1.3.1 Compression, Encryption, Hashing

• Process used to Reduce file size to take up less storage space or for faster transfer over a network • increases the number of files that can be transferred in a given time • allows faster download • downloading a compressed file is faster than downloading the full version

• reduces file size while also removing some information • slightly reduces quality but significantly reduced file size [1] e.g images can tolerate some reduction in quality • reduces image quality e.g more pixelated images or less clear audio recording • suitable for image audio and video • not suitable for text documents as important info may be lost and text may become unreadable

• reduces file size without losing any information maintaining the original data [1] • the original file can be recovered/recreated when it is uncompressed • suitable for executable files and documents or vector style images, cartoons and logos • maintains data integrity

• run length encoding • dictionary encoding

• a method of lossless compression • Condenses identical elements to a single occurrence with its count next to it • used in bitmap images to condense sequences of the same colour • relies on consecutive pieces of data being the same so more effective when data has a lot of repetition • doesn’t offer a great reduction in file size if there is little repetition

• a method of lossless compression • that replaces frequently occurring pieces of data with a shorter, unique code/index • compressed data is stored alongside a dictionary • the dictionary matches frequently occurring data to an index/code • original data can be restored using the dictionary • this method is effective for both text binary data

• is used to keep data secure from unauthorised access when it’s being transmitted • is used to convert readable data into an unreadable format • uses keys which are specialised programs designed to scramble and unscramble data

• is a method of encryption where the same key is used for both encryption and decryption of data [1] • requires both parties to have a copy of the key [1] • the sender encrypts information using a private key before transmission • the receiver uses the same key to decrypt the data • if the key is intercepted, any communications can also be intercepted

• usually faster making it ideal for encrypting large amounts of data/ faster and efficient for bulk data • easier to implement as it uses a single key for both encryption and decryption/ same person encrypts and decrypts e.g when backing up • less resource incentive

• loss/interception of the key means the data encrypted with it can be compromised and intercepted • so requires key to be shared securely with the other party

• uses 2 keys, a public key for encryption and a private key for decryption [1] • public keys are shared openly allowing anyone to encrypt data [1] • the private key is kept securely/locally on the receivers side and is kept private [1] • use case: used when exchanging confidential data /secret communications .eg. Credit card details over the internet

• use of a separate private private key makes it more secure • if the public key is lost or stolen data won’t be compromised

• the process in which an input (e.g string of characters) is turned into a fixed sized value (called a hash) using a hash function • even a slight change in the input message produces a totally different hash value [1] • unlike encryption the hash function can’t be reversed to form the input message. [1]

• hashing is useful for storing passwords • a password entered by a user can be hashed and checked against the stored hash value to see if it’s correct [1] • only the hash values are stored, not the actual passwords itself so a successful hacker would only gain access to hash values which cant be reversed to gain the passwords [1]

• Hash tables are data structures that store key-value pairs using a hash function to compute an index for fast data retrieval. • They offer average-case O(1) time complexity for insertion, deletion, and lookup. • Collisions are handled using techniques like chaining or linear probing

• Compute the hash of the key, using a hash function • map it to an index, and place the key-value pair at that index.

• low chance of collisions [1] • quick to calculate (as lots of data needs to hashed) [1] • provides an output that is smaller than its input, so quicker to compare hashes than original data [1]

When two keys/inputs produce the same hash

• Chaining: items are stored together in a linked list under the hash value/index • linear probing : the algorithm checks the next sequential slots until an empty one is found.

• it is slower than symmetric encryption • uses more reasources • if the private key is lost, there is no way to decrypt the info