Section 3 Chapter 18 - Data Compression and Encryption

Question 1

Why compression is used

Answer 1

To reduce the size of data so that it takes up less storage space or less bandwith

Question 2

The two types of compression

Answer 2

Lossy and lossless

Question 3

Lossy compression

Answer 3

Unnecessary information is removed from the original file

Question 4

Lossless compression

Answer 4

All information required to exactly replicate the file is retained in a more space efficient way

Question 5

How mp3 uses compression

Answer 5

Lossy compression. Removes sounds at too high frequencies for humans to ear and removes quiet sounds that occur at the same time as loud sounds

Question 6

How lossless compression works

Answer 6

By recording patterns in the data

Question 7

Where lossy and lossless compression are used

Answer 7

Lossy - Images, sounds, videos

Lossless - Program files

Question 8

Run length encoding

Answer 8

Rather than recording every value in a sequence it records the value and how many times it continuosly repeats

Question 9

Dictionary based compression

Answer 9

A “Dictionary” is sent alongside the data and instead of the actual data being sent the indexes of the dictionary are sent. (lossless)

Question 10

Encryption

Answer 10

The transformation of data from one form to another so that an unauthorised third party cannot understand it

Question 11

Plaintext

Answer 11

The original unencrypted data

Question 12

Ciphertext

Answer 12

The encrypted data

Question 13

Cipher

Answer 13

A method or algorithm for encrypting data

Question 14

Caeser Cipher

Answer 14

Each character is shifted along by a certain number

Question 15

Vernam Cipher

Answer 15

A one time pad of equal or longer length to the plaintext is used and the ciphertext is found by using XOR on the one time pad and the plaintext.

Question 16

Why the vernam cipher is special

Answer 16

It is the only method of encryption which is unbreakable

Question 17

Cryptanalysis

Answer 17

The study of ciphers and with a view of finding a weakness that will allow the plaintext to be retrieved from the ciphertext without possessing the key

Question 18

Key

Answer 18

The data which can be used to convert ciphertext to plaintext and vice-versa

Question 19

Why the caesar cipher is easily broken

Answer 19

There very few (26) different possibilities for the key. This means it can be easily brute forced. Also, since it is a type of substitution cipher, the frequencies of characters could be analysed

Question 20

Why the vernam cipher has perfect security

Answer 20

Since the key is chosen uniformly at random, the ciphertext is distributed randomly and therefore there is no way to determine patterns or glean any information from the ciphertext. It cannot be brute forced since a ciphertext of length n could literally be any string with that length making a brute force meaningless

Question 21

Vernam ciphers vs the rest

Answer 21

Vernam is the only cipher proven to be completely secure. The worth of all other ciphers is based on computational security and theoretically all other ciphers could be broken given enough ciphertext and time.