Data Rep

what is encryption?

the process of turning plaintext into scrambled ciphertext

what is decryption?

the process of deciphering encrypted data

why is encryption used?

to secure sensitive data (e.g. bank transactions, health infomation and personal details)

what is caeser cipher?

uses a specific pattern to substitute one character for another

what is frequency analysis?

studying commonly used letters to speed up decryption

what is vernam cipher?

Uses a one-time pad technique - each key that is only used once to encrypt and decrypt a message and then be discarded (not linked to message)

The key has to be longer/ the same length as the plaintext

 

1. Align plaintext and key

2. Convert characters to binary

3. Apply XOR operation to the two bit patterns

4. Convert back to a character

what is computational security?

theoretically breakable but not with current technology

what is the concept of computational security?

ciphers are theoretically breakable but now when using current technology

ways to crack code:

- identifying commonly used techniques

- reverse engineering

- dictionary attacks with common words/phrases

- brute force

what is a parity bit?

a single bit that is added to data to check for errors in transmission

how does even parity work?

extra bit is set to either 1 or 0 in order to make the total number of 1s even

how does odd parity work?

extra bit is set to either 1 or 0 in order to make the total number of 1s odd

advantages of parity bit

very efficient

disadvantages of parity bit

doesn't detect two-bit errors

what is hamming code?

a way to locate and correct errors using multiple parity bits

advantages of using hamming code over parity bit

- can detect two-bit errors

- can correct single-bit errors

what is majority voting?

data is sent multiple times and the most commonly value taken is correct

advantages of majority voting

can detect errors and correct errors

disadvantages of majority voting

- increased transmission time

- increased bandwidth

what is checksum?

adding a value to the data from a calculation that is used to checked for errors

advantages of checksum

- can detect errors

- mostly efficient

disadvantages of checksum

- cant correct errors

- cannot detect multiple errors

- could be complex

what is checkdigit?

a type of checksum with only a single digit (more efficient)

ascii

a 7-bit character set consiting of 128 characters

extended ascii

an 8-bit character set consisting of 256 characters

Unicode

a character set (starting with 16 bits) to represent loads of characters, languages and symbols

signed binary

used to represent negative numbers

how do computers subtract

by adding negative numbers

fixed point binary

uses a specified number of bits before and after the binary point

floating point binary

uses a mantissa and an exponent to find where the binary point should go

advantages of floating point

- allows for a representation of a greater range of numbers with a given number of bits

disadvantges of floating point

- complexity

- precision errors (rounding)

- uses more memory

mantissa

significant digits

exponent

the power of 2 applied to the mantissa

rounding error

when decimal numbers cannot be represented exactly in binary

underflow error

when very small numbers are not able to be represented so is approximated as zero

overflow error

when a number is too large to be represented with the available bits

absolute error

the actual difference between the given value and the actual value

relative error

relative error is a measure of uncertainty comparing the actual value and absolute error
(=absolute error/true value)

normalisation

adjusting the mantissa so the first two bits are different (0.1 or 1.0) to allow for more at the end

adding binary

0 + 0 = 0

0 + 1 = 1

1 + 1 = 10

1 + 1 + 1 = 11

multiplying binary

same as usual, different addition

how to show number of permutations from a bit (n)

2^n

prefix for binary?

kibi (1024)

prefix for decimal?

kilo (1000)

Natural number

N, all positive whole numbers and zero

Integer

Z, both positive and negative whole numbers, including zero

Rational numbers

Q, any number that can be written as a fraction

Irrational numbers

any number that cannot be written as a fraction

Real numbers

R, all real numbers (not imaginary)

Ordinal numbers

Integers to describe the position of an object in relation to others

What number system is used for counting

Natural

What number system is used for measuring

Real

what is a bitmap?

values of binary assigned to pixels within an image containing colours

what is colour depth?

the number of bits per pixel in an image

what is a vector graph?

representing an image through geometric objects, shapes and lines

advantage of vector graphs over bitmaps

- can be scaled without losing quality

- uses less storage

disadvantage of vector graphs

- only suited for simple images, not photos

what is metadata?

data about data

examples of metadata

- width

- height

- data created

- file type

- colour depth

what is analogue data?

continuous and variable, representing real-world values (e.g. temperature and sound)

what is digital data?

stores values in distinct steps as binary (0s and 1s)

what are analogue signals?

continuous waveforms that change smoothly over time (e.g. sound waves)

what are digital signals?

step-like waveforms consisting of high and low values

what is sampling rate?

number of samples per second

what is sampling resolution?

number of bits allocated to representing the sound

what is nyquist's theorem?

that sampling rate should be at least twice the frequency of the sound for accuracy

what is MIDI?

sends event messages between electronic instruments to a computer showing how to recreate the event

(musical instrument digital interface)

examples of messages sent by midi

- duration of a note

- instrument playing the note

- volume of the note

- if a note is sustained

what is lossy compression?

reducing file size the most by removing ‘unnoticable data’ but file quality is affected which could make it unreproducable

what is lossless compression?

reduces file size without impacting important information

methods of lossless compression

- run length encoding

- dictionary-based methods

how does lossy compression work?

eliminates repeating data that is least likely to affect the human perception of the image

how does run length encoding work?

replaces repeated information with the value and count

how do dictionary-based methods work?

places repeated data in a dictionary, using a key to recall it

ADC

converts analogue to digital

DAC

converts digital to analogue