You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: decrypt.rb
    path of file: ./repos/combine_pdf/lib/combine_pdf
    
    the code of the file until where you have to start completion: # -*- encoding : utf-8 -*-
########################################################
## Thoughts from reading the ISO 32000-1:2008
## this file is part of the CombinePDF library and the code
## is subject to the same license.
########################################################

module CombinePDF
  #:nodoc: all

  protected

  # @private
 
    
    def decrypt_RC4(encrypted, encrypted_id, encrypted_generation, _encrypted_filter)
      ## start decryption using padding strings
      object_key = @key.dup
      object_key << [encrypted_id].pack('i')[0..2]
      object_key << [encrypted_generation].pack('i')[0..1]
      # (0..2).each { |e| object_key << (encrypted_id >> e*8 & 0xFF ) }
      # (0..1).each { |e| object_key << (encrypted_generation >> e*8 & 0xFF ) }
      key_length = object_key.length < 16 ? object_key.length : 16
      rc4 = ::RC4.new(Digest::MD5.digest(object_key)[(0...key_length)])
      rc4.decrypt(encrypted)
    end