_transfer in EncryptedERC20

pjhnocegood · March 6, 2024, 9:30am

zama-ai/fhevm/blob/main/examples/EncryptedERC20.sol

// SPDX-License-Identifier: BSD-3-Clause-Clear

pragma solidity ^0.8.20;

import "../abstracts/Reencrypt.sol";
import "../lib/TFHE.sol";
import "@openzeppelin/contracts/access/Ownable2Step.sol";

contract EncryptedERC20 is Reencrypt, Ownable2Step {
    event Transfer(address indexed from, address indexed to);
    event Approval(address indexed owner, address indexed spender);
    event Mint(address indexed to, uint64 amount);

    uint64 private _totalSupply;
    string private _name;
    string private _symbol;
    uint8 public constant decimals = 0;

    // A mapping from address to an encrypted balance.
    mapping(address => euint64) internal balances;

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If you look at the _transfer function, it operates with + and -.

From what I understand, the value entered in balances[key] is not the ciphertext itself, but only the hash value of the ciphertext. Shouldn’t I use TFHE.and or TFHE.sub instead of + or - as above?

Jat · March 6, 2024, 9:47am

Hi, under the hood we are overloading the + and - operators in the TFHE.sol library as you can see here : fhevm/lib/TFHE.sol at main · zama-ai/fhevm · GitHub so these are equivalent to TFHE.add(euintXX,euintXX) and TFHE.sub(euintXX,euintXX). Notice however that it works only if both operands are encrypted types, overloading is not working if we have mixed types , i.e if the first operand is a ciphertext and the second is in cleartext, you still must use TFHE.add and TFHE.sub explicitly.