SUPER CHALLENGE:

Use the automatic "Super EnDec Simulator"
to encrypt and decrypt some top secret documents from World War II.

ACTIVITY

In English and other alphabetic languages certain letters appear with a greater frequency than others. Anyone who has watched Wheel of Fortune knows that the consonants r, s, and t are guessed before others, and vowel e is the most popular purchase. In English, the ranking of letters from most frequently occurring to least frequently occurring is:

E T O A N I R S H D L C W U M F Y G P B V K X Q J Z

The NetAdventure Super decoder operates on this principle. It assumes that text has been encoded with a simple random replacement scheme (in other words, each letter is replaced with another randomly selected letter, everywhere it appears in the text). The NetAdventure Super decoder finds the number of times each letter appears in the encoded text and ranks the letters according to their relative frequencies. Then it replaces the most common letter in the text with E, the most frequently occurring letter in English, the second most common letter in the text with T, and so on.

This decoding method would work perfectly if the encoded document had exactly the same relative letter frequencies as the English language as a whole. If the sample of encoded text is small, the chance that it would have exactly the same letter frequencies as the English language as a whole is not very good.

For example, if the encoded text is an article on the migratory habits of quail, the relative frequency of the letter that replaced q in the code will be higher than is typical for English. The decoder will then replace that letter with the letter of corresponding frequency in English, which will not be correct. The larger the encoded sample, however, the more likely for the frequencies to parallel those in English, and thus the more likely the decoder will decode correctly.

We wrote a program for NetAdventure called "Super EnDec Simulation," which encodes and decodes messages using the simple replacement code scheme we've been talking about. There are two parts to the EnDec Simulation: an Encoder mode and a Decoder mode. When the simulation starts the EnDec will be in the Encoder mode, which allows you to either load a sample message into the Encoder or type in a message of your own. By swapping letters around you can create your own replacement code - the resulting encoded message will appear in the lower window.

1) Encryption

The EnDec Simulation has three sample uncoded messages which you will need to encrypt to complete this challenge. Before attempting to do this, however, you may wish to use the EnDec to create your own encoded messages, so you can get a feel for the entire encryption / decryption process.

When you have encoded the three sample messages using the Decoder portion of the EnDec Simulator answer the following questions:

a) Enter the translation keys for the three encoded messages.

b) Explain any strategies you used.

c) Explain any problems you had with encryption.

2) Decryption

The EnDec Simulation has three sample coded messages which you will need to decode to complete this challenge. Before attempting to do this, however, you may wish to use the EnDec to create your own encoded messages, so you can get a feel for the entire encryption/decryption process.

When you have decoded the three sample messages using the Decoder portion of the EnDec Simulator answer the following questions:

a) Enter the translation keys for the three coded messages.

b) Explain any strategies you used.

c) Explain any problems you had with decryption.

Clue 1

Technical tip: to run this game you need the ShockWave plug-in. Click below and follow the directions.

http://sdc.shockwave.com/shockwave/download/ Get ShockWave plug-in.

Clue 2

When the EnDec starts running, you will be in Encoder mode. You can switch to the Decoder mode by pressing the Switch To Decoder button on the lower right side of the EnDec.

Load a sample coded message into the Decoder by pressing the "Load Code" button at the upper left. Press the "1", "2", or "3" button, followed by the "Done" button to load in a sample message.

Be patient - the Decoder needs to count how many times each letter appears in the coded message, so it might take a little while to load the code into the Decoder.

The coded message should appear in the top window. Above the two rows of letters you will see green "frequency" bars. The number of green bars show how often the corresponding letter in the top row appears in the coded text. You'll notice that the order of the top row of letters represents the frequency of each letter. The most frequent letter in the code will be the first letter in the top row. The least frequent letter will be the last letter.

Sometimes some of the letters will have the same frequency. The EnDec will rank the letters sharing the same frequency in alphabetic order. The bottom row of letters is the substitution code the Decoder will use. It is initally set to the general English language letter frequency we mentioned above. The bottom window displays the decoded message.

Clue 3:

Click the help button on the Super EnDec Simulator for more specific instructions on how to use the Simulator.