Chapter 20: Cryptographic Algorithms

Question 1

What is symmetric encryption?

Answer 1

Encryption based on substitution and transposition. Earliest form from ancient Egypt. Secret key shared by both parties.

Question 2

What is asymmetric encryption ?

Answer 2

Encryption using a matermaticwlly related public and private key pair for encryption. Public key of the receiver is used to encrypt the senders message. Private key of receiver is used to decrypt the message.

Question 3

What is DES?

Answer 3

Data encryption standard. 1970. Block cipher (takes several blocks at a time and encrypt block by block) based on lucifer cipher

Question 4

What did the NSA add to DES?

Answer 4

S-boxes (substitution boxes). 1976. This decreased the key length.

Question 5

Why was 3DES created?

Answer 5

To prevent attacks. Used 1,2 or 3 56b keys. It was designed to strengthen the algorithm and be backward compatible.

Question 6

What is 3DES?

Answer 6

Strengthened DES. effective key sizes of 56, 112 or 168b. Two rounds of encryption. One round of decryption. Backward compatible.

Question 7

What are the steps of the DES function?

Answer 7

1. Expansion
2. Key mixing
3. Substitution
4. Permutation

Question 8

What is the expansion process of DES?

Answer 8

32b input to a 48b output. On nibble, duplicate partition bits. All other bits shift one to the right. Circular. Nibble partition bits duplicate twice in order.

Question 9

What is key mixing?

Answer 9

Output of 48b expansion. Phase XORed with the particular 48b round key.

Question 10

What is substitution?

Answer 10

Every 6bits of the phase 3 output is substituted for a 4b output from sbox 1-8 depending on location of the 6b. Uses s-boxes. Results in 32bit output.

Question 11

What is permeatation ?

Answer 11

32b block permeated using p matrix

Question 12

What does AES use that DES doesn’t ?

Answer 12

Does not use a deist all structure.

Question 13

When did AES replace DES?

Answer 13

2001

Question 14

What is AES based on ?

Answer 14

Rinjdael algorithm. (Suite of algorithms that operate on different block and key sizes)

Question 15

How many rounds in AES?

Answer 15

Number of rounds is based on key size.

128b - 10 rounds
192b - 12 rounds
256b - 14 rounds

Question 16

What differentiates the key sizes in AES?

Answer 16

Each key size algorithm is slightly different. Substitutions and permeatations operate on byte level.

Question 17

What are the steps of AES?

Answer 17

1. Key expansion
2. Initial round
3. Sub steps for rounds 1-9, 11, 13
   A. Sub bytes
   B. Shift rows
   C. Mix columns
   D. Add round key
4. Make algorithm reversible
   A. Sub bytes
   B. Shift rows
   C. Add round key.

Question 18

What is the finite field theorem ?

Answer 18

Finite field exists only if they have p^m elements, where p is prime and m is a positive integer