1.3 Compression, Encryption, Hashing

• 2 different methods of compression:

  • Lossy
  • Lossless
• Purpose of compression:

  • Reduce size of files
  • Reduce download times
  • Reduce storage requirements
  • Make best use of bandwith
• Lossy compression:

  • Some data is permanently removed
  • Reduces quality - significantly reduces file size
  • Suitable for images, audio and video
  • Not able to recreate original file
  • Compression ratio may be adjusted depending on bandwith
• Lossless compression:

  • None of the original data is lost - rewrites original data in more efficient format
  • The original file can be recreated when uncompressed
  • Suitable for executable files and documents
• The 2 different methods of lossless compression:

  • Dictionary coding
  • Run length encoding
• • Dictionary encoding is ideal for compression of text-based documents
  • Run-length encoding is more suited to the compression of images
• Dictionary coding:

  • Works by building an index that we visualize as a table
  • Every item of entry is encoded and is indexed
  • The compressed file consists of: The dictionary index, and the sequence of occurrences

  
• In run-length encoding:

  • The information is stored in what we call frequency/data pairs
  • See the example attached

  
• Encryption:

  • The process of encoding/scrambling a message
  • So it can be read only be the sender and the intended recipient
• Symmetric encryption:

  • Where a single key is used to both encrypt and decrypt a message
  • Both parties need to know the key.
• Disadvantages of symmetric encryption:

  • Interception of the key
  • Duplication of the key production process to acquire a copy of the key
• Asymmetric encryption:

  • Where 2 different keys are used - Public and Private key
  • Public key is used to encrypt the message
  • The other key is used to decrypt the message
• Asymmetric encryption:

  • Encryption with Public Key: User A uses User B's public key to encrypt the message
  • Decryption with Private Key: User B uses their private key to decrypt the message
  • Authentication with Private Key: User A signs the message with their private key
  • Verification with Public Key: User B verifies the signature using User A's public key
• Hashing is different from encryption:

  • As it a one-way process
  • You cannot get back to the original value even from the hashing algorithm
• Hashing:

  • Prevents information like passwords from being read by hacker
  • one-way transformation of data
  • The details are stored by using an hashing function
  • If the hacker gains access to the file contain the passwords - they will only be able to read the hashing values
• On average hashing:

  • Provides a constant O(1) time complexity
• Advantages of asymmetric encryption:

  • No one else will be able to read the message
  • The message is authentic
  • Can be sure the message hasn't been modified
• Characteristics of a good hashing algorithm:

  • Low chance of collision to reduce risk of different files being marked as the same
  • Quick to calculate
  • Provide a smaller output then input so quicker to compare hashes than original data
• Symmetric Encryption

  • both the sender and the receiver share the same private key which they distribute to eachother in a key exchange
  • key is used for both encrypting and decrypting data
  • private key is kept secret. if key is intercepted during key exchange, then any communications sent can be decrypted
• Asymmetric Encryption

  • 2 keys used: one public and one private
  • public key can be published anywhere, private kye must be kept secret. together known as a key pair and mathematically related
  • single key cant be used, messages encrypted with recipient's public key can only be decrypted with recipient's private key
  • if someone wants to send a message, they must find your public key and then message is encrypted with your public key