Modern Symmetric Key

AlgorithmBy Prof : Rupesh Mishra

Algorithm Type

Stream Cipher

Block Cipher

Stream Cipher

• Bit by Bit encryption and Decryption

• Encoding and Decoding

• XOR Logic Operation

• RC4

Block Cipher

• Block of text encrypted and decrypted

Confusion

• Substitution Technique

• No clue about Key from ciphertext

Diffusion

• No clue about plaintext from ciphertext

• Transposition Technique

Modes of Algorithm

• Electronic Code Book (ECB)

• Cipher Block Chaining (CBC)

• Cipher Feedback Mode (CFB)

• Output Feedback Mode (OFB)

• Counter Mode

Electronic Code Book

Drawback of ECB

• Identical blocks of ciphertext.

• An eavesdropper notices that blocks 23 and

95 of the ciphertext are equal.

• If, in addition, he knows (or can guess) the

value of block 23 of plaintext, he can then

deduce the content of block 95 of the

plaintext

Cipher Block Chaining

Cipher Feedback Mode

Cipher Feedback Mode

Output Feedback Mode

Counter Mode

Counter Mode • A b-bit counter is initialized to a random value.

This value is encrypted with the secret key.

• Encrypted value is XORed with the first block of

plaintext

• The counter is then incremented.

• the incremented value is encrypted and XORed

with the next block of plaintext to create the

ciphertext and so on.

Original ECB CBC

DES

• Data Encryption Standard/Algorithm

• Adopted in 1977 by National Institute of

Standards and Technology (NIST)

• Plaintext 64 bit/Block

• Key 56 bit

64 bit

Plaintext

DES

64 bit

Ciphertext

56 bit

Key

L0 R0

F

L1 R1

𝑲𝒆𝒚

Steps in DES1. Initial Permutation

2. 16 Round of Feistel Function

3. Final Permutation

Feistel Function

1. Expansion Permutation

2. S – Box Substitution

3. P – Box Permutation

Key Generation

• Key Discarding Process

• Key Transformation

• Sub - Key Generation

• DES Decryption

• DES Cryptanalysis

o Keyspace

o Diffrential and Linear cryptanalysis

• Variations of DES

o Double DES

o Triple DES

Advanced Encryption Standard (AES)

Introduction

• Regular DES is not very secure and triple

DES is slow. The block size in both is 64 bits

• A more secure secret key algorithm with a

larger block size is desirable

• In 2001, Rijndael was selected by NIST from

among 5 finalists as the new standard

AES features

• Block size = 128 bits

• Key size = 128/192/256 bits

• No of rounds = 10, 12 or 14

• Does not have a Fiestal structure

AES Encryption• Each round includes

– Byte substitution

– Row shift

– Column mixing

– Subkey Addition

Byte Substitution

S-Box Operation

B

(Input)

GF(28) Inverse

Affine Mapping

B’

(Output)

Row Shift

MixCol

Mix Column

Add Round Key

Add Round Key

Symmetric Key Cipher

• Single Shared Key is used for Encryption and

Decryption.

• Fast and Effective

• DES , AES , Salsa20

• Limitation

o Key Exchange

o Repetition of Key