AQA A-Level Computer Science Paper 2 - Data Compression, Graphics, Ciphers

lossy compression

some information is lost in the process of reducing the file’s size.

* e.g. reducing the resolution of an image or lowering the sample resolution of a sampled audio file.

lossless compression

no loss of information when using lossless compression.

* e.g. size of a file can be reduced without decreasing its quality.
* RLE, dictionary-based.

run length encoding (RLE)

reduces file size by replacing repeated information with one occurance of the repeated information followed by the number of times it is to be repeated.

RLE in practice

* replaces repeated pixel with one pixel value and the number of repetitions as a subscript.
* third row of pixels has no repeated value so can’t be compressed.
* shows this compression is not suitable for all data.
* effective on files that contain a lot of repeated data rather than short files.

dictionary-based methods

reduces file size by appending a dictionary containing repeated data the file.

dictionary-based methods in pratice

* the image can be represented just using 1234251.
* significant reduction in size.
* dictionary has to be present in the file so this will increase the file size.
* effective on files that contain a lot of repeated data rather than short files.

bitmapped graphics

image broken down into pixels, each of which has a binary value assigned to it.

resolution of bitmap

number and depth of pixels in an image.

* number of pixels = pixel width x pixel height

colour depth of bitmap

number of bits assigned to a pixel.

* binary representation - each pixel is assigned 1 bit allowing for 2 different colours to be represented.

storage requirement calculation

no. of pixels (width x height) x bit depth.

* this is the minimum as file may contain metadata.

\

this image has a resolution of 49 bits.

* 7 x 7 = 49

\
this image has a colour depth of 2 bits.

* 4 different colours are represented.
* 00 = white, 01 = black, 10 = red, 11 = yellow

\
minimum storage required is 98 bits

* 7 x 7 x 2 = 98

this image has a resolution of 4 bits.

* 2 x 2

\
this image has a colour depth of 1 bits.

2 different colours are represented

* 0 = white, 1 = black

\
minimum storage required is 4 bits

* 2 x 2 x 1 = 4

vector graphics

represent images using geometric objects and shapes such as rectangles, circles and lines.

the properties of each geometric object or shape in the image are stored in a list.

* e.g. fill colour, fill style and dimensions

vector graphics vs bitmapped graphics

* enlarging a bitmap image = blurry or even pixelated image.


* enlarging a vector graphic = in no loss of clarity.


* vector graphics for simple images which use shapes, like company logos.
* not good for photographs/detailed images.
* bitmapped graphics for both simple and detailed images, like company logos or photographs.
* vector graphics use less storage as information is stored for each shape.
* bitmapped graphics store information for every pixel.

vernam ciphers

* invented in 1917 by Gilbert Vernam
* one-time pad cipher
* only cipher considered perfectly secure because:
* each key only used once then destited
* encryption key is truly random and as long as the plaintext that is to be encrypted.
* key is securely shared in person

vernam cipher steps

1. align the characters of the plaintext and the key.
2. convert each character to binary (using an information coding system).
3. applying a logical XOR operation to the two bit patterns.
4. converting the result back to a character

computational security

* **all other ciphers** are, in theory, crackable, but not within a *reasonable timeframe* given current computing power.
* ciphers that use this form of security are said to rely on computational security.

symmetric encryption

the same *private key* is used to both encrypt and decrypt the data.

disadvantages of symmetric encryption

* keys must be kept secret and it is necessary to cycle through keys.
* keys must be shared in advance. can’t be done with individuals you haven’t previously communicated with.
* if you send key across the network unencrypted, an eavesdropper could intercept the communication and retrieve the key.

asymmetric encryption

a *public key* is used *encrypts* any data and a separate *private key* is used *decrypt.*

public key

key is shared publicly and used by sender to encrypt the data they wish to send to receiver.

private key

key is kept secret by receiver and is never shared as it is the key that is used to decrypt the encrypted data.