Compression, encryption and hashing

Question 1

Define compression

Answer 1

The process of making files smaller by representing then using less data. Making data quicker to transmit over a connection

Question 2

What is lossy compression

Answer 2

• Info is removed
• Usually used in video/audio files (changes might include removing frequencies of sound the human ear won’t notice)

The higher the compression ratio the smaller the file has been made

Question 3

What is lossless compression

Answer 3

• No info is removed
• The reconstructed version of the compressed file will be the same bit for bit

Question 4

What are the two types of lossless compression

Answer 4

• Run-length encoding (RLE)
• Dictionary coding

Question 5

Describe how run length encoding works

Answer 5

This makes use of redundant data
- If a data item occurs more than once consecutively the item is stored once in an index along with how many times it is repeated

Question 6

Describe how dictionary coding works

Answer 6

An index is built where each data item is recorded along with an index reference

The compressed file contains the dictionary and the sequence of occurrences

Question 7

Define encryption

Answer 7

The process of making data unreadable to third parties

Question 8

What is a key

Answer 8

A value that needs to be provided to encrypt/decrypt data

Question 9

How does encryption generally work

Answer 9

• Apply the encryption algorithm to the original data using the key so it is no longer readable
• The recipient applies the decryption algorithm using the key to decrypt the data

The key is kept secret

Question 10

Define symmetric encryption

Answer 10

Encryption where the same key is used to encrypt and decrypt the data

Question 11

Define asymmetric encryption

Answer 11

Encryption, where one key (the public key) is used to encrypt the data and another (the private key), Is used to decrypt it

Question 12

What is a Hash function

Answer 12

A function that takes in data of any size and returns a fixed-length output

Question 13

What are the properties of a hash function

Answer 13

• Irreversibility
• Determinism
• Collision resistance

Question 14

Define Irreversibility (hashing)

Answer 14

It is impossible to calculate what data was put into the hash function given its output

Question 15

define Determinism (hashing)

Answer 15

The same input will always give the same output

Question 16

Define Collision resistance (hashing)

Answer 16

Reduced probability that two or more inputs will give the same output

Question 17

describe some of the uses of hashing

Answer 17

Protects passwords stored on a database in case it gets hacked
- The password entered and compared to the stored hash
- If they are the same access is granted

Proof of state
- This can be used in police investigations to see if a database has been tampered with

Question 18

What are hash tables

Answer 18

A data structure designed for fast access of data. Each piece of data is stored using a key

• The key is hashed into a memory location
• When they need to retrieve the data they hash the key again to get the location in which the data is stored