Caesar cipher puzzles: read and write three messages

Three cards from a tiny museum

Imagine a miniature museum made from paper. Its keeper has left three message cards: one to read, one with a missing key, and one for you to write. These are independent word puzzles; you do not need a map or objects hidden around your home.

Use paper and a pencil, or work directly from the alphabet below. Try all three tasks before scrolling to the hints and answers. You can also take turns with someone: one person changes the letters and the other checks them.

Agree on the alphabet first

This game uses only the 26 letters A–Z. Turn the message into capitals before starting. Keep spaces, punctuation and digits exactly as written; shift only letters. One key applies to every letter in a message.

For a Spanish message, first replace Á, É, Í, Ó, Ú with A, E, I, O, U; replace Ü with U and Ñ with N. For example, “Mañana, pingüino.” becomes “MANANA, PINGUINO.” This is a deliberately simplified game alphabet, not the Spanish alphabet. Decoding returns this simplified text; it cannot restore the removed accents or distinguish an original N from Ñ.

How a shift works

The key is the number of steps forward when you encode. With key 3, A becomes D. After Z, continue at A: X becomes A, Y becomes B, and Z becomes C. The starting letter counts as zero steps.

To decode, move backward by the same number. In the two rows below, encode from top to bottom and decode from bottom to top. Find the letter first, then take its partner in the same column.

Task 1: read the first card

The keeper used key 3. Decode the whole card. If a letter repeats, you can reuse the result you already found.

Task 2: find the missing key

The next card uses a different key from 1 to 25. You know its first word was OPEN before encoding. Find the key, then decode the entire message. The key stays the same throughout this card.

Task 3: write the reply

Encode the following reply with key 11. Preserve the spaces and the full stop. Write the complete coded message, then try decoding your own result to check it.

Hints: take only as much help as you need

• Task 1, small hint: when reading a coded letter, move backward, not forward. Stronger hint: the first four letters become LOOK.
• Task 2, small hint: compare the first letter of OPEN with the first coded letter. Stronger hint: O → P is one step; keep counting until V. Check the same shift on P → W before decoding the rest.
• Task 3, small hint: make a new pair of alphabet rows; put L below A. Stronger hint: the first word becomes ESP. Remember: B → M and Z → K use the same key.

Answer 2: recover and check

The key is 7. Count forward from O to V: P, Q, R, S, T, U, V. The other known letters agree: P → W, E → L and N → U. Now subtract seven steps from each coded letter. V becomes O; A wraps backward through Z and becomes T.

Make a card someone else can solve

Write a new message of four to eight words and choose a key from 1 to 25. Keep a copy of your plain message. Give the solver either the key or the complete first word; say which one you are providing. Use the same alphabet rules on both sides.

Check the finished card by decoding every letter, including repeats and letters near Z. If one word looks wrong, compare it letter by letter instead of changing the key halfway through. When swapping Spanish cards, agree that the answer is checked in the simplified A–Z form.

About the method

Harvard’s CS50 Caesar exercise explains fixed letter shifts, wrapping at the end of the alphabet and reversing the direction to decode. Coru created the museum messages, tasks, hints and solutions here. Our uppercase-only alphabet and Spanish normalization are rules for this particular paper game.

Sources & further reading

• Harvard CS50x — Caesar