chal17

def challenge17
  server = CBCPaddingServer.new
  iv,session_cookie = server.get_session_cookie()
  block_size=16
  puts session_cookie.blocks(block_size).length
  blocks=session_cookie.blocks(block_size)
  blocks=[iv]+blocks
  decoded_byte_string=[]
  1.upto(blocks.length-1) do |bi| 
    decoded_bytes=[]
    block=blocks[-bi-1]
    target_block=blocks[-bi]
    1.upto(block_size) do |pad_byte| 
      256.times do |byte| 
        if byte == pad_byte
          next
        end
        decoded_bytes[pad_byte-1]=byte
        edited_block=create_edit_text(block,decoded_bytes,pad_byte)
        if server.valid_decrypt(edited_block+target_block,iv)
          break
        end
      end
    end
    decoded_byte_string+=decoded_bytes
  end
  puts decoded_byte_string.pack("C*").reverse.inspect
end