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Hacker Professional Ebook part 152

Tham khảo tài liệu 'hacker professional ebook part 152', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Logically a stream cipher can be seen as the general concept of repeatedly using a block transformation to handle more than one block of data. I would say that even the simple repeated use of a block cipher in ECB mode would be streaming the cipher. And use in more complex chaining modes like CBC are even more clearly stream meta-ciphers which use block transformations. One common idea that comes up again and again with novice cryptographers is to take a textual key phrase and then add or exclusive-OR the key with the data byte-by-byte starting the key over each time it is exhausted. This is a very simple and weak stream cipher with a short and repeatedly-used running key and an additive combiner. I suppose that part of the problem in seeing this weakness is in distinguishing between different types of stream cipher key In a real stream cipher even a single bit change in a key phrase would be expected to produce a different running key sequence a sequence which would not repeat across a message of any practical size. In the weak version a single bit change in the short running key would affect only one bit each time it was used and would do so repeatedly as the keying sequence was re-used over and over again. In any additive stream cipher the re-use of a keying sequence is absolutely deadly. And a real stream cipher would almost certainly use a random message key as the key which actually protects data. Public Key Ciphers Public key ciphers are generally block ciphers with the unusual property that one key is used to encipher and a different apparently unrelated key is used to decipher a message. So if we keep one of the keys private we can release the other key the public key and anyone can use that to encipher a message to us. Then we use our private key to decipher any such messages. It is interesting that someone who enciphers a message to us cannot decipher their own message even if they want to. The prototypical public key cipher is RSA which uses the .