Below is work example of joining two bytes arrays. I wonder how I can join two bytes32 using assembly or solidity? two bytes32 values should serve as an input and a joined bytes32 is an output.

function mergeBytes(bytes memory a, bytes memory b) public pure returns (bytes memory c) {
    // Store the length of the first array
    uint alen = a.length;
    // Store the length of BOTH arrays
    uint totallen = alen + b.length;
    // Count the loops required for array a (sets of 32 bytes)
    uint loopsa = (a.length + 31) / 32;
    // Count the loops required for array b (sets of 32 bytes)
    uint loopsb = (b.length + 31) / 32;
    assembly {
        let m := mload(0x40)
        // Load the length of both arrays to the head of the new bytes array
        mstore(m, totallen)
        // Add the contents of a to the array
        for {  let i := 0 } lt(i, loopsa) { i := add(1, i) } { mstore(add(m, mul(32, add(1, i))), mload(add(a, mul(32, add(1, i))))) }
        // Add the contents of b to the array
        for {  let i := 0 } lt(i, loopsb) { i := add(1, i) } { mstore(add(m, add(mul(32, add(1, i)), alen)), mload(add(b, mul(32, add(1, i))))) }
        mstore(0x40, add(m, add(32, totallen)))
        c := m
  • 2
    Are you trying to merge two arrays ? Dec 11, 2018 at 17:56

3 Answers 3


You cannot concatenate two bytes32 into one bytes32, because there is not enough space.

If you just want a way to combine two bytes32 into one without preserving the original data, you can use XOR:

bytes32 c = a ^ b;

... or you can use a hashing function:

bytes32 c = keccak256(abi.encodePacked(a, b));

If you want to concatenate two bytes32 into one bytes, you can do something like this:

function concat(bytes32 b1, bytes32 b2) pure external returns (bytes memory)
    bytes memory result = new bytes(64);
    assembly {
        mstore(add(result, 32), b1)
        mstore(add(result, 64), b2)
    return result;

As stated by Jesse you cannot join two bytes32 and obtain a bytes32 but you should be able to concatenate both to obtain a bytes array.

One option is to use abi.encodePacked for such endeavor

pragma solidity >=0.4.22 <0.6.0;

contract Ballot {
    function concatenate(bytes32 x, bytes32 y) public pure returns (bytes memory) {
        return abi.encodePacked(x, y);

As of Solidity v0.8.12 the string.concat and bytes.concat methods are available

So this problem can be easily solved with

bytes32 memory b1;
bytes32 memory b2;
bytes.concat(b1, b2);

Source: Solidity docs

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