A Level Computer Science Paper 1: 1.3.1 - Compression, Encryption, Hashing

• Reduces download time

• Uses less bandwidth

• Takes up less storage

• Takes time to compress

- Uses processor time

Where compression removes non-essential data. Compresses to a smaller size

Compresses pixels in an image by removing colours the human eye cannot distinguish between

Removes audio frequencies the human ear can’t detect

Only records changes in differences between picture frames of video rather than each entire frame

Patterns of data are summarised and recorded in a shorter format without removing any data
The original file can be recovered

Summarises data by saying the data and how many times repeated data is repeated

Spots patterns of data that are repeated and stores them in a dictionary, stores in the main file
Any occurence of the repeated data will be replaced by its index

Identifies repeated file content and replaces every occurence with a repeated code

The ciphertext is only decoded when it reaches the device of the recipient

A way of making sure data cannot be understood if you don’t possess the means to decrypt it.
The message is encrypted using a cipher algorithm and key and decrypted when received

The text produced by the cipher algorithm and key

The text produced when the ciphertext is decoded

A cipher in which each letter of the alphabet is shifted by the same amount

A random sequence equal than or greater in length than the plaintext is used once.
Works by using a XOR between the ASCII codes of the encrypted characters and the characters of the one-time pad.

The encryption key (one-time pad) is shared with the recipient by hand and is destroyed immediately.
The one-time pad is generated from physical and unpredictable phenomena.

Also known as private key encryption

The same key is used to encrypt and decrypt data so it must be shared with the recipient

The recipient’s public key is used to send data securely

Only the recipient’s private key can be used to decrypt the ciphertext

Uses a hashing algorithm to map an input to a shorter length of hex characters
You cannot convert the encrypted message back to the original

Storing PINS/passwords securely

Adding random data to the input to change the hashing output

The process to reduce the storage space required by a file