Compression and Encryption

• Why is Caesar cipher not secure enough for wireless networks?

  Because it can be easily cracked by unintended users.
• What is a public key?

  Keys that are available to all users and are used to encrypt messages.
• What is a private key?


  Keys that are only available to the intended recipient and are used to decrypt messages.
• Where can a sender find public keys?

  Senders can find public keys in a directory.
• What is compression used for?

  To reduce the size of the files and reduce the download time of an image, audio and video from the Internet.
• What are the two types of compression?

  Lossless compression and lossy compression.
• What is lossless compression?

  When the file is compressed, the quality of the image remains the same and the file can be reconstructed to its original form.
• What is lossy compression?

  When a file is compressed, the unnecessary bits of information are removed permanently.
• What are the file formats for images?

  PNG, JPEG and GIF
• Which file format is used to store animated graphics?

  GIF
• What is an MPEG?


  MPEG (Moving Pictures Experts Group) is a standard to encode audio-video files that uses lossy compression technique.
• What is the file format for DVD movies?

  MPEG
• How is a video created?

  By playing a series of images at high speed.
• What attributes of a video file are affected due to compression?

  Compression reduces the resolution, dimensions and bit rate.
• What are artefacts?


  Compressing a video file may also lead to poor quality and random coloured blocks on the screen. These blocks are called artefacts.
• List a few lossy video file formats.

  MP4 and MOV
• What are codecs?


  Codecs are programs that encode or decode an audio, image or video file.
• What are compression codecs?

  Codecs are programs that encode or decode an audio, image or video file. Compression codecs reduce the size of a file without affecting its quality.
• What is run-length encoding?

  Run-length encoding (RLE) is a compression algorithm that converts the consecutive similar values into a code.
• True or False: Run-length encoding is a lossy compression technique.

  False. Run-length encoding is a lossless compression technique.
• What is MP3?


  A file format used for audio compression. This is a lossy compression technique.
• Name an uncompressed audio file format.

  WAV
• Name a few uncompressed image file formats.

  TIFF (.tif) and BMP (.bmp)
• Give an example of a substitution algorithm.

  Caesar cipher
• Give an example for an encryption algorithm that uses a one-type pad.

  Vernam cypher
• In Vernam cypher, what operation takes place between ASCII codes and one-time pad?

  XOR operation
• What is hashing function?

  A hashing function maps input of arbitrary length to a fixed-length or smaller output.
• What is a digital certificate?

  An electronic document that confirms the ownership of public keys.
• What is a digital signature?


  An electronic signature that authenticates documents and websites.
• What is hash total?


  A mathematical value calculated by applying hash function to original messages.