[Ethereum] How to unpack/decode parameters encoded with web3.eth.abi.encodeParameters in Solidity

decodingencodingsolidityweb3js

My client app (nodejs) packs some arguments to pass to a smart contracts as bytes _data (similar to ERC223 calls with fallback). I pack the arguments in my JS client like this:

const packedArgs = web3.eth.abi.encodeParameters(['uint256', 'string'], ['123', 'Hello world']);`

When the solidity function gets that bytes argument, how does it unpack it back to uint256 and string?

Best Answer

Ok I have finally figured it out. web3.eth.abi.encodeParameters encodes each parameter as one or more 32 bit words, so to get each parameter we need to grab the next 32 bit from the bytes parameter's memory. Something like:

// Assume that _encodedParams is encoded from ([address, uint256])
function (bytes _encodedParam) public returns (address a, uint256 n) {
    uint256 btsptr;
    /* solium-disable-next-line security/no-inline-assembly */
    assembly {
        btsptr := add(_encodedParam, /*BYTES_HEADER_SIZE*/32)
        a := mload(btsptr)
        btsptr := add(_encodedParam, /*BYTES_HEADER_SIZE*/64)
        n := mload(btsptr)
    }
}

That's it really. I haven't figured out yet how to unpack a string, but probably you keep unpacking word by word and concatenating them bite-wise. Something for the next time.

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I don't believe there's any built-in function for doing this, but here's a toBase58 implementation: https://github.com/MrChico/verifyIPFS/blob/master/contracts/verifyIPFS.sol#L28.

Depending on your use case, it might be better to have Solidity deal with the raw bytes and convert to/from base58 outside of the contract (e.g. in JavaScript or whatever client language you're using).

Solidity – How to Decode Raw Data Using ABI Bytes in Solidity 0.8.x

You have to remove the first four bytes first. The first four bytes of a contract call is the function selector, which specifies which function you want to call, in this case balanceOf(address). I wrote a more detailed explanation about transaction data, which you can find here.

Assuming you're using Solidity 0.6.0 or newer, you can use data[4:] to slice off the first four bytes of the input, or additionally just remove them from the input if you're hardcoding the data.

function getBalance(bytes calldata data) public pure returns (uint256 balance) {
  (balance) = abi.decode(data, (uint256));
}

// or

function getBalance() public pure returns (uint256 balance) {
  bytes memory data = hex'000000000000000000000000278261c4545d65a81ec449945e83a236666b64f5';
  (balance) = abi.decode(data, (uint256));
}

Best Answer

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