Decode input data on transaction

Question

I have an ABI and the input value of the transaction and want to know what function has been called on a contract and with what arguments (one or multiple of different types), preferably using geth, JSON-RPC and some pseudo code that I can then translate into Ruby.

There are a couple of similar questions on SO but none of them really answered the question in a way that's understandable to someone new to Ethereum.

As an example, I use the MetaCoin contract generated by Truffle. I call the method getBalance('0xfd46f749f9d916122fe958d7f8d5ad033b187472') and get the input 0xf8b2cb4f000000000000000000000000fd46f749f9d916122fe958d7f8d5ad033b187472.

truffle(development)> web3.eth.getTransaction('0x5d7a91c9f068d723ac52480c0ef61b9bf3f7b52dc726046e0b47f1beaa2f44ff')
{ hash: '0x5d7a91c9f068d723ac52480c0ef61b9bf3f7b52dc726046e0b47f1beaa2f44ff',
  nonce: 5,
  blockHash: '0xd150b520eb69a621a7218722c0de5b270c9abc499a90a81ca96340e274599c28',
  blockNumber: 6,
  transactionIndex: 0,
  from: '0xfd46f749f9d916122fe958d7f8d5ad033b187472',
  to: '0x8dec2f9cccbe718c7c51e9154f223eb226bb7942',
  value: { [String: '0'] s: 1, e: 0, c: [ 0 ] },
  gas: 4712388,
  gasPrice: { [String: '100000000000'] s: 1, e: 11, c: [ 100000000000 ] },
  input: '0xf8b2cb4f000000000000000000000000fd46f749f9d916122fe958d7f8d5ad033b187472' }

The ABI is as follows:

[
    {
      "constant": false,
      "inputs": [
        {
          "name": "addr",
          "type": "address"
        }
      ],
      "name": "getBalanceInEth",
      "outputs": [
        {
          "name": "",
          "type": "uint256"
        }
      ],
      "payable": false,
      "type": "function"
    },
    {
      "constant": false,
      "inputs": [
        {
          "name": "receiver",
          "type": "address"
        },
        {
          "name": "amount",
          "type": "uint256"
        }
      ],
      "name": "sendCoin",
      "outputs": [
        {
          "name": "sufficient",
          "type": "bool"
        }
      ],
      "payable": false,
      "type": "function"
    },
    {
      "constant": false,
      "inputs": [
        {
          "name": "addr",
          "type": "address"
        }
      ],
      "name": "getBalance",
      "outputs": [
        {
          "name": "",
          "type": "uint256"
        }
      ],
      "payable": false,
      "type": "function"
    },
    {
      "inputs": [],
      "payable": false,
      "type": "constructor"
    },
    {
      "anonymous": false,
      "inputs": [
        {
          "indexed": true,
          "name": "_from",
          "type": "address"
        },
        {
          "indexed": true,
          "name": "_to",
          "type": "address"
        },
        {
          "indexed": false,
          "name": "_value",
          "type": "uint256"
        }
      ],
      "name": "Transfer",
      "type": "event"
    }
  ]

How can I decode any input to know what method was called and what arguments were passed in?

Update

I've added all data of the block with the input field at the very bottom in the logs.

{"number"=>"0x09",
 "hash"=>"0xf543eba0c80d49beea1f934c0f0ad9f0babe272a8318312545d43e6b612e4006",
 "parentHash"=>
  "0xde7a61e8dd84b5e080da8e16430951f7f138347c77ed263d0093e367c9d30775",
 "nonce"=>"0x0",
 "sha3Uncles"=>
  "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
 "logsBloom"=>
  "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
 "transactionsRoot"=>
  "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
 "stateRoot"=>
  "0x269cfa04ca1fe73202f788397ece62c520c64d5da7225ba2af2d673a2e9892f2",
 "receiptRoot"=>
  "0x56e81f171bcc55a6ff8345e692c0f86e5b48e01b996cadc001622fb5e363b421",
 "miner"=>"0x0000000000000000000000000000000000000000",
 "difficulty"=>"0x0",
 "totalDifficulty"=>"0x0",
 "extraData"=>"0x0",
 "size"=>"0x03e8",
 "gasLimit"=>"0x47e7c4",
 "gasUsed"=>"0x5a3e",
 "timestamp"=>"0x58c83fda",
 "transactions"=>
  [{"hash"=>
     "0xce1eaf03dd1dfa5898243711ebcc1dc5c6357701f5a6c19f0b1f84130cb651fa",
    "nonce"=>"0x08",
    "blockHash"=>
     "0xf543eba0c80d49beea1f934c0f0ad9f0babe272a8318312545d43e6b612e4006",
    "blockNumber"=>"0x09",
    "transactionIndex"=>"0x0",
    "from"=>"0xfd46f749f9d916122fe958d7f8d5ad033b187472",
    "to"=>"0x8dec2f9cccbe718c7c51e9154f223eb226bb7942",
    "value"=>"0x0",
    "gas"=>"0x47e7c4",
    "gasPrice"=>"0x174876e800",
    "input"=>
     "0xf8b2cb4f000000000000000000000000fd46f749f9d916122fe958d7f8d5ad033b187472"}],
 "uncles"=>[]}

