The History and Mathematics of Codes and Code Breaking

Month: September 2010 Page 1 of 2

Problem Set 4

Here's Problem Set 4 in Word and PDF formats. It's due at the beginning of class on Tuesday, October 5th.

Resources for September 30th - Breaking the Vigenere, More Movies

Wiretapping the Internet

Breaking the Vigenere

  • Below you'll find my Prezi from today illustrating the cryptanalysis of the Vigenere.
  • Here's the Excel file I used in class today: Vigenere Cryptanalysis (4 Letter).
  • And here's a version that uses five-letter keyword: Vigenere Cryptanalysis (5 Letter).
  • Between the two of these files, you have what you need to extrapolate and handle other keyword lengths. However, if you have any Excel questions, don't hesitate to ask.


As I mentioned today, Prezi is a great presentation tool, one I find myself using more and more frequently. I like the ability to arrange the ideas in my presentation in any configuration I like. PowerPoint only gives me one configuration: strictly linear. Prezi lets me go in different directions, group related ideas on the canvas, and zoom in and out to see things at different scales. You can sign up for a free account. (Use your .edu email address for bonus features.) The free account lets you design Prezis online, share your Prezis online, and download your Prezis. The paid version gives you an offline editor and more online storage space, but I've found the free version works well for my needs.

Cryptography in the Movies

  • Several of the movies featuring cryptography I mentioned today were listed in an earlier blog post. Movies mentioned today not in that earlier post were Rendezvous (1935) with William Powell (from The Thin Man) and Sebastian (1968).
  • Also, here's the trailer for Pi (1998) I showed today:

Facebook Page Working?

I'm trying out a new technique for importing these blog posts into our Facebook page. If this post shows up in Facebook, then I'm pretty sure the new technique works.

Reading Questions for September 30th

In preparation for class on Thursday, September 30th, please read the rest of the second chapter in the Simon Singh book (page 78 and following) and respond to the following questions.

  1. Singh notes that in the latter half of the 19th century, there was “an enormous growth of interest in ciphers among the general public.” (p. 79)  What factors do you think led to this growth?  Would you say there is interest in ciphers among the general public today?
  2. The Beale Ciphers have remained unbroken for over a hundred years.  Given that hundreds if not thousands of professional and amateur cryptanalysts have tried to break them without success, why do you think there are still people who attempt to break them?  What motivates people like that?

Resources from September 21st - Plagiarism, Babbage, Lovelace


Babbage and Lovelace

Image: "The Vigenere Cypher" by Sydney Padua

Essay #1 - Opinion Paper

Here's your assignment for the first essay and a rubric that should help you understand my expectations for the essay. I think these two files will tell you what you need to know about the assignment, but if you have any questions, feel free to ask.

Reading Questions for September 21st

In preparation for class on Tuesday, September 21st, please read pages 63 to 78 in the second chapter in the Simon Singh book and respond to the following questions.

  1. Prior to the work of Babbage and Kisiki, “most cryptanalysts had given up all hope of ever breaking the Vigenère cipher.”  Given that the Vigenère cipher was well-known, what might lead a cryptanalyst of that time to give up hope in cracking it?
  2. If the rows of the Vigenère square Singh uses (p. 48) were not shifts of the standard alphabet but were instead other arrangements of the standard alphabet (such as keyword cipher alphabets or keyword columnar cipher alphabet), how would that impact Babbage’s cryptanalysis technique?

Problem Set 3

Here's Problem Set 3 in Word (.doc) and PDF formats. It's due at the beginning of class on Tuesday, September 21st.

Resources from September 14th - Monoalphabetic Ciphers, Academic Integrity

First, here are my PowerPoint slides from today's class as a PDF. (I kept the black background this time, so you may use a lot of ink if you print them.) You'll find all the clicker questions today--the ones on frequency analysis graphs and on academic integrity--as well as the two slides I included on the Argenti family's work as papal cipher secretaries.

I shared a few of the "ethical or not?" clicker questions on Twitter and invited folks there to respond with their opinions. Here are some of the results:

  • "The student next to you drops his test and you accidentally see the answers. This leads you to change one of your answers." You all said this was unethical (69 to 31). My "tweeps" agreed, 3-to-1.
  • "Suppose that one of your spring semester professors assigns a 5-page paper on a topic identical to one assigned by one of your fall semester professors. You received an A- on the fall paper, so you turn the same paper in to your spring professor." You all were split (53% ethical, 47% unethical). My tweeps were not; they said it was unethical 4-to-1.
  • "You find a copy of the instructor’s solutions manual to one of your textbooks online. You use it to check your homework before turning your homework in." You all said unethical by a big margin (87% to 13%). My tweeps were split; one said ethical, one said unethical, one said it was a gray area.

Regarding that second scenario (reusing a fall paper in a spring course), I pointed out the section in the Vanderbilt Honor Code that explicitly forbids this without prior authorization from both instructors. However, we didn't talk about why this would be considered a violation of academic integrity. My Twitter friends had some useful things to say about this...

  • @jbj said: In a university setting, the idea is that you get your credits for doing work. You're essentially stealing credits.
  • @jbj continued: & so (for your student), it's not necessarily unethical in some abstract, contemplative sense, but mostly in that univ. context.
  • @derekbruff (that's me) replied: Which is one of the points I was making today: There are sometimes complex relationships btw individual and group ethics.
  • @TSindelar said: I've run into this issue a lot with students, the school's academic honesty policy specifically addresses it but Ss dont get why
  • @TSindelar continued: likely it connects to underlying issues about why students are in school and what credit means...
  • @derekbruff (me again) said: The "self-plagiarism" issue does seem to imply that course credit = amount of work completed. Which is better than = time w/butt in seat.
  • @derekbruff continued: But it's a long way away from course credit = evidence of mastery of particular learning goals. Which would permit evidence to be reused.
  • @idbeatty said: Q2 is unethical 'cuz the point was for the student to LEARN something by engaging in the PROCESS (now bypassed).
  • @idbeatty continued: How about course credit = evidence of differential increase in student's knowledge/skills? => Reused evidence doesn't show gain.

My take on the question is pretty much the same as @jbj's take in the first couple of tweets above. Take this to the extreme: What if you could use the same paper for every single course you took at Vanderbilt? Should you get credit for all those courses? If so, might people dramatically misinterpret your transcript?

Receiving credit for two courses is likely to be interpreted as you having learned (roughly) twice as much as you would have learned just taking a single course. But if you didn't learn nearly as much in the second course (because you recycled work from the first course), that interpretation would be wrong.

Questions or further comments about this topic?

Image: "Victory!" by Flickr user Xjs-Khaos, Creative Commons licensed.

A Better Frequency Analysis Excel File

One of your classmates, John H., noticed that I had hard-coded the length of the ciphertext into the frequency analysis Excel file I shared with you on Thursday. He's fixed the file to calculate the length of the ciphertext automatically while computing frequencies. You'll want to use the revised version of the file as you work on your homework.

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