https://github.com/distributed-lab/spv-contracts

SPV Contract: Bitcoin Light Client on EVM
https://github.com/distributed-lab/spv-contracts

bitcoin evm smart-contracts spv

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SPV Contract: Bitcoin Light Client on EVM

• Host: GitHub
• URL: https://github.com/distributed-lab/spv-contracts
• Owner: distributed-lab
• License: mit
• Created: 2025-04-08T13:51:41.000Z (11 months ago)
• Default Branch: master
• Last Pushed: 2025-08-07T12:16:56.000Z (7 months ago)
• Last Synced: 2025-08-07T13:04:02.257Z (7 months ago)
• Topics: bitcoin, evm, smart-contracts, spv
• Language: TypeScript
• Homepage: https://distributed-lab.github.io/spv-contracts/
• Size: 5.55 MB
• Stars: 4
• Watchers: 2
• Forks: 0
• Open Issues: 1
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

          
# 🛡️ SPV Contract: Bitcoin Light Client on EVM

Welcome to the **SPV Contract**, a robust and efficient Solidity implementation for verifying Bitcoin block headers directly on an EVM-compatible blockchain. This contract empowers dApps to act as a **Simplified Payment Verification (SPV)** client, allowing them to validate the existence and inclusion of Bitcoin transactions without needing to run a full Bitcoin node.

# ✨ Why this SPV Contract?

In the decentralized world, connecting different blockchain ecosystems securely is paramount. This SPV contract provides a trust-minimized bridge, enabling smart contracts on EVM chains to cryptographically verify the state of the Bitcoin blockchain. This opens doors for exciting use cases like:

- **Cross-chain bridges** for Bitcoin-backed assets

- **Light clients** for dApps that need to confirm Bitcoin transaction finality

- **Decentralized custodianship** solutions

- **Oracle services** for Bitcoin data on EVM

# 🚀 Key Features

- **Block Header Submission:** Efficiently add individual or batches of Bitcoin block headers to the contract.

- **Mainchain Tracking:** Automatically identifies and updates the "main" Bitcoin chain based on accumulated work.

- **Block Validation:** Verifies block headers against Bitcoin's consensus rules, including:

  - Proof-of-Work (target difficulty)

  - Block time validity (median time past)

  - Chain continuity (previous block hash)

- **Block Information Retrieval:** Query detailed information about any stored block, such as:

  - Its Merkle root

  - Its height

  - Its inclusion status in the mainchain

  - Its cumulative work (difficulty)

  - Its confirmation count relative to the mainchain head

- **Difficulty Adjustment:** Integrates Bitcoin's precise difficulty adjustment algorithm to accurately calculate current and future targets.

# ⚙️ How it Works (Under the Hood)

The contract operates by receiving raw Bitcoin block headers, which are then parsed and validated against Bitcoin's strict consensus rules.

1. **Header Parsing:** Raw 80-byte Bitcoin block headers are parsed into a structured *BlockHeaderData* format. This involves handling Bitcoin's unique little-endian byte ordering.

2. **Double SHA256 Hashing:** Each block header is double SHA256 hashed to derive its unique block hash, which is then byte-reversed for standard representation.

3. **Proof-of-Work Verification:** The calculated block hash is checked against the current network difficulty target (derived from the *bits* field in the header).

4. **Chain Extension & Reorganization:** New blocks are added to a data structure that allows for tracking multiple chains. When a new block extends a chain with higher cumulative work, the *mainchainHead* is updated, reflecting potential chain reorganizations.

5. **Difficulty Adjustment:** Every 2016 blocks, the contract calculates a new difficulty target based on the time taken to mine the preceding epoch. This ensures the 10-minute average block time is maintained.

# 📊 Flow Diagrams

These diagrams outline the step-by-step process for adding block headers to the SPV Contract.

