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WTF Solidity: 27. ABIEncode&Decode

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ABI(Application Binary Interface) is the standard for interacting with Ethereum smart contracts. Data is encoded based on its type, and because the encoded result doesn't contain type information, it is necessary to indicate their types when decoding.

In Solidity, ABI encode has four functions: abi.encode, abi.encodePacked, abi.encodeWithSignature, abi.encodeWithSelector. While ABI decode has one function: abi.decode, which is used to decode the data of abi.encode.

In this chapter, We will learn how to use these functions.

ABI encode​

We will encode four variables, their types are uint256 (alias uint), address, string, uint256[2]:

    uint x = 10;
    address addr = 0x7A58c0Be72BE218B41C608b7Fe7C5bB630736C71;
    string name = "0xAA";
    uint[2] array = [5, 6];

abi.encode​

Use ABI rules to encode the given parameters. ABI is designed to interact with smart contract by filling each parameter with 32 bytes data and splicing them together. If you want to interact with contracts, you should use abi.encode.

    function encode() public view returns(bytes memory result) {
        result = abi.encode(x, addr, name, array);
    }

The result of encoding is0x000000000000000000000000000000000000000000000000000000000000000a0000000000000000000000007a58c0be72be218b41c608b7fe7c5bb630736c7100000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000005000000000000000000000000000000000000000000000000000000000000000600000000000000000000000000000000000000000000000000000000000000043078414100000000000000000000000000000000000000000000000000000000. Since abi.encode fills each data with 32 bytes data, there are a lot of 0 in middle

abi.encodePacked​

Encode given parameters according to their minimum required space. It is similar to abi.encode, but omits a lot of 0 filled in. For example, only 1 byte is used to encode the uint type. You can use abi.encodePacked when you want to save space and don't interact with contracts. For example when computing hash of some data.

    function encodePacked() public view returns(bytes memory result) {
        result = abi.encodePacked(x, addr, name, array);
    }

The result of encoding is0x000000000000000000000000000000000000000000000000000000000000000a7a58c0be72be218b41c608b7fe7c5bb630736c713078414100000000000000000000000000000000000000000000000000000000000000050000000000000000000000000000000000000000000000000000000000000006. Because abi.encodePacked compacts encoding, the length of result is much shorter than abi.encode.

abi.encodeWithSignature​

Similar with abi.encode function, but the first parameter is function signatures, such as "foo(uint256, address, string, uint256[2])". It can be used when calling other contracts.

    function encodeWithSignature() public view returns(bytes memory result) {
        result = abi.encodeWithSignature("foo(uint256,address,string,uint256[2])", x, addr, name, array);
    }

The result of encoding is0xe87082f1000000000000000000000000000000000000000000000000000000000000000a0000000000000000000000007a58c0be72be218b41c608b7fe7c5bb630736c7100000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000005000000000000000000000000000000000000000000000000000000000000000600000000000000000000000000000000000000000000000000000000000000043078414100000000000000000000000000000000000000000000000000000000. This is equivalent to adding 4 bytes function selector to the front of result of abi.encodenote. note: Function selectors identify functions by signature processing(Keccak–Sha3) using function names and arguments, which can be used for function call between different contracts.

abi.encodeWithSelector​

Similar to abi.encodeWithSignature, except that first argument is a function selector which is the first 4 bytes of function signature Keccak hash.

    function encodeWithSelector() public view returns(bytes memory result) {
        result = abi.encodeWithSelector(bytes4(keccak256("foo(uint256,address,string,uint256[2])")), x, addr, name, array);
    }

The result of encoding is0xe87082f1000000000000000000000000000000000000000000000000000000000000000a0000000000000000000000007a58c0be72be218b41c608b7fe7c5bb630736c7100000000000000000000000000000000000000000000000000000000000000a00000000000000000000000000000000000000000000000000000000000000005000000000000000000000000000000000000000000000000000000000000000600000000000000000000000000000000000000000000000000000000000000043078414100000000000000000000000000000000000000000000000000000000. The result is the same as abi.encodeWithSignature

ABI decode​

abi.decode​

abi.decode is used to decode the binary code generated by abi.encode and restore it to its original parameters.

    function decode(bytes memory data) public pure returns(uint dx, address daddr, string memory dname, uint[2] memory darray) {
        (dx, daddr, dname, darray) = abi.decode(data, (uint, address, string, uint[2]));
    }

We input binary encoding of abi.encode into decode, which will decode the original parameters:

在remix上验证​

  • deploy contract to check the encoding result of abi.encode

  • compare and verify the similarities and differences of four encoding functions

  • check the decoding result of abi.decode

ABI的使用场景​

  1. In contract development, ABI is often paired with call to implement low-level call to contract.
    bytes4 selector = contract.getValue.selector;

    bytes memory data = abi.encodeWithSelector(selector, _x);
    (bool success, bytes memory returnedData) = address(contract).staticcall(data);
    require(success);

    return abi.decode(returnedData, (uint256));
  1. ABI is often used in ethers.js to implement contract import and function call.
    const wavePortalContract = new ethers.Contract(contractAddress, contractABI, signer);
    /*
        * Call the getAllWaves method from your Smart Contract
        */
    const waves = await wavePortalContract.getAllWaves();
  1. After decompiling a non-open source contract, some functions cannot find function signature but can be called through ABI.
  • 0x533ba33a() is a function which show after decompiled, we can only get function encoded results, and can't find function signature.
  • in this case we can't call through constructing an interface or contract

In this case, we can call through ABI function selector.

    bytes memory data = abi.encodeWithSelector(bytes4(0x533ba33a));

    (bool success, bytes memory returnedData) = address(contract).staticcall(data);
    require(success);

    return abi.decode(returnedData, (uint256));

Summary​

In Ethereum, data must be encoded into bytecode to interact with smart contracts. In this chapter, we introduced four abi encoding functions and one abi decoding function.