1. Advanced Encryption Standard
Network Security

2. Advanced Encryption Standard
Objectives of the Topic
After completing this topic, a student will be able to
describe Advanced Encryption Standard.

3. Advanced Encryption Standard
Figures and material in this topic have been
adapted from “Network Security Essentials: Applications and Standards”, 2014, by William Stallings.

4. Advanced Encryption Standard
The principal drawbacks of 3DES:
1.It has three times as many rounds as DEA and is correspondingly slower.
2.Both DEA and 3DES use a 64-bit block size.
Its not a reasonable candidate for long term use.

5. Advanced Encryption Standard
In 1997 NIST issued a call for proposals for a new AES:
1.Should have a security strength equal to or better than 3DES and significantly improved efficiency.

6. Advanced Encryption Standard
2. Must be a symmetric block cipher with a block length of 128 bits and support for key lengths of 128, 192, and 256 bits.

7. Advanced Encryption Standard
3. Evaluation criteria included security, computational efficiency, memory requirements, hardware and software suitability, and flexibility

8. Advanced Encryption Standard
NIST selected Rijndael as the proposed AES algorithm
Developers were two cryptographers from Belgium: Dr. Joan Daemen and Dr. Vincent Rijmen
published as a final standard (FIPS PUB 197) in 2001.

9. Advanced Encryption Standard
AES uses a block length of 128 bits and a key length that can be 128, 192, or 256 bits.
For our discussion, we assume 128 bits in this topic.

10. Advanced Encryption Standard
The input to the encryption and decryption algorithms is a single 128-bit block.
In FIPS PUB 197, this block is depicted as a square matrix of bytes.

11. Advanced Encryption Standard
The block is copied into the State array, which is modified at each stage of encryption or decryption.
After the final stage, State is copied to an output matrix.

12. Advanced Encryption Standard
Similarly, the 128-bit key is depicted as a square matrix of bytes.
This key is then expanded into an array of key schedule words: Each word is four bytes and total key schedule is 44 words for 128-bit key.

13. Advanced Encryption Standard
Ordering of bytes in a matrix is by column.
First four bytes of a 128-bit plaintext input to the encryption cipher occupy the first column of the in matrix, the second four bytes occupy the second column, and so on.

14. Advanced Encryption Standard
Similarly, the first four bytes of the expanded key, which form a word, occupy the first column of the w matrix.

15. Advanced Encryption Standard
AES’s Working:
Four different stages are used, one of permutation and three of substitution

16. Advanced Encryption Standard
Substitute bytes: Uses a table, referred to as an S-box, to perform a byte-by- byte substitution of the block.
Shift rows: A simple permutation that is performed row by row.

17. Advanced Encryption Standard
Mix columns: A substitution that alters each byte in a column as a function of all of the bytes in the column.
Add round key: A simple bitwise XOR of the current block with a portion of the expanded key.

18. Advanced Encryption Standard

19. Advanced Encryption Standard
Internal Details of a round.

20. Advanced Encryption Standard

21. Advanced Encryption Standard
Some comments:
AES structure is not a Feistel structure.
For both encryption and decryption, the cipher begins with an Add Round Key stage, followed by nine rounds that each includes all four stages

22. Advanced Encryption Standard
Each stage is easily reversible.
The final round of both encryption and decryption consists of only three stages.
The decryption algorithm is not identical to the encryption algorithm.

23. Advanced Encryption Standard
The decryption algorithm makes use of the expanded key in reverse order.
End