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.
Which cipher should a beginner 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.
What is the best way to learn a cipher?
Encode a message of your own before decoding anybody else's, because writing it is where the rule sticks. Then swap with somebody and decode each other's: the arguments about who made the mistake are the lesson, not an interruption. Finish on a short puzzle so you 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 you can actually use by hand?
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 the worksheets free to print?
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 anyone else. No account, no sign-up, nothing collected.
What can you build at home?
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
Starting out: pigpen, then the Caesar shift, then a scytale made from a pencil. Print the first booklet and work one sheet at a time.
Once those are easy: the rail fence and the Polybius square, then the challenges. This is the point where breaking a cipher by counting letters lands properly.
Then the harder ones: Playfair, the one-time pad, and the code-breaking pages. After that, the National Cipher Challenge in the autumn, or picoCTF.
Start with pigpen The printable booklets For teachers