rand_hash.ex

defmodule HashData do
  def open_file do
    File.read! "rand.txt"
  end

  def hash_contents(n) do
    open_file()
    |> hash_contents(n)
  end

  def hash_contents(hash, n) when n < 1 do
    hash
    |> Base.encode16
    |> String.downcase
  end

  def hash_contents(hash, n) do
    :crypto.hash(:sha256, hash)
    |> hash_contents(n - 1)
  end
end

defmodule Benchmark do
  def measure(function) do
    function
    |> :timer.tc
    |> elem(0)
    |> Kernel./(1_000_000)
  end
end

Bench with:
Benchmark.measure(fn -> HashData.hash_contents(100_000_000) end)