defmodule DecryptPayload do
  @moduledoc """
  Decrypts payload
  """

  def run(payload_hex, key_hex, iv_hex) do
    payload_bytes = payload_hex |> Base.decode16!
    key_bytes = key_hex |> Base.decode16!
    iv_bytes = iv_hex |> Base.decode16!

    state = :crypto.stream_init(:aes_ctr, key_bytes, iv_bytes)

    {_new_state, decrypted} = :crypto.stream_decrypt(state, payload_bytes)

    decrypted
    |> Base.encode16
    |> IO.inspect
  end
end

[payload_hex, key_hex, iv_hex] = System.argv
DecryptPayload.run(payload_hex, key_hex, iv_hex)