Compression, encryption and hashing

File Handling

• File handling is one of the most important functions of a computer system.
• Different file formats exist to optimise data storage and interpretation.
• Each file format occupies a different amount of space, varied further by the content of the file.

Compressing Files

• Lossy compression
  • File size is decreased significantly, but quality is sacrificed.
  • The image cannot be reconstructed into its original form.
  • Removes any necessary information, stuff that is unlikely to be noticed by a human, but is stored in the file.
• Lossless compression
  • No change in the quality of the image.
  • File size is decreased, but less so than lossy.
  • The image can be reconstructed into its original form.
  • Useful if all information is vital, but storage or bandwidth is limited.
  • In a text file, you could created a dictionary, where you replace certain recurring strings in the file with a single symbol that references the string in the dictionary. This allows for reduced storage size.
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Uncompressed Image Files

• TIFF (.tif) and bitmap (.bmp) are examples of uncompressed image files.

Different types of image files

• PNG—Lossless - Used for transferring images over the internet
• JPEG—Lossy - Higher compression rate than a PNG. Used in digital cameras and webpages.
• GIF—Lossy - A sequence of frames that can be replayed to produce a short animation.

Compressing video files

• A typical HD video has a frame rate of 60 fps. The sampling rate of the video is given in frames per second.
• Video files also have a bit rate that defines the quality of video and audio.
• Compressing video files allows you to do more with less bandwidth when streaming videos.

Streaming audio files : MP3

• Compression is very helpful when streaming audio and video files.
• MP3 file format is used for audio compression.
• MP3 allows for up to 90% compression.
• Even though MP3 files cannot match original CD quality, they provide a satisfactory experience.
• Mp3 is lossy compression for an audio file.
• A user does not hear a difference between MP3 and CD because only a few parts of the file are removed.
• FLAC and ALAC are examples of open source lossless compression. Up to 50% size reduction with no noticable difference to quality.

Streaming video files : MP4

• MP4 files allow the storage of audio and video.
• Video files are compressed into MP4 format to stream online. This file is used for transmission over digital channels, cables and satellites.
• DVD movies are also available in this format, coming over from MP2.

Run-length encoding (RLE)

A form of lossless compression where repeated bits are stored together. Good if there is high repetition, but it can sometimes be inefficient.

In video files, RLE works between frames, allowing for unchanging colours to stay as they are without using additional bandwidth.

This can cause screen tearing when it isn’t working properly.

Huffman coding

• A compression technique used to reduce the number of bits that represent each letter.
• A binary tree is used to encode letters.
• A binary tree is a data structure made of nodes and is constructed based on a hierarchy. A parent node in a binary tree has up to 2 child nodes.

So to say AB in this tree, you would use: 1110

In Huffman coding, the more frequently used characters require less bits to represent.

Another, more complex tree, as an example:

How to build a binary tree

• We need the lowest number of bits for letters with a higher frequency
• So start with the least frequent characters, and work upwards.
• When two nodes are close together, create a root node above to link them together.
• When following a tree down, each possible path should terminate at a valid “leaf”.

Encryption

• The process of changing data into another form or code so that only people with access to a secret key can read it. For others, the message would be unreadable.

Caesar Cypher

• A very simple substitution cypher. Each letter of the alphabet is shifted by a certain number of characters. A shift left of 2, would make “abc” become “cde”.
• A shift of 5 on the message “INITIATE PLAN A” would become “NSNYFJ UQFS F.”
• Easy to decrypt, as it is vulnerable to brute force attacks and frequency analysis.
• Longer messages are easier to decode than shorter ones.

The Vernam cypher

• The Vernam cypher uses a one-time pad or an encryption key.

• Encryption key: its length is equal to the length of characters, or greater than that. They keys are only used once.

• Sender and receiver meet in person to exchange the keys and destroy them after the exchange of messages.

• The sole mathematically unbreakable code found so far: RSA, PGP etc can be broken given enough time and computing power.

• Keys can also be called salts

• Vernam cypher works with the ASCII codes of characters. Each ASCII code is taken in binary form.

• The one-time key is also taken in binary form.

• An XOR operation is performed between the key and the data to produce cypher text. An XOR operation is reversible, and therefore the same method can be used to decrypt the message.

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