Cryptography & Network Security: Block Ciphers PDF

Cryptograph y and Network Security Seventh Edition by William Stallings © 2017 Pearson Educat...

Cryptograph y and Network Security Seventh Edition by William Stallings © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Chapter 4 Block Ciphers and the Data Encryption Standard © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Stream Cipher For practical reasons In the ideal case, a the bit-stream one-time pad version of generator must be Encrypts a digital data the Vernam cipher implemented as an stream one bit or one would be used, in algorithmic procedure byte at a time which the keystream is so that the as long as the plaintext cryptographic bit bit stream stream can be produced by both users Examples: If the It must be Autokeyed Vigenère cryptographic computationally cipher keystream is impractical to Vernam cipher random, then this predict future cipher is portions of the bit unbreakable by stream based on any means other previous portions than acquiring the of the bit stream keystream Keystream must be provided to both users in advance via some The two users need independent and secure channel only share the This introduces generating key and insurmountable each can produce logistical problems if the keystream the intended data traffic is very large © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Block Cipher A block of plaintext is treated as a Typically a block whole and used size of 64 or to produce a 128 bits is used ciphertext block of equal length The majority of As with a network-based stream cipher, symmetric the two users cryptographic share a applications symmetric make use of encryption key block ciphers © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved. © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Feistel Cipher Feistel proposed the use of a cipher that alternates substitutions and permutations Substitutio Each plaintext element or group of elements is uniquely replaced by a ns corresponding ciphertext element or group of elements Permutatio No elements are added or deleted or replaced in the sequence, rather the n order in which the elements appear in the sequence is changed Is a practical application of a proposal by Claude Shannon to develop a product cipher that alternates confusion and diffusion functions Is the structure used by many significant symmetric block ciphers currently in use © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Diffusion and Confusion Terms introduced by Claude Shannon to capture the two basic building blocks for any cryptographic system Shannon’s concern was to thwart cryptanalysis based on statistical analysis Diffusion The statistical structure of the plaintext is dissipated into long-range statistics of the ciphertext This is achieved by having each plaintext digit affect the value of many ciphertext digits Confusion Seeks to make the relationship between the statistics of the ciphertext and the value of the encryption key as complex as possible 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 © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved. © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Feistel Cipher Design Features Block size Round function F Larger block sizes mean greater security but reduced Greater complexity generally encryption/decryption speed for a means greater resistance to given algorithm cryptanalysis Key size Fast software encryption/decryption Larger key size means greater security but may decrease In many cases, encrypting is embedded in applications or utility encryption/decryption speeds functions in such a way as to preclude a hardware Number of rounds implementation; accordingly, the speed of execution of the The essence of the Feistel cipher is algorithm becomes a concern that a single round offers inadequate security but that multiple rounds offer increasing security Ease of analysis If the algorithm can be concisely and clearly explained, it is easier Subkey generation algorithm to analyze that algorithm for Greater complexity in this algorithm cryptanalytic vulnerabilities and should lead to greater difficulty of therefore develop a higher level of cryptanalysis assurance as to its strength © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Data Encryption Standard (DES) Issued in 1977 by the National Bureau of Standards (now NIST) as Federal Information Processing Standard 46 Was the most widely used encryption scheme until the introduction of the Advanced Encryption Standard (AES) in 2001 Algorithm itself is referred to as the Data Encryption Algorithm (DEA) Data are encrypted in 64-bit blocks using a 56-bit key The algorithm transforms 64-bit input in a series of steps into a 64-bit output The same steps, with the same key, are used to reverse the encryption © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved. © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Initial Permutation IP The first step of the data computation IP reorders the input data bits even bits to LH half, odd bits to RH half quite regular in structure (easy in h/w) example: IP(675a6967 5e5a6b5a) = (ffb2194d 004df6fb) What is the purpose of this? © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved. Initial Permutation IP View of a Single Round  Single Round in Words uses two 32-bit L & R halves as for any Feistel cipher can describe as: Li = Ri–1 Ri = Li–1  F(Ri–1, Ki) F takes 32-bit R half and 48-bit subkey: expands R to 48-bits using perm E adds to subkey using XOR passes through 8 S-boxes to get 32-bit result finally permutes using 32-bit perm P F is not a permutation! DES Round Function F  Substitution Boxes S have eight S-boxes which map 6 to 4 bits each S-box is actually 4 little 4 bit boxes outer bits 1 & 6 (row bits) select one row of 4 possible 4-bit permutations inner bits 2-5 (col bits) are substituted result is 8 lots of 4 bits, or 32 bits row selection depends on both data & key feature known as autoclaving (autokeying) example: S(18 09 12 3d 11 17 38 39) = 5fd25e03  Table 4.2 DES Exampl e (Table can be found on page 114 in textbook) Note: DES subkeys are shown as eight 6-bit values in hex format © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Table 4.3 Avalanche Effect in DES: Change in Plaintext © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Table 4.4 Avalanche Effect in DES: Change in Key © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Table 4.5 Average Time Required for Exhaustive Key Search © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.  Avalanche Effect key desirable property of encryption alg where a change of one input or key bit results in changing approx half output bits making attempts to “home-in” by guessing keys impossible DES exhibits strong avalanche  Summary Traditional Block Cipher The strength of Structure DES Stream ciphers Use of 56-bit keys Block ciphers Nature of the DES Motivation for the algorithm Feistel cipher Timing attacks structure Feistel cipher Block cipher design principles The Data Encryption Number of rounds Standard (DES) Design of function Encryption F Decryption Key schedule Avalanche effect algorithm © 2017 Pearson Education, Inc., Hoboken, NJ. All rights reserved.