This is a guide for students attempting encryption (one of the three encoding subtopics) in digital technologies achievement standard 2.44 (AS91371)

This is an achieved level guide.

Remember that you only need to do one of the three encoding topics (compression, encryption, and error control coding) to the excellence level. If you are either not interested in getting more than achieved, or are doing either compression or error control coding to the excellence level, then this is the right guide for you. If you were wanting to do encryption up to the excellence level, then you should select the RSA cipher assessment guide instead. Note that there is no excellence level assessment guide for caesar cipher - while it is great for the achieved criteria, it is unsuitable for merit/ excellence due to not being used in the real world.

In order to fully cover the standard, you will also need to have done projects covering the topics of compression and error control coding to at least the achieved level (with one of them to the excellence level if you are attempting to get more than achieved), and projects covering the topics of representing data using bits and human computer interaction, and include these in your report.

Encoding has the following achieved bullet points in achievement standard 2.44 which this guide covers.

Achieved: “describing the concept of encoding information using compression coding, error control coding, and encryption; and typical uses of encoded information”

As with all externally assessed reports, you should base your explanations around personalised examples.

You should read and work through the interactives in the following sections of the CS Field Guide in order to prepare yourself for the assessed project.

Read all of these sections, as they give the necessary introduction of the topics

What’s the Big Picture?

Substitution Ciphers

If you are really keen, you might like to read further into the problems with substitution ciphers, although note that this is optional because it is not necessary for the project in this guide.

Problems with Substitution Ciphers

Start this section by writing an introduction to the topic of encryption. Briefly explain what encryption is, what it is used for, and what kinds of problems would exist if there was no such thing as encryption. This introduction only needs to be a few sentences - you are just showing the marker that you understand the bigger picture of what encryption is, and some of the typical uses of it.

Now you are going to make an example of encryption being used to encrypt a sentence. For this example, you should use Caesar Cipher. Despite it not having being used in practice for a very long time, it is ideal for illustrating the basic ideas of encryption. Your example should consist of the following components, all of which should be included in your report.

  1. Start by writing a short sentence (a few words will be enough, and keep it appropriate for your report). You will be encrypting this sentence.
  2. Choose a number between 1 and 25 that will be your encryption key.
  3. Make a conversion table that shows how each letter in your original sentence should be changed using your encryption key (format the conversion table to 3 columns in your report to make it compact and easy to read)
  4. Encrypt your original sentence using the conversion table you have generated.

Briefly describe what you have done throughout the various parts of your example, and be sure your descriptions include the terms "plain text", "key", and "cipher text" in the relevant places.

Briefly describe what information a classmate would need to decrypt your message (assuming they decrypt it the "proper" way, as opposed to trying to break the cipher).

  • Put encryption in its own section (your report should have suitable headings and subheadings for each topic to make it clear for the marker) and ensure that you briefly introduce the topic. It is important that your report clearly demonstrates that you know the difference between encryption, error control coding, and compression, and what their different purposes are.
  • Display the conversion table in a way that doesn't waste space. Format it to 3 columns, if not 4.
  • Overall, this project should take up to 1 page, possibly 1 ½ pages of your report.