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Non-interactive swaps allow users to exchange assets without requiring both parties to be online simultaneously. This pattern leverages Arkade Script’s advanced capabilities to create secure, trustless swap mechanisms.

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Overview

Unlike traditional atomic swaps that require coordination between parties, non-interactive swaps allow:
  • One party to create a swap offer that can be taken by anyone
  • Takers to complete the swap at their convenience without the maker being online
  • Automatic verification of swap conditions without trusted third parties

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Contract Architecture

The non-interactive swap contract uses a combination of:
  1. Hash Preimage Verification - To ensure the correct asset is being swapped
  2. Transaction Introspection - To verify output amounts and destinations
  3. Timelock Mechanisms - To allow the maker to reclaim their assets after expiration

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Example Implementation

// Contract configuration options
options {
  server = server;
  exit = 144;
}

contract NonInteractiveSwap(
  pubkey maker,
  pubkey server,
  bytes32 assetIdHash,
  int amount,
  int expiryTime
) {
  // Maker can cancel the swap after expiry
  function cancel(signature makerSig) {
    require(tx.time >= expiryTime, "Swap has not expired yet");
    require(checkSig(makerSig, maker), "Invalid maker signature");
  }
  
  // Anyone can complete the swap by providing the correct asset
  function swap(bytes32 assetId, signature takerSig, pubkey taker) {
    // Verify the asset being provided matches what the maker requested
    require(sha256(assetId) == assetIdHash, "Asset ID doesn't match requested asset");
    
    // Verify the output contains the correct amount going to the maker
    require(tx.outputs[0].value >= amount, "Output amount too small");
    require(tx.outputs[0].asset == assetId, "Output asset incorrect");
    
    // Verify the output is spendable by the maker
    bytes makerScript = new P2PKH(maker);
    require(tx.outputs[0].scriptPubKey == makerScript, "Output not spendable by maker");
    
    // Verify the taker signature
    require(checkSig(takerSig, taker), "Invalid taker signature");
  }
}

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Advanced Features

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Partial Fills

Non-interactive swaps can be extended to support partial fills, allowing multiple takers to each fulfill a portion of the swap: 
function partialSwap(bytes32 assetId, int partialAmount, signature takerSig, pubkey taker) {
  // Verify the asset being provided matches what the maker requested
  require(sha256(assetId) == assetIdHash, "Asset ID doesn't match requested asset");
  
  // Verify the partial amount is reasonable
  require(partialAmount > 0, "Partial amount must be positive");
  require(partialAmount <= amount, "Partial amount too large");
  
  // Calculate the proportional output based on the partial amount
  int proportionalOutput = (partialAmount * totalOutput) / amount;
  
  // Verify the output contains the correct amount going to the maker
  require(tx.outputs[0].value >= proportionalOutput, "Output amount too small");
  require(tx.outputs[0].asset == assetId, "Output asset incorrect");
  
  // Verify the output is spendable by the maker
  bytes makerScript = new P2PKH(maker);
  require(tx.outputs[0].scriptPubKey == makerScript, "Output not spendable by maker");
  
  // Verify the change output returns to a new swap contract with updated amount
  int remainingAmount = amount - partialAmount;
  if (remainingAmount > 0) {
    // Verify there's a change output with the remaining swap contract
    // Implementation details omitted for brevity
  }
  
  // Verify the taker signature
  require(checkSig(takerSig, taker), "Invalid taker signature");
}

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Price Oracles

For swaps that need to execute at market price rather than a fixed rate, oracle integration can be added: 
function marketSwap(bytes32 assetId, int price, signature oracleSig, pubkey oracle, signature takerSig, pubkey taker) {
  // Verify the asset being provided matches what the maker requested
  require(sha256(assetId) == assetIdHash, "Asset ID doesn't match requested asset");
  
  // Verify the oracle signature on the price data
  bytes message = sha256(assetId + int2bytes(tx.time));
  require(checkSigFromStack(oracleSig, oracle, message), "Invalid oracle signature");
  
  // Calculate the expected amount based on the oracle price
  int expectedAmount = amount * price / 10000; // Assuming price is in basis points
  
  // Verify the output contains the correct amount going to the maker
  require(tx.outputs[0].value >= expectedAmount, "Output amount too small");
  require(tx.outputs[0].asset == assetId, "Output asset incorrect");
  
  // Verify the output is spendable by the maker
  bytes makerScript = new P2PKH(maker);
  require(tx.outputs[0].scriptPubKey == makerScript, "Output not spendable by maker");
  
  // Verify the taker signature
  require(checkSig(takerSig, taker), "Invalid taker signature");
}

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Security Considerations

When implementing non-interactive swaps, consider these security aspects:
  1. Frontrunning Protection - Consider mechanisms to prevent frontrunning of attractive swap offers
  2. Fee Management - Ensure the contract accounts for transaction fees to avoid dust outputs
  3. Replay Protection - Implement nonces or other mechanisms to prevent replay attacks
  4. Timelock Selection - Choose appropriate timelocks that balance security with capital efficiency

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Future Directions

The Arkade ecosystem is exploring several enhancements to non-interactive swaps:
  • Multi-asset Swaps - Supporting swaps involving more than two assets
  • Conditional Swaps - Swaps that execute only when certain external conditions are met
  • Privacy Enhancements - Techniques to improve the privacy of swap participants
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