1.3.1 Compression, Encryption & Hashing

Question 1

What is compression?

Answer 1

A process used to reduce the storage space required by a file

Question 2

What is the purpose of compression?

Answer 2

Increases the number of files that can be transferred in a given time
Reduce download time

Question 3

Name two types of compression.

Answer 3

Lossy
Lossless

Question 4

Describe lossless compression.

Answer 4

Reduces the size of a file less significantly as data isn’t removed
The original file can be recovered

Question 5

Describe lossy compression.

Answer 5

Reduces the size of a file significantly by removing some data
The original file cannot be recovered

Question 6

What is lossy compression suitable for?

Answer 6

Media
Not text/program files as characters will be removed

Question 7

What is lossless compression suitable for?

Answer 7

Media but won’t reduce file size as significantly
Text/program files as can be recovered

Question 8

What is Dictionary Encoding?

Answer 8

Method of lossless compression
Frequently occurring pieces of data are replaced with an index
Compressed data is stored with a dictionary which can be used to restore original data

Question 9

What is Run Length Encoding?

Answer 9

Method of lossless compression
Repeated values replaced with one occurrence followed by number of repititions

Question 10

Give two disadvantages of Run Length Encoding.

Answer 10

Relies on consecutive pieces of data being the same
Doesn’t offer much reduction in file size if little repitition

Question 11

What is encryption?

Answer 11

A process used to ensure data is kept secure during its transmission
Data is scrambled before transmission and deciphered at its destination

Question 12

What is asymmetric encryption?

Answer 12

Encryption used two keys

Question 13

What is symmetric encryption?

Answer 13

Encryption using a single key

Question 14

Describe what is meant by a key pair.

Answer 14

A public key that can be published anywhere
A private key that must remain secret
Both keys mathematically related

Question 15

How does asymmetric encryption work?

Answer 15

Messages encrypted with the public key can only be decrypted with the corresponding private key

Question 16

How does symmetric encryption work?

Answer 16

Both sender and receiver share the same private key
The key is distributed by key exchange and is used for both encryption/decryption

Question 17

Why can symmetric encryption be less secure?

Answer 17

If the key is intercepted, communications can be intercepted

Question 18

What is hashing?

Answer 18

An input is turned into a fixed size value (hash) by an algorithm

Question 19

Why is storing password using hash functions secure?

Answer 19

The output cannot be reserved to gain the initial input

Question 20

What is a salt?

Answer 20

A random value added to the input of a hash function to create an unique output

Question 21

What is a hash table?

Answer 21

A data structure that holds key-value pairs that can be used to look up data in an array in constant time

Question 22

What is constant time?

Answer 22

The time complexity O(1)

Question 23

List the 4 features needed to be a good hash function.

Answer 23

Take any length input & produce fixed length output
Be efficient & fast to compute
Be computationally infeasible to invert
Have a low chance of collision

Question 24

What is a collison?

Answer 24

A collison is said to have occurred if two keys produce the same hash

Question 25

Describe two methods to overcome collisons.

Answer 25

Storing items together in a list under the hash value
Using a second hash function to get a new hash