18. Data compression and encryption algorithms

Question 1

What is compression?

Answer 1

The act of making file size smaller.

Question 2

What does it mean when compression is lossless?

Answer 2

No data is lost in compression and original can be recreated exactly.

Question 3

What does it mean when compression is lossy?

Answer 3

Data deemed non-essential is deleted.

An exact replica can not be recovered.

Question 4

Run length encoding is a compression technique.

Explain how it is performed?

Answer 4

Searches for patterns in code that can be shortened.

Question 5

What is encryption?

Answer 5

The conversion of data into a form that is difficult to read by an unauthorised third party

Question 6

What is decryption?

Answer 6

The process of turning scrambled data into data that can be understood

Question 7

What is a cipher?

Answer 7

The algorithm for encryption

Question 8

What is a key? - In terms of encryption and decryption

Answer 8

The secret information used to encrypt / decrypt the data

Question 9

What is plaintext?

Answer 9

The original form of data

Question 10

What is cipher text?

Answer 10

The encrypted data

Question 11

How does a caesar (shift) cipher work?

Answer 11

Shifts letters across a few places in the alphabet to create a cipher text

Question 12

In which direction would a positive shift go?

Answer 12

A –> B

Question 13

In which direction would a negative shift go?

Answer 13

A

Question 14

How does a keyword cipher work?

Answer 14

Use of a keyword to fill in the first few spaces in the alphabet, then fill the rest in in alphabetical order

Question 15

Explain dictionary based compression?

Answer 15

Lossless.
Uses common strings of characters.
Identify common patterns and create a dictionary value based upon this.

Question 16

what is encryption and why is it used?

Answer 16

encryption is the conversion of data into a form that cannot be read by a third party

Question 17

why are substitution ciphers, caesar cipher relatively easy to crack?

Answer 17

frequency analysis can be used to identify patterns in cipher text
(brute force methods)
e.g E and T are the most commonly used letters in the english alphabet

Question 18

explain how vernam ciphers work

Answer 18

based on the principle that each plaintext character from the message to be encrypted is combined with one character from a key (XOR)
if a random key is used the result will be a random cipher text which bears no relation to the original plaintext

Question 19

why is it said that a vernam cipher is theoretically unbreakable?

Answer 19

‘one time’ pads are used in vernam ciphers.
as long as the pads are unique and never re used, no statistical analysis or pattern matching techniques can be applied.
making a vernam cipher theoretically unbreakable

Question 20

identify the two commonly used methods to crack codes

Answer 20

frequency analysis and dictionary attacks

Question 21

what are public and private keys and how are they used?

Answer 21

in public key encryption, the recipient generates two keys together, a public encryption key and a private decryption key
the message is encrypted with the public key and can only be decrypted with the private key

Question 22

strengths of lossy compression

Answer 22

uses less space when stored

quicker to transmit because smaller

Question 23

limitations of lossy compression

Answer 23

permanently loses quality when compressed that cannot be restored when decompressed
cannot be used on data where every bit is integral to the end product, i.e. text documents or source code

Question 24

strengths of lossless compression

Answer 24

user has the ability to keep all the original data and revert to the original

Question 25

limitations of lossless compression

Answer 25

Larger files than if you were to use lossy compression

lossless compression can only be used once, whereas lossy compression can be used multiple times on a piece of data