A cipher is only fun if the solver can get into it without being told how. That one rule rules out most of the ciphers people reach for first, and it is the reason a shift cipher beats a clever one.
The test is simple. Hand the message to someone who has never seen it, say nothing, and count how long before they try something. Under a minute and the cipher is doing its job. Three minutes of staring and you have given them a locked door with no handle.
What you need
- Paper and a dark pen. Pencil rubs out in a pocket.
- A decoder they can hold: a strip of card, a printed grid, a wheel
- One short message per clue. Four or five words, no more
- A pen for them too, because every one of these is solved by writing
Four that teach themselves
- Shift by a number. Give the number in the clue and it explains itself the moment one word lands.
- Backwards writing. Trivially solvable, and still satisfying for anyone under about nine.
- Letters as numbers. A=1 through Z=26. No key needed; people guess the system from a short word.
- Pigpen. Looks impressively cryptic and is pure lookup once the grid is in front of them.
Shift by a number
Move every letter the same distance along the alphabet. DOG shifted by three is GRJ. The solver needs only the number, and the number is the easiest thing in the world to hide in plain sight: "count the candles", "the answer is on the front door", a five written large in the corner of the card.
It is the best one for a mixed group, because the moment one person gets one word the whole table can finish it. That is a feature. A cipher only one person can solve empties the room.
Backwards writing
Write the message right to left. GARAGE becomes EGARAG. It looks like nonsense for about four seconds, which is exactly the right amount of time for a six year old, and holding it up to a mirror is a genuinely satisfying way to find out you were right.
Letters as numbers
A is 1, B is 2, through to Z is 26. 3-1-20 is CAT. Nobody needs to be told the system: give them one short word and they work it out, which is the whole difference between a puzzle and an instruction. Use hyphens between letters and a slash between words, or 11-9-20-3-8-5-14 is a wall.
Pigpen
Letters drawn as the corner of a grid they sit in. It looks impressively like a real code and is pure lookup once the grid is in front of them, which is the combination you want: hard to look at, easy to do.
The catch is that it cannot be solved without the grid, so the grid is not a secret. Print it on the back of the same card. Hiding it turns a five minute puzzle into a twenty minute search for a piece of paper.
YNRJW — shift each letter back by five.Getting the decoder to them
This is where most home ciphers fail, and it fails quietly: the message is found first and the key is somewhere two clues away. For as long as that gap lasts, the group is holding something they cannot act on, and that is the feeling you are trying to avoid.
Three ways around it, in the order I would try them. Hand the decoder out at the very start as part of the kit. Print it on the reverse of the card the message is on. Or make the first clue the decoder itself, so finding it is the first small win.
Matching it to the age
The mistake is reaching for a harder cipher when a group is doing well. Difficulty in the cipher is just more arithmetic. Keep the cipher easy and make the message cleverer.
- Five to seven. Backwards words, and pictures standing in for letters. Keep it to one word.
- Eight to ten. Letters as numbers, and a shift of three or five. This is the age where a cipher first feels like a real one.
- Eleven to thirteen. Pigpen, bigger shifts, and two words instead of one. They will want the grid and they will be right.
- Fourteen and up. Two steps: a shift whose number has to be worked out from somewhere else in the room.
And one to avoid
A keyword cipher, where the alphabet is scrambled by a secret word, cannot be solved without the keyword. Handing over the keyword is handing over the answer, so the puzzle is really just the search for a piece of paper.
Before anybody plays
- Encode every message, then decode them yourself from the card alone.
- Check every letter survived. A single wrong letter is unsolvable and reads as your handwriting rather than as a mistake.
- Give one message to somebody in the house and watch. If they do not try something within a minute, the decoder is not close enough to hand.
