Cracking a Simple Cipher

Home ] Up ] [ Cracking a Simple Cipher ] Cracking Classic Ciphers ] Probability ] Team Research Project ]



If you're a fan of puzzles (or even if you're not), this should be a fun assignment!

The textbook for the course (Network Security: Private Communication in a Public World by Charlie Kaufman, Radia Perlman, Mike Speciner, 2nd edition) contains two ciphers:

  • On the page immediately following the title page, there is the following ciphertext:

    Si spy net work, big fedjaw iog link kyxogy

  • Page 44 contains the following ciphertext:

    Cf lqr'xs xsnyctm n eqxxqgsy iqul qf wdcp eqqh, erl lqrx qgt iqul!

These ciphers are simple substitution ciphers of the type that many people like to amuse themselves trying to solve.  Newspapers often publish a daily cryptopuzzle (along the lines of a daily crossword puzzle) which readers try to solve -- often during their daily commutes

The Assignment

The assignment is twofold:

  1. Read Edgar Allan Poe's The Gold Bug, and Sir Arthur Conan Doyle's The Adventure of the Dancing Men 

    Note: Those of you who are not native speakers of English may have some trouble reading parts of these stories, because they use archaic English words (I had to look several words up myself), and attempt to represent characters' accents and dialects (in particular, the freed slave in The Gold Bug has a strong Southern black accent).   You may have to ask a native English speaker to "translate" for you.   If you can't  understand something, please just send me an email and I'll try to make sense of it for you.  Remember, for the purposes of this assignment, not all aspects of the stories are germane.

  2. Solve the above two ciphers -- that is, determine the English cleartext from which the ciphertext was derived.

There are many resources you can use to accomplish the above, including (in no particular order):

  • Your own brain, with paper and pencil.  
    • In particular, think about what you know about each cipher, in its own context.  You can infer an amazing amount of information, based on intelligent guesswork, and knowledge of, say, the typical form you would expect for the plaintext version, based on what function it is performing, in its own context.
    • Also, you can look at the grouping of the letters and infer which words are likely to be verbs, or nouns, or whatever.  In the case of 2- or 3-letter words, you can probably make a few guesses, so see whether you can make further progress based on those guesses.  What word(s) might start the sentence or phrase, given its context?
    • Punctuation can often be a dead giveaway, and lead to interesting insights.
  • Collaboration with others, to share ideas, and divide up the work.  Don't forget to give credit where it's due!

Once you've tried the above (please try it first, and spend some time on it), you can resort to other methods:

  • Various deciphering tools and programs available from many sources, including the World-Wide Web.  This course web site has some Java applets which may help.
  • Various accounts of deciphering methods and techniques.  This also includes short stories such as ones referred to above.

What to Submit

I am more interested in how you solved the ciphers than the actual answers (which I already know).  Please submit a description of what you did, which should include:

  • Who you worked with, and specifically how you divided up the work of deciphering the ciphertexts (who did what;  how much of that work was done by whom)
  • What plan of attack you used.  This may actually be more than one plan of attack, if one or more plans failed to produce results.  I want to hear about all the unsuccessful attempts as well as the successful one(s) -- this is often more illuminating than just discussing the successful approaches.  (You learn a lot more from your mistakes than from your successes!)
  • What assumptions you made; what deductions you made, etc.
  • How long it took you to solve each of the puzzles

Edgar Allan Poe's The Gold Bug shows an example of an explanation of how one of the characters solved his cipher.  Have your explanation be of a similar nature, so that I can  understand how you went about solving the problems.


This page was last changed on February 18, 2007