Encryption and compression - 96 - 99 Flashcards

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1
Q

What is the trade-off between when compressing files?

A

The quality of the file and the amount of processing power that will be needed to compress/decompress

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2
Q

What does lossy compression involve?

A
  • Reducing file size by removing non essential data
  • original can’t be reconstructed
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3
Q

What does lossless compression involve?

A

Reducing file size in a way which results in no data loss

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4
Q

when is lossy compression used

A

sound and images
- in executable file, where all data is necessary

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5
Q

advantages of lossy compression

A

we can get a considerable reduction in file size and the user will notice no difference in quality

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6
Q

examples of lossy file formats

A

JPEG
MPEG
MP3

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7
Q

advantages of lossless compression

A

allows the original file to be recreated exactly

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8
Q

how does lossless compression work

A
  • by using redundant data
  • for example, if an item occurs multiple times, then the actual item will only be stored and sensed once
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9
Q

example of dictionary coding formats

A
  • ZIP
    GIF
    PNG
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10
Q

examples of lossless compression

A

Dictionary coding and run length encoding

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11
Q

How does dictionary coding work?

A

substrings of original data are represented by
single tokens;
A dictionary is formed using the tokens as the keys;
The strings of symbols are used as the entries;

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12
Q

how does run length encoding work

A

The image is analysed to identify runs of the same value
- The values and counts of values are
stored

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13
Q

when is run length encoding used

A

source code,images

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14
Q

what is encryption

A

converting a message into a form that is only understandable by the intended parties

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15
Q

summarise the method of encryption

A

the sender will apply an encryption key to a plaintext message, the recipient receives the encrypted message and uses a decryption key to see the plaintext

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16
Q

disadvantages of the caesar cypher

A
  • easy cipher to crack
  • only 25 possible shifts
  • Very easy to break with modern techniques; brute-force or frequency analysis can quickly reveal the key.
17
Q

how to provide perfect security

A
  • if the encryption key (one time pad) is equal to or longer than the plaintext message
  • if the key is truly random
  • if the key is used only once and then destroyed
18
Q

whats the difference between the Vernam cipher and all the others

A

nothing can be learnt about the plaintext using the ciphertext
- others are based on computational theory and can be cracked

19
Q

what is compression

A

reducing file size

20
Q

advantages of dictionary based compression

A

a pre existing dictionary may exist so the encoder might not have to make a new one

21
Q

Why is the Vernam cipher considered as a cipher with perfect security?

A
  • because if the key (one-time pad) is truly random, at least as long as the message, and never reused, then the cipher
22
Q

How do you encrypt a plaintext message using the Caesar cipher?

A

each letter in the plaintext is shifted 5 places down the alphabet.

23
Q

ciphertext

A

encrypted version of plaintext

24
Q

plaintext

A

the original, readable text or message

25
Q

difference between symmetric and asymmetric cyphers

A
  • symmetric means the same key is used to encrypt and decrypt
  • asymmetric is when different but related keys are used for encryption and decryption
26
Q

explain the key difference between lossy and lossless compression

A

the original data can be fully recovered if lossless has been used
lossy compression cannot be reversed

27
Q

how to compress text using dictionary based

A
  • a dictionary is built that maps sequences of characters in the text onto tokens
  • the characters are then replaced by the corresponding tokens in the dictionary
28
Q

why would dictionary based compression not be very effective for compressing small amounts of text

A
  • for small pieces of text, there is little repetition
  • the dictionary will require storage space
29
Q

what does it mean if a cipher is computationally secure

A

the cipher cannot be cracked in a polynomial amount of time

30
Q

explain what the key exchange problem is

A

how to pass the key to the receiver without it being intercepted

31
Q

explain how a computer (computer b) would decrypt a message using asymmetric encryption and verify it was sent by another computer (comp A)

A
  • Bs private key is used to decrypt the message
  • the message is rehashed
  • As public key is used to decrypt the digital signature
  • if both gashes match then the sender can be authenticated