Added keccak256 hash

lib/exw3.ex

@@ -1,4 +1,11 @@
 defmodule ExW3 do
+
+  @spec keccak256(binary()) :: binary()
+  @doc "Returns a 0x prepended 32 byte hash of the input string"
+  def keccak256(string) do
+    Enum.join(["0x", ExthCrypto.Hash.Keccak.kec(string) |> Base.encode16(case: :lower)], "")
+  end
+  
   @spec bytes_to_string(binary()) :: binary()
   @doc "converts Ethereum style bytes to string"
   def bytes_to_string(bytes) do

test/exw3_test.exs

@@ -41,6 +41,18 @@ defmodule EXW3Test do
     assert ExW3.block_number() == block_number + 5
   end
 
+  test "keccak256 hash some data" do
+    hash = ExW3.keccak256("foo")
+    assert String.slice(hash, 0..1) == "0x"
+
+    num_bytes =
+      hash |>
+      String.slice(2..-1) |>
+      byte_size
+
+    assert trunc(num_bytes / 2) == 32
+  end
+
   test "starts a Contract GenServer for simple storage contract", context do
     ExW3.Contract.start_link(SimpleStorage, abi: context[:simple_storage_abi])