encrypt (PDF::Writer)

         # File lib/pdf/writer.rb, line 710
    710:   def encrypt(user_pass = nil, owner_pass = nil, permissions = [])
    711:     perms = ["11000000"].pack("B8")
    712:
    713:     permissions.each do |perm|
    714:       perms += ENCRYPT_OPTIONS[perm] if ENCRYPT_OPTIONS[perm]
    715:     end
    716:
    717:     @arc4 ||= PDF::ARC4.new
    718:     owner_pass ||= user_pass
    719:
    720:     options = {
    721:       :owner_pass   => owner_pass,
    722:       :user_pass    => user_pass,
    723:       :permissions  => perms,
    724:     }
    725:     @encryption = PDF::Writer::Object::Encryption.new(self, options)
    726:   end