1.3.1 Compression, Encryption and Hashing

Compression is the process of reducing the size of a file so that it takes up less storage space and so that it uses lower bandwidth in transmission, meaning that the file can be transferred more quickly over a network.

Lossy compression reduces the size of a file but also results in some data being permanently removed. The removed data is usually less important to human perception. It’s most commonly used on photos or audios (e.g. JPEG and MP3)

• It greatly reduces the file size, more so than lossless compression

• It results in a loss in quality, causing issues such as pixelated images, this may be especially noticeable on slower connections • Any data removed is removed permanently

This is a form of compression in which the file size is reduced without losing any information. Instead, data is rewritten into a more efficient format and the original file can be recovered from the compressed version. It’s most commonly used on programs, text files and images (e.g. PDF and GIF)

• Lossless compression cannot reduce a file’s size as much as lossy compression can, this generally means losslessly compressed data will be slower to load or transfer

• Lossless compression does not permanently remove data • After lossless compression has been used, the original data can be fully reconstructed • Quality is not lost • Lossless compression rewrites data in an efficient format

Encryption converts data into data that cannot be understood (called cipher text) using a key. The data can only be restored to its original form if its decryption key is utilised.

It’s a form of lossless compression, often used on bitmap images, in which repeated values are removed and replaced with one occurrence of the data followed by the number of times it should be repeated. It therefore only offers great reduction in file size if the file contains lots of repeated data. e.g. AAAAABBBBCCC = A5B4C3

It’s a form of lossless compression often used on text files. Frequently occurring data is matched to a unique index value. Any instances of this frequently occurring data is replaced with the corresponding unique code. A dictionary which contains the index and matching data must be stored alongside the compressed file, otherwise the compressed data cannot be used. In files which frequently contain repeated phrases this can greatly reduce the file size.

This form of encryption uses the same key for both encryption and decryption. Therefore, no keys need to be transmitted. This creates the issue that if the key is intercepted or duplicated then any communications be decrypted using this key.

Hashing is a one-way mathematical operation that produces a value from an input value. This is useful for data that needs to be verified but does not need to be known, such as passwords, PINs, or digital signatures. For instance, when a user enters their password for the first time, the hash function is applied to this input, this data is stored and the next time the user inputs their password it has the hash function applied and compared to the hashed password value that was previously stored.

This form of encryption uses 2 keys in a key pair: one for encryption and one for decryption. This form of encryption is more secure than symmetric encryption as it’s virtually impossible to derive one key from another. It can also be used to verify the identity of the sender, as if the sender encrypts a message using their private key and sends it out, it can only be decrypted using the other key in the pair, the publicly available public key.