3.6 - Data Compression & Encryption

Question 1

Why are files compressed?

Answer 1

To reduce their size so they can be transferred faster between devices

Question 2

What is lossy compression?

Answer 2

Some information is lost in the process of reducing the file’s size -> reducing resolution if an image or lowering sample resolution of an audio file

Question 3

What is lossless compression?

Answer 3

Patterns in the data are spotted and summarised in a shorter format without permanently removing any information so it can be reduced without decreasing its quality

Question 4

What is run length encoding (RLE)?

Answer 4

Method of lossless compression that reduces the file’s size size by removing repeated information and replacing it with one occurrence of the repeated information followed by the number of times it has to be repeated

Question 5

What is dictionary compression?

Answer 5

Another method of lossless compression where a dictionary containing repeated data is appended to the file. It results in a significant reduction in size but the dictionary does need to be present in the file for the image to be reproduced

Question 6

What is encryption?

Answer 6

The process of scrambling data so that it cannot be understood if intercepted in order to keep it secure during transmission
Easily cracked

Question 7

What is plaintext and ciphertext?

Answer 7

Plaintext = unencrypted information
Ciphertext = encrypted information

Question 8

What is a Caesar cipher?

Answer 8

Encrypts information by replacing characters, one character is always replaced by the same character

Question 9

What are the two different Caesar ciphers and how do they work?

Answer 9

Shift = all letters in the alphabet are shifted by the same amount, the amount by which characters are shifted form the key
Substitution = letters are randomly placed

Question 10

When is a one-time pad considered to have perfect security?

Answer 10

• the encryption key is equal to or longer than the plaintext message
• the key is totally random
• the key is used only one and then destroyed

Question 11

What is a one time pad?

Answer 11

A random key that is used to encrypt a message once

Question 12

Describe the rough steps for a Vernam cipher?

Answer 12

XOR the ASCII binary version for the plaintext letters with the ASCII binary version of the one time pad

Question 13

How do other ciphers rely on computational security?

Answer 13

Every cipher other than Vernam is in theory crackable but not within a reasonable timeframe given current computing power