Tham khảo tài liệu 'hacker professional ebook part 158', công nghệ thông tin, kỹ thuật lập trình phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | A message of arbitrary size can always be partitioned into some number of whole blocks with possibly some space remaining in the final block. Since partial blocks cannot be ciphered some random padding can be introduced to fill out the last block and this naturally expands the ciphertext. In this case it may also be necessary to introduce some sort of structure which will indicate the number of valid bytes in the last block. Block Partitioning without Expansion Proposals for using a block cipher supposedly without data expansion may involve creating a tiny stream cipher for the last block. One scheme is to reencipher the ciphertext of the preceding block and use the result as the confusion sequence. Of course the cipher designer still needs to address the situation of files which are so short that they have no preceding block. Because the one-block version is in fact a stream cipher we must be very careful to never re-use a confusion sequence. But when we only have one block there is no prior block to change as a result of the data. In this case ciphering several very short files could expose those files quickly. Furthermore it is dangerous to encipher a CRC value in such a block because exclusive-OR enciphering is transparent to the field of mod 2 polynomials in which the CRC operates. Doing this could allow an Opponent to adjust the message CRC in a known way thus avoiding authentication exposure. Another proposal for eliminating data expansion consists of ciphering blocks until the last short block then re-positioning the ciphering window to end at the last of the data thus re-ciphering part of the prior block. This is a form of chaining and establishes a sequentiality requirement which requires that the last block be deciphered before the next-to-the-last block. Or we can make enciphering inconvenient and deciphering easy but one way will be a problem. And this approach cannot handle very short messages its minimum size is one block. Yet any general-purpose .
