Download - Modern symmetric cipher
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