Encryption and compression - 96 - 99

Question 1

What is the trade-off between when compressing files?

Answer 1

The quality of the file and the amount of processing power that will be needed to compress/decompress

Question 2

What does lossy compression involve?

Answer 2

• Reducing file size by removing non essential data
• original can’t be reconstructed

Question 3

What does lossless compression involve?

Answer 3

Reducing file size in a way which results in no data loss

Question 4

when is lossy compression used

Answer 4

sound and images
- in executable file, where all data is necessary

Question 5

advantages of lossy compression

Answer 5

we can get a considerable reduction in file size and the user will notice no difference in quality

Question 6

examples of lossy file formats

Answer 6

JPEG
MPEG
MP3

Question 7

advantages of lossless compression

Answer 7

allows the original file to be recreated exactly

Question 8

how does lossless compression work

Answer 8

• by using redundant data
• for example, if an item occurs multiple times, then the actual item will only be stored and sensed once

Question 9

example of dictionary coding formats

Answer 9

• ZIP
  GIF
  PNG

Question 10

examples of lossless compression

Answer 10

Dictionary coding and run length encoding

Question 11

How does dictionary coding work?

Answer 11

substrings of original data are represented by
single tokens;
A dictionary is formed using the tokens as the keys;
The strings of symbols are used as the entries;

Question 12

how does run length encoding work

Answer 12

• a run is a sequence of the same colour
• the number of consecutive pixels of the same colour would need to be counted
• eg 7 Yellow

Question 13

when is run length encoding used

Answer 13

source code,images

Question 14

what is encryption

Answer 14

converting a message into a form that is only understandable by the intended parties

Question 15

summarise the method of encryption

Answer 15

the sender will apply an encryption key to a plaintext message, the recipient receives the encrypted message and uses a decryption key to see the plaintext

Question 16

disadvantages of the caesar cypher

Answer 16

• easy cipher to crack
• only 25 possible shifts
• Very easy to break with modern techniques; brute-force or frequency analysis can quickly reveal the key.

Question 17

how to provide perfect security

Answer 17

• if the encryption key (one time pad) is equal to or longer than the plaintext message
• if the key is truly random
• if the key is used only once and then destroyed

Question 18

whats the difference between the Vernam cipher and all the others

Answer 18

nothing can be learnt about the plaintext using the ciphertext
- others are based on computational theory and can be cracked

Question 19

what is compression

Answer 19

reducing file size

Question 20

advantages of dictionary based compression

Answer 20

a pre existing dictionary may exist so the encoder might not have to make a new one

Question 21

Why is the Vernam cipher considered as a cipher with perfect security?

Answer 21

• because if the key (one-time pad) is truly random, at least as long as the message, and never reused, then the cipher

Question 22

How do you encrypt a plaintext message using the Caesar cipher?

Answer 22

each letter in the plaintext is shifted 5 places down the alphabet.

Question 23

ciphertext

Answer 23

encrypted version of plaintext

Question 24

plaintext

Answer 24

the original, readable text or message

Question 25

difference between symmetric and asymmetric cyphers

Answer 25

- symmetric means the same key is used to encrypt and decrypt - asymmetric is when different but related keys are used for encryption and decryption

Question 26

explain the key difference between lossy and lossless compression

Answer 26

the original data can be fully recovered if lossless has been used lossy compression cannot be reversed

Question 27

how to compress text using dictionary based

Answer 27

- a dictionary is built that maps sequences of characters in the text onto tokens - the characters are then replaced by the corresponding tokens in the dictionary

Question 28

why would dictionary based compression not be very effective for compressing small amounts of text

Answer 28

- for small pieces of text, there is little repetition - the dictionary will require storage space

Question 29

what does it mean if a cipher is computationally secure

Answer 29

the cipher cannot be cracked in a polynomial amount of time

Question 30

explain what the key exchange problem is

Answer 30

how to pass the key to the receiver without it being intercepted

Question 31

explain how a computer (computer b) would decrypt a message using asymmetric encryption and verify it was sent by another computer (comp A)

Answer 31

- Bs private key is used to decrypt the message - the message is rehashed - As public key is used to decrypt the digital signature - if both gashes match then the sender can be authenticated

Question 32

if a picture contains a lot of different colours, why might run length encoding not be appropriate to compress the image

Answer 32

- runs will be of shorter length because colour depth is higher - the additional run length data may cancel out the reduction in storage of pixel colour data

Question 33

disadvantage of lossy

Answer 33

Less compression compared to lossy