1.3.1 Encryption Compression Hashing

Question 1

Compression

Answer 1

Reducing the storage space required by a file.

Question 2

Benefits of compression

Answer 2

• Store more files with the same amount of storage space
• Reduces the time taken to share files over networks or the internet, as larger files require a longer time to transfer

Question 3

Lossy Compression

Answer 3

Reduces the size of a file while permanently removing of its information. This could result in a more pixelated image, or less clear audio recording.

Question 4

Lossless compression

Answer 4

Reducing the size of a file without losing any information.

Question 5

How can lossy compression be used on an audio file?

Answer 5

Removing the very high or very low frequencies which are less noticeable to the ear.

Question 6

Which compression method allows you to still be able to recover the original file?

Answer 6

Lossless

Question 7

How does Run Length Encoding work?

Answer 7

Repeated values are replaced with one occurrence of the data, followed by the number of times it should be repeated.

Question 8

Is RLE a form of lossy or lossless compression?

Answer 8

Lossless.

Question 9

Apply RLE to the following string.
AAAAABBCCC

Answer 9

A5B2C3

Question 10

In order for RLE to work well, what does it rely on?

Answer 10

Consecutive pieces of data being the same. If there’s little repitition, RLE doesn’t offer a great reduction in file size.

Question 11

How does Dictionary Encoding work?

Answer 11

• Frequently occuring pieces of data are replaced with an index.
• Compressed data is stored alongside a dictionary which matches the frequently occuring data to an index.
• The original data can then be restored using the dictionary.

Question 12

Is Dictionary Encoding a method of lossy compression or lossless compression?

Answer 12

Lossless.

Question 13

What is an important thing to remember about data compressed using dictionary compression?

Answer 13

The data must be transferred alongside its dictionary.

Question 14

What is encryption useful for?

Answer 14

Keeping data secure when its being transmitted.

Question 15

Symmetric encryption

Answer 15

Both the sender and receiver share the same private key, which they distribute to each other in a process called a key exchange. This key is used for both encrypting and decrypting data.

Question 16

For symmetric encryption, it is important to keep the private key secret. Why?

Answer 16

If the key is intercepted during the key exchange, then any communications sent can be intercepted and decrypted using the key.

Question 17

Asymmetric Encryption

Answer 17

A public and private key are used. The public key can be published anywhere, but the private key must be kept secret. Messages encrypted with the recipient’s public key can only be decrypted with the recipient’s private key.

Question 18

Person A wants to send a message to Person B using asymmetric encryption. What must Person A do?

Answer 18

They need to find Person B’s public key, so they can encrypt their message with it.

Question 19

How can asymmetric encryption be used to prove that a message has been sent by you?

Answer 19

Encrypt the message using your private key. Anyone can decrypt it using your public key. This shows you encrypted the message as only you have your private key.

Question 20

Hashing

Answer 20

A process in which an input (called a key) is turned into a fixed size value (called a hash).

Question 21

What is the difference between hashing and encryption?

Answer 21

The output cant be reversed to form a key. This makes hashing useful for storing passwords.

Question 22

Where are hash tables used?

Answer 22

In situations where lots of data needs to be stored with constant access times.
(Databases, caches)

Question 23

Two pieces of data produce the same hash. What is this called?

Answer 23

A collision.

Question 24

What are qualities of a good hash function?

Answer 24

• Low chance of collision
• Quick to calculate
• Provides output smaller than the input

Question 25

Why is it important that a good hash function provides an output smaller than the input?

Answer 25

If it wasn’t, then searching for the hash could take longer than simply searching for the key.