OCR A Level Computer Science (H446) - Component 1.3.1 - Compression, Encryption and Hashing

What are the key reasons for compressing data?

• Less space needed for storage

• Less time to transmit

• Higher quality files

• Reduced amount of processing power

• Limited RAM

Lossless compression...

• Is used when not all the information is needed

• Is not as effective as lossy compression

• Can fully regenerate the original file

• Includes files such as .ZIP

• Would be a sensible choice for a music download site

Lossy compression...

• Permanently removes data from a file

• Always leads to a noticeable loss of quality

• Aims to remove redundant data

• Can return the original file to its original quality

• Would be a sensible choice for a music download site.`

Dictionary based compression...

• Is a lossless compression format

• Creates a dictionary of common letter combinations to encode words with fewer characters

• Creates a dictionary of smaller letters so less space is used

• Sends the dictionary with the data

• Doesn't need to send the dictionary with the data

Run-length encoding...

• is Lossless

• is Lossy

• Can only be used for text

• Records the number of repeats of an individual element

• Can be used for a wide variety of file types

Encryption...

• Uses at least one key to cipher data

• Is only used on text

• Can be symmetric or asymmetric

• Is impossible to break

• Has developed a lot as processing power has increased

Symmetrical encryption techniques...

• Are the most secure types of encryption

• Use a single key to encode/decode

• Use multiple keys to encode/decode

• Include XOR, Caesar and Route

• Include RSA, Diffie-Hellman and DSS

Asymmetrical encryption techniques...

• Are the most secure types of encryption

• Use a single key to encode/decode

• Use multiple keys to encode/decode

• Include XOR, Caesar and Route

• Include RSA, Diffie-Hellman and DSS

Asymmetrical encryption...

• Often requires considerable processing power

• Is impossible to break

• Is being threatened by the possibility of quantum computing

• Can be brute-force attacked

• Is very widely used, even in small businesses