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A Level Computing > Chapter 15 Compression,Encryption,Hashing > Flashcards

Chapter 15 Compression,Encryption,Hashing Flashcards

1
Q

Why were file compression techniques created?

A

Reduce storage space on secondary storage, easier and quicker transmission, often easier to work with compressed files

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2
Q

Why are compression techniques useful on the internet?

A

Quicker transmission and less bandwidth used. This means less charge for using mobile phone networks. Also websites require quick loading to reduce buffering.

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3
Q

Why does compression do?

A

Reduces file sizes?

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4
Q

What are the two types of file compression techniques?

A

Lossy and lossless

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5
Q

What is lossy compression?

A

involves removing unnecessary data to reduce file size. The original data cannot be recreated

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6
Q

Examples of lossy compression? - file types

A

MP3,MP4 - audio

JPG,GIF - images

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7
Q

How is MP3 a type of lossy compression?

A

As frequencies outside of human hearing range are removed and quiet sounds played at the same time as loud ones are also removed

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8
Q

Where is lossy compression used on the internet?

A

Voice chat, quality noticeably removed but we can still hear the person

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9
Q

Advantages of lossy compression?

A
  • compressed files uses less bandwidth when shared over internet
  • takes up less storage space
  • overall output quality is usually acceptable
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10
Q

Disadvantages of lossy compression?

A
  • depending on degree of compression quality may be of very low standards
  • cannot recreate original quality/file
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11
Q

What is lossless compression?

A

involves reducing file size in a way that means it can be reconstructed without any data loss

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12
Q

Examples of lossless file types?

A

WAV FLAC - audio

PNG BMP - images

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13
Q

Why is lossless compression required in the world?

A

As some programs, text files require all data to work or be useful

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14
Q

How does lossless compression work?

A

By recording patterns in data. It then uses patterns and set of information on how to use these patterns to assemble compressed data with exact accuracy

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15
Q

Advantages of lossless compression?

A
  • orginal copy can be reassembled

- takes up less storage space and bandwidth (although more than lossy)

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16
Q

Disadvantages of lossless compression?

A
  • a lot less space is saved compared to lossy

- a program of some sort is required to reconstruct the compressed data

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17
Q

How does lossless compression work in sound files?

A

Waveforms are condensed and reconstructed by algorithm.

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18
Q

What are the two types of lossless compression we have to know about?

A

Run Length Encoding (RLE)

Dictionary Based Compression

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19
Q

What does RLE stand for?

A

Run Length Encoding

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20
Q

What is RLE ?

A

form of lossless compression where a sequence containing repeated value of data is replaced by a single value of that data and a count of how many times it occurs

21
Q 
How would this example be represented by RLE? 
 pppxxx 
pxxxxx 
pppppp  
where p  = 1 in binary  
where x = 0 in binary
A

row RLE description
1 3,1 3,0 3 x binary 1 then 3 x binary 0
2 1,1 5,0 1 x binary 1 then 5 x binary 0
3 6,1 6 x binary 1

22
Q

How does RLE reduce file size?

A

As 2 binary values have to be stored (bytes) instead of x number

23
Q

In this example of how many bytes are we using with RLE compared to normally?

A

10 bytes compared to 18 bytes

24
Q

What is dictionary based compression?

A

a substitution method of lossless compression. It uses a search to match compressed data (tokens) to that stored in a library

25
Q

How does it work?

A

When decompressing the compressed data is used to find matching data in a library to find matching data. It then translates it.

26
Q

What is used to find matching data in library?

A

tokens , they’re used as an index

27
Q

Why does this reduce size?

A

As tokens take up less space than data

28
Q

xxxxxxxxxx

A

xx

29
Q

xxxxxxxxxxx

A

xx

30
Q

What is encryption?

A

The transformation of data from one form to another to prevent third party access

31
Q

Wha is the plaintext?

A

original non-encoded data

32
Q

What is ciphertext?

A

encrypted data

33
Q

What is a cipher?

A

The enryption method or algorithm

34
Q

What is a key?

A

information which can be used to decrpyt ciphertext and change it into plaintext

35
Q

What are the two methods of ciphers (encryption methods) I have to know about?

A

caesar cipher

vernam cipher

36
Q

What is the caesar cipher?

A

method of encryption in which words are shifted x number of characters in the alphabet

37
Q

Is the Vernam Cipher breakeable?

A

NO

38
Q

What does the vernam cipher use?

A

A one time pad

39
Q

What is a one time pad?

A

A technique where a plaintext character is mixed with a truly random character from the key stream.

40
Q

How does the vernam cipher work?

A

A one time pad is used and the mixed characters (a pair of bytes) is used in an XOR operation to create cipher text

41
Q

What needs to be ensured to keep vernam unbreakeable?

A
  • only 2 copies of key
  • both sides have the key
  • key is used once
  • key is destroyed after use
  • use one time pad to ensure truly random key
42
Q

What does the ciphertext need to be unbreakable?

A

truly random, not computer made as computer uses an algorithm to randomise

43
Q

What is a cryptanalytic attack?

A

A study of systems in order to study hidden apsects, used to breach security and gain access to contents of encrypted messages

44
Q

What is a symmetric encryption? What is another name for it?

A

Encryption form where the same key is used for encryption and decryption
private key

45
Q

What is asymmetric encryption? What is another name for it?

A

Enryption form where 2 seperate but related keys are used. There is a public key used for encryption and private key used for decryption?

46
Q

Why is there a public key(cipher) and private key in asymmetric encryption?

A

Public key is used for encryption and can be used by anyone. Private Key is only known by receiver to decrypt.

47
Q

What can we do to a message to authenticate the sender?

A

a message can be digitally signed

48
Q

What is the main advantage of assymetric encryption compared to symmetric?

A

Key to decrypt is not transmitted/sent so cannot be intercepted. So even if a message is intercepted it cant be decrypted

49
Q

What is suitable for text encryption?

A

Dictionary based encryption

A Level Computing (21 decks)

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