Easiest to hardest, with a puzzle in each
Substitution ciphers
The ciphers that turn up in scavenger hunts, camps, treasure hunts and lessons, each with its rule in one sentence, a puzzle already encoded and the answer hidden. The puzzles were encoded by the same programs the site's decoders use, so a puzzle and its answer cannot disagree.
What a substitution cipher is
A substitution cipher swaps each letter of the text for another symbol under a fixed rule. The symbol can be another letter, as in the Caesar cipher, a number, a group of ones and zeros, as in binary code, or a cell of dots, as in braille. What changes from one cipher to the next is the rule, and the rule is the key: whoever knows it reads the message, whoever does not has to work it out.
That is why they teach so well. The whole rule fits in a sentence, it can be applied with pencil and paper, and working out the key of someone else's message is a real problem, sized for a child. The list below runs from the cipher with no key at all to the one that held out against adults for three centuries.
AtbashEasy
The alphabet backwards: A becomes Z, B becomes Y, and so on to Z, which becomes A. No key; the rule is the key.
GSV GIVZHFIV RH RM GSV TZIWVM
Apply the same rule twice and the text comes back.
Answer
THE TREASURE IS IN THE GARDEN
A1Z26 (numbers for letters)Easy
Each letter becomes its position in the alphabet: A is 1, B is 2, Z is 26. Letters of a word are joined with hyphens and words with spaces.
12-15-15-11 2-5-8-9-14-4 20-8-5 4-15-15-18
Count along the alphabet, or write it out numbered on a sheet.
Answer
LOOK BEHIND THE DOOR
Caesar cipherMedium
Each letter moves a fixed number of places along the alphabet. The number is the key; with three, A becomes D.
YMJ UFXXBTWI NX YMJ ITLX SFRJ
The key here is five. Without the key, try all 25 in the decoder.
Answer
THE PASSWORD IS THE DOGS NAME
Binary codeMedium
Each letter becomes its number in the ASCII table, written with eight ones and zeros. Upper and lower case differ by one digit.
01000011 01001100 01001001 01001101 01000010
Eight digits per letter. The ASCII table on the site has each number.
Answer
CLIMB
BrailleHard
Each letter is a cell of six dots. The first ten letters use the four upper dots; the next ten add dot three.
⠇⠕⠕⠅⠀⠥⠏
The braille table on the site shows the dots of each letter.
Answer
look up
Vigenère cipherHard
Like Caesar, but each letter of a keyword decides the shift of one letter of the text, and the keyword repeats. Without the key it is the hardest on this page.
VHO PCSD GNUO MU IX XJE UMVCRIP
The key is a four-letter dessert you bring to a party.
Answer
THE LAST CLUE IS IN THE KITCHEN
How to find the key
In the Caesar and Vigenère ciphers the key is a secret separate from the rule, and the secret is what a child wants to crack. For Caesar there are only 25 possibilities, and the honest method is to try them all: the decoder has a brute-force list that does exactly that and puts the most English-looking reading at the top. The clever method is to count letters. The commonest letter in English is E, and the commonest letter in the ciphertext is almost always E shifted; the distance between the two is the key.
In Vigenère the simple count fails, because the same E of the text becomes different letters depending on its position. What remains is to look for repeated fragments in the ciphertext and measure the distance between them, which tends to be a multiple of the key length. With the length found, each position of the key is a Caesar cipher and falls like one. That is work for a patient adult, which is why the Vigenère puzzle on this page comes with a hint about the key.
Roman numerals and the ASCII table are not ciphers, but they turn up in hunts in the same role: a writing system the child does not yet know and can learn in an afternoon. Both pages have the full table and a converter to check against.
Frequently asked
Where should children start?
With Atbash, because the rule fits in one sentence and there is no key to lose. Then numbers for letters, which drills the alphabet. The Caesar cipher is the first step with a key, and the conversation about how to find the key without being told is the best part of the activity.
How do I set up a treasure hunt with ciphers?
Write the clues in plain text, one per hiding place, and encode each with the decoder of the cipher you chose, raising the level as the game goes on. Print the encoded clue and, if you like, the rule beside it. Keep the plain text with you to check. Every puzzle on this page was encoded that way.
How do I know the answer to a puzzle is right?
Each puzzle here was encoded by the same program that decodes it, and the hidden answer is the text that went in; the two cannot disagree. For your own puzzles, paste the encoded text into the cipher's decoder and compare.
What is a substitution cipher solver?
A tool that tries the possible keys for you. For Caesar there are 25, and the decoder lists them; for a general substitution, where any letter can stand for any other, the solver counts letter frequencies and guesses E for the commonest symbol. The cipher decoder on this site does the keyless part: paste a text and it tries every cipher that needs no key, all at once.
What is cryptography, in the end?
The art of writing so that only the holder of the key can read. The ciphers on this page are the version from two thousand years ago, breakable with a pencil and patience, and that is exactly why they teach well: the whole rule is visible and so is why it fails. The ciphers protecting a password today use the same idea with keys so large that no patience reaches them.