This document discusses modern symmetric key algorithms. It describes stream ciphers and block ciphers, with RC4 and DES provided as examples. The concepts of confusion and diffusion are introduced. Modes of operation for block ciphers are defined, including ECB, CBC, CFB and OFB. DES encryption uses a 64-bit block size and 56-bit key, operating over 16 rounds of feistel network and key schedule. AES was selected to replace DES due to larger block size and key size, operating over 10 to 14 rounds of byte substitution, row shifting, column mixing and subkey addition. Symmetric key ciphers are fast but require secure key exchange and management to prevent repetition.

1. Modern
Symmetric Key
Algorithm
By Prof : Rupesh Mishra

3. Algorithm
Type
Stream
Cipher
Block
Cipher

4. Stream Cipher
• Bit by Bit encryption and Decryption
• Encoding and Decoding
• XOR Logic Operation
• RC4

5. Block Cipher
• Block of text encrypted and decrypted

7. Confusion
• Substitution Technique
• No clue about Key from ciphertext

8. Diffusion
• No clue about plaintext from ciphertext
• Transposition Technique

9. Modes of Algorithm
• Electronic Code Book (ECB)
• Cipher Block Chaining (CBC)
• Cipher Feedback Mode (CFB)
• Output Feedback Mode (OFB)
• Counter Mode

10. Electronic Code Book

11. 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

12. Cipher Block Chaining

13. Cipher Feedback Mode

14. Cipher Feedback Mode

15. Output Feedback Mode

17. Counter Mode

18. 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.

19. Original ECB CBC

21. DES
• Data Encryption Standard/Algorithm
• Adopted in 1977 by National Institute of
Standards and Technology (NIST)
• Plaintext 64 bit/Block
• Key 56 bit

22. 64 bit
Plaintext
DES
64 bit
Ciphertext
56 bit
Key

23. L0 R0
F
L1 R1
𝑲𝒆𝒚

24. Steps in DES
1. Initial Permutation
2. 16 Round of Feistel Function
3. Final Permutation

27. Feistel Function
1. Expansion Permutation
2. S – Box Substitution
3. P – Box Permutation

31. Key Generation
• Key Discarding Process
• Key Transformation
• Sub - Key Generation

34. • DES Decryption
• DES Cryptanalysis
o Keyspace
o Diffrential and Linear cryptanalysis
• Variations of DES
o Double DES
o Triple DES

35. Advanced Encryption
Standard (AES)

36. 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

37. 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

38. AES Encryption
• Each round includes
– Byte substitution
– Row shift
– Column mixing
– Subkey Addition

39. Byte Substitution

41. S-Box Operation
B
(Input)
GF(28)
Inverse
Affine
Mapping
B’
(Output)

43. Row Shift

44. MixCol

45. Mix Column

46. Add Round Key

47. Add Round Key

49. 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