Select a partner with whom you will work on this project. Only groups of two will be allowed, unless one group of three is necessary to see that each person in the class is in a group. As when Caesar cyphers were discussed in class, begin with the scheme
| Letter | Code |
| A | 0 |
| B | 1 |
| C | 2 |
| D | 3 |
| . . . | . . . |
| Z | 25 |
produces all 26 letters as substitutes for other letters. Submit to the professor (via email) the names of the people in the group and the choice of a and b.
Phase 2: Due Fri., Feb. 25
Find a passage of text to encode. This could well be a favorite passage from a book or from the Bible. The passage needs to be long enough to contain at least 500 letters, though it is all right if it is significantly longer. It would be best if it is a passage that is likely unfamiliar to most others in the class. Submit your encrypted message by typing it into this encryption webform along with other relevant information such as the name of a contact person in your group, the choice of a and b you are using for encryption, etc. When you submit the form, your passage will be stripped of punctuation and spaces and then encrypted using your scheme from above. Your message should now be one long string of letters (alphabetic) containing no meaningful spaces. (The form itself will take care of submitting the message to the professor, so you need not send it to me separately.) Your encrypted message will be forwarded on to another group in the class whose task it will be to decrypt it.
Phase 3: Due Mon., Feb. 28
This task is one you are to perform on your own (i.e., your partner must do it separately). You are to find four texts in electronic form (so you may cut and paste them into a browser), all of which contain 3000 characters (letters) or more. Seek these from non-obvious locations, so as to minimize the chance that you and someone else in MATH 100 Sections A or B might choose texts in common. (Any documents associated with our class website are off limits!) Your texts should all be in English, and exactly two of them (no more nor less) should have been written prior to 1900. If you cannot ascertain the date of a text, do not use it.
Submit, one at a time, your texts from this text analysis webform. The analysis will consist of counting in the text's first 3000 letters the number of occurrences of each letter from the alphabet. Along with allowing you to paste your texts into a form for analysis, the website will ask for your name; the date, author and title of the text; and the website (if applicable) where you found it. These character counts will serve us both as a guide in the decryption process, and as a means for learning and using some basic statistical concepts.
Phase 4: Due Mon., Mar. 28
Prior to this phase, you will receive a handout summarizing the results of letter usage in the texts you and your classmates submitted during Phase 3. As these texts were unencrypted, they serve as a sample for the relative frequencies that letters appear in all natural English texts.
Your group will be receiving an encrypted text from some other MATH 100 group. You will not be told how the text was encrypted (that is, the choices of a and b used in the process), but you are required to decrypt it. There are several programs available to assist you. The first is accessed through this webform for counting letter occurrences. It will help you determine, from highest to lowest frequencies, how the various letters appear in the text you are to decrypt.
The other webform accepts both your encrypted text and a list of letter substitutions you think will restore the text to its unencrypted form. It then runs a program to make those letter substitutions, showing you the results. If your approach to decryption is that of determining the values of a and b which were used in the affine mapping used for encryption, then you will find this webform more useful, as it allows you to enter a choice of a and b and then shows what the letter substitutions of the resulting encryption would be.
Your report on this final phase of the project should be typed up. It should include what you believe is the unencrypted text (with spaces and punctuation added by you as you see fit in order to make it more readable), the output from the first of the two webforms (in this phase) showing the frequencies of letters in the text's encrypted form, and the letter substitutions you made that ultimately led to the text's decryption. Along with this, you should explain how you arrived at the letter substitutions you used. Specifically, what was your first attempt at letter substitutions, and why did you choose it? Was the successful list of substitutions different than this first attempt? If so, explain the process that led from your initial guess of substitutions to the one that decrypted the text.
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This page maintained by:
Thomas L. Scofield
Department of Mathematics and Statistics
Calvin College