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UNIT-I Q.1 Explain Differential and Linear Cryptanalysis of DES

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UNIT-I Q.1 Explain Differential and Linear Cryptanalysis of DES UNIT-I Q.1 Explain Differential And Linear Cryptanalysis Of DES. Ans: The Data Encryption Standard (DES) is a symmetric-key block cipher.DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm (function as check bits only). General Structure of DES is depicted in the following illustration − Since DES is based on the Feistel Cipher, all that is required to specify DES is − • Round function • Key schedule • Any additional processing − Initial and final permutation Initial and Final Permutation The initial and final permutations are straight Permutation boxes (P-boxes) that are inverses of each other. They have no cryptography significance in DES. The initial and final permutations are shown as follows − Round Function The heart of this cipher is the DES function, f. The DES function applies a 48-bit key to the rightmost 32 bits to produce a 32-bit output. • Expansion Permutation Box − Since right input is 32-bit and round key is a 48-bit, we first need to expand right input to 48 bits. Permutation logic is graphically depicted in the following illustration − • The graphically depicted permutation logic is generally described as table in DES specification illustrated as shown − • XOR (Whitener). − After the expansion permutation, DES does XOR operation on the expanded right section and the round key. The round key is used only in this operation. • Substitution Boxes. − The S-boxes carry out the real mixing (confusion). DES uses 8 S- boxes, each with a 6-bit input and a 4-bit output. Refer the following illustration − • The S-box rule is illustrated below − • There are a total of eight S-box tables. The output of all eight s-boxes is then combined in to 32 bit section. • Straight Permutation − The 32 bit output of S-boxes is then subjected to the straight permutation with rule shown in the following illustration: Key Generation The round-key generator creates sixteen 48-bit keys out of a 56-bit cipher key. The process of key generation is depicted in the following illustration − The logic for Parity drop, shifting, and Compression P-box is given in the DES description. DES Analysis The DES satisfies both the desired properties of block cipher. These two properties make cipher very strong. • Avalanche effect − A small change in plaintext results in the very great change in the ciphertext. • Completeness − Each bit of ciphertext depends on many bits of plaintext. The pragmatic approach was not to abandon the DES completely, but to change the manner in which DES is used. This led to the modified schemes of Triple DES (sometimes known as 3DES). Incidentally, there are two variants of Triple DES known as 3-key Triple DES (3TDES) and 2- key Triple DES (2TDES). Linear cryptanalysis is an approach where we aim to find affine approximations to the action of a cipher. Letter frequency analysis is one of the simplest forms of linear cryptanalysis. Differential cryptanalysis is an approach to cryptanalysis whereby differences in inputs are mapped to differences in outputs and patterns in the mappings of plaintext edits to ciphertext variation are used to reverse engineer a key. Linear and differential cryptanalysis are most often applied to block ciphers (encryption functions operating on messages that are split into blocks). They are symmetric key algorithms. Linear Cryptanalysis The paradigm of linear cryptanalysis was originally designed in 1993 as a theoretical attack on DES. It is now used widely on block ciphers across the field of cryptanalysis and is an effective starting point for developing more complex attacks. Linear cryptanalysis posits a linear relationship between the elements (characters or individual bits) of plaintext, the cipher text, and the key. It therefore tries to find a linear approximation to the action of a cipher, i.e. if "ciphertext = f(plaintext, key)", then we are trying to find a linear approximation of f. The approach in linear cryptanalysis is to determine expressions of the form above which have a high or low probability of occurrence. (No obvious linearity such as above should hold for all input and output values or the cipher would be trivially weak.) If a cipher displays a tendency for [the] equation [above] to hold with high probability or not hold with high probability, this is evidence of the cipher’s poor randomization abilities. Consider that if we randomly selected values for [...] bits and placed them into the equation above, the probability that the expression would hold would be exactly ½. It is the deviation or bias from the probability of ½ for an expression to hold that is exploited in linear cryptanalysis: the further away that a linear expression is from holding with a probability of ½, the better the cryptanalyst is able to apply linear cryptanalysis. This quote tells us the fundamental