1.3.1 Compression, Encryption, Hashing

Question 1

What is compression

Answer 1

Reducing the size of a file by identifying repetitions of data

Question 2

What are the attributes of Lossy Compression

Answer 2

Reduces File size,
Permanently deletes some data,
Useful for compressing image files

Question 3

What are the attributes of Lossless Compression

Answer 3

Reduces File size,

Patterns in data are summarised in a shorter file format,

No data deleted,

Useful for compressing text documents

Question 4

What’s dictionary compression

Answer 4

Splits files up into repeated data patterns,

Stores these in a dictionary

Question 5

What’s a dictionary

Answer 5

A data structure that allow you to store key-value pairs. Each word can be stored as an ASCII-code of 8 bits

Question 6

What’s encryption

Answer 6

The process of converting a message from plain text to cipher text,

Requires a key

Prevents data from being understood if intercepted..

Question 7

What’s decryption

Answer 7

Process of converting cipher text back into plain text requires a key

Question 8

What’s a message

Answer 8

Data that will be communicated between 2 parties

Question 9

What’s plaintext

Answer 9

A message in an easily human readable form

Question 10

What’s cipher text

Answer 10

An encrypted message

Question 11

What’s a cipher

Answer 11

A set of instructions (algorithm) for encrypting plain text

Question 12

What’s authentication

Answer 12

Proving the identity of the sender

Question 13

What’s symmetric encryption

Answer 13

The same private key is used to encrypt and decrypt,

The key must be shared between the sender and receiver,

Key can be easily intercepted when sharing

Question 14

What’s Asymmetric Encryption

Answer 14

Two different ( yet mathematically linked) keys are used to encrypt and decrypt,

Keys don’t need to be shared

Public key used to encrypt

Private key used to decrypt

Question 15

Why do most systems use asymmetric encryption

Answer 15

To generate a symmetric key, securing limited communication sessions

Question 16

What type of key exchange is used to generate a symmetric key

Answer 16

Diffie Hellman key exchange is used to generate it over transport layer security

Question 17

What is Transport Layer Security

Answer 17

An encryption that’s used to secure TCP/IP protocols

Question 18

What’s Caesar cypher encryption

Answer 18

Encryption that shifts each letter of plain text by an amount in a cyclical manner,

The amount is specified by the key

Question 19

Advantages of Caesar cypher

Answer 19

Quick to reveal message

Question 20

Cons of Caesar cypher

Answer 20

Easily crackable - only 25 possible keys

Question 21

What’s the one time pad

Answer 21

A substitution cipher, in which each character is encrypted using its own key,

It’s theoretically impossible to crack and used by the vernam cipher

Question 22

Define computational security

Answer 22

When a scheme cannot be cracked in reasonable time

Question 23

What are the prerequisites of computational security

Answer 23

Key must be at least the same length as the plain text,

Characters in the key must be truly random,

The key must only be used ONCE,

Must only be two copies of the key, and it must be kept secret,

Key must be destroyed after use

Question 24

What’s a hash table

Answer 24

A data structure that implements an associative array

Question 25

What’s an associative array

Answer 25

An array in which data is stored as collection of key-value pairs,

E.g. a dictionary

Question 26

Why must an array be used in a hash table

Answer 26

You have to be able to access each position of the array directly

Question 27

What’s hashing

Answer 27

Applying a hashing algorithm to the key, to determine the position of data within the array

Question 28

What’s the formula for the load factor of a hashing table

Answer 28

Number of occupied buckets divided by the total number of buckets,

Optimal load factor = 0.75

Question 29

What’s a hash function

Answer 29

An algorithm that converts a hash key to a hash value

Question 30

What are the requirements needed to use a hash function

Answer 30

Always produce the same hash value for the same key,

To provide a uniform distribution of hash values. This means every value has an equal probability of generation,

Minimise cluttering

Question 31

What causes cluttering

Answer 31

When many different keys produce the same hash value, a collision has occurred

Question 32

What’s the purpose of a hashing function

Answer 32

It provides a mapping between an arbitrary length input and a fixed length or smaller output,

Its one way - so it can’t be reverted

Question 33

What’s rehashing

Answer 33

When you rehash, a new hash table is created (larger if needed).

The key for each item in the existing table will be rehashed and the item will be inserted into the new hash table.

If the new table is larger the hashing algorithm will need to be modified as it will need to generate a larger range of hash values.

Question 34

Why might rehashing be needed

Answer 34

If the hash table starts to fill up, or a large number of items are in the wrong place, the performance of the hash table will degrade.