Compression, Encryption, Hashing

Purpose of compression

• Reduce the size of files

• Reduce download times

• Reduce storage requirements

• Make best use of bandwidth

Lossy compression

• Unnoticeable by naked eye

• Removes data

Lossless compression

• Can be fully restored to original

• Good for images with large areas of one colour

Lossless - Dictionary encoding

• Suitable for text text documents

• Builds an index, visualised as a table, for data entries

• List of references to the index replaces data

Lossless - Run Length Encoding

• Suitable for images

• Stores colours in an image as frequency/data pairs

Encryption

Encoding a message so that only the sender and intended recipient can read it

Caesar cypher

Encodes a message by replacing letters with letters a fixed distance away in the alphabet

Symmetric encryption

• Both sender and recipient have the same key

• Same key is used to encrypt and decrypt the message

• Possibility of intercepting or duplicating the key

Asymmetric encryption

• Two separate keys

• One key to encrypt

• One key to decrypt

• Works both ways

• Public key allows people to send you messages that only you can decrypt with your private key

Hashing

• Translates a string into a fixed length value

• One way process, cannot get the original string

• Data stored with hashing function, compares if new entry is the same by applying hashing algorithm and comparing output