# File lib/omniauth/strategies/windows_live/windowslivelogin.rb, line 1067
  def derive(secret, prefix)
    begin
      fatal("Nil/empty secret.") if (secret.nil? or secret.empty?)
      key = prefix + secret
      key = OpenSSL::Digest::SHA256.digest(key)
      return key[0..15]
    rescue Exception => e
      debug("Error: derive: #{e}")
      return
    end
  end