First Codebreak — Facilitator’s Answers

1. The library notice

LOOK FOR THE BOOK OF MAPS IN THE SCHOOL ROOM. DO NOT GO TO THE OLD DOOR. BRING THE BOOK TO THE READING ROOM AT NOON.

Plain alphabet: ABCDEFGHIJKLMNOPQRSTUVWXYZ Cipher alphabet: QWERTYUIOPASDFGHJKLZXCVBNM

Read this table left to right for encryption; reverse its mappings for the worksheet’s cipher-to-plaintext key. Only letters that appear in the notice can be recovered from this exercise.

O occurs 24 times, so G is the most frequent cipher letter. G → E is wrong here. FGGF matches NOON’s doubled middle and repeated outside letter. ZIT → THE, WGGA → BOOK and KGGD → ROOM provide mutually consistent mappings. NOON alone is a crib hypothesis, not a unique solution to an arbitrary four-letter cryptogram.

Ask learners to identify a rejected guess and two supporting clues. The supplied plaintext check verifies this fixture; readability alone would not establish the authenticity of a historical message.

2. The repeating-key dispatch

Generating key: LAMP. Its minimal period is four. Repeating that keyword twice produces an equivalent eight-position stream.

THE READING ROOM WILL OPEN AT NINE. BRING THE MAP AND THE LETTER TO THE LIBRARY. THE READING ROOM WILL CLOSE AT FIVE. CHECK THE NAME ON THE LETTER BEFORE YOU GIVE IT TO THE READER. KEEP A COPY OF THE MESSAGE AND NOTE THE TIME IT ARRIVED. THE MAP SHOWS THE ROAD TO THE OLD SCHOOL AND THE BRIDGE OVER THE RIVER. THE READER WILL WAIT IN THE LIBRARY UNTIL THE OTHER COPY ARRIVES. COMPARE THE TWO COPIES AND RECORD EVERY CHANGE. A CLEAR MESSAGE IS ONLY PART OF THE EVIDENCE.

The repeated group TTT begins at positions 34 and 54: a distance of 20 suggests factors including 2, 4, 5 and 10. Its occurrence at position 98 provides a distance of 64 from position 34. Their common factors still do not prove a key; test the columns and full plaintext. IZT at positions 26 and 52 has distance 26 and illustrates an accidental repeat that does not align with the generating period.

At period four, this exercise’s best English chi-square shifts happen to recover LAMP. That is a property of this fixture, not a promise for short columns or other languages. The online tool shows all 26 candidates and checks the reconstructed text.

3. Language and homophone observations

Results depend on the selected sample. The five displayed texts are original, short modern-language samples normalized to A–Z. Word choice, topic, length and normalization can change their frequencies. They must not be presented as historical or population-wide language models.

There are 24 O letters. With four equally likely alternatives, each O-token has expected count 24/4 = 6. Actual counts fluctuate; one draw need not flatten the chart. Other letters retain fixed tokens, and higher-order structure remains. Uniform-looking counts do not prove secrecy.

4. Facilitation and scope

For a 15-minute session, use the first activity only: introduce the mapping rule, let pairs try clues, then discuss a rejected guess. Add the remaining activities when time permits. Never require a learner to reveal their own private information for a puzzle.

Use the topic guide for related exhibits. The format was inspired by Simon Singh’s Black Chamber; the text, examples and implementation are original. Technical reading is listed on the lesson page.