Branching in FHE

The result of comparison operations is of type ebool. Typical boolean operations are not supported for this type, because it is an encrypted boolean.

Condition with encrypted boolean

fhEVM provides a method which acts as a ternary operator on encrypted integers. This method is called select.

function bid(einput encryptedValue, bytes calldata inputProof) public onlyBeforeEnd {
  euint64 bid = TFHE.asEuint64(encryptedValue, inputProof);
  ebool isAbove = TFHE.lt(highestBid, bid);

  // Replace highest bid
  highestBid = TFHE.select(isAbove, bid, highestBid);
  TFHE.allow(highestBid, address(this));
}

It is important to keep in mind that each time we assign a value using TFHE.select, the value changes, even if the plaintext value remains the same.

Error handling

If a condition is not satisfied, the transaction will not be reverted, potentially posing a challenge when attempting to communicate issues to users. A recommended approach to address this is by implementing an error handler in which the contract stores the latest error information for all wallets.

struct LastError {
  euint8 error;
  uint timestamp;
}

euint8 internal NO_ERROR;
euint8 internal NOT_ENOUGH_FUND;

constructor() {
  NO_ERROR = TFHE.asEuint8(0);
  NOT_ENOUGH_FUND = TFHE.asEuint8(1);
}

function setLastError(euint8 error, address addr) private {
  _lastErrors[addr] = LastError(error, block.timestamp);
  emit ErrorChanged(addr);
}

function _transfer(address from, address to, euint32 amount) internal {
  // Make sure the sender has enough tokens.
  ebool canTransfer = TFHE.le(amount, balances[from]);
  setLastError(TFHE.select(canTransfer, NO_ERROR, NOT_ENOUGH_FUND), msg.sender);

  // Add to the balance of `to` and subract from the balance of `from`.
  balances[to] = TFHE.add(balances[to], TFHE.select(canTransfer, amount, TFHE.asEuint32(0)));
  TFHE.allow(balances[to], address(this));
  TFHE.allow(balances[to], to);

  balances[from] = TFHE.sub(balances[from], TFHE.select(canTransfer, amount, TFHE.asEuint32(0)));
  TFHE.allow(balances[from], address(this));
  TFHE.allow(balances[from], from);
}