tailieunhanh - Hacker Professional Ebook part 151

Ebook giới thiệu các công cụ và đồ nghề cần thiết cho việc Hack-Security. Ebook gồm có 1177 bài viết trong 28 chủ đề chính Phần 15: 10 part gồm Club Showoff - Hacked Sites & Domain,Tools and toys for Hacking. | It is fairly easy to design a complex cipher program to produce a single complex intermediate form. In this case the program itself becomes the key. But this means that the deciphering program must be kept available to access protected information. So if someone steals your laptop they probably will also get the deciphering program which -- if it does not use keys -- will immediately expose all of your carefully protected data. This is why cryptography generally depends upon at least one remembered key and why we need ciphers which can produce a multitude of different ciphertexts. Keyspace Cryptography deliberately creates the situation of a needle in a haystack. That is of all possible keys only one should recover the correct message and that one key is hidden among all possible keys. Of course The Opponent might get lucky but probably will have to perform about half of the possible decipherings to find the message. To keep messages secret it is important that a cipher be able to produce a multitude of different intermediate forms or ciphertexts. Clearly no cipher can possibly be stronger than requiring The Opponent to check every possible deciphering. If such a brute force search is practical the cipher is weak. The number of possible ciphertexts is the design strength of a cipher. Each different ciphertext requires a different key. So the number of different ciphertexts which we can produce is limited to the number of different keys we can use. We describe the keyspace by the length in bits of the binary value required to represent the number of possible ciphertexts or keys. It is not particularly difficult to design ciphers which may have a design strength of hundreds or thousands of bits and these can operate just as fast as our current ciphers. However the . Government generally does not allow the export of data ciphers with a keyspace larger than about 40 bits which is a very searchable value. Recently a 56-bit keyspace was searched with special hardware