1.3.1 Compression, Encryption and Hashing

Question 1

what is the purpose of compression?

Answer 1

• reduce file size
• reduce download times
• reduce storage requirements
• make best use of bandwidth

Question 2

why is making the best use of bandwidth important?

Answer 2

• large amounts of data are streamed and sent over the internet so using bandwidth efficiently is critical

Question 3

what is compression?

Answer 3

reduction of the overall size of a file

Question 4

what are two methods of compression?

Answer 4

• lossy
• lossless

Question 5

what needs to happen to a compressed file to read it?

Answer 5

it needs to be uncompressed

Question 6

what is lossy compression?

Answer 6

reduction of file size by permanently deleting data/reducing the quality of the data

Question 7

what is lossless compression?

Answer 7

reduction of the file size by storing the data in a different, more compressed way
this means the overall quality of the data does not change

Question 8

which types of data cannot be compressed with lossy compression?

Answer 8

text documents
executable programs

Question 9

what are two methods of lossless compression?

Answer 9

• dictionary coding
• run-length encoding

Question 10

what is dictionary coding good for?

Answer 10

text-based documents

Question 11

what is run-length encoding more suited for?

Answer 11

images

Question 12

how does dictionary coding work?

Answer 12

• builds an index
• every data item or entry in the file is recorded, along with an indexed reference or unique code
• file consists of dictionary index + sequence of occurrences needed to recreate original file

Question 13

how does run-length encoding work?

Answer 13

• if there are lots of contiguous pixels of the same colour, the number of pixels of that colour can be stored rather than storing them individually
• it is stored in frequency/data pairs

Question 14

what is encryption?

Answer 14

the process of encoding a message so that it can be read only by the sender and the intended recipient

Question 15

how does symmetric encryption work?

Answer 15

• a single key is used to both encrypt and decrypt a message
• both parties need to know the key and keep it secret

Question 16

how can the message be cracked with symmetric encryption?

Answer 16

• interception of the key
• duplication of the key production process to acquire a copy of key

Question 17

How does asymmetric encryption work?

Answer 17

• use two different keys
• encrypt the message with the first key
• message is then decrypted with the second key after it is sent

Question 18

What makes asymmetric encryption more secure?

Answer 18

• the two keys are not the same
• it is virtually impossible to derive one key from the other

Question 19

what are ‘key pairs’?

Answer 19

keys that are generated so anything encrypted with one of the keys can be decrypted with the other

Question 20

what is a public key as opposed to a private key?

Answer 20

• public key can be made public (give it out or publish online)
• anyone can have access to the public key
• PRIVATE key - not sent/shared with anyone

Question 21

how does public/private key encryption work?

Answer 21

• each party has their own public/private key pairs
• sender + recipient exchange copies of their public keys
• sender uses the recipient’s public key to encrypt before sending
• recipient uses their private key to decrypt

Question 22

what is another advantage of the public/private key method for encryption?

Answer 22

• someone can encrypt with their private key
• it can then be decrypted by their public key
• as the message can be decrypted with the public key, it must have originally been encrypted with the private key
• this confirms the message as authentic

Question 23

what is a combined encryption key?

Answer 23

• when the sender’s private key and the receiver’s public key are used to encrypt a message
• it is then decrypted using the receiver’s private key and the sender’s public key

Question 24

what 3 things do you know about a message when it is encrypted using asymmetric encryption?

Answer 24

1. the sender and receiver know no one else can read the message
2. you can be sure the message is authentic
3. you can be sure the message hasn’t been modified

Question 25

what is hashing/hashing function?

Answer 25

transforms a string of characters into a fixed-length value or key that represents the original input string

Question 26

what does a hashing function contain?

Answer 26

an algorithm that converts the input data
eg. SHA-1, MD5

Question 27

what happens to the hash value there is a slight change in the input message?

Answer 27

• produces a totally different hash value
• this is because the hash value is generated from the entire input message

Question 28

what is the difference between hashing and encryption?

Answer 28

hashing is a one-way process.
you cannot get back to the original value from the hashed value, even if you have access to the original hashing algorithm

Question 29

why is applying a hashing function to passwords secure?

Answer 29

• you do not have to send the passwords in plain text to the online system
• OR store the passwords in plain text in the online system
• any hacker who accesses the password file will only be able to see the hashed values (will not be able to obtain actual passwords as hashing is a one way process)

Question 30

How are passwords verified if they have been hashed?

Answer 30

• hashing function is applied to the password entered
• system compares the hashed value to the one stored

Question 31

how can hashing be used in data manipulation?

Answer 31

• used for quick searching, insertion and deletion of data in data structures
• there is no need to search the data structure for an entry
• just input what you are searching for and apply the hashing function to find item

Question 32

what is the time complexity of hashing?

Answer 32

average - O(1)