Answer 1

You can refer to a packaged developed by Consensys https://github.com/ConsenSys/abi-decoder

Answer 2

Ah ok, I understand what you are asking. You can do this web3.toAscii(transactionID.input) to get a human readable format.

See web3.toAscii https://github.com/ethereum/wiki/wiki/JavaScript-API#web3toascii

So with your code it would be:

transaction = web3.eth.getTransaction('0x5d7a91c9f068d723ac52480c0ef61b9bf3f7b52dc726046e0b47f1beaa2f44ff')
input = web3.toAscii(transaction.input)
console.log(input)

Answer 3

You can now use the web3.eth.abi.decodeLog function (web3 1.0).

Example from the documentation:

web3.eth.abi.decodeLog([{
    type: 'string',
    name: 'myString'
},{
    type: 'uint256',
    name: 'myNumber',
    indexed: true
},{
    type: 'uint8',
    name: 'mySmallNumber',
    indexed: true
}],
'0x0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000748656c6c6f252100000000000000000000000000000000000000000000000000',
['0x000000000000000000000000000000000000000000000000000000000000f310', '0x0000000000000000000000000000000000000000000000000000000000000010']);
> Result {
    '0': 'Hello%!',
    '1': '62224',
    '2': '16',
    myString: 'Hello%!',
    myNumber: '62224',
    mySmallNumber: '16'
}

Answer 4

You can use the tool ethereum-input-data-decoder for decoding transaction input data given the ABI.

Example:

const InputDataDecoder = require('ethereum-input-data-decoder');
const decoder = new InputDataDecoder(`${__dirname}/abi.json`);

const data = `0x67043cae0000000000000000000000005a9dac9315fdd1c3d13ef8af7fdfeb522db08f020000000000000000000000000000000000000000000000000000000058a20230000000000000000000000000000000000000000000000000000000000040293400000000000000000000000000000000000000000000000000000000000000a0f3df64775a2dfb6bc9e09dced96d0816ff5055bf95da13ce5b6c3f53b97071c800000000000000000000000000000000000000000000000000000000000000034254430000000000000000000000000000000000000000000000000000000000`;

const result = decoder.decodeData(data);

console.log(result);

{
  "name": "registerOffChainDonation",
  "types": [
    "address",
    "uint256",
    "uint256",
    "string",
    "bytes32"
    ],
    "inputs": [
      <BN: 5a9dac9315fdd1c3d13ef8af7fdfeb522db08f02>,
      <BN: 58a20230>,
      <BN: 402934>,
      "BTC",
      <Buffer f3 df ... 71 c8>
    ]
}

Answer 5

I have developed a simple ruby gem for decoding and encoding transactions input data in EVM based blockchains: https://github.com/rkotov93/evm-tx-input-decoder

Here's usage example regarding your case:

# abi = parsed JSON ABI you provided
input = '0xf8b2cb4f000000000000000000000000fd46f749f9d916122fe958d7f8d5ad033b187472'
EvmTxInput::Decoder.new(abi).decode_input(input)

# Result:
# 
# #<EvmTxInput::Function:0x000000010b29a638
#  @arguments=[#<EvmTxInput::Argument:0x000000010b29ba88 @name="addr", @type="address", @value="0xfd46f749f9d916122fe958d7f8d5ad033b187472">],
#  @id="f8b2cb4f",
#  @name="getBalance">

Answer 6

On new web3.js 1.0 you must use: web3.utils.toAscii(transaction.input).

Some times you should do a let input = transaction.input.replace(/^0x/, ``); to remove these useless characters from string start.

See docs.