How are we able to send 512 bit hash as a bytes32 variable?

Question

I have a code that is working:

solidity

function checkSignature(bytes32 h, uint8 v, bytes32 r, bytes32 s) public pure returns (address signer) {
    bytes memory prefix = "\x19Ethereum Signed Message:\n32";    
    bytes32 prefixedHash = keccak256(abi.encodePacked(prefix,h));  
    signer = ecrecover(prefixedHash, v, r, s); 
}

javascript

    const message = "Hello";
    const h = web3.utils.soliditySha3(message);
    console.log(h.length);
    let signature = await web3.eth.sign(h, second);

    var r = signature.slice(0, 66);
    var s = "0x" + signature.slice(66, 130);
    var v = "0x" + signature.slice(130, 132);
    v = web3.utils.toDecimal(v);
    v = v + 27;

    const result = await multisig.checkSignature(h, v, r, s);
    //result === second

The logged length of h is 66. 512bit hash should be 64bytes. The first 2 bytes(0x)denotes this is a hex value.

I am sending it as a solidity bytes32 variable which has a fixed length of 32. How is the code working?

I might be missing something very obvious. Please point it out. Thanks in advance.

Answer 1

You are using soliditySha3, which produces a 256-bit (32 bytes) hash, because of that your code works with bytes32.

As you can see in the docs, the output of soliditySha3 is something like the following, with a length of 66, 64 without 0x. Check that one byte corresponds to two chars, so the result is 32 bytes.

var x = web3.utils.soliditySha3('234'); // auto detects: uint256
> "0x61c831beab28d67d1bb40b5ae1a11e2757fa842f031a2d0bc94a7867bc5d26c2