What is a cipher, and how is it different from a code?
A cipher changes every letter by a rule, so MEET becomes something unreadable. A code swaps whole words or sentences for other words, using a book both sides own. A cipher hides how a message is written. A code hides what it says. Children usually meet ciphers first, because a cipher can be worked out with a pencil.
What cipher should a 7 year old start with?
Pigpen. Every letter becomes the shape of the box it sits in, so there is no arithmetic and no alphabet to count along. It looks like magic on the page, which is most of the reason it sticks. After that, the Caesar shift, then the rail fence.
How do you teach codes and ciphers to children?
Let them encode a message of their own before decoding anybody else's, because writing it is where the rule sticks. Then swap and decode each other's, and let the arguments about who made the mistake happen: that is the lesson, not an interruption. Finish with a short puzzle so they end on a win.
What is frequency analysis?
Counting how often each letter appears. In English, E is far commoner than anything else, then T, A, O, I and N. A cipher that swaps one letter for another hides which letter is which, but it does nothing to hide how often each one turns up, so counting breaks it. Al-Kindi wrote this down in Baghdad in the ninth century.
What is the hardest cipher a child can actually use?
A one-time pad, and it cannot be broken at all. Both people need the same page of random numbers, each number is used once, and the page is destroyed afterwards. The DIANA table makes it usable without any arithmetic. Its weakness is not mathematics: it is that both people need the same pad.
Are these worksheets free to print for a classroom?
Yes. Every page on this site prints as a worksheet, there are five printable booklets covering 51 pages, and all of it is free to photocopy for a class, a club or a family. No account, no sign-up, nothing collected.
What can you make at home to teach ciphers?
A scytale from a pencil and a paper strip takes ten minutes. A Caesar wheel needs card, scissors and a split pin. A Cardano grille needs a craft knife and an adult. Invisible ink needs a lemon and a warm lamp. Each one on this site comes with a template printed at exact size.
How do you know if something is a real hidden message or a coincidence?
Fix the rule before you look, and then keep it. Take every seventh letter of any long text and words will appear, because there are hundreds of chances for a short word to turn up. If you are allowed to change the rule until something appears, you will always make something appear, and it will mean nothing.
Where to start with a child
Age seven or eight: pigpen, then the Caesar shift, then a scytale made from a pencil. Print the first booklet and do one sheet at a time.
Age nine or ten: the rail fence and the Polybius square, then the challenges. This is the age where breaking a cipher by counting letters lands properly.
Age eleven and up: Playfair, the one-time pad, and the code-breaking pages. Then the National Cipher Challenge in the autumn, or picoCTF from thirteen.
Start with pigpen The printable booklets For teachers