When a hard disk drive is new – files get added onto the disk in order – very much like starting with a blank piece of paper and adding to it.

As files are deleted – this leaves gaps.

When new files are saved – the files fill the gaps and become fragmented.

Defragmentation software groups fragmented files back together.

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Reducing the size of the file by performing an algorithm on the original data

• Lossless

  • The original file can be re-created as no data is lost

• Lossy

  • Some of the original data is lost and the original file can not be re-created

Encryption means to scramble data in a way that it is unreadable to anybody who doesn’t have a key to be able to unscramble this.

Data is encrypted using a key and then decrypted using a key.

Data is encrypted so that it is unreadable if anyone is to intercept it.

You must understand what encryption means, and be able to work out an encrypted message used the Caesar cipher.

For a challenge, click on the image on the right to learn about the Vernam Cipher, a much more complex cipher.