The output of hash function SHA256 is a hex string of size 64. For example: 3ed54831f488a22b28398de0c567a3b064b937f54f81739ae9bd545967f3abab

I want to call the function of a smart contract with the following structure:

function register(bytes32 hash)

and pass the hex as an argument.

I use the web3 JavaScript library.

First I tried to pass the argument without any conversion. The result was a truncation of the last 32 bytes

Then I tried web3's fromAscii function with the same results.

I also tried hexToBytes and bytesToHex functions from the 1.x version. hexToBytes returns a byte array of size 32. When I pass the array to the smart contract function the results are totally different from the expected ones.

So how do I convert a hex of 32 bytes (64 character hex) in a format suitable for a bytes32 function argument ?

I know I can use bytes or string instead of bytes32 but I would rather avoid it as the hex of SHA256 is indeed 32 bytes.


I create a test contract and test it in remix. I call set function with input 0x3fd54831f488a22b28398de0c567a3b064b937f54f81739ae9bd545967f3abab. The value is not truncated and I get the expected results.

pragma solidity ^0.4.24;

contract Test {
    bytes32 public hash;

    function set(bytes32 h) public {
        hash = h;

So, the problem should be in the web3 library or JS itself right ?


I use the truffle-contract library to call my smart contract's functions

  • 1
    Total guess, but did you try prefixing the 'hex' string with '0x'? Jul 6, 2018 at 14:18
  • Yes, I did for all cases. Same results.
    – Laxmana
    Jul 6, 2018 at 14:38
  • I know it is not answer to your question, but if you are at the beginning of your project I suggest you to use ethers.js - github.com/ethers-io/ethers.js Truffle contract and web3 really often has strange and unexpected behaviors.
    – Kristian
    Jul 6, 2018 at 15:17

1 Answer 1


Pass it as a string "0x3fd54831f488a22b28398de0c567a3b064b937f54f81739ae9bd545967f3abab" It won't get truncated.

If this not solve read this: passing String object with "0x.." value as bytes32 to a solidity function

  • Thanks. Very useful links. Passing as a string with prefix "0x" is not working. I tried the other methods but I am getting a out of gas exception.
    – Laxmana
    Jul 13, 2018 at 11:15
  • Check all intermediate inputs and outputs on the console log, the error must be somewhere else
    – user38075
    Jul 14, 2018 at 11:37
  • I was converting the input to bytes before passing it as: web3.fromAscii(value). Removing the conversion and pass the input as 0x... worked.
    – Laxmana
    Jul 23, 2018 at 10:27

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