Slide an toàn và hệ thống bảo mật thông tin chapter 4 cryptography symmetric modern dec

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SYMMETRIC CIPHERS

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Contents
Block Cipher Principles

2)

Feistel Cipher

3)

Data Encryption Standard (DEC)

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1)

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CRYPTOLOGY

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CRYPTOGRAPHY

Transposition

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Substitution

ASYMMETRIC

MODERN

Block ciphers

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CLASSICAL

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SYMMETRIC

CRYPTANALYSIS

Stream ciphers

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1. Block Cipher Principles

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Stream Ciphers and Block Ciphers
 A stream cipher is a type pf symmetric encryption in which input data is

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encrypted one bit (byte) at a time.

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Stream Ciphers and Block Ciphers
 Block Ciphers is one in which the plaintext is divided in blocks and one

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 Block size?

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block is encrypted at a time producing a ciphertext of equal length

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2. Feistel Cipher

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The Feistel Cipher
 Horst Feistel devised the feistel cipher

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 Most symmetric block ciphers are based on a feistel cipher structure.

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 Feistel proposed the use of a cipher that alternates substitutions and

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permutations, where these terms are defined as follows:

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 Substitution: Each plaintext element or group of elements is uniquely replaced

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by a corresponding ciphertext element or group of elements
 Permutation: A sequence of plaintext elements is replaced by a permutation of

that sequence

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Feistel Cipher Structure

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 The Plaintext block: 2w bits

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 The plaintext block is divided into two halves,

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L0 and R0.

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 L, R pass through n rounds of processing and

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then combine to produce the ciphertext block.

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 Each round i (1->16):

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Feistel Cipher Principles

The exact realization of a Feistel network depends on the choice of the

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following parameters and design features:

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 Block size: Larger block sizes mean greater security but reduced

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encryption/decryption speed for a given algorithm.
 Key size: Larger key size means greater security but may decrease

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encryption/decryption speed.
 Number of rounds: increase number improves security, but slows cipher

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Feistel Cipher Principles (cont.)
 Subkey generation algorithm: Greater complexity in this algorithm

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should lead to greater difficulty of cryptanalysis.

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 Round function F: greater complexity generally means greater resistance

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to cryptanalysis.

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Feistel decryption algorithm

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Feistel decryption algorithm

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 On the decryption side

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 Consider the encryption process

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 Thus, we have:

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 The XOR has the following properties:

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Feistel decryption algorithm

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Feistel Example

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3. Data Encryption Standard - DEC

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Data Encryption Standard - DEC
 The Data Encryption Standard (DES) is a symmetric-key block cipher

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published by the National Institute of Standards and Technology (NIST).

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 A block cipher

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 Data Encrypted in 64-bits blocks using a 56-bit key; ciphertext is of 64-bit

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 Number of rounds: 16

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Description
 The 64-bit plaintext passes through an initial permutation (IP) that rearranges the

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bits to produce the permuted input.

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 Sixteen rounds of the same function, which involves both permutation and

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substitution functions. The output of the last (sixteenth) round consists of 64 bits

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that are a function of the input plaintext and the key. The left and right halves of

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the output are swapped to produce the preoutput.

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 Finally, the preoutput is passed through the inverse of the initial permutation

function, to produce the 64-bit ciphertext

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Initial Permutation (IP)

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 The ouput bit-1 for example is the input bit 58

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 Find the output of the initial permutation box when the input is given in

hexadecimal as: 0x0002 0000 0000 0001

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Inverse Initial Permutation (IP-1)

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Find the output of the inverse initial permutation box when the input is given in
hexadecimal as: 0x0000 0080 0000 0002

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Details Of single round
 The left and right halves of each 64-bit

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value

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separate 32-bit quantities, labeled L

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(left) and R (right)

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 Expansion Permutation: 32bit → 48 bit

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E table

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Details Of single round (cont.)
 After the expansion permutation, DES uses the XOR operation on the

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expanded right section and the round key

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S-Boxes

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 Input: 6-bit

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 Output: 4-bit

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The 48-bit data from the second operation is divided into eight 6-bit chunks,

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and each chunk is fed into a box. The result of each box is a 4-bit chunk

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