Medern Symmetric -Key Ciphers (AES, DES)

AES & DES- working, need & difference between them.

Data Encryption Standard
Advanced Encryption Standard

1. Modern Symmetric-Key Ciphers(AES, DES) B P Poddar Institute of Management & Technology Presented by Guided by Maharaj Dey (11500117099) Sandeep Kumar(11500117059) Pratap Chandra Mondal Term paper presentation Computer Science Engineering April – 2022 Cryptography and Network Theory 1

2. 2 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 2 Medern Symmetric-key ciphers

3. 3 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 3 Why Was the AES Encryption Algorithm necessary?

4. 4 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 4 What are the Features of AES? SP Network: It works on an SP network structure rather than a Feistel cipher structure Key Expansion: It takes a single key up during the first stage, which is later expanded to multiple keys used in individual rounds. Byte Data: The AES encryption algorithm does operations on byte data instead of bit data. So it treats the 128-bit block size as 16 bytes during the encryption procedure. Key Length: The number of rounds to be carried out depends on the length of the key being used to encrypt data. The 128-bit key size has ten rounds, the 192-bit key size has 12 rounds the 256-bit key size has 14 rounds.

5. 5 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 5 How Does AES Work? a single block is 16 bytes, a 4x4 matrix holds the data in a single block, with each cell holding a single byte of information. The matrix shown in the image above is known as a state array. Similarly, the key being used initially is expanded into (n+1) keys, with n being the number of rounds to be followed in the encryption process. So for a 128-bit key, the number of rounds is 16, with no. of keys to be generated being 10+1, which is a total of 11 keys.

6. 6 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 6 The steps to be followed in AES 

7. 7 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 7 Add Round Key You pass the block data stored in the state array through an XOR function with the first key generated (K0). It passes the resultant state array on as input to the next step.

8. 8 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 8 Sub-Bytes it converts each byte of the state array into hexadecimal, divided into two equal parts. These parts are the rows and columns, mapped with a substitution box (S-Box) to generate new values for the final state array.

9. 9 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 9 Shift Rows It swaps the row elements among each other. It skips the first row. It shifts the elements in the second row, one position to the left. It also shifts the elements from the third row two consecutive positions to the left, and it shifts the last row three positions to the left.

10. 10 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 10 Mix Columns It multiplies a constant matrix with each column in the state array to get a new column for the subsequent state array. Once all the columns are multiplied with the same constant matrix, you get your state array for the next step. This particular step is not to be done in the last round.

11. 11 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 11 Add Round Key The respective key for the round is XOR’d with the state array is obtained in the previous step. If this is the last round, the resultant state array becomes the ciphertext for the specific block; else, it passes as the new state array input for the next round.

12. 12 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 12 Example

13. 13 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 13 Example

14. 14 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 14 Example Add Round Key

15. 15 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 15 Example Sub-Bytes: It passes the elements through a 16x16 S-Box to get a completely new state array.

16. 16 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 16 Example Shift Rows

17. 17 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 17 Example Mix Columns

18. 18 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 18 Example Add Round Key This state array is now the final ciphertext for this particular round. This becomes the input for the next round. Depending on the key length, you repeat the above steps until you complete round 10

19. 19 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 19 Example

20. 20 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 20 What Are the Applications of AES?

21. 21 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 21 What are the Features of AES? The Data Encryption Standard (DES) is a symmetric-key block cipher published by the National Institute of Standards and Technology (NIST). DES is an implementation of a Feistel Cipher. It uses 16 round Feistel structure. The block size is 64-bit. Though, key length is 64-bit, DES has an effective key length of 56 bits, since 8 of the 64 bits of the key are not used by the encryption algorithm (function as check bits only).

22. 22 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 22 General Structure of DES

23. 23 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 23 Content Since DES is based on the Feistel Cipher, all that is required to specify DES is − Round function Key schedule Any additional processing − Initial and final permutation

24. 24 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 24 Initial and Final Permutation The initial and final permutations are straight Permutation boxes (P-boxes) that are inverses of each other. They have no cryptography significance in DES

25. 25 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 25 Round Function The heart of this cipher is the DES function,  f . The DES function applies a 48-bit key to the rightmost 32 bits to produce a 32-bit output.

26. 26 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 26 Expansion Permutation Box Since right input is 32-bit and round key is a 48-bit, we first need to expand right input to 48 bits. The graphically depicted permutation logic is generally described as table

27. 27 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 27 XOR (Whitener)  After the expansion permutation, DES does XOR operation on the expanded right section and the round key.

28. 28 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 28 Substitution Boxes The S-boxes carry out the real mixing (confusion). DES uses 8 S-boxes, each with a 6-bit input and a 4-bit output

29. 29 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 29 The S-box rule There are a total of eight S-box tables. The output of all eight s-boxes is then combined in to 32 bit section.

30. 30 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 30 Straight Permutation The 32 bit output of S-boxes is then subjected to the straight permutation with rule shown in the following illustration

31. 31 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 31 Key Generation

32. 32 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 32 DES Analysis The DES satisfies both the desired properties of block cipher. These two properties make cipher very strong. Avalanche effect  − A small change in plaintext results in the very great change in the ciphertext. Completeness  − Each bit of ciphertext depends on many bits of plaintext. During the last few years, cryptanalysis have found some weaknesses in DES when key selected are weak keys. These keys shall be avoided.

33. 33 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 33 Differences Between AES & DES DES Algorithm AES Algorithm Key Length - 56 bits Key Length - 128, 192, 256 bits Block Size - 64 bits Block size - 128 bits Fixed no. of rounds No. of rounds dependent on key length Slower and less secure Faster and more secure

34. 34 Modern Symmetric-Key Ciphers(AES, DES) April – 2022 34 Conclusion This tutorial explores the need & working of AES & DES Encryption, its origin and process of encryption, all the way up to its applications, and a direct comparison in both the algorithm. Thank You