## Presentation ~ above theme: "Chapter3: Block Ciphers and the Data Encryption Standard"— Presentation transcript:

1 **Chapter3: Block Ciphers and also the Data Encryption Standard**NUIST college of Computer and also Software Jian Shen, PhD, Professor

2 development A block cipher is one encryption/decryption scheme in which a block that plaintext is treated as a whole and used to produce a ciphertext block of same length. Plenty of block ciphers have actually a Feistel structure. Together a structure is composed of a variety of identical ring of processing. In every round, a substitution is perform on one half of the data being processed, complied with by a permutation that interchanges the two halves. The original an essential is expanded so that a different key is supplied for every round. The Data Encryption conventional (DES) has actually been the many widely provided encryption algorithm till recently. It exhibits the standard Feistel structure. DES supplies a 64-bit block and a 56-bit key. Two essential methods the cryptanalysis room differential cryptanalysis and linear cryptanalysis. DES has been shown to be very resistant to these two varieties of attack. Is designed to detect, prevent, or recuperate from a protection attack. Security services include authentication, access control, data confidentiality, data integrity, nonrepudiation, and also availability.

3 **3.1. Block Cipher Principles**Stream Ciphers and Block Ciphers A currently cipher is one that encrypts a digital data currently one bit or one byte at a time. An ideas for the Feistel Cipher framework A block cipher operates top top a plaintext block of n bits to produce a ciphertext block of n bits.

4 **3.1. Block Cipher Principles**Figure 3.1 illustrates the reasonable of a basic substitution cipher for n = 4. A 4-bit entry produces one of 16 possible input states, which is mapped by the substitution cipher right into a distinct one that 16 feasible output states, every of i beg your pardon is stood for by 4 ciphertext bits.

5 **3.1. Block Cipher Principles**Table 3.1. This is the many general type of block cipher and can be supplied to define any type of reversible mapping between plaintext and also ciphertext.

6 **3.1. Block Cipher Principles**The Feistel Cipher risk Feistel suggest

7 **3.1. Block Cipher Principles**Diffusion and also Confusion assume the attacker has some understanding of the statistical characteristics of the plaintext. In diffusion, the statistical structure of the plaintext is dissipated into long-range statistics the the ciphertext. This is accomplished by having each plaintext digit influence the value of plenty of ciphertext digits; typically this is tantamount to having each ciphertext digit be impacted by plenty of plaintext digits.

8 **3.1. Block Cipher Principles**Feistel Cipher Structure number 3.2 depicts the structure proposed by Feistel. The inputs come the encryption algorithm room a plaintext block of length 2w bits and a crucial K. The plaintext block is separated into two halves, L0 and also R0. The 2 halves of the data pass v n rounds of processing and then integrate to create the ciphertext block. Every round i has as input Li-1 and Ri-1, derived from the ahead round, as well as a subkey Ki, derived from the in its entirety K. In general, the subkeys Ki are various from K and also from every other.

9 **3.1. Block Cipher Principles**The exact realization that a Feistel network counts on the an option of the following parameters and also design features: Block size: larger block sizes mean better security (all other things being equal) but reduced encryption/decryption speed for a given algorithm. Key size: Larger vital size way greater security however may decrease encryption/decryption speed. Variety of rounds: The significance of the Feistel cipher is that a solitary round offers insufficient security yet that multiple rounds offer increasing security. Subkey generation algorithm: Greater intricacy in this algorithm should cause greater an obstacle of cryptanalysis. Ring function: Again, greater complexity generally method greater resistance to cryptanalysis.

10 **3.1. Block Cipher Principles**There room two other considerations in the architecture of a Feistel cipher: rapid software encryption/decryption lull of evaluation

11 **3.1. Block Cipher Principles**Feistel Decryption Algorithm figure 3.3 reflects the encryption procedure going under the left-hand side and the decryption process going up the right-hand side for a 16-round algorithm.

12 **3.2. The Data Encryption Standard**Feistel Decryption Algorithm for DES, data room encrypted in 64-bit blocks making use of a 56-bit key. The algorithm transforms 64-bit intake in a collection of steps into a 64-bit output. The exact same steps, v the very same key, are offered to turning back the encryption. DES Encryption The overall scheme because that DES encryption is depicted in number 3.4. As with any kind of encryption scheme, there are two inputs to the encryption function: the plaintext to it is in encrypted and also the key. In this case, the plaintext should be 64 bits in length and the an essential is 56 bits in length.

13 **3.2. The Data Encryption Standard**Initial Permutation The initial permutation and its inverse are characterized by tables, as presented in Tables 3.2a and also 3.2b, respectively. .

14 **3.2. The Data Encryption Standard**Initial Permutation To view that these two permutation attributes are indeed the station of each other, think about the adhering to 64-bit input M:

15 **3.2. The Data Encryption Standard**Initial Permutation whereby Mi is a binary digit. Climate the permutation X = IP(M) is as follows: If us then take it the inverse permutation Y = IP-1(X) = IP-1(IP(M)), it can be watched that the original ordering of the bits is restored.

