SLR09 - Compression encryption & hashing

Purpose of compression

• ↓ size file

• ↓ download time

• ↓ storage requirements

• make best use of bandwidth

Lossy compression

• Some data lost

• Slightly ↓ quality

• Significantly ↓ file size

• Suitable for: image, audio, video

Lossless compression

• None orig data lost

• Orig file can be recreated when it is uncompressed

• Suitable for: executable files & documents

Methods of lossless compression

• Dictionary coding: text based documents

• Run-length coding: images

Dictionary coding

• Builds index using every data item/entry in file 

• File is now made up of:

  • Dictionary index

  • Sequence of occurrences needed to recreate orig file

Run length coding

• ideal for bitmap images

• There may be continuous pixels of the same colour 

• So instead of storing each one individually, information stored in frequency/data pairs:

  • 22 0: 22 continuous white pixels

  • 6 1: 6 continuous black pixels

Explain why compression is important for the streaming of high resolution films

Symmetric encryption

• Uses a single key during encryption & decryption

• Both parties know the key

How can a symmetrically encrypted message be cracked?

• Interception of key

• Duplication of key production process to acquire a copy of the key

Asymmetric encryption

• Each party has a pair of keys

• Message encrypted by one key, decrypted by the other

• One key is public - anyone can access, the other is private

How sending a file using asymmetric encryption works

• Sender & recipient exchange public keys

• Sender uses recipients public key to encrypt message

• Recipient uses private key to decrypt

How to confirm that a message is authen

• Encrypt message with private key

• Anyone can access public key & decrypt - confirmed authentic

Sending direct message

• Send using:

  • Own private key

  • Recipients public key

• Decrypted using:

  • Their private key

  • Own public key

Hashing function

• Transforms string of characters into fixed length value or key

• One way

How hashing function used to check password

• Apply function on inputted password

• Compare hashed vale to stored value

How can hashing be used for quick data storage & retrival

• When storing, apply, function on string being stored

• Store the string in the address corresponding to the value generated

• When retrieving, run function again and retrieve value held in the given value - remove need to search