Compression, Encryption, and Hashing

• Compression:

  • Reduce file size
  • Reduce download time
  • Reduce storage requirements
  • Makes best use of bandwidth

  Two methods: Lossy and Lossless

  The human eye cannot tell when an image has lost a little bit of detail so lossy is fine

  Lossless is good for images with large areas of the same colour such as cartoons but not suited for full-colour photography
• Lossy - best for images, audio, video

  Lossless - best for executables and text documents
• Two types of lossless compression:

  • Dictionary coding (text)
  • Run-length encoding (images)
• Dictionary coding builds an index we can visualise as a table.

  Each data item in the file is recorded in the table along with a unique code

  There is then a sequence of occurrences which tells you in what order items appear
• Run-length encoding that is useful on images with a lot of the same colour in a row

  We store these as sequences instead of storing each individual binary code for each pixel

  So instead of storing 000000010000

  (0 = white, 1 = black)

  we can store 7W 1B 4W
• Two types of encryption:

  • Symmetric encryption
  • Asymmetric encryption
• In symmetric encryption, a single key is used to encrypt and decrypt a message.

  This means both parties need to know the key and keep it secret

  An alternate can be generated when a new message is sent

  This is vulnerable to:

  • Interception of the key
  • Duplication of the key production process
• Asymmetric encryption is much more popular as it is virtually impossible to break

  Each person has a key pair, a public key which is available to the public, and a private key, which is only held by the person themselves

  If you were to send someone a message, you would encrypt it using their public key, send it, and then they would decrypt it using their private key
• Hashing is a function that transforms a string of characters into a fixed-length value or key that represents the original input

  You can never get the original message back after hashing, even with the algorithm

  Ideal for storing passwords so they cannot be stolen

  Also used for quick searching, insertion, and deletion of data from a data structure