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Compression, Encryption and hashing

Define 'Sampling'

At a given time point in a sound, a measurement is taken

Give the formula for the size of the sampled resolution

Sampling rate(no of bits) x sample resolution x length of the sound(seconds)

Define 'Compression' and give the two types of compression

The process of reducing a file's size.
Lossy compression

Lossless compression

Give two benefits of compression

- Reduces file size
- Files are quicker to be transferred

What does 'Lossy compression' and name files where it is used on

Reduces file size by permanently removing data from the file.
JPEG

MP3

MPEG-1

MPEG-2

What does 'Lossless comprssion' do and give examples of where it is used

Reduces the size of the file however doesn't remove data from the file.
PNG

ZIP

GIF

Give 2 ways of reducing file size of an image

- Store a lower number of colours
- Store a larger area of pixels as a single colour

Name 2 types of lossless compression

Dictionary coding
Run-length encoding

What does 'Dictionary coding' do

Replaces recurring sequences with shorter unique codes

Where would dictionary coding be useful and where would it not be useful?

Useful: Large text documents with repeating data
Not Useful: text documents with no repeating data, takes up more space

What does 'Run-Length encoding ' do

Condenses identical data into single values with a count

Where would you use run-length encoding?

In bitmap images. Where you have contiguous pixels of the same colour so you store the binary for one of the pixels and multiply it by the number of pixels with that colour

What is the purpose of encryption?

To make the original code impossible to crack without the key

What is the difference between Cipher and Plain text

Plain text - text that is reabdable by anyone
Cipher text - text that can only be read by someone with a decryption key

What is 'Symmetric encryption' and give some drawbacks of it

Where a single key is used to both encrypt and decrypt a message.
- Messages can be cracked by someone intercepting the key

- Messages can be crakced by someone duplicating the production of the key to get a copy

What is 'Asymmetric Encryption'

Where the key used to encrypt the message (public key) is different to the key needed to decrypt (private key) the message (key pair)

Give benefits of 'Asymmetric encryption'

- The sender and recipient are sure no one else can read their message
- You're sure the message hasn't been modified

- You can be sure the message is authentic

How does asymmetric encryption work

The sender gives the recipient their public key. The recipient uses the public key to encrypt a message and send back to the sender. The sender then uses their private key to decrypt the message

Where would you use symmetric and asymmetric encryption

Symmetric: On large files and databases
Asymmetric: Confidential data

Define 'Hashing'

Transforms a string into a fixed length value or a key that represents the input string

Give 2 common hashing algorithms

MDA
SHA-1

SHA-256

SHA-3

Define a 'hash table'

A data structure used to remove & insert key value pairs

What makes hashing a different process to encryption

Hashing is a one-way process, meaning you can't get the original value from the hashed value even if you have the hashing algorithm

Why do hash tables need to be large in size

In order to reduce the chance of a collision occurring

Define a "Collision"

Where two different inputs produce the same hashed value

Give 3 features of a good hashing function

- Be calculated quickly
- Result in as little collisions as possible

- Use as little memory as possible

Define "Open Addressing"

A process which searches for the next available empty space in a hash table and stores the item there.

Define "Linear probing"

Searching for an empty spot by checking the next slot sequentially

Give 2 disadvantages of "linear probing"

- It prevents certain items being stroed in their correct location
- Can cause clustering

Define "Clustering"

Where many inputs have the same hashed value or similar hashed values causing large blocks of occupied slots, which increases search time (reduces efficiency)

Give another way of handling collisions

2D hash tables, this can allow more than one item to be placed in the same index (chaining)

Give a 3rd way of handling collisions

Overflow tables, a second table used to handle collisions when the first table is either full or items can't fit in their expected location

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