If you buy something using links in our stories, we may earn a commission. This helps support our journalism. Learn more. Please also consider subscribing to WIRED
A few months ago, GeekDad's Challenge of the Week was a lovely cipher about famous novels beginnings.
I enjoyed solving it and that reminded me of a time where I was teaching younger students (12 to 15 years old) and had once used ciphers as a way to teach languages (French, in my case).
It is quite obvious (at least for our geek readers) that one can use ciphers to teach math.
But simple shift codes such as Caesar Shift Cipher used in the GeekDad Challenge are about letters and words, and the way we had to decipher them before computer programs depended of our knowledge of language.
This activity was once designed for 12 years old students. I'm sure younger children can manage it, too. Or older children, if you increase the difficulty by choosing another language than their mothertongue.
The cipher was in French in my first version, I changed the examples to pick them in GeekDad challenge cipher.
Here was the challenge :
The first paragraph gave the key of the code ("The word “words” appears 6 times in the paragraph, 6 is the key to decrypt these clues using the good old Caesar Shift Cipher.")
But what if you don't have the key (which is the case in most spies stories) ?
You'll still be able to decipher the message. We'll mainly use the method known as frequency analysis. You can decipher all substitution ciphers using that method, not only simple shift ones like Caesar's. That will only take longer but will be more interesting, too, as you will need more than one confirmed hypothesis to decipher the text.
1. Look for single-letter words. Now, think about frequent single-letter words in the language you work with.
By example, in GeekDad challenge, you find G as one-letter word.
Most frequent single-letter words in English obviously are "a" (the indefinite article) and "I" (the first person singular nominative pronoun). You'll have to try both.
2. Do the same thing with two-letters and three-letters words. Be smart, remember to connect your results between them, remember to consider the place of the words in the sentence. It may confirm or infirm your 1# hypothesis.
By example, you will notice the frequency of OZ at the beginning of the sentences. Ask your geeklings to wonder: which two-letters words are more likely to be found at the beginning of sentences ? (In this case, the word happens to be "it".)
If your #1 hypothesis was that G stood for "a", you will notice two frequent three-words letters with a G : CGY and GTJ. Ask your geeklings to find frequent three-letters words like ?a? and a??. (In this case, the words are "was" and "and".)
In French, you may observe the single letters just before an apostrophe, likely to be "l" or "j" or "d".
You may use Word Frequency files as an help for your geekling. Such lists are usually boring, but here they will become wonderfully useful, a real spy's tool.
- For most languages and from various sources
- For American English, but they offer a Spanish and Portuguese version, too
3. Work with the letter frequencies in your encrypted message. The counting would be quite tedious and not really interesting so you can use this program. This other tool will also give you two-letters sequences, three-letters sequences…
Now you can show your geekling the letter frequencies in the language (s)he's working with and let him/her try some hypothesis.
4. Be smart, remember ? When you have to decrypt a cipher, you sometimes have some clue of what it may be about. Use it to work faster.
With GeekDad Challenge, once you understood the sentence are famous novels beginnings, you could deduce some words without actually needing to decrypt them.
Now feel free to design some lovely ciphers of various difficulties (and in various languages) with your kids !
A few tools you might need or enjoy :
Need to change something from lowercase to uppercase? Count the letters, numbers, and punctuation? Remove spaces or add spaces at every X characters? This can help. Just type some text into the box and click on the links to change things around.
Lazy ? Want a program to encrypt a message for you ? This one even offers the possibility to use Sherlock Holmes' Dancing Men or Bionicle alphabet to encrypt a message (as well as more standard substitution ciphers, of course).
Here's another famous beginning (in English, I promise) using Dancing Men alphabet. Have fun decrypting it.
- If you and your kids are more into paper crafts than computer programs, you may design your own cipher disks with NSA insignia !
You may directly download the templates and instructions.
One last thing : as GeekDad did, use books sentences as encrypted messages. Books you love and you'd like to share with your kids. They're probably be teased by these mysterious messages.
And if they really love ciphers, you may propose them further literary readings like :
- Edgar Allan Poe, "The Gold-Bug"
- Arthur Conan Doyle, "The Dancing Men"
- and of course Neal Stephenson, Cryptonomicon, the adventures of some World War II codebreakers and their modern day progeny.
Two other books I haven't read but potentially of interest :
- Ken Follett, The Key to Rebecca (1980), World War II spy novel whose plot revolves around the heroes' efforts to cryptanalyze a book cipher with time running out.
- Clifford B. Hicks, Alvin's Secret Code (1963), a children's novel which introduces some basics of cryptography and cryptanalysis (recommended for age 9-12). About this last one, you will enjoy this very geek dad/geek son review on Amazon by Dave Hicks :
