FHE × DeFi

FHE 系統: TFHE, BFV, BGV, CKKS

パフォーマンス比較 (CPU、2024 年)

When to Use Each Variant

性能進化 Timeline

FHE-MEV: Blind Arbitrage Without Seeing Transactions

The Core Problem: Searchers Must See Tx to Profit (Today)

Protocol Flow — Blind Arbitrage

TFHE Gate 操作 Required

// Pseudocode: FHE gate count for ブラインドアービトラージ
// 1. Extract token pair field (64-bit comparison in 暗号文 space)
enc_pair = TFHE.select_bits(enc_tx, offset=0, len=64)   // ~64 gates

// 2. Check validity constraint: amount > minimum
enc_valid = TFHE.compare_ge(enc_amount, MIN_AMOUNT)     // ~1,000 gates (128-bit)

// 3. Compute price impact on encrypted reserves
enc_impact = TFHE.div(enc_amount, enc_reserve_in)       // ~10,000 gates

// 4. Determine if opportunity profitable
enc_opp = TFHE.and(enc_valid, TFHE.compare_ge(enc_impact, THRESHOLD))

// Total: ~50,000-100,000 gate operations
// At ~1ms/gate (CPU 2024): 50-100 seconds total
// With GPU (10-100x): 0.5-10 seconds — still too slow for MEV

Current Limitations and Improvement Path

Zama Ecosystem: fhEVM, Fhenix, Inco

fhEVM Contract Example: Encrypted ERC-20

// Traditional ERC-20 — balances are PUBLIC
contract PublicToken {
    mapping(address => uint256) public balance; // visible to everyone

    function transfer(address to, uint256 amount) external {
        balance[msg.sender] -= amount; // readable by MEV bots
        balance[to] += amount;
    }
}

// fhEVM ERC-20 — balances are ENCRYPTED on-chain
contract PrivateToken {
    mapping(address => euint256) private encBalance; // encrypted 暗号文

    function transfer(address to, euint256 amount) external {
        // All operations are FHE — validators can't see amount
        encBalance[msg.sender] = FHE.sub(encBalance[msg.sender], amount);
        encBalance[to]         = FHE.add(encBalance[to], amount);
        // No 平文 is ever revealed during execution
    }

    function balanceOf(address owner) external view
        returns (euint256) {
        require(msg.sender == owner); // only owner can decrypt
        return encBalance[owner];     // returns 暗号文; owner decrypts locally
    }
}

Ecosystem: Who's Building on FHE

Concrete Performance Numbers (2024)

FHE vs MPC vs ZKP vs Differential Privacy

Different cryptographic privacy primitives serve different roles in the DeFi privacy stack. Understanding when to choose each is critical for system design.

Decision Guide: When to Use Each

DeFi Privacy Stack (Layered View)

Roadmap to FHE-Practical DeFi

Near-Term (1-3 years): What's Viable Now

Medium-Term (3-5 years): Hardware Acceleration Required

Long-Term (5-10+ years): Breakthrough Required

Current Research Frontiers (2024)