It seems like when you copied the keys from the documentation example, you might have missed the starting %, i.e. instead of %{.......} you copied {.......}.

Can you show us the value of auth_keys and secret_index?

Can you change your snippet

  def list_authorized_public_keys_at(%__MODULE__{authorized_keys: auth_keys}, secret_index)
      when is_integer(secret_index) and secret_index >= 0 do
    auth_keys
    |> Enum.at(secret_index)
    |> Map.keys()
  end

to

  def list_authorized_public_keys_at(%__MODULE__{authorized_keys: auth_keys}, secret_index)
      when is_integer(secret_index) and secret_index >= 0 do
    auth_keys
    |> IO.inspect()
    |> Enum.at(secret_index)
    |> Map.keys()
  end

And share the results of the inspection?