1.3.1 Compression, Encryption and Hashing Flashcards
1
Q
What is Compression
A
- Reducing the amount of storage space (bits) a given file takes up
- Removes unnecessary bits to reduce the overall size of the file
2
Q
What are the advantages of compressing files
A
- Reduces the file size
- Reduces download times
- Reduces storage requirements
- Makes better use of bandwidth
3
Q
How is the level of compression measured
A
- Compression ratio
- Size of the compressed file/ size of original file
- Number between 0 and 1. The closer to 0 the better
4
Q
What are the 2 types of compression
A
- Lossy
- Lossless
5
Q
What is Lossy Compression
A
- File is compressed to a smaller size bust some information is lost during the process
- The original file can’t be perfectly reconstructed
6
Q
What is Lossless Compression
A
- Able to reduce a file’s size without losing any information.
- Original file can be perfectly restored
7
Q
When is lossy and lossless sutiable
A
- Lossy provides better compression ratio - better if lost data isnt important - good for compressing images. unnoticeable reduction in quality
- Text documents and executable programs - can’t lose data during compression - use lossless
8
Q
What is Run length encoding
A
- Method of lossless compression
- When a single character will be repeated many times
- R.L.E replaces a sequence of repeated characters with a flag character, (£,$) character itself, number of times its repeated
9
Q
What is Dictionary Encoding
A
- Another form of lossless compression best for text-based documents
- Works by building an index that we visualise as a table - dictionary index
- Sequence of occurences of characters is created
10
Q
What is Huffman Encoding
A
- One of the easiest dictionary encoding methods.
- Huffman encoding uses a dictionary to store all the letters that occur in text.
- Each letter is given a unique code, most common letters given a shorter code
11
Q
What is Encryption and Decryption and name an example of a encryption algorithm
A
- Encryption: The act of scrambling a piece of plain text into cipher text so it cant be easily understood
- Decryption: Unscrambling encrypted message to reveal original text.
- Piece of information needed to decrypt message is the key.
- Caeser Cipher
12
Q
Why is Caeser Cipher bad
A
- The shift cipher is weak
- Modern computers could use brute force hacking to crack it.
13
Q
What is Symmetric Encryption
A
- The process of decryption is simply the opposite of the process used to encrypt
- e.g Caeser Cipher : replacing letters with letters a set number of places down the alphabet
- They are quick to set up and easy to execute but they are very weak (easily cracked)
14
Q
What is Asymmetric Encryption
A
- The encryption and decryption keys are different
- If someone knows the encryption key they can encrypt the information but not decrypt it
- If someone has decryption key - can read info but not encrypt their response using the same algorithm.
- Also known as public key encryption
15
Q
What is a key pair
A
- In assymetric encryption - the keys are generated so anything encrypted with 1 key can be decrypted with the other - key pair
- 1 key needs to be made into a public key and the other becomes the private key
- The public key can be published online and the private key should not be given to anyone.
16
Q
What is a problem of using public key encryption and how should you solve this
A
- Someone could intercept Adam’s message and randomly remove chunks of the encoded message before passing it on to Charlotte. She wouldnt know if she had recieved the right message
17
Q
Describe an example of using the asymmeric encryption
A
- Paul wants to send a message to Sarah
- Sarak keeps her decryption key secret (private key) and publishes her encryption key (public key)
- Paul uses the public key to encrypt the message
- Sarah uses her private key to decrypt the message
18
Q
What is the RSA cipher and who invented it
A
- The asymmetric key was invented by American Computer Scientist Whitfield Diffie in 1975
- Ron Rivest, Adi Sharmir and Leonard Adelman developed the function for the encryption called the RSA Cipher and became the foundation for all ancryption techniques
19
Q
What is Hashing
A
- A hash is a unique string of data
- Transforms a string of characters into a fixed length value or key that represents the original input string.
- In public key encryption, the hash is unique to the original message sent and can be used to confirm the integrity of information
20
Q
What is the difference between encryption and hashing
A
- The difference is that hashing is a one-way process
- you can’t get back to the original value from the hashed value even if you have access to the original hashing algorithm.
- A slight change in the input message generates a different hash value
21
Q
What are the 2 algorithms that a hash uses and what are the advantages of hashing
A
- SHA-1 and MD5
- Preventing information like passwords from being read
- Harder to access as they are unreadable
- As hash is 1 way, the hacker can’t obtain any actual passwords from the information in the file.
22
Q
What is a digital signature
A
- Commonly used for software distribution, financial transactions
- 1 of the most common digital signature mechanisms is the digital signature algorithm (DSA)
- DSA lets 1 person with the private key sign the document so that others with a matching public key can verify that it was signed by the holder of the private key
- Digital signatures are usually applied to the hash of a message to add more security.
23
Q
Exam Question) Explain why asymmetric encryption is more suited to transactions over the internet that symmetric encryption
A
- Symmetric encryption would require both parties to have a copy of the key
- This couldn’t be transmitted over the internet or an eavesdropper monitoring the message may see it
- Asymmetric gets round this requirement as there are 2 different keys.
- One key encrypts which can be publicly distributed
- And a different key to decrypt it which is kept private.
24
Q
Exam Question) Explain the difference between lossy and lossless compression
A
- Lossy compression achieves a higher compression ratio but some data is lost
- Lossless compression retains all the original data but compression ratios are lower
25
Q
Exam Question) What is meant by Symmetric Encryption and give an example of a symmetric encryption algorithm
A
- Type of encryption where the same key is used to encrypt and decrypt the data.
- The substitution or Ceaser Cipher
26
Q
Exam Question) What is meant by asymmetric encryption
A
- The encryption and decryption keys are different
- For example RSA Cipher
27
Q
Exam Question
- Other than encryption give 1 use of a hash function
- Describe what is meant by the term “hash”
A
- Hash functions can be used to create digital signatures
- A result generated by applying an algorithm/ numeric process to a value
28
Q
Exam Question) Describe 1 advantage to storing passwords as a hash
A
- Hash functions are 1 way/ can’t be reversed
- If someone gains access to the database they cant access user’s password
29
Q
Exam Question: Explain 2 characteristics of the hashing algorithm
A
- Low chance of collision (i.e. different inputs giving same output) (1 – AO1.2)
- to reduce risk of different files being marked as the same (1 – AO2.1)
- Quick to calculate (1 – AO1.2)
- as lots of files need to be hashed / needs to be quicker than a bitwise comparison to make it worthwhile (1 – AO2.1).
- Provides a smaller output than input (1 – AO1.2)
- so quicker to compare hashes than original data (1 – AO2.1).
