4b - Encryption

Question 1

What is encryption?

Answer 1

Using an algorithm to convert a message into a form that is not understandable without the key to decrypt it

Question 2

Why is data often encrypted when transmitted over a network?

Answer 2

To prevent unauthorised users understanding any intercepted data and to prevent the message being altered

Question 3

What is the purpose of a checksum?

Answer 3

To check if the contents of the packet/data have been corrupted/changed during transmission

Question 4

How is the checksum’s value determined?

Answer 4

Calculated from the payload/data/contents of the packet.

Question 5

What does it mean for a cipher to be described as being computationally secure?

Answer 5

The cipher cannot be cracked by any known method in a reasonable/practical/useful amount of time.

Question 6

What are two conditions for the Vernam cipher to offer perfect security?

Answer 6

• The key must be at least as long as the data to be encrypted/plaintext
• The key must not be reused and must be truly random

Question 7

How could a text be compressed using a dictionary-based method?

Answer 7

A dictionary is built that maps sequences of characters/substrings/words in the text onto tokens/values/numbers

Question 8

Why is dictionary-based compression not very effective for small amounts of text?

Answer 8

For small pieces of text there is little repetition, making the compressed text similar in size to the original.

Question 9

What principle does RLE use to compress a file?

Answer 9

• Identifies sequences of identical data values/color pixels
• Represents these as one data value/pixel color together with a count of how many such values are in the sequence.