🛡️ 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

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

Loading

addBlockHeaderBatch(bytes[] calldata blockHeaderRawArr_) Sequence Diagram

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

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

Loading

📦 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:

npm run compile

Test

To run the tests, execute the following command:

npm run test

Or to see the coverage, run:

npm run coverage

Local deployment

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

npm run private-network
npm run deploy-localhost

Bindings

The command to generate the bindings is as follows:

npm run generate-types