File Name: william stalling cryptography and network security solution .zip
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A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. One possible attack under these circumstances is the brute-force approach of trying all possible keys. If the key space is very large, this becomes impractical. Thus, the opponent must rely on an analysis of the ciphertext itself, generally applying various statistical tests to it.
Known plaintext. The analyst may be able to capture one or more plaintext messages as well as their encryptions. With this knowledge, the analyst may be able to deduce the key on the basis of the way in which the known plaintext is transformed.
Chosen plaintext. If the analyst is able to choose the messages to encrypt, the analyst may deliberately pick patterns that can be expected to reveal the structure of the key. An encryption scheme is said to be computationally secure if: 1 the cost of breaking the cipher exceeds the value of the encrypted information, and 2 the time required to break the cipher exceeds the useful lifetime of the information.
Plaintext is encrypted two letters at a time using this matrix. There is the practical problem of making large quantities of random keys. Any heavily-used system might require millions of random characters on a regular basis. Supplying truly random characters in this volume is a significant task. Even more daunting is the problem of key distribution and protection. For every message to be sent, a key of equal length is needed by both sender and receiver.
Thus, a mammoth key distribution problem exists. The first letter t corresponds to A, the second letter h corresponds to B, e is C, s is D, and so on. Second and subsequent occurrences of a letter in the key sentence are ignored.
It is a monalphabetic cipher and so easily breakable. The last sentence may not contain all the letters of the alphabet. If the first sentence is used, the second and subsequent sentences may also be used until all 26 letters are encountered. The first entry, , refers to page The second entry, C2, refers to column two.
The remaining numbers are words in that column. We need an even number of letters, so append a "q" to the end of the message. We first perform a matrix inversion.
Once the inverse matrix has been determined, decryption can proceed. Source: [LEWA00]. Lay the message out in a matrix 8 letters across. Each integer in the key tells you which letter to choose in the corresponding row. Result: He sitteth between the cherubims.
The isles may be glad thereof. As the rivers in the south. Quite secure. In each row there is one of eight possibilities. So if the ciphertext is 8n letters in length, then the number of possible plaintexts is 8n.
Not too secure. Lord Peter figured it out. Therefore, a study of the Feistel structure reveals the principles behind these more recent ciphers. For small n, such systems are vulnerable to a statistical analysis of the plaintext. This is achieved by having each plaintext digit affect the value of many ciphertext digits, which is equivalent to saying that each ciphertext digit is affected by many plaintext digits.
Confusion seeks to make the relationship between the statistics of the ciphertext and the value of the encryption key as complex as possible, again to thwart attempts to discover the key. Thus, even if the attacker can get some handle on the statistics of the ciphertext, the way in which the key was used to produce that ciphertext is so complex as to make it difficult to deduce the key. This is achieved by the use of a complex substitution algorithm.
Number of rounds: The essence of the Feistel cipher is that a single round offers inadequate security but that multiple rounds offer increasing security. Subkey generation algorithm: Greater complexity in this algorithm should lead to greater difficulty of cryptanalysis. Round function: Again, greater complexity generally means greater resistance to cryptanalysis.
Accordingly, the speed of execution of the algorithm becomes a concern. Ease of analysis: Although we would like to make our algorithm as difficult as possible to cryptanalyze, there is great benefit in making the algorithm easy to analyze. The difference patterns of the resulting ciphertext provide information that can be used to determine the encryption key. Linear cryptanalysis is based on finding linear approximations to describe the transformations performed in a block cipher.
The math works out that for decryption in these cases, the encryption function must also be used. This adds nothing to the security of the algorithm. There is a one-to-one reversible relationship between the bit key and the output of the P10 function. If we consider the output of the P10 function as a new key, then there are still different unique keys. By the same reasoning as a , this adds nothing to the security of the algorithm.
But a permutation on N — t objects with t' fixed points is equal to the number of ways t' out of N — t objects can be fixed, while the remaining N — t — t' are not fixed. Then using Problem 3. First, pass the bit input through PC-1 Table 3. Then perform a left circular shift separately on the two bit halves. Finally, pass the bit result through PC-2 Table 3. L0 , R0 are derived by passing the plaintext through IP Table 3.
The E table Table 3. Using Table 3. The ciphertext is the concatenation of L1 and R1. Source: [MEYE82] 3. For each input, there is an n-bit output. We only have to show the effect of the IP and IP—1 functions. The output of that stage is the ciphertext. On decryption, the first step is to take the ciphertext and pass it through IP.
Let us work this from the inside out. This would enable a similar type of implementation. Beyond that, there does not appear to be any particular cryptographic significance. Now, consider the two XOR operations in Figure 3. If the plaintext and key for an encryption are complemented, then the inputs to the first XOR are also complemented. The output, then, is the same as for the uncomplemented inputs. Further down, we see that only one of the two inputs to the second XOR is complemented, therefore, the output is the complement of the output that would be generated by uncomplemented inputs.
Now, pick a test value of the key T and perform ET [X]. If the result is Y1 , then we know that T is the correct key. If the result is Y2 ', then we know that T' is the correct key. If neither result appears, then we have eliminated two possible keys with one encryption. An easy, but not necessary, way to see this is to number the 64 bits of the key as follows read each vertical column of 2 digits as a number : The first bit of the key is identified as 21, the second as 10, the third as 13, and so on.
The eight bits that are not used in the calculation are unnumbered. The numbers 01 through 28 and 30 through 57 are used. The reason for this assignment is to clarify the way in which the subkeys are chosen. With this assignment, the subkey for the first iteration contains 48 bits, 01 through 24 and 30 through 53, in their natural numerical order. It is easy at this point to see that the first 24 bits of each subkey will always be from the bits designated 01 through 28, and the second 24 bits of each subkey will always be from the bits designated 30 through For example, suppose C 1 is corrupted.
A block cipher is one in which a block of plaintext is treated as a whole and used to produce a ciphertext block of equal length. One possible attack under these circumstances is the brute-force approach of trying all possible keys. If the key space is very large, this becomes impractical. Thus, the opponent must rely on an analysis of the ciphertext itself, generally applying various statistical tests to it. Known plaintext. The analyst may be able to capture one or more plaintext messages as well as their encryptions. With this knowledge, the analyst may be able to deduce the key on the basis of the way in which the known plaintext is transformed.
Follow StallingsBooks. Go here for Prentice Hall instructor support Websites for my other books. Project Support : This document describes support available to instructors for assigning projects to students. This textbook places greater emphasis on computer security issues as opposed to cryptography and network security issues. For instructors and students, there is a technical resource and course page to supplement the book. Errata sheet : Latest list of errors, updated at most monthly.
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All books are in clear copy here, and all files are secure so don't worry about it. This site is like a library, you could find million book here by using search box in the. William Stallings' Cryptography and Network Security: Principles and Practice, 5e is a practical survey of cryptography and network security with unmatched support for instructors and students. In this age of universal electronic connectivity, viruses and hackers, electronic eavesdropping, and electronic fraud, security is paramount.
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