Frequently Asked Questions

// Bad: leaks information
if (FHE.decrypt(balance) > 100) {
    // ...
}

// Good: no leakage
ebool isAboveThreshold = FHE.gt(balance, FHE.asEuint64(100));
result = FHE.select(isAboveThreshold, valueIfTrue, valueIfFalse);

euint32 newBalance = FHE.add(balance, amount);
balances[user] = newBalance;
FHE.allowThis(newBalance);  // CRITICAL: allow contract to read newBalance later

euint8 a = FHE.asEuint8(200);
euint8 b = FHE.asEuint8(100);
euint8 sum = FHE.add(a, b);  // sum = 44 (300 mod 256), no revert

ebool willOverflow = FHE.gt(FHE.add(a, b), FHE.asEuint8(255));
result = FHE.select(willOverflow, a, FHE.add(a, b));

FHE.makePubliclyDecryptable(encryptedValue);

// Contract grants ACL permission
FHE.allow(encryptedValue, user);

// User decrypts client-side via Relayer SDK
const decrypted = await instance.userDecrypt(encryptedValueHandle);

import { createInstance } from "@fhevm/hardhat-plugin";

const instance = await createInstance();
const input = instance.createEncryptedInput(contractAddress, userAddress);
input.add32(42);
const encryptedInput = input.encrypt();

await contract.myFunction(encryptedInput.handles[0], encryptedInput.inputProof);

// User deposits 100 USDT (plaintext ERC-20)
usdt.transferFrom(user, address(this), 100);

// Contract mints 100 cUSDT (encrypted ERC-7984)
_mint(user, FHE.asEuint64(100));

euint32 newValue = FHE.add(oldValue, amount);
balances[user] = newValue;
FHE.allowThis(newValue);  // REQUIRED
FHE.allow(newValue, user); // If user needs to read it

import { createInstance } from "@fhevm/hardhat-plugin";

beforeEach(async function () {
    this.instance = await createInstance();
    // ... deploy contract
});