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Hacker Professional Ebook part 166

Tham khảo tài liệu 'hacker professional ebook part 166', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Group In abstract algebra a nonempty set G with one dyadic two-input one-output operation which we choose to call multiplication and denote as usual. If elements not necessarily numbers a b are in R then ab or a b is also in R. The following properties hold 1. Multiplication is associative ab c a bc 2. There is a multiplicative identity for e in G ea ae a 3. There is a multiplicative inverse for a in G there is an a-1 in G such that a-1a e aa-1 A group is basically a mapping from two elements in the group through the group operation m into the same group m G x G - G Hamming Distance A measure of the difference or distance between two binary sequences of equal length in particular the number of bits which differ between the sequences. This is the weight or the number of 1-bits in the exclusive-OR of the two sequences. Hardware The physical realization of computation. Typically the electronic digital logic power supply and various electro-mechanical components such as disk drives switches and possibly relays which make up a computer or other digital system. As opposed to software. See system design and debug. Hash A classic computer operation which forms a fixed-size result from an arbitrary amount of data. Ideally even the smallest change to the input data will change about half of the bits in the result. Often used for table look-up so that very similar language terms or phrases will be well-distributed throughout the table. Also often used for error-detection and known as a message digest authentication. A hash of data will produce a particular hash value which then can be included in the message before it is sent or stored . When the data are received or read and the hash value computed this should match the included hash value. So if the hash is different something has changed and the usual solution is to request the data be sent again. But the hash value is typically much smaller than the data so there must be many different data sets which will produce that .