Ch15-Compression, Encryption and Hashing

What is Encryption?

• It is to scramble the original input so that its meaning is hidden.

• To encode the original message in a way to reduce the data size (and the number of bytes it takes to store)

• To process the entire input message into a fixed length (bits) integer

• To verify the trustworthiness of the sender

What is Compression?

• It is to scramble the original input so that its meaning is hidden.

• To process the entire input message into a fixed length (bits) integer

• To encode the original message in a way to reduce the data size (and the number of bytes it takes to store)

• To discard less important information to save storage space

What is Hashing? (multiple)

• It is to scramble the original input so that its meaning is hidden.

• To process the entire input message into a fixed length (bits) integer

• Its output is the hash total, also known as checksum, or digest

• It is a one-way function, i.e. not able to work out the input from the output.

Why do Hashing? (multiple)

• To hide the meaning of the original message

• To use the hash to work out the original information

• To direct-access record in memory

• To validate information sent and received

Why do Encryption? (multiple)

• To hide the meaning of the original message

• To direct-access record in memory

• To reduce the storage space and transmission time

• To reduce the damage of interception

Why do Compression? (multiple)

• To hide the meaning of the original message

• To reduce the damage of interception

• To reduce the storage space need (file size)

• To reduce transmission time

Select all correct statements for LOSSY compression

• To discard the less important information to reduce file size

• The original file will be restored from the compressed file

• The data loss is permanent after compression.

• It cannot be used for text-based files

• The data is encoded in a way to reduce the file size, like using patterns.

Select all correct statements for LOSSLESS compression

• To discard the less important information to reduce file size

• The original file will be restored from the compressed file

• The data loss is permanent after compression.

• It cannot be used for text-based files

• The data is encoded in a way to reduce the file size, like using patterns.

Run Length Encoding (RLE) is (multiple)

• an example of lossless compression utilising patterns

• to use code for repeating patterns, instead of the patterns in the code, e.g. 1 for 1010101010, 2 for 1111111111.

• To use code for repeating data groups, e.g. 5 x 1000, rather than 1000 1000 1000 1000 1000

• an example of lossy compression which removes least important data

Dictionary-based compression is (multiple)

• an example of lossless compression utilising patterns

• to use code for repeating patterns, instead of the patterns in the code, e.g. 1 for 1010101010, 2 for 1111111111.

• To use code for repeating data groups, e.g. 5 x 1000, rather than 1000 1000 1000 1000 1000

• an example of lossy compression which removes least important data