### `addBlockHeader(bytes calldata blockHeaderRaw_)` Sequence Diagram

```mermaid

sequenceDiagram

  participant Caller

  participant SPVContract

  participant BlockHeaderLib

  participant TargetsHelperLib

  Caller->>SPVContract: addBlockHeader(blockHeaderRaw)

  activate SPVContract

  SPVContract->>BlockHeaderLib: 1. Parse blockHeaderRaw_ (parseBlockHeaderData)

  activate BlockHeaderLib

  BlockHeaderLib-->>SPVContract: 1.1. Check length (80 bytes) & LE to BE

  alt Length Invalid

      BlockHeaderLib--xSPVContract: Error: InvalidBlockHeaderDataLength

      SPVContract--xCaller: Revert

  end

  BlockHeaderLib-->>SPVContract: 1.2. Return BlockHeaderData & blockHash

  deactivate BlockHeaderLib

  SPVContract->>SPVContract: 1.3. Check blockHash existence

  alt BlockHash Exists

      SPVContract--xCaller: Error: BlockAlreadyExists

  end

  SPVContract->>SPVContract: 2. Check prevBlockHash existence

  alt Prev Block Missing

      SPVContract--xCaller: Error: PrevBlockDoesNotExist

  end

  SPVContract->>SPVContract: 3. Calculate newBlockHeight = prevBlockHeight + 1

  SPVContract->>SPVContract: 4. Get Current Target

  SPVContract->>SPVContract: 4.1. Get target from prevBlockBits

  SPVContract->>TargetsHelperLib: Check if newBlockHeight is Recalculation Block (isTargetAdjustmentBlock)

  activate TargetsHelperLib

  alt Recalculation Block

      SPVContract->>SPVContract: Recalculate target & Save lastEpochCumulativeWork

      TargetsHelperLib-->>SPVContract: Return newNetworkTarget

  else Not Recalculation Block

      TargetsHelperLib-->>SPVContract: Use prevBlockTarget as networkTarget

  end

  deactivate TargetsHelperLib

  SPVContract->>SPVContract: 5. Check Block Rules

  SPVContract->>TargetsHelperLib: 5.1. Check Header Target == Contract Target

  activate TargetsHelperLib

  TargetsHelperLib-->>SPVContract: Result

  deactivate TargetsHelperLib

  alt Invalid Target

      SPVContract--xCaller: Error: InvalidTarget

  end

  SPVContract->>SPVContract: 5.2. Check newBlockHash <= networkTarget (PoW)

  alt Invalid Block Hash

      SPVContract--xCaller: Error: InvalidBlockHash

  end

  SPVContract->>SPVContract: 5.3. Check newBlockTime >= medianTime

  alt Invalid Block Time

      SPVContract--xCaller: Error: InvalidBlockTime

  end

  SPVContract->>SPVContract: 6. Add Block To Chain

  SPVContract->>SPVContract: 6.1. Save newBlockHeader & newBlockHash to Storage

  SPVContract->>SPVContract: 6.2. Update Mainchain

  alt 6.2.1. prevBlockHash == mainchainHead?

      SPVContract->>SPVContract: Move mainchainHead to newBlockHash

  else

      SPVContract->>SPVContract: 6.2.2. Calculate New Block & Current Head Cumulative Work

      SPVContract->>SPVContract: 6.2.3. newBlock Cumulative Work > Current Head?

      alt New Block Has Higher Work

          SPVContract->>SPVContract: Set New Block as mainchainHead

          SPVContract->>SPVContract: Recursively update mainchain path backwards (do-while loop)

      end

  end

  SPVContract->>SPVContract: Emit BlockHeaderAdded

  SPVContract-->>Caller: Transaction Complete

  deactivate SPVContract