paradigm of linear (and indeed differential) cryptanalysis. The cryptanalyst aims to exploit the fact that encryption is non-random, attaining information through the measurement of deviations from random behavior. Steps to perform Linear Cryptanalysis In the most common use case, we assume that everything about the encryption algorithm is known apart from the private key. Performing linear cryptanalysis on a block cipher usually consists of three steps. Find linear approximations of the non-linear parts of the encryption algorithm (usually only the substitution boxes, known as S-boxes). Combine linear approximations of S-boxes with the rest of the (linear) operations done in the encryption algorithm, to obtain a linear approximation of the entire encryption algorithm. This linear approximation is a function which relates the plaintext bits, the ciphertext bits, and the bits of the private key. Use the linear approximation as a guide for which keys to try first. This leads to substantial computational savings over trying all possible values of the key. Multiple linear approximations may be used to further cut down the number of keys that need to be tried. Differential Cryptanalysis Differential cryptanalysis preceded linear cryptanalysis having initially been designed in 1990 as an attack on DES. Differential cryptanalysis is similar to linear cryptanalysis; differential cryptanalysis aims to map bitwise differences in inputs to differences in the output in order to reverse engineer the action of the encryption algorithm. It is again aiming to approximate the encryption algorithm looking to find a maximum likelihood estimator of the true encryption action by altering plaintexts or (looking at different plaintexts) and analysing the impact of changes to the plaintext to the resulting ciphertext. Differential cryptanalysis is therefore a chosen plaintext attack. The description of differential cryptanalysis is analogous to that of linear cryptanalysis and is essentially the same as would be the case of applying linear cryptanalysis to input differences rather than to input and output bits directly. OR Q.1 (A) What is Shannon’s Theory of Confusion and Diffusion? Explain Fiestel Structure Of Block Ciphers. Ans: Shannon's Theory: Confusion and diffusion area used for creating a secure cipher. Each Confusion and diffusion area unit wont to stop the secret writing key from its deduction or ultimately for preventing the first message. Confusion is employed for making uninformed cipher text whereas diffusion is employed for increasing the redundancy of the plain text over the foremost a part of the cipher text to create it obscure. The stream cipher solely depends on confusion, or else, diffusion is employed by each stream and block cipher. In Shannon's definitions, confusion refers to making the relationship between the ciphertext and the symmetric key as complex and involved as possible; diffusion refers to dissipating the statistical structure of plaintext over the bulk of ciphertext. This complexity is generally implemented through a well-defined and repeatable series of substitutions and permutations. Substitution refers to the replacement of certain components (usually bits) with other components, following certain rules. Permutation refers to manipulation of the order of bits according to some algorithm. To be effective, any non-uniformity of plaintext bits needs to be redistributed across much larger structures in the ciphertext, making that non-uniformity much harder to detect. In particular, for a randomly chosen input, if one flips the i-th bit, then the probability that the j-th output bit will change should be one half, for any i and j—this is termed the strict avalanche criterion. More generally, one may require that flipping a fixed set of bits should change each output bit with probability one half. One aim of confusion is to make it very hard to find the key even if one has a large number of plaintext-ciphertext pairs produced with the same key. Therefore, each bit of the ciphertext should depend on the entire key and in different ways on different bits of the key. In particular, changing one bit of the key should change the ciphertext completely. The simplest way to achieve both diffusion and confusion is to use a substitution-permutation network. In these systems, the plaintext and the key often have a very similar role in producing the output, hence the same mechanism ensures both diffusion and confusion. S.NO CONFUSION DIFFUSION Confusion is a cryptographic technique which is used to create While diffusion is used to create 1. faint cipher texts. cryptic plain texts. This technique is possible through While it is possible through 2. substitution algorithm. transportation algorithm. In confusion, if one bit within the While in diffusion, if one image secret’s modified, most or all bits within the plain text is modified, within the cipher text also will be many or all image within the cipher 3.
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