Tham khảo tài liệu 'hacker professional ebook part 178', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | our guys provides no information about the strength of the cipher as seen by our Opponents. Increasing Probable Strength and Reducing Possible Loss Technical strength is just one of the many possibilities for weakness in a cipher system and perhaps even the least likely. It is surprisingly difficult to construct a cipher system without holes despite using good ciphers and The Opponents get to exploit any overlooked problems. Users must be educated in security and must actively keep secrets or there will be nothing to protect. In contrast cryptanalysis is very expensive success is never assured and even many of the known attacks are essentially impossible in practice. Nevertheless it is a disturbing fact that we do not know and cannot guarantee a true strength for any cipher. But there are approaches which may reduce the probability of technical weakness and the extent of any loss 1. We can extrapolate various attacks beyond weakness levels actually shown and thus possibly avoid some weak ciphers. 2. We can use systems that change ciphers periodically. This will reduce the amount of information under any one cipher and so limit the damage if that cipher is weak. 3. We can use multiple encryption with different keys and different ciphers as our standard mode. In this way not just one but multiple ciphers must each be penetrated simultaneously to expose the protected data. 4. We can use systems that allow us to stop using ciphers when they are shown weak and switch to others. Kinds of Cipher Strength In general we can consider a cipher to be a large key-selected transformation between plaintext and ciphertext with two main types of strength One type of strength is an inability to extrapolate from known parts of the transformation . known plaintext to model -- or even approximate -- the transformation at new points of interest message ciphertexts . Another type of strength is an inability to develop a particular key given the known cipher and a large number of .