Estimate gas

FHE operations are typically more computationally expensive than classical operations due to their inherent complexity. As a reference, here is an approximation of the gas cost associated with each operation.

function name	ebool/euint8	euint16	euint32
TFHE.add	120,000	150,000	180,000
TFHE.add (scalar)	120,000	150,000	180,000
TFHE.sub	120,000	150,000	180,000
TFHE.sub (scalar)	120,000	150,000	180,000
TFHE.mul	200,000	260,000	380,000
TFHE.mul (scalar)	135,000	140,000	170,000
TFHE.div (scalar)	450,000	500,000	550,000
TFHE.rem (scalar)	450,000	500,000	550,000
TFHE.and	30,000	33,000	36,000
TFHE.or	30,000	33,000	36,000
TFHE.xor	30,000	33,000	36,000
TFHE.shr	150,000	180,000	210,000
TFHE.shr (scalar)	32,000	32,000	32,000
TFHE.shl	150,000	180,000	210,000
TFHE.shl (scalar)	32,000	32,000	32,000
TFHE.eq	56,000	67,000	89,000
TFHE.ne	56,000	67,000	89,000
TFHE.ge	56,000	67,000	89,000
TFHE.gt	56,000	67,000	89,000
TFHE.le	56,000	67,000	89,000
TFHE.lt	56,000	67,000	89,000
TFHE.min	220,000	280,000	340,000
TFHE.max	220,000	280,000	340,000
TFHE.neg	29,000	31,000	33,000
TFHE.not	29,000	31,000	33,000
TFHE.cmux	60,000	65,000	70,000
TFHE.decrypt()	500,000	500,000	500,000
TFHE.randEuintX()	100,000	100,000	100,000

Estimate gas

When you call estimate gas method, we can’t determine accurately the gas usage if your function uses TFHE.decrypt. During gas estimation, all TFHE.decrypt() will return 1.

What does it mean?

• require(TFHE.decrypt(ebool)); will be ok but require(!TFHE.decrypt(ebool)); will fail during estimation (revert transaction)

• A loop, where you expect a decrypt to be false to break, will never end in gas estimate method (and fails), since the decrypt will always return 1 (true)

• On the other hand, if your loop should last 2 or 3 cycles, until the value is 1, the estimation will be below.

• If you have branches (if/else) based on a decryption, the estimation will use the branch running when the decryption is 1

While it’s challenging to accurately estimate gas consumption when using TFHE.decrypt, we strongly encourage you to take this into consideration.

What can I do?

A possible solution is to overestimate your gas estimation. You can take this function (with ethers.js) as an example where we multiply the gas limit by 1.2.

export const createTransaction = async <A extends [...{ [I in keyof A]-?: A[I] | Typed }]>(
  method: TypedContractMethod<A>,
  ...params: A
) => {
  const gasLimit = await method.estimateGas(...params);
  const updatedParams: ContractMethodArgs<A> = [
    ...params,
    { gasLimit: Math.min(Math.round(+gasLimit.toString() * 1.2), 10000000) },
  ];
  return method(...updatedParams);
};

Gas limit

The current devnet has a gas limit of 10,000,000. If you send a transaction exceeding this limit, it won't be executed. Consequently, your wallet won't be able to emit a new transaction. To address this, emit a new transaction with the same nonce but the correct gas limit. In Metamask, you can enforce the use of a specific nonce by enabling the feature in 'Advanced Settings'.