Cracking Classic Ciphers

Shift Substitution Ciphers

the following represents a Caesar cipher with a shift of 3:

Plaintext: ABCDEFGHIJKLMNOPQRSTUVWXYZ 

Ciphertext: XYZABCDEFGHIJKLMNOPQRSTUVW 

(Notice that we are doing a circular shift, by wrapping the end of the alphabet around to the beginning.)

using this substitution table, we can take the message:

 Once more unto the breach, dear friends 

and encipher into the following:

 Lkzb jlob rkql qeb yobxze, abxo cofbkap 

Of course, to decipher the text, we simply reverse the process -- or equivalently, use the negative of the original shift value.

Question 1: Here's the ciphertext for a message enciphered in the same way as above (this is a shift substitution cipher): Kyzj vetipgkzfe dvkyfu nrj ljvu sp Ifdrej ze Tvrjri'j nrij.

What is the plaintext for this message? (This should be really easy! -- you can solve it manually, or use one of the Java Tools, e.g, http://pages.central.edu/emp/LintonT/classes/spring01/cryptography/java/Shift.html )

Question 2: Given the approach described above, for a Shift Substitution Cipher, how many possibilities are there for a shift value? Is this a feasible task? You can try this Java applet (http://pages.central.edu/emp/LintonT/classes/spring01/cryptography/java/Shift.html) [1] that implements this, if you'd like.

Analyze the letter frequency of the ciphertext, and try to deduce the shift value. MonoAlphabetic Substitution Ciphers & Frequency Analysis: Here's an example of a MonoAlphabetic Substitution Cipher. Here's a letter substitution table for such a cipher:

ABCDEFGHIJKLMNOPQRSTUVWXYZ OEXPVYQNAZGCDKURHJLBIFMSWT where the top row represents the plaintext letters, and the bottom row represents the corresponding ciphertext letters.

Question 3: Given the approach described above, for a MonoAlphabetic Substitution cipher, how many possibilities are there for character mappings? Is this a feasible task?

So, here's what you need to do for this assignment:

Answer questions 1, 2, and 3 (see above). Explore the Java Tools and report on your findings:

First, I recommend that you download the code for the Java Tools, Security.jar file, so you can run them locally on your own machine.

Bring up the Java Tools:

and select "MonoAlphabetic Substitution Cipher".

This will bring up a window which provides you with a lot of tools for cracking a monoalphabetic substitution cipher.

Play with this tool for a while, familiarizing yourself with its capabilities.

Try cutting and pasting some (fairly long) samples of English text into the Input Text area, and seeing how closely the letter frequencies match (or don't match) the typical English frequencies.

Try using the Autogenerate feature to see what happens to your text when you translate (encipher) it.

Optional Question 4: What would you expect to happen in this case?

Try really encrypting some text, and then see how successful you are at decrypting it, using the tool and its features (as opposed to you doing it -- you already know what the plaintext is!)

Note: The Input Text area is editable; that is, you can enter text into it, edit that text, paste into it, etc. On the other hand, the Output Text area is not editable; you cannot enter text into it directly. However, you can select text from the Output Text area and copy from that selection. You can use the standard Ctrl/A and Ctrl/C key combinations to select the entire contents of the area, and copy those contents into the paste buffer, respectively.

Next, download the ciphertext Ciphertext for HW2:

In this ciphertext, I have retained the original word spacing, punctuation, etc.

Note: In Internet Explorer, you should right-click on these links, and choose "Save Target As...". This way, you prevent Internet Explorer from trying to put HTML tags around the files when you save them.

Bring this ciphertext up in an editor (a plain text editor, not something like Microsoft Word)

Copy all of the ciphertext from your editor, and paste it into the Input Text area of the Java tool.

You can now use the Java tool to determine letter frequencies, and come up with a first approximation of the substitution table, based on those frequencies. Also, try: http://www.richkni.co.uk/php/crypta/freq.php

Translate the text, based on the auto-generated table, and see if you can see anything that resembles recognizable plaintext.

Using the tool, change individual letter substitutions until you believe you have the complete plaintext.

Hint: You will probably find that you will have to look at the generated "plaintext" and make educated guesses at what the words might be, using clues like partially-formed words, punctuation, and a knowledge of the common two-letter and three-letter words that exist in the English language. If you get stuck, talk to someone else -- your wife, husband, girlfriend, boyfriend, parents, friends, etc. It is often the case that when you get stuck, just interacting with someone can lead you to overcome the problem, even if the other person doesn't actually come up with the solution.

Note: Just tell me where the text comes from, who the author is, etc.

Copy the plaintext from your Output Text area (Ctrl/A, followed by Ctrl/C), and paste it back into a new document in your editor.

Print the result from your editor, or copy and paste it into your Homework-2 assignment report.

What to Submit in your individual Homework-2 Report:

First, refresh your knowledge of what I expect in an assignment report (and the format of that report).

I want to see the following:

Answers to all the questions 1-2-3 I pose, above.

A description of what you did.

The results of your deciphering of the Ciphertext for HW2.

While it would be nice to see you come up with a correctly deciphered result in each case, I am much more interested in a detailed description of what you did, what you tried, what worked, and (especially) what didn't work, what your thought processes were, your frustrations, any suggestions you might have for improvements to the tool, and so on. How hard was it to solve these ciphers? How much work did you have to contribute, compared to what the tool contributed?