NWEN - Revision

Question 1

Name Three Crypt Analysis Techniques

Answer 1

1. Brute Force
2. Frequency Analysis
3. Relationship Analysis

Question 2

How does Polyalphabetic Cipher work

Answer 2

Uses more than one alphabet for the encryption and decryption process

Question 3

Explain how relationship analysis works

Answer 3

Adjacent letters in some languages may have certain personality traits. E.g. ‘Q’ is most likely always followed by a ‘U’

Question 4

How do you break the Polyalphabetic Cipher

Answer 4

1. Determine key length (X)
2. Split into X seperate cipher texts
3. Apply frequency analysis
4. Recombine results

Question 5

Name a way to make it harder to crack the Polyalphabetic Cipher

Answer 5

Change the pattern the cipher uses for encryption.

Question 6

Breaking Vigenere Cipher

Answer 6

You only care about the length of the keyword used. Finding the correct word length allows you to split the text into separate texts which can then have frequency analysis performed to break the cipher.

Question 7

Estimating key length of Vigenere

Answer 7

Must estimate based on any repeated sequences that can be viewed. More specifically the distance between repeated patterns that can be viewed helps estimate the length.

Question 8

Explain One-Time Pad Key Length

Answer 8

Must have the same number of bits as the plaintext

Question 9

Explain the operation performed for One-Time Pad

Answer 9

Exclusive or (XOR)

Question 10

Polyalphabetic Cipher’s main purpose

Answer 10

To decrease the footprint left by a language used for encryption.

Question 11

What is impractical about One-Time Pad

Answer 11

Long messages need an equally long key and so may prove to be impractical

Question 12

Name the two types or classes of symmetric encryption techniques

Answer 12

1. Stream Cipher (One time pad)

2. Block cipher

Question 13

Name the two block cipher main operations

Answer 13

1. S-Box (substitution mapping on n bit input to n bit output)
2. P-Box (permutation taking in a number of bits and rearranging them)

Question 14

Two main approaches of Cryptography

Answer 14

1. Substitution
2. Transposition
   Block cipher uses both

Question 15

Block cipher aim

Answer 15

For every bit of input to effect every bit within the output.

Question 16

Why are multiple S-boxes used

Answer 16

To try and decrease the overall complexity. S boxes for N inputs can have 2^n outputs, which can get very large very quickly. `

Question 17

What condition must be satisfied by P-Box

Answer 17

Must be reversible. And so uses XOR to perform the scrambling operation

Question 18

What is an issue with Block Ciphers

Answer 18

The same plaintext input produces the same output (which can in turn be used to break the encryption).

Question 19

How to overcome Block Cipher limitations

Answer 19

Perform XOR and use a random key to encrypt the messages as well. Random key is derived through the use of CBC.