I need a function (Java/Kotlin) that returns hashMessage which corresponds to Solidity function:
function getMessageHash(address _signer, uint _amount, uint _id) public pure returns (bytes32) {
return keccak256(abi.encodePacked(_signer, _amount, _id));
}
This function should return value of format: hash = "0xcf36ac4f97dc10d91fc2cbb20d718e94a8cbfe0f82eaedc6a4aa38946fb797cd"
I've tried using Hash.sha3(String hexInput). from the org.web3j.crypto.Hash package but it only accepts one parameter. I can't figure out what does abi.encodePacked() is doing with parameters under the "hood".
I've finally found the solution, so I'll post it here if anyone needs it.
The main question was, what is the abi.encodePacked() method doing "under the hood".
HERE is a detailed explanation how some of the parameters are encoded in this method.
For the particular question and method with next parameters:
encodedMessage = abi.encodePacked("0x1b77882d1e55ffff0bc1c4215b869a2a36f7054d", 1, 1) the result from this method is:
encodedMessage = 1b77882d1e55ffff0bc1c4215b869a2a36f7054d00000000000000000000000000000000000000000000000000000000000000010000000000000000000000000000000000000000000000000000000000000001
For the _signer (string) the prefix (0x) was removed, for _amount and _id (uint) I used TypeEncoder.encode(Uint256(1)).
Encoded values for each parameter is appended to the previous one, and at last hashed together with method: val hashedMessage = Hash.sha3(encodedMessage) which result is: 0xe06176482c2b2619110be196a3d999dedf922782c0fe85603b19044bf30b3856
Later on hashedMessage can be used for signing or verification of signature. But that is out of the scope of this question.