Question

1) How large are the blocks that get fed through the AES Encryption algorithm?

2) What are the three possible key sizes for AES? How many rounds are there for each key size? How large is the keyspace?

3) List five distinct differences between the AES and DES algorithms.

4) List the 16 elements of GF(16) as polynomials. What is 3x^2+6x+1 equal to in GF(16), when the coefficients are reduced appropriately?

5) Use the table on slide 17 of the Chapter 4 slides to compute the inverse of x^4+x^3+1 in GF(256). Then verify that the product of x^4+x^3+1 and the inverse you computed is, in fact, 1 modulo the polynomial indicated in red on slide 17.

6) List three separate known attacks on AES. Include the weakness(es) of AES that they exploit and include the year in which the attack was first discovered

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Answer 1

1. The AES has three fixed 128-bit block ciphers with the cryptographic size of 128, 192 and 256 bits, the maximum block size that gets fed through the AES Encryption Algorithm is 256 bits.

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2. Three possible key sizes of AES are 128, 192 and 256 bits.

Round can be defined as that convert the input, called the plaintext, into the final output, called the ciphertext. The number of rounds is as follows:  

• 10 rounds for 128-bit keys.
• 12 rounds for 192-bit keys.
• 14 rounds for 256-bit keys.

AES can use a symmetric key of 256 bits, resulting in a keyspace containing 2²⁵⁶ (or 1.1579 × 10⁷⁷) possible keys.

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3.

5 Difference between AES and DES

AES	DES
AES works on Substitution and Permutation Principle.	DES work on Feistel Cipher structure.
AES has larger key size as compared to DES	DES in comparison to AES has smaller key size
10 rounds for 128-bit algo 12 rounds for 192-bit algo 14 rounds for 256-bit algo	16 rounds
AES has large secret key comparatively hence, more secure.	DES has a smaller key which is less secure.
AES is faster.	DES is comparatively slower.

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