Week 4: Symmetric Key Encryption

Question 1

What is Symmetric Key Encryption?

Answer 1

One key is used to encrypt and decrypt message.

Question 2

How do Block Ciphers work?

Answer 2

Message is split into fixed length block each block is encrypted separately.

Question 3

Name the requirements of an Ideal Block Cipher.

Answer 3

1. A plaintext block to a ciphertext block should be a one to one mapping.
2. Encryption/decryption should be easy with key, hard otherwise.
3. Computing the mapping should be efficient.

Question 4

What is ideally used for the mapping between one block and another block?

Answer 4

A Pseudo Random Permutation (PRP).

Question 5

Why is it not possible to use a Pseudo Random Permutation as a mapping?

Answer 5

Too many entries to write.

Question 6

How many possible mapping exist for a message of 128 bits?

Answer 6

2^128 mappings.

Question 7

What is a Pseudo Random Function (PRF)?

Answer 7

A function to map input to output space (usually one-to-one).

Question 8

How to you turn a strong PRF (Pseudo Random Function) into a PRP (Pseudo Random Permutation)?

Answer 8

Use Feistel network (at least 4 rounds needed)

Question 9

How does a Feistel round work?

Answer 9

Consists of 4 steps

1. Divide message into two parts: a & b
2. PRF takes input b and key k.
3. PRF output is xored with a to get a’.
4. a’ and b are switched places.

Question 10

How do you decrypt a message encrypted with a Feistel structure?

Answer 10

Do process backwards.

1. Switch a’ and b.
2. Feed b through PRF.
3. xor output of PRF with a’ to get a.
4. You have a and b.

Question 11

What are some Block Cipher modes?

Answer 11

• Electronic Code Book (ECB)
• Cipher Block Chaining (CBC)
• Counter (CTR)

Question 12

How does Electronic Code Book (ECB) work?

Answer 12

Every block is encoded in same way. Major security risk as identical blocks will be encoded same way.

Question 13

How does Cipher Block Chaining (CBC) work?

Answer 13

First message block is xored with IV and encrypted. The ciphertext is used to xor next message.

Addresses issue of ECB.

Question 14

How does Counter Block Cipher work?

Answer 14

First plaintext block has IV, second block has IV+1, etc.

Each IV is encrypted and then xored with plaintext to get ciphertext.

Question 15

What are the advantages of a Counter Block Cipher?

Answer 15

1. Can be computed in parallel.

2. Allows for random access.

Question 16

What type of key does AES have?

Answer 16

Supports 128, 192, and 256 bit keys.

Question 17

How big is each block in AES?

Answer 17

128 bits.

Question 18

How many rounds does AES need?

Answer 18

10 rounds for 128 bit key

14 rounds for 256 bit key

Question 19

What happens in each round of AES?

Answer 19

1. SubBytes - substitution using lookup table (Galois Field used to generate table) [PRP].
2. ShiftRows - last three rows cyclically shifted.
3. MixColumns - linear mixing operation.
4. AddRoundkey - xor with round key.

Question 20

How does AES work at high level?

Answer 20

For each round a new key from original key is generated.

Key and message is xored and then message goes through multiple AES rounds. Each round output is xored with key.