1.3.1 - Compression, Encryption and Hashing

Question 1

Define compression and when it is useful

Answer 1

• The process of making a file smaller/take up less storage.
• Particularly useful when transferring data via the internet

Question 2

Define lossy compression and what types of files it would and wouldn’t be suited to.

Answer 2

• Takes away some information from the original - Information is permanently lost in the process (irreversible)
• Used on images as some loss of detail is acceptable and unlikely to be noticeable
• It is unsuitable for text as the loss of small amounts of information will make it unreadable.

Question 3

Define lossless compression and what types of files it would and wouldn’t be suited to.

Answer 3

• Preserves all the information from the original (No data is lost)
• Used on files where losing data would not be an option, such as code and written texts.
• Rewrites data in a more efficient format.

Question 4

State the two examples of lossless compression

Answer 4

Run Length Encoding

Dictionary Coding

Question 5

Define run-length encoding and what types of scenarios it would and wouldn’t be suited to.

Answer 5

• Not suited to natural language
• More likely to be used in simple images
• Relies on consecutive pieces of data
• Each set of consecutive symbols can be represented by the symbol and its number of occurrences e.g. AAAABBBBBCCC could be represented as 4A5B3C

Question 6

Define dictionary coding

Answer 6

• Frequently occurring pieces of data or groups of characters are replaced by tokens.
• A dictionary is then used to say which tokens match which groups of characters.
• When decompressed the dictionary is used to replace the tokens with the original text.
• The dictionary needs to be transmitted with the file.

Question 7

Define encryption

Answer 7

When data is scrambled so that if it is intercepted, it cannot be understood

Question 8

Define symmetric encryption and its drawbacks

Answer 8

• The same key is used to decrypt and encrypt data
• Key exchange is required
• The key couldn’t be transmitted over the internet as an eavesdropper monitoring the message may see it.

Question 9

Define asymmetric encryption and give an example of where it could be used

Answer 9

• One public key encrypts the data, and a different private key is used to decrypt
• No key exchange required
• Good for internet traffic as it gets around the requirement of the key being sent as there are two different keys. Used for secure online transactions (e.g. Banks)

Question 10

Define the term hash

Answer 10

A result that is generated by applying a hashing algorithm to a value.

Question 11

Define the term hashing and give an example of where it could be used.

Answer 11

• An algorithm used to convert data into a hash - one-way process
• Used for storing passwords in databases

Question 12

What main criteria can be used to judge the suitability of a hashing algorithm?

Answer 12

1. Low chance of collisions
2. Quick to calculate

Where possible, have a shorter output than input