Solidity struct exactly
4. Index vs Hash: GetProof() has overloads for item, index, or hash
5. Sparse Tree Keys: Keys must fit within the tree depth (depth 256 = 256-bit keys)

Sparse Merkle Tree Storage

In-Memory (testing only):

var storage = new InMemorySparseMerkleTreeStorage<string>();

Database (production):

public class MyRepository : ISparseMerkleRepository
{
    // Implement database access
}

var storage = new DatabaseSparseMerkleTreeStorage<string>(
    new MyRepository(),
    new HexStringByteArrayConvertor()
);

Database storage is critical for:

• Persisting tree across restarts
• Handling millions of records
• Enabling horizontal scaling

Sparse Merkle Binary Tree (ZK-Optimized)

SparseMerkleBinaryTree<T> is a high-performance binary sparse Merkle tree designed for zero-knowledge circuits. It supports pluggable hash strategies (Poseidon for ZK, Celestia-compatible SHA-256, or generic hash providers), optional persistent storage with lazy node loading, and both synchronous and asynchronous APIs.

Quick Start

using Nethereum.Merkle.Sparse;
using Nethereum.Util.ByteArrayConvertors;

// Create a Poseidon-based SMT for ZK circuits
var smt = new SparseMerkleBinaryTree<byte[]>(
    new PoseidonSmtHasher(),
    new ByteArrayToByteArrayConvertor(),
    new IdentitySmtKeyHasher(256));

smt.Put(key1, value1);
smt.Put(key2, value2);
var root = smt.ComputeRoot();

// Retrieve
var value = smt.Get(key1);

// Delete
smt.Delete(key1);

Async API with Persistent Storage

using Nethereum.Merkle.Sparse;

var storage = new InMemorySmtNodeStorage();
var smt = new SparseMerkleBinaryTree<byte[]>(
    new CelestiaSmtHasher(),
    new ByteArrayToByteArrayConvertor(),
    storage: storage);

await smt.PutAsync(key1, value1);
await smt.PutAsync(key2, value2);
var root = await smt.ComputeRootAsync();
await smt.FlushAsync();  // Persist to storage

// Load from storage in a new instance
var smt2 = new SparseMerkleBinaryTree<byte[]>(
    new CelestiaSmtHasher(),
    new ByteArrayToByteArrayConvertor(),
    storage: storage);
await smt2.LoadRootAsync(root);  // Lazy-loads nodes on demand
var value = await smt2.GetAsync(key1);

ISmtHasher — Hashing Strategies

The ISmtHasher interface defines how leaves and internal nodes are hashed:

public interface ISmtHasher
{
    bool MsbFirst { get; }              // Bit ordering for key path traversal
    bool UseFixedEmptyHash { get; }     // Fixed vs per-level empty hash
    bool CollapseSingleLeaf { get; }    // Skip hashing single leaf up to root
    byte[] EmptyLeaf { get; }           // Hash of empty/zero leaf

    byte[] HashLeaf(byte[] path, byte[] valueBytes);
    byte[] HashNode(byte[] leftHash, byte[] rightHash);
}

Built-in implementations:

Hasher	Hash Function	Bit Order	Use Case
PoseidonSmtHasher	Poseidon (CircomT3 leaf, CircomT2 node)	LSB-first	ZK circuits (Circom, Privacy Pools)
CelestiaSmtHasher	SHA-256 with domain prefixes	MSB-first	Celestia namespace tree compatibility
DefaultSmtHasher	Any IHashProvider (Keccak, SHA-256)	LSB-first	Generic use

PoseidonSmtHasher

Poseidon-based hasher optimized for zero-knowledge circuits:

var hasher = new PoseidonSmtHasher();
// Leaf: Poseidon(key, value, 1) using CircomT3 (3 inputs)
// Node: Poseidon(leftHash, rightHash) using CircomT2 (2 inputs)

var smt = new SparseMerkleBinaryTree<byte[]>(
    hasher,
    new ByteArrayToByteArrayConvertor(),
    new IdentitySmtKeyHasher(140));  // 140-bit key paths

CelestiaSmtHasher

SHA-256-based hasher compatible with Celestia's sparse Merkle tree:

var hasher = new CelestiaSmtHasher();
// Leaf: SHA256(0x00 || path || SHA256(value))
// Node: SHA256(0x01 || leftHash || rightHash)

var smt = new SparseMerkleBinaryTree<byte[]>(
    hasher,
    new ByteArrayToByteArrayConvertor());

ISmtKeyHasher — Key Path Computation

Controls how keys are mapped to tree paths:

public interface ISmtKeyHasher
{
    byte[] ComputePath(byte[] key);  // Key → bit path
    int PathBitLength { get; }       // Tree depth (1-256)
}

Implementation	Description
IdentitySmtKeyHasher(n)	Direct key as path (no hashing), n-bit depth
Sha256SmtKeyHasher	SHA-256(key) → 256-bit path

ISmtNodeStorage — Persistence

public interface ISmtNodeStorage
{
    Task<byte[]> GetAsync(byte[] hash);
    Task PutAsync(byte[] hash, byte[] data);
    Task DeleteAsync(byte[] hash);
}

InMemorySmtNodeStorage provides a thread-safe in-memory implementation using ConcurrentDictionary.

SmtNodeCodec — Node Serialization

Encodes/decodes nodes for storage:

• Leaf: [0x00][pathLen:2][path][valueLen:2][value]
• Branch: [0x01][leftHash][rightHash]

byte[] encoded = SmtNodeCodec.EncodeLeaf(path, valueBytes);
SmtNodeCodec.DecodeLeaf(encoded, out var path, out var value);

byte[] branch = SmtNodeCodec.EncodeBranch(leftHash, rightHash);
SmtNodeCodec.DecodeBranch(branch, 32, out var left, out var right);

bool isLeaf = SmtNodeCodec.IsLeaf(data);
bool isBranch = SmtNodeCodec.IsBranch(data);

Additional Resources

Ethereum & Merkle Trees

• Merkle Trees on Ethereum.org
• OpenZeppelin Merkle Tree JavaScript Library
• OpenZeppelin MerkleProof.sol

Use Case Examples

• Uniswap Merkle Distributor - Token airdrop pattern
• ENS Airdrop - Real-world example

Research Papers

• Certificate Transparency (Merkle Trees in Practice)
• Sparse Merkle Trees

Nethereum Documentation

• Nethereum Documentation
• Nethereum GitHub