1.3.1 Compression, Encryption, Hashing

Question 1

Compression

Answer 1

• Process used to Reduce file size to take up less storage space or for faster transfer over a network
• increases the number of files that can be transferred in a given time
• allows faster download
• downloading a compressed file is faster than downloading the full version

Question 2

Lossy compression

Answer 2

• reduces file size while also removing some information
• slightly reduces quality but significantly reduced file size [1] e.g images can tolerate some reduction in quality
• reduces image quality e.g more pixelated images or less clear audio recording
• suitable for image audio and video
• not suitable for text documents as important info may be lost and text may become unreadable

Question 3

Lossless compression

Answer 3

• reduces file size without losing any information maintaining the original data [1]
• the original file can be recovered/recreated when it is uncompressed
• suitable for executable files and documents or vector style images, cartoons and logos
• maintains data integrity

Question 4

Lossless compression types

Answer 4

• run length encoding
• dictionary encoding

Question 5

Run length encoding

Answer 5

• a method of lossless compression
• Condenses identical elements to a single occurrence with its count next to it
• used in bitmap images to condense sequences of the same colour
• relies on consecutive pieces of data being the same so more effective when data has a lot of repetition
• doesn’t offer a great reduction in file size if there is little repetition

Question 6

Dictionary encoding

Answer 6

• a method of lossless compression
• that replaces frequently occurring pieces of data with a shorter, unique code/index
• compressed data is stored alongside a dictionary
• the dictionary matches frequently occurring data to an index/code
• original data can be restored using the dictionary
• this method is effective for both text binary data

Question 7

Encryption

Answer 7

• is used to keep data secure from unauthorised access when it’s being transmitted
• is used to convert readable data into an unreadable format
• uses keys which are specialised programs designed to scramble and unscramble data

Question 8

Symmetric encryption

Answer 8

• is a method of encryption where the same key is used for both encryption and decryption of data [1]
• requires both parties to have a copy of the key [1]
• the sender encrypts information using a private key before transmission
• the receiver uses the same key to decrypt the data
• if the key is intercepted, any communications can also be intercepted

Question 9

Symmetric encryption advantages / sutability

Answer 9

• usually faster making it ideal for encrypting large amounts of data/ faster and efficient for bulk data
• easier to implement as it uses a single key for both encryption and decryption/ same person encrypts and decrypts e.g when backing up
• less resource incentive

Question 10

Symmetric encryptions CONS

Answer 10

• loss/interception of the key means the data encrypted with it can be compromised and intercepted
• so requires key to be shared securely with the other party

Question 11

Asymmetric encryption

Answer 11

• uses 2 keys, a public key for encryption and a private key for decryption [1]
• public keys are shared openly allowing anyone to encrypt data [1]
• the private key is kept securely/locally on the receivers side and is kept private [1]
• use case: used when exchanging confidential data /secret communications .eg. Credit card details over the internet

Question 12

Asymmetric encryption pros

Answer 12

• use of a separate private private key makes it more secure
• if the public key is lost or stolen data won’t be compromised

Question 13

Hashing

Answer 13

• the process in which an input (e.g string of characters) is turned into a fixed sized value (called a hash) using a hash function
• even a slight change in the input message produces a totally different hash value [1]
• unlike encryption the hash function can’t be reversed to form the input message. [1]

Question 14

Advantage of storing password/data as a hash

Answer 14

• hashing is useful for storing
  passwords
• a password entered by a user can be hashed and checked against the stored hash value to see if it’s correct [1]
• only the hash values are stored, not the actual passwords itself so a successful hacker would only gain access to hash values which cant be reversed to gain the passwords [1]

Question 15

Hash tables

Answer 15

• Hash tables are data structures that store key-value pairs using a hash function to compute an index for fast data retrieval.
• They offer average-case O(1) time complexity for insertion, deletion, and lookup.
• Collisions are handled using techniques like chaining or linear probing

Question 16

Hash table : data insertion

Answer 16

• Compute the hash of the key, using a hash function
• map it to an index, and place the key-value pair at that index.

Question 17

Characteristic of good hash function

Answer 17

• low chance of collisions [1]
• quick to calculate (as lots of data needs to hashed) [1]
• provides an output that is smaller than its input, so quicker to compare hashes than original data [1]

Question 18

Collision

Answer 18

When two keys/inputs produce the same hash

Question 19

Collision resolution

Answer 19

• Chaining: items are stored together in a linked list under the hash value/index
• linear probing : the algorithm checks the next sequential slots until an empty one is found.

Question 20

Asymmetric encryption cons

Answer 20

• it is slower than symmetric encryption
• uses more reasources
• if the private key is lost, there is no way to decrypt the info