Tham khảo tài liệu 'hacker professional ebook part 150', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | which is not much strength. A 56 bit keyspace represents about 7 x 1016 different keys and was recently broken by special brute force hardware in 56 hours this is also not much strength. The current strength recommendation is 112 to 128 bits and 256 is not out of the question. 128 bits is just 16 bytes which is the amount of storage usually consumed by 16 text characters a very minimal amount. A 128 bit key is strong enough to defeat even unimaginably extensive brute force attacks. Huge Keys Under the theory that if a little is good a lot is better some people suggest using huge keys of 56 000 bits or 1 000 000 bits or even more. We can build such devices and they can operate quickly. We can even afford the storage for big keys. What we do not have is a reason for such keys a 128 bit key is strong enough to defeat even unimaginably extensive brute force attacks. While a designer might use a larger key for convenience even immense keys cannot provide more strength than strong enough. And while different attacks may show that the cipher actually has less strength a huge keyspace is not going to solve those problems. Some forms of cipher need relatively large key values simply to have a sufficiently large keyspace. Most number-theory based public key ciphers are in this class. Basically these systems require key values in a very special form so that most key values are unacceptable and unused. This means that the actual keyspace is much smaller than the size of the key would indicate. For this reason public key systems need keys in the 1 000 bit range while delivering strength comparable to 128 bit secret key ciphers. Naive Ciphers Suppose we want to hide a name We might think to innovate a different rule for each letter. We might say First we have T but t is the 3rd letter in bottle so we write 3. We can continue this way and such a cipher could be very difficult to break. So why is this sort of thing not done There are several reasons 1. First any cipher .
