3.2 Symmetric Cryptography

Question 1

What is DES?

Answer 1

Data Encryption Standard is one of the most well-known symmetric encryption algorithms implemented by IBM in the 1970s that was intended to serve as the federal encryption standard.

Question 2

What does each F-box do in DES?

Answer 2

Each F-box performs a combination of substitution and transposition operations using the Feistel function.

1. They each take half a block of input (32bits) and combines it with a piece of the encryption key (48bits).
2. Then, that input is broken into 8 segments and fed into 8 different functions called S-boxes. Each of these boxes contains a different substitution cipher.
3. The results of all those substitutions are then combined together again and fed into a P-box. The p-box scrambles up the output of the S-boxes to produce the output.
4. This is repeated 16 times.

Question 3

How does DES work?

Answer 3

Data Encryption Standard uses an encryption operation
called the Feistel function for 16 rounds of encryption.

The algorithm takes 64 bits of plaintext as input in the top and then runs it through an encryption operation known as the Feistel function.

Question 4

What does S stand for in S-box?

Answer 4

Substitution.

Question 5

What does F stand for in F-box?

Answer 5

Feistel.

Question 6

What does P stand for in P-box?

Answer 6

Permutation.

Permutation is just another word for transposition.

Question 7

Is DES considered secure?

Answer 7

No. It is no longer being considered secure.

Question 8

What are the four key facts about DES?

Answer 8

1. DES is a symmetric encryption algorithm.
2. DES is a block cipher operating on 64-bit blocks.
3. DES has a key length of 56 bits.
4. DES is now considered insecure.

Question 9

What is Triple DES (3DES)?

Answer 9

It’s the application of DES three times on the same input.

Question 10

What are the three keying options for 3DES?

Answer 10

1. Key 1, Key 2, and Key 3 are all different.
2. Key 1 and Key 3 are the same, Key 2 is different.
3. All the keys are the same.

Question 11

Why 3DES? Why not just use DES twice?

Answer 11

Double DES is no more secure than DES, due to the meet-in-the-middle attack.

Question 12

What are the four key facts about 3DES?

Answer 12

1. 3DES is a symmetric encryption algorithm.
2. 3DES is a block cypher operating on 64-bit blocks.
3. 3DES has an effective key length of 112 bits.
4. 3DES is considered weak and being phased out.

Question 13

What does AES stand for?

Answer 13

Advanced Encryption Standard

Question 14

What’s the Rijndael algorithm?

Answer 14

It’s the algorithm that won a competition to become the AES.

Question 15

What does AES use at a high level?

Answer 15

Transposition and substitution.

Question 16

What are the four key facts about AES?

Answer 16

1. AES is a symmetric encryption algorithm.
2. AES is a block cipher operating on 128-bit blocks.
3. AES uses key lengths of 128, 192, or 256 bits.
4. AES is considered secure.

Question 17

What is Blowfish?

Answer 17

Blowfish is a public domain algorithm that is designed as a DES replacement. It uses a Feistel network to combine substitution and transposition.

Question 18

What are the four key facts about Blowfish?

Answer 18

1. Blowfish is a symmetric encryption algorithm.
2. Blowfish is a block cipher operating on 64-bit blocks.
3. Blowfish has any key length between 32 and 448 bits.
4. Blowfish is no longer considered secure.

Question 19

What is Twofish?

Answer 19

Twofish is a public-domain encryption algorithm that was designed as a replacement for DES. It relies on a Feistel network and relies on substitution and transposition.

Question 20

What are the four key facts about Twofish?

Answer 20

1. Twofish is a symmetric encryption algorithm.
2. Twofish is a block cipher operating on 128-bit blocks.
3. Twofish uses key lengths of 128, 192, or 256 bits.
4. Twofish is considered secure.