Compression and Encryption Flashcards
(25 cards)
What are the main benefits of compression?
- Amount of data required to store data is reduced so more files can be stored on a computer system or on external storage
- Amount of data required to transmit data is reduced so less bandwidth is required, reduced congestion on the network, increased transmission speeds
- **improves playback **of video and audio with less buffering due to reduced size
What are the other benefits of compressions?
- Using compressed images in web pages reduces loading times
- Improves playback of media streams with less buffering
What is lossy compressions?
Form of compression where unnecessary detail is removed permanently from the original data.
What are the +ive and -ive of lossy compression?
+Greatest reduction in file size
-Permanent loss of quality
Ideal for image and audio
ideal for compressing images and audio
What is lossless compression?
Form of compression that uses algorithms to efficiently represent repeating patterns within the original data.
What are the properties of lossless compression?
+No original data is lost so can be perfectly reconstructed
-Less reduction in file size
This is the only option where data cannot be lost such as text and program files
What is the JPEG image file?
- Form of lossy compression that averages similar colours across adjacent pixels (permanently removes details)
- Suitable for photographs which do not have sharp edges
What is a PNG image file?
- Lossless compression that efficiently encodes repeating patterns
- Higher quality images with sharp edges such as logos, icons and texts.
- Larger file sizes
What is an MP3 file?
- Uses lossy compression to remove detail from audio recordings that we cannot perceive then other algorithms are used
- Results in files that are around 10 times smaller
How is text compressed?
Text can only be compressed using lossless compression. Otherwise text files such as source code could fail to be interpreted/compiled otherwise.
Therefore we use Dictionary Compression or Huffman Tree Encoding.
What is Run Length Encoding?
Simple form of lossless compression that represents repeating patterns of data more efficiently. E.g. 6 white pixels in a row could be encoded into 6w
not all data suitable if no repeating patterns
What is Dictionary Based Compression?
A form of lossless compression where frequently occurring pieces of data are replaced by an index of a dictionary.
- Words are stored in a dictionary, each with an index number pointing to it
- The text is then encoded using the dictionary index values to represent each word
- Dictionary must also be sent with the original file
- Works best for long documents with many common words (not short texts with little repetition)
How does Dictionary Based Compression work?
- Each word in a piece of text is stored in a dictionary along with an index number
- Message is then encoded using the dictionary index values in place of the original data
- The resultant message will be significantly smaller, although the dictionary must also be sent/stored with the file.
What is encryption?
The process of making information unreadable to unauthorised parties through the use of an encryption algorithm.
How does encryption work?
A cipher is applied to plaintext using a key to transform it into ciphertext so that it is not understandable if intercepted during transmission without the key
What is computational security?
A cipher is considered computationally secure if it cannot be broken using current technologies within a reasonable timeframe.
What are methods for cracking ciphers?
- Frequency analysis
- Looking for other patterns
- Dictionary attacks
What is frequency analysis?
It involves looking for frequent symbols within the cipher text and mapping them to common letters in the language of the original message.
What is a dictionary attack?
A list of common words or phrases that can be compared to the cipher text to see if any patterns arise
What is a Caesar cipher and how does it work?
A basic substitution cipher where each letter is substituted for another. A numeric key is used to determine the distance that each letter should be shifted by.
Why is a Caesar cipher not computationally secure?
- It can be cracked almost instantly using basic brute force attacks as there are only 25 possible keys
- Frequency analysis is very effective (common letters)
- Each character is always converted to the same cipher text letter
- As it is only a shift cipher you only need to map one of the letters between plaintext and cipher text to deduce mapping for all letters
When is a Vernam cipher perfectly secure?
- key used is truly random (e.g. using atmospheric radiation)
- key is at least as long as the plaintext
- key used only once and then discarded
- key itself must be transferred securely
How do you use a Vernam cipher?
To encrypt a message, each character in the plaintext and key are encoded into their binary representations in a given character set
- bitwise XOR binary representations of plaintext and key together
- convert binary result back to characters
Why is a Vernam cipher perfectly secure?
- Brute force would not work as key is randomly generated, -there are too many possible keys (not possible) to try all of them (and multiple plaintexts can be derived)
- Different plain text letters can map onto the same plaintext letter, so frequency analysis does not happen as cipher text letters have uniform probability