```

### `addBlockHeaderBatch(bytes[] calldata blockHeaderRawArr_)` Sequence Diagram

This function processes multiple block headers in a single transaction, iterating through the array and validating each sequentially.

```mermaid

sequenceDiagram

  participant Caller

  participant SPVContract

  participant BlockHeaderLib

  participant TargetsHelperLib

  Caller->>SPVContract: addBlockHeaderBatch(blockHeaderRawArray_)

  activate SPVContract

  SPVContract->>SPVContract: Check if Header Array is Empty

  alt Array Empty

      SPVContract--xCaller: Error: EmptyBlockHeaderArray

  end

  SPVContract->>BlockHeaderLib: 1. Parse Block Headers Array (_parseBlockHeadersRaw)

  activate BlockHeaderLib

  BlockHeaderLib-->>SPVContract: Returns BlockHeaderData[] & bytes32[]

  deactivate BlockHeaderLib

  loop For each blockHeader in parsed array (from i=0 to length-1)

      SPVContract->>SPVContract: 2. Check prevBlockHash for current block

      alt First block in batch

          SPVContract->>SPVContract: Check prevBlockHash existence (like addBlockHeader)

          alt Prev Block Missing

              SPVContract--xCaller: Error: PrevBlockDoesNotExist

          end

      else Subsequent blocks

          SPVContract->>SPVContract: Check prevBlockHash == blockHash of (i-1)th block

          alt Order Invalid

              SPVContract--xCaller: Error: InvalidBlockHeadersOrder

          end

      end

      SPVContract->>SPVContract: 3. Calculate currentBlockHeight = prevBlockHeight + 1

      SPVContract->>SPVContract: 4. Get Current Target (like addBlockHeader)

      SPVContract->>TargetsHelperLib: Check for Recalculation Block & Recalculate if needed

      activate TargetsHelperLib

      TargetsHelperLib-->>SPVContract: Return networkTarget

      deactivate TargetsHelperLib

      SPVContract->>SPVContract: 5. Get Median Time

      alt 5.1. Num blocks added < 12

          SPVContract->>SPVContract: Use _getStorageMedianTime (like addBlockHeader)

      else 5.2. Num blocks added >= 12

          SPVContract->>SPVContract: Use _getMemoryMedianTime (from batch data)

      end

      SPVContract->>SPVContract: 6. Validate Block Rules (_validateBlockRules)

      alt Validation Fails

          SPVContract--xCaller: Error: InvalidTarget / InvalidBlockHash / InvalidBlockTime

      end

      SPVContract->>SPVContract: 7. Add Block To Chain (_addBlock)

      SPVContract->>SPVContract: Emit BlockHeaderAdded

  end

  SPVContract-->>Caller: Transaction Complete

  deactivate SPVContract

```

# 📦 Contract Components

The solution is primarily composed of the main SPV contract and two essential helper libraries that manage the intricacies of Bitcoin's block structure and difficulty adjustments.

## SPVContract

This is the central contract that users will interact with. It serves as the primary interface for managing Bitcoin block headers on the EVM. It handles the core logic for adding and validating blocks, tracking the main Bitcoin chain, and providing querying functionalities. All custom errors and events related to the SPV operations are defined here, ensuring clear feedback and transparency during contract execution.

## BlockHeader Library

This is a pure utility library specifically designed to handle the low-level details of Bitcoin block headers. It's responsible for the precise parsing of raw 80-byte Bitcoin block header data into a structured format that Solidity can easily work with. Crucially, it manages the byte order conversions, translating Bitcoin's little-endian format to Solidity's big-endian, and vice-versa. It also provides the essential function for calculating the double SHA256 hash of a block header, which is fundamental for verifying Proof-of-Work.

## TargetsHelper Library

This library encapsulates all the complex mathematical and logical operations related to Bitcoin's difficulty targets. It provides functions to accurately calculate new difficulty targets based on elapsed time between blocks, ensuring the contract adheres to Bitcoin's dynamic difficulty adjustment rules. Additionally, it offers utilities for converting between the compact "bits" format (as found in Bitcoin block headers) and the full 256-bit target value, and it calculates the cumulative work associated with a given block or epoch, which is vital for determining the most valid chain.

# 💻 Dev Info

## Compilation

To compile the contracts, use the next script:

```bash

npm run compile

```

## Test

To run the tests, execute the following command:

```bash

npm run test

```

Or to see the coverage, run:

```bash

npm run coverage

```

## Local deployment

To deploy the contracts locally, run the following commands (in the different terminals):

```bash

npm run private-network

npm run deploy-localhost

```

## Bindings

The command to generate the bindings is as follows:

```bash

npm run generate-types

```