1.3.1: Compression, Encryption, and Hashing

Question 1

What is Lossy?

Answer 1

• Compressing files by deleting unnecessary metadata

Question 2

What are the disadvantages of Lossy?

Answer 2

• Data is lost permanently
• Quality is sacrificed

Question 3

What is an advantage of Lossy?

Answer 3

• File size reduces dramatically

Question 4

How is Lossy implemented?

Answer 4

• Reducing number of colours or cropping an image

Question 5

What is Lossless?

Answer 5

• Compressing files by temporarily reducing file size

Question 6

What is an advantage of Lossless?

Answer 6

• No data is lost
• Quality is not sacrificed
• The original data can be retrieved

Question 7

What is a disadvantage of Lossless?

Answer 7

• File size isn’t as reduced as with Lossy as all the original data is still present when the file is accessed

Question 8

How is Lossless implemented?

Answer 8

• Run Length Encoding
• Dictionary Encoding

Question 9

How does Run Length Encoding work?

Answer 9

• An algorithm will identify sequences of colours and replace them

Question 10

How does Dictionary Encoding work?

Answer 10

• Repetitive characters or words are replaced

Question 11

Where would Lossy be most beneficial?

Answer 11

• When uploading, downloading, or streaming off the internet

Question 12

Where would Lossless be most beneficial?

Answer 12

• Compressing high quality photos over the internet, where the quality would be required to be uncompromised but the file would be originally too large to efficiently send over the internet

Question 13

What is Run Length Encoding?

Answer 13

• A method of Lossless compression in which repeated values are removed and replaced with one occurrence of the data followed by the number of times it should be repeated
• Example: AAAAAABBBBBCCC could be represented as A6B5C3

Question 14

What is required for Run Length Encoding to work well?

Answer 14

• Consecutive pieces of data being the same
• If there is little repetition, RLE doesn’t offer a great reduction in file size

Question 15

What is Dictionary Encoding?

Answer 15

• A method of Lossless compression where frequently occurring pieces of data are replaced with an index and compressed data is stored alongside a dictionary matching frequently occurring data to an index
• The original size can be restored using the dictionary

Question 16

What is Encryption?

Answer 16

• The process by which plaintext (data) is transformed into an unreadable file which can be deciphered using a key authorised by users

Question 17

Why is Encryption used?

Answer 17

• To prevent unauthorised viewing of private and confidential files
• Used for protecting data when being sent across the internet

Question 18

What is Symmetric Encryption?

Answer 18

• The process by which plaintext is encrypted is the method by which the ciphertext is decrypted

Question 19

How does Symmetric Encryption work?

Answer 19

• Both the sender and receiver share the same private key distributed via key exchange

Question 20

What is a disadvantage of Symmetric Encryption?

Answer 20

• Low security because of how easy it is to crack
• Even if the key doesn’t get intercepted during exchange, the patterns in the ciphertext render the scrambled message easy to crack

Question 21

What is Asymmetric Encryption?

Answer 21

• The process of Encryption utilising 2 keys: A public and private key

Question 22

How does Asymmetric Encryption work?

Answer 22

• The public key can be published anywhere so anyone can see
• The private key must be kept secret
• Together the keys are known as a key pair and are mathematically related to one another

Question 23

What is an advantage of Asymmetric Encryption?

Answer 23

• A single key cannot be used to both encrypt and decrypt communication
• Messages encrypted with the recipient’s public key can only be decrypted with the recipient’s private key (that should only be in the possession of the recipient)

Question 24

What is Hashing?

Answer 24

• The process by which an input (a ‘key’) is turned into a fixed size value (a ‘hash’) by one of many hash functions (algorithms that turn keys into hashes)

Question 25

What is an advantage of Hashing?

Answer 25

- The output of a Hash function cannot be reversed to form the key: Useful for storing passwords

Question 26

Why is Hashing useful for storing passwords?

Answer 26

- A password entered by a user can be hashed and checked against the key to check if it is correct, but a hacker would only gain access to the keys, which can't be reversed to gain the passwords

Question 27

What is a Hash Table?

Answer 27

- A data structure that holds key-value pairs

Question 28

How is a Hash Table formed?

Answer 28

- From a bucket array and a Hash function

Question 29

What are Hash Tables used for?

Answer 29

- Looking up data in an array in constant time - Used extensively in situations where a lot of data needs to be stored with constant access (e.g. caches and databases)

Question 30

How do Hash Tables work?

Answer 30

- When data needs to be inserted, it is used as the key for the Hash function and stored in the bucket corresponding to the Hash

Question 31

What is a Collision?

Answer 31

- If two pieces of data (keys) produce the same Hash

Question 32

How can Collisions be overcome?

Answer 32

- Storing items together in a list under the Hash value - Using a second Hash function to generate a new Hash

Question 32

What are the features of a good Hash Function?

Answer 32

- Low chance of collision - Quick to calculate - Produce an output smaller than the input provided, otherwise searching for the Hash could take longer than searching for the key

Question 33

What is a Hash Total?

Answer 33

- A mathematical value calculated from unencrypted message data

Question 34

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