how to generate a unique id which can be validated to be authentic

id.rb

require 'digest'

module Id

  SECRET = 'nobody knows the trouble'

  def self.generate
    a = digest("#{Time.now.to_f}:#{rand}")
    b = digest(SECRET + a)
    id = a + b
  end

  def self.valid?(id)
    return false if id.nil? or id.length != 80
    a, b = id[0,40], id[40,40]
    b == digest(SECRET + a)
  end
  
private
  
  def self.digest(text)
    Digest::SHA1.hexdigest(text)
  end

end


id = Id.generate
puts "#{Id.valid?(id)}:  #{id}"

fake_id = id.reverse
puts "#{Id.valid?(fake_id)}: #{fake_id}"