Web Technologies (Compression/Encryption)

compression

• reduce size of files as much as possible

• reduce download times

• reduce storage requirements

• make best use of bandwidth

• quicker transfer time

lossy compression

• reduces file size by storing a lower number of colours, or storing larger areas of pixels as a single colour

• some data is lost when the file is compressed

• slightly reduces quality, but significantly reduces file size

• suitable for images, audio, video

lossless compression

• instead of storing every pixel with the same binary pattern, it stores the binary for white followed by the number of pixels in a row

• none of the original data is lost

• the original file can be recreated when it is uncompressed

• suitable for executable files and documents

dictionary coding

• ideal for compressing text-based documents

• builds an index that we visualise as a table

• every data item or entry in the file is recorded, along with an indexed reference or unique code

run-length coding

• ideal for compressing bitmap images

• with two colours, we can encode the image with a colour depth of 1 bit — 0 for white, 1 for black

encryption

• the process of encoding data so it can be read only by the sender and the intended recipient

symmetric encryption

• a single key is used to both encrypt and decrypt a message

• both parties need to know the key

• the same key can be used multiple times

asymmetric encryption

• keys are generated so anything encrypted with one of the keys can be decrypted with the other

• these form a key pair

• one public key, one private key