16 **3.2. The Data Encryption Standard**Details of single Round figure 3.5 mirrors the internal structure that a single round.

17 **3.2. The Data Encryption Standard**Details of solitary Round The duty of the S-boxes in the role F is portrayed in number 3.6.

18 **3.2. The Data Encryption Standard**Details of solitary Round These revolutions are identified in Table 3.3, i m sorry is taken as follows: The an initial and critical bits of the input to box Si type a 2-bit binary number to select one of four substitutions defined by the four rows in the table because that Si. The middle 4 bits choose one the the sixteen columns. The decimal worth in the cabinet selected through the row and also column is climate onverted come its 4-bit representation to develop the output.

19 **3.2. The Data Encryption Standard**Key Generation

20 **3.2. The Data Encryption Standard**DES Decryption as with any Feistel cipher, decryption offers the same algorithm together encryption, except that the application of the subkeys is reversed.

21 **3.2. The Data Encryption Standard**The Avalanche effect A desirable property of any type of encryption algorithm is that a small change in one of two people the plaintext or the crucial should produce a far-ranging change in the ciphertext. DES exhibits a strong avalanche effect. Table 3.5 mirrors some results taken indigenous

22 **3.3. The toughness of Des The usage of 56-Bit Keys**The Nature the the DES Algorithm Timing assaults

23 **3.4. Differential and also Linear Cryptanalysis**Differential Cryptanalysis background Differential Cryptanalysis attack Figure 3.7, based upon a number in

24 **3.4. Differential and Linear Cryptanalysis**We now offer a brief an introduction of the rule on which direct cryptanalysis is based. For a cipher v n-bit plaintext and also ciphertext blocks and also an m-bit key, permit the plaintext block be labeling P<1>, ... P

25 **3.5. Block Cipher design Principles**DES architecture Criteria No output little of any S-box must be as well close a linear function of the input bits. Each row of an S-box (determined through a fixed value that the leftmost and rightmost intake bits) should encompass all 16 possible output bit combinations. If 2 inputs come an S-box different in specifically one bit, the outputs need to differ in at the very least two bits. If 2 inputs to an S-box differ in the two middle bits exactly, the outputs have to differ in at the very least two bits. If two inputs to an S-box differ in their very first two bits and are identical in your last 2 bits, the 2 outputs must not it is in the same. For any kind of nonzero 6-bit difference in between inputs, no much more than 8 that the 32 pairs of entry exhibiting that difference may result in the same output differ This is a criterion similar to the previous one, yet for the situation of 3 S-boxes.ence.

26 **3.5. Block Cipher architecture Principles**DES design Criteria The criteria for the permutation P space as follows: The 4 output bits from every S-box in ~ round ns are spread so that 2 of them impact (provide entry for) "middle bits" of round (i + 1) and the various other two influence end bits. The two center bits the input come an S-box are not shared with surrounding S-boxes. The end bits room the 2 left-hand bits and also the two right-hand bits, which are common with adjacent S-boxes. The 4 output bits from every S-box influence six different S-boxes ~ above the next round, and no two affect the exact same S-box. For 2 S-boxes j, k, if an output bit from Sj affects a middle little of Sk ~ above the following round, climate an output bit from Sk cannot affect a middle little of Sj. This indicates that for j = k, an output bit from Sj must not affect a middle little of Sj.

27 **3.5. Block Cipher style Principles**Number of rounds The cryptographic strength of a Feistel cipher derives native three facets of the design: the variety of rounds, the duty F, and also the an essential schedule algorithm. Style of function F style Criteria for F S-Box architecture For bigger S-boxes, such as 8 x 32, the inquiry arises regarding the best method of choosing the S-box entries in stimulate to meet the form of criteria we have been discussing. Nyberg, who has written a lot about the theory and practice that S-box design, argues the adhering to approaches: arbitrarily Random with trial and error Human-made Math-made

28 **3.5. Block Cipher architecture Principles**Key Schedule Algorithm A last area of block cipher design, and also one that has received much less attention 보다 S-box design, is the an essential schedule algorithm. With any kind of Feistel block cipher, the crucial is used to generate one subkey because that each round. In general, us would like to choose subkeys come maximize the difficulty of deducing separation, personal, instance subkeys and also the an obstacle of working back to the key key. No general values for this have actually yet to be promulgated.

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29 **Review inquiries 3.1 Why is it important to research the Feistel cipher?**3.2 What is the difference between a block cipher and also a present cipher? 3.3 Why is it not helpful to use an arbitrarily reversible substitution cipher the the kind displayed in Table 3.1? 3.4 What is a product cipher? 3.5 What is the difference between diffusion and confusion? 3.6 which parameters and also design selections determine the yes, really algorithm the a Feistel cipher? 3.7 What is the function of the S-boxes in DES? 3.8 explain the avalanche effect. 3.9 What is the difference between differential and linear cryptanalysis?