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I have a Solidity 0.4.26 smart contract. I'm using a commit-reveal pattern to hide to first user's commitment move.

I have a helper contract Hasher that hashes the user's commitment with a hash() function:

contract Hasher{
    /** @dev Give the commitement. Must only be called locally.
     *  @param _c The move.
     *  @param _salt The salt to increase entropy.
     */
    function hash(uint8 _c, uint256 _salt) constant returns(bytes32) {
        return keccak256(_c,_salt);
    }
}

The Solidity 0.4.26 implementation of a native keccak256 function allows 2 arguments: a message and a salt phrase.

For some reason, I cannot find an equivalent of that function in any JS-based library.

I'm ideally looking to code a Javascript keccak256 function that would be compatible with the Solidity's 0.4.26 keccak256 in order to later decrypt the commitment move with the salt:

require(keccak256(_c1,_salt)==c1Hash); // Verify the value is the commited one.

Please give me any advice on how I can achieve that.

Thank you!

1 Answer 1

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To do the equivalent hashing in JavaScript, you have to encodePacked the values _c and _salt before hashing. In ethers there is a helper method solidityKeccak256 which does exactly what you are looking for.

const _c = 5;
const _salt = 10;
const hash = ethers.utils.solidityKeccak256(['uint8', 'uint256'], [_c, _salt]);
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  • Works like a charm! Thanks!
    – Mila A
    Commented Jun 27, 2023 at 8:13

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