Compression, encryption and hashing

Question 1

Define compression

Answer 1

The process of making files smaller by representing them using less data

Question 2

Describe lossy compression

Answer 2

Removes non-critical data from the data set. These may be frequencies of sound the human ear won’t notice or changing very similar colours to the same colour e.g. JPEG, MP3

Question 3

Describe lossless compression

Answer 3

No data is removed from the data set. The original file that is reconstructed from the compressed version will be exactly the same before. e.g. ZIP, PNG

Question 4

Name 2 methods of lossless compression

Answer 4

Run-length encoding and Dictionary coding

Question 5

What is run-length encoding

Answer 5

Makes use of redundant data, so that if a data item occurs multiple times consecutively, the item is stored once in an index along with the number of repetitions

Question 6

What is dictionary coding

Answer 6

An index is built where each data item is recorded along with an index reference. The compressed file then consists of the dictionary and the sequence of occurrences

Question 7

Define encryption

Answer 7

The process of making data unreadable to third parties

Question 8

What is symmetric encryption

Answer 8

Encryption where the same key is used to encrypt and decrypt data

Question 9

What is asymmetric encryption

Answer 9

Encryption where the public key is used to encrypt data and the private key is used to decrypt it

Question 10

What is a hash function

Answer 10

A function that takes in data of any size and returns a fixed-length output

Question 11

Name the properties hash functions should have

Answer 11

• Irreversibility
• Determinism => Same input will always give the same output
• Collision resistance => A collision is when two or more inputs give the same outputs.

Question 12

Name uses of hashes

Answer 12

Passwords and proof of state