1.3: Exchanging Data

Question 1

What is compression?

Answer 1

Compression is the process used to reduce the storage space required by a file, meaning you can store more files with the same amount of storage space.

Question 2

Why is compression important?

Answer 2

Compression is particularly important for sharing files over networks or the Internet. The larger a file, the longer it takes to transfer and so compressing files increases the number of files that can be transferred in a given time.

E.g. Apps like Google Photos compress files so that they can quickly be searched for and downloaded. Downloading a compressed file over the Internet is faster than downloading the full version of the file.

Question 3

Benefits of compression?

Answer 3

• Data is sent more quickly
• Less bandwidth is used as transfer limits may apply
• Buffering on audio and video streams is less likely to occur
• Less storage is required

Question 4

What is lossy compression?

Answer 4

As the name suggests, lossy compression reduces the size of a file by permanently removing data deemed non-essential. This could result in a more pixelated image or less clear audio recording.

Question 5

Example of lossy file type?

Answer 5

.JPG, .MP3, .MP4

Question 6

What is lossless compression?

Answer 6

Lossless compression reduces the size of a file without losing any information by spotting and summarising patterns in the data.

Question 7

Example of lossless file type?

Answer 7

.zip, .png

Question 8

Difference between lossy and lossless?

Answer 8

When using lossless compression, the original file can be recovered from the compressed version. Something which is not possible when using lossy compression which reduces the size of the file by completely disregarding some information.

Question 9

How can audio files be compressed using lossy compression?

Answer 9

For example, audio files can be compressed lossily by removing the very high or very low frequencies which are least noticeable to the ear and by removing quiet sounds that are overlapped with louder sounds. There’s no way to go from the lossy version of the recording back to the full version as there’s no record of what the high and low frequencies were.

Question 10

What is RLE?

Answer 10

Run length encoding is a method of lossless compression in which consecutive values are removed and replaced with one occurrence of the data followed by the number of times it should be repeated.

For example, the string AAAAAABBBBBCCC could be represented as A6B5C3.

Question 11

When does RLE work well?

Answer 11

In order to work well, run length encoding relies on consecutive pieces of data being the same - if there’s little repetition, run length encoding doesn’t offer a great reduction in file size. Image and sound data often have a lot of repetition.

Question 12

What is dictionary encoding?

Answer 12

Dictionary encoding is another example of a method of lossless compression. Frequently occurring pieces of data are replaced with an index and compressed data is stored alongside a dictionary which matches the frequently occurring data to an index. The original data can then be restored using the dictionary. The dictionary produces additional overheads but the space saving negates this problem.

Question 13

Can dictionary compressed data be used without the dictionary?

Answer 13

No - It’s important to remember that data compressed using dictionary compression must be transferred alongside its dictionary. Without a dictionary, the data cannot be used.

Question 14

What is encryption?

Answer 14

A way of making sure data cannot be understood if you don’t possess the means to decrypt it

Question 15

What is encryption used for?

Answer 15

Encryption is used to keep data secure when it’s being transmitted. There are a variety of different methods which can be used to scramble data before it’s transmitted and then decipher it once it arrives at its destination.

Question 16

What are two methods of encryption?

Answer 16

• Ceasar cipher

- Vernam cipher

Question 17

Ceasar cipher?

Answer 17

The Caesar cipher is most basic type of encryption and the most insecure

Letters of the alphabet are shifted by a consistent amount

Question 18

Methods of cracking a ceasar cipher?

Answer 18

• Brute force attackA brute force attack attempts to apply every possible key to decrypt ciphertext until one works

-Frequency analysis

Frequency analysis consists of counting the occurrence of each letter in a text. Frequency analysis is based on the fact that, in any given piece of text, certain letters and combinations of letters occur with varying frequencies. This will help us decrypt some of the letters in the text.

Question 19

Vernam cipher (+what should the key be)?

Answer 19

The encryption key, also known as the one-time pad, is the only cipher proven to be unbreakable.

The key must be:

• a truly random sequence greater or equal in length than the plaintext and only ever used once
• Shared with the recipient by hand, independently of the message and destroyed immediately after use

Question 20

How are messages encrypted and decrypted? (Vernam)

Answer 20

Encryption and decryption of a message is performed bit by bit using an exclusive or (XOR) operation with the shared key. XOR means that there can only be one occurence, two occurences does not count.

Question 21

Sources where the one-time pad can be generated from?

Answer 21

Sources may include: atmospheric noise, radioactive decay, the movements of a mouse or snapshots of a lava lamp.

Question 22

Why is the one-time pad so effective?

Answer 22

A truly random key will render any frequency analysis useless as it would have a uniform distribution.

Question 23

Why can you not use computer generated random sequences for the one-time pad?

Answer 23

Computer generated ‘random’ sequences are not actually random.

Question 24

What are the things needed to crack a cipher?

Answer 24

Given enough ciphertext, computer power and time, any key (except the one-time pad) can be determined and the message cracked.

Question 25

What are the two types of encryption?

Answer 25

Symmetric Encryption, Asymmetric Encryption

Question 26

Symmetric Encryption?

Answer 26

In symmetric encryption (also known as private key encryption), both the sender and receiver share the same private key, which they distribute to each other in a process called a key exchange. This key is used for both encrypting and decrypting data.

It’s important that the private key is kept secret. If the key is intercepted during the key exchange then any communications sent can be intercepted and decrypted using the key.

Question 27

Asymmetric Encryption?

Answer 27

When sending information using asymmetric encryption, two keys are used: one public and a second, private, key. The public key can be published anywhere, free for the world to see, while the private key must be kept secret. Together, these keys are known as a key pair and are mathematically related to one another.

In contrast to symmetric encryption, a single key cannot be used to both encrypt and decrypt communication. Instead, messages encrypted with the recipient’s public key can only be decrypted with the recipient’s private key, which should only be in the possession of the recipient.

Question 28

What does someone need to do to send you a message using asymmetric encryption?

Answer 28

If someone wants to send you a message, they must first find your public key. There are a variety of websites which can do this for you. The message is then encrypted with your public key meaning that only you can decrypt it.

Question 29

What are digital signatures?

Answer 29

If you want to prove that a message has been sent by you, you can encrypt it using your private key. This means that anyone can decrypt it (as your public key is available to anyone) and by doing so, can guarantee that you encrypted the message, as only you have access to the private key. This forms the basis of a system called digital signatures.

Question 30

What is hashing?

Answer 30

Hashing is the name given to a process in which an arbitrary length input (called a key) is turned into a fixed size value (called a hash). There are a vast number of algorithms, called hash functions, which do this.

Question 31

Difference between hashing and encryption? (And give an example of when this difference might be useful)

Answer 31

Unlike encryption, the output of a hash function can’t be reversed to form the key. This quality makes hashing useful for storing passwords. A password entered by a user can be hashed and checked against the key’s hash to see if it is correct, but a successful hacker would only gain access to the hashes, which can’t be reversed to gain the passwords.