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A level Computer Science > 1.3.1 Compression, Encryption and Hashing > Flashcards

1.3.1 Compression, Encryption and Hashing Flashcards

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1
Q

Why do we use compression?

A

Reduce space used up in storage
Load online images and video faster
Reduce download times and bandwidth

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2
Q

What is lossy compression?

A

When non-essential information is removed from the file so that the data can still be understood, but the quality of it is degraded.

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3
Q

On what sort of data can we use lossy compression?

A
  • Images
  • Audio
  • Video
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4
Q

How may a sound file be compressed using lossy compression?

A

Frequencies that are too high for us to hear are removed.

Quiet sounds that play at the same time as louder sounds are removed.

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5
Q

What percentage of the original file size can lossy compression reduce an audio file to?

A

10%

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6
Q

How may an image be compressed using lossy compression?

A

Reducing bit depth - reduce the number of bits that are used to store each colour.

e.g. five different shades of yellow all now look the same

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7
Q

What is lossless compression?

A

When patterns in data are used to store the file in a compressed format which can then be reversed to display the original data with no loss of quality.

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8
Q

What technique, lossy or lossless, results in a smaller file size?

A

Lossy

But both reduce file size significantly

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9
Q

What compression method would be used for a word document and why?

A

Lossless - data cannot be removed as this would make the document unreadable

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10
Q

What is run length encoding?

A

When rather than recording every pixel in the sequence, the value of the pixel is recorded with the number of times that it repeats.

RED, RED, RED, RED, RED, RED, BLUE

RED(6), BLUE

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11
Q

What is a dictionary-based compression technique?

A

When instead of a whole text file being stored, a dictionary is stored with the location of each word in the text file being stored instead of the word. (see example in textbook, p77)

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12
Q

What does it mean if a text file uses its own dictionary?

A

The dictionary is comprised of the words in the text file.

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13
Q

What does it mean if a text file uses a known dictionary?

A

The dictionary is predetermined and does not depend on the words in the file.

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14
Q

When would using a dictionary-based compression technique not be adequate?

A

If the text file is very short or if it does not contain any repeating words.

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15
Q

Why is dictionary-based compression suitable for very long text files?

A

Because it saves so much space compared to the original file that the extra space of storing a dictionary is insignificant in proportion.

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16
Q

What is encryption?

A

The transformation of data from one form to another in order to prevent an unauthorised third party from being able to understand it.

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17
Q

What is the original data / message known as before encryption?

A

Plaintext

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18
Q

What is the encrypted data known as?

A

Ciphertext

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19
Q

What is the “cipher” in encryption?

A

The encryption method or algorithm

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20
Q

What is the “key” in encryption?

A

The secret information required to lock or unlock the message.

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21
Q

What is the Caesar cipher?

A

A cipher where the letters of the alphabet are shifted along a given number of characters.

22
Q

What is the “key” in a Caesar cipher?

A

The number of places along that the letters have been shifted.

23
Q

What is a Caesar cipher also known as?

A

Shift cipher

24
Q

What is the main issue with a Caesar cipher?

A

It is extremely easy to crack and therefore is insecure.

25
Q

What is the Vernam cipher?

A

When plaintext is compared to a key using an XOR operation to produce a ciphertext.

26
Q

What must the encryption key be in a Vernam cipher?

A
  • Completely random
  • Only known by the sender and recipient
  • Only used once
27
Q

What effect does using a truly random key in a Vernam cipher have?

A

The distribution of characters will be completely random and therefore, unbreakable.

28
Q

What is the encryption key also sometimes referred to in a Vernam cipher?

A

One time pad.

Due to the way that pads were used to punch holes in paper to store the message. Hole = 1, no hole = 0

29
Q

Why is XOR used in the Vernam cipher?

A

Because it is reversible whereas other operations are not.

30
Q

What is a truly random key?

A

A truly random sequence collected from a physical and unpredictable phenomenon such as white noise, timing of a hard disk, radioactive decay.

31
Q

Why is a computer generated random key not truly random?

A

Because it follows an algorithm to generate the key and can therefore be worked out and solved.

32
Q

What is symmetric encryption also known as?

A

Private key encryption

33
Q

What is symmetric encryption?

A

When the same key is used to encrypt the data as used to decrypt the data.

34
Q

What is a major issue with symmetric encryption?

A

The key must be transferred to the same destination as the ciphertext and can easily be intercepted and used to decrypt the data.

35
Q

What is asymmetric encryption?

A

When a public key that anyone can access is used to encrypt the data but a separate, private but related key is used to decrypt the data.

36
Q

What is a benefit of asymmetric encryption?

A

It has higher security than symmetric encryption because unauthorised users cannot intercept the private key to decrypt the message as it is not transferred.

37
Q

How do you prevent a malicious third party from using your public key to send a malicious message, impersonating a trusted individual?

A

Digital signing.

38
Q

What is hashing?

A

Hashing is generating a value or values from a string of text using a mathematical function.

39
Q

What makes hashing different to other encryption techniques?

A

It is one way - you cannot get back to the original.

40
Q

What is a hash total?

A

A mathematical value, calculated from unencrypted message data.

41
Q

What is a hash total also known as?

A
  • Checksum

- Digest

42
Q

What effect does the hash total being calculated from the entire message have?

A

Even the slightest change in the message will produce a different total.

43
Q

What is a digital signature?

A

When a sender generates a hash total from the message they are sending and encrypts it using their own private key. When the reciever gets the message, they can decrypt the hash total using the senders public key and calculate their own hash value from the message. If the two hash keys are the same, then the message is guaranteed to have come from the sender.

44
Q

What does it mean if the hash total that the reciever generates does not match the has key that the sender sent?

A

The message may have been altered some time during transmission OR the message did not come from that sender but from someone with a different private key impersonating them.

45
Q

How can we be sure that a message has not been copied and resent at a later date?

A

The time and date can be included in the message - affecting the hash total if different.

46
Q

Where can digital signatures be used?

A

When any kind of message is transmitted, with any method of encryption.

47
Q

Where are digital signatures often used?

A
  • Browsers

- Email clients

48
Q

What is a digital certificate?

A

Verification that a sender’s public key is formally registered to that particular sender.

49
Q

What does a digital certificate allow?

A

The holder to use the Public Key Infrastructure.

50
Q

What does a digital certificate contain?

A

Serial number, expriy date, name of holder, copy of public key, digital signature of CA.

51
Q

Who issues digital certificates?

A

Certificate Authorities (CAs)

e.g. Symantec, Verisign

52
Q

How does a digital certificate work alongside a digital signature to verify a sender’s validity?

A

The digital signature proves that the message is the original message coming from the sender and has not been tampered with.

The digital certificate proves that the sender is valid and trustworthy and not a hoax with a fake private key.

A level Computer Science (16 decks)

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