ExABI

The Application Binary Interface (ABI) of Solidity describes how to transform binary data to types which the Solidity programming language understands. For instance, if we want to call a function bark(uint32,bool) on a Solidity-created contract contract Dog, what data parameter do we pass into our Ethereum transaction? This project allows us to encode such function calls.

Installation

The latest version (>= 0.5.0) of ex_abi requires Rust because it uses a Rust NIF for KECCAK-256 hash. You can also try using 0.4.0, it doesn't have a Rust requirement because it uses a C NIF. But 0.4.0 does not support OTP 23.

If available in Hex, the package can be installed by adding ex_abi to your list of dependencies in mix.exs:

def deps do
  [
    {:ex_abi, "~> 0.5"}
  ]
end

Documentation can be generated with ExDoc and published on HexDocs. Once published, the docs can be found at https://hexdocs.pm/ex_abi.

Usage

Encoding

To encode a function call, pass the ABI spec and the data to pass in to ABI.encode/1.

iex> ABI.encode("baz(uint,address)", [50, <<1::160>> |> :binary.decode_unsigned])
<<162, 145, 173, 214, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 50, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, ...>>

That transaction can then be sent via JSON-RPC Client ethereumex.

Decoding

Decode is generally the opposite of encoding, though we generally leave off the function signature from the start of the data. E.g. from above:

iex> ABI.decode("baz(uint,address)", "00000000000000000000000000000000000000000000000000000000000000320000000000000000000000000000000000000000000000000000000000000001" |> Base.decode16!(case: :lower))
[50, <<0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1>>]

Function selectors

Both ABI.encode/2 and ABI.decode/2 can accept a function selector as the first parameter. For example:

selector = %ABI.FunctionSelector{
          function: "startInFlightExit",
          input_names: [
            "inFlightTx",
            "inputTxs",
            "inputUtxosPos",
            "inputTxsInclusionProofs",
            "inFlightTxWitnesses"
          ],
          inputs_indexed: nil,
          method_id: <<90, 82, 133, 20>>,
          returns: [],
          type: :function,
          types: [
            tuple: [
              :bytes,
              {:array, :bytes},
              {:array, {:uint, 256}},
              {:array, :bytes},
              {:array, :bytes}
            ]
          ]
        }

ABI.encode(selector, params)

To parse function selector from the abi json, use ABI.parse_specification/2:

iex> [%{
...>   "inputs" => [
...>      %{"name" => "_numProposals", "type" => "uint8"}
...>   ],
...>   "payable" => false,
...>   "stateMutability" => "nonpayable",
...>   "type" => "constructor"
...> }]
...> |> ABI.parse_specification
[%ABI.FunctionSelector{function: nil, input_names: ["_numProposals"], inputs_indexed: nil, method_id: <<99, 53, 230, 34>>, returns: [], type: :constructor, types: [uint: 8]}]

Decoding output

By default, decode and encode functions try to decode/encode input (params that passed to the function). To decode/encode output pass :output as the third parameter:

      selector = %FunctionSelector{
        function: "getVersion",
        input_names: [],
        inputs_indexed: nil,
        method_id: <<13, 142, 110, 44>>,
        returns: [:string],
        type: :function,
        types: []
      }

      data =
        "0000000000000000000000000000000000000000000000000000000000000020000000000000000000000000000000000000000000000000000000000000000d312e302e342b6136396337363300000000000000000000000000000000000000"
        |> Base.decode16!(case: :lower)

      expected_result = ["1.0.4+a69c763"]

      assert expected_result == ABI.decode(selector, data, :output)
      assert data == ABI.encode(selector, expected_result, :output)

Support

Currently supports:

• uint<M>
• int<M>
• address
• uint
• int
• bool
• fixed<M>x<N>
• ufixed<M>x<N>
• fixed
• bytes<M>
• <type>[M]
• bytes
• string
• <type>[]
• (T1,T2,...,Tn)

Docs

• Solidity ABI
• Solidity Docs