[Ethereum] Web3js and keccak different values

keccaksolidity

In solidity there is the keccak256 function which hashes its arguments.
I use the keccak256 to compute and use the result to check if some entries exist in my array.

I want to compute the same value in javascript and check if the entry exists on the blockchain, although the web3.sha gives different value than the keccak256.

How can I compute the same value for the same arguments?

In console I run this:

web3.sha3("test1");

and my contract:

function hashVal(bytes32 val) public returns(bytes32) {
     return keccak256(val);
 }

Best Answer

Try this:

web3.sha3(web3.padRight(web3.fromAscii("test1"), 66), { encoding: 'hex' });

Explanation:

A bytes32 representation of "test1" is 32 bytes long, while "test1" is only 5 bytes long.

web3.fromAscii("test1") converts the string to its hexadecimal representation with a leading "0x": 0x7465737431.

web3.padRight pads the value with zeros on the right-hand side, like the conversion to bytes32 does. 32 bytes is 64 hexadecimal characters, but to take into account the leading "0x", I'm padding to 66 characters. This producets 0x7465737431000000000000000000000000000000000000000000000000000000, which is a bytes32 representation of the string.

Finally, we pass this value to web3.sha3 and specify the { encoding: 'hex' } option so sha3 interprets the value passed in as hexadecimal rather than ASCII.

Related Solutions

Keccak256 – How to Pass a String to be Hashed Using Keccak256

try this snipet

function bytes32ToString(bytes32 x) constant returns (string) {
    bytes memory bytesString = new bytes(32);
    uint charCount = 0;
    for (uint j = 0; j < 32; j++) {
        byte char = byte(bytes32(uint(x) * 2 ** (8 * j)));
        if (char != 0) {
            bytesString[charCount] = char;
            charCount++;
        }
    }
    bytes memory bytesStringTrimmed = new bytes(charCount);
    for (j = 0; j < charCount; j++) {
        bytesStringTrimmed[j] = bytesString[j];
    }
    return string(bytesStringTrimmed);
}

 function someFunction(bytes32 _string) returns(bool){
    bool result = false;
    bytes32 hashparameter = sha3(bytes32ToString(_string));
    bytes32 hashstring = sha3("hello");
    if(hashstring == hashparameter) {
        result = true;
    }

    return result;

}

the first function converts bytes32 to string then passes the result to hashing function. I've tried it gives true for "hello".

[Ethereum] keccak3 in SOLIDITY and in WEB3 different hashes for an address type field

web3.sha3() takes an encoding parameter to specify that you are passing in a hex address:

address_string = '0x5b2063246f2191f18f2675cedb8b28102e957458';
web3.sha3(address_string, {encoding: 'hex'});

As a general side-note: you have to pay gas costs to do any custom work in Solidity so, when possible, you are best off working on the javascript side.

Best Answer

Related Solutions

Keccak256 – How to Pass a String to be Hashed Using Keccak256

[Ethereum] keccak3 in SOLIDITY and in WEB3 different hashes for an address type field

Related Topic