secure string comparison in ruby

secure_compare.rb

# plain ruby
def secure_compare(a, b)
  return false if a.empty? || b.empty? || a.bytesize != b.bytesize
  
  l = a.unpack "C#{a.bytesize}"

  res = 0
  b.each_byte { |byte| res |= byte ^ l.shift }
  
  res == 0
end

# RAILS
ActiveSupport::SecurityUtils.secure_compare(a, b)