Add CoinJoin functionality to improve transaction privacy

[nejos97]

We want to implement CoinJoin in Grimm App to enhance user privacy by allowing multiple users to combine their Bitcoin transactions into a single transaction, making it difficult to trace inputs to outputs.

This feature should be designed for both beginner-friendly usage and advanced user control, aligned with Grimm App’s goal of being simple yet powerful.

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Objectives

• Improve on-chain privacy for users
• Support collaborative transaction building
• Provide a seamless UX for privacy features

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Scope

1. CoinJoin Strategy

• Choose implementation model:

  • Centralized coordinator (e.g., similar to Wasabi Wallet)
  • Decentralized (P2P, more complex)

• Define anonymity set target (e.g., 5, 10, 50 participants)

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2. Transaction Flow

• Allow user to opt-in to CoinJoin
• Queue system for participants
• Build PSBT (Partially Signed Bitcoin Transaction)
• Handle signing flow securely (client-side only)
• Broadcast final transaction

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3. Backend / Coordinator (if applicable)

• Create CoinJoin coordinator service
• Manage session lifecycle (register inputs/outputs)
• Prevent double-spending
• Ensure no custody of user funds

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4. Privacy & Security

• Ensure no logs linking inputs ↔ outputs
• Integrate Tor support (optional but recommended)
• Protect against Sybil attacks
• Validate equal output amounts (standard denomination)

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5. UX/UI

• Simple toggle: “Enhance Privacy (CoinJoin)”

• Display:

  • Estimated delay
  • Fees
  • Anonymity level

• Status states:

  • Waiting for participants
  • Signing
  • Broadcasting

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6. Fees Handling

• Define fee model:

  • Mining fee split
  • Coordinator fee (if any)

• Show clear fee breakdown to user

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Acceptance Criteria

• User can opt-in to CoinJoin from send flow
• Transactions are combined with multiple participants
• Final transaction breaks deterministic input-output linkage
• No private keys leave the device
• Feature works on testnet before mainnet

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Risks / Considerations

• Legal/regulatory implications (important given past cases like Samourai Wallet)
• UX complexity for beginners
• Liquidity problem (not enough participants)
• Potential DoS / Sybil attacks

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References

• Bitcoin CoinJoin concept (BIP-related docs)
• Wasabi / Whirlpool design patterns

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Nice-to-have (Future)

• CoinJoin rounds automation
• Custom anonymity sets
• Integration with PayJoin (BIP78)
• Post-mix wallet segregation