First Codebreak — Student Activities

Use your browser’s Print command to print or save as PDF. Open the interactive lessons · Facilitator’s answers (spoilers)

Name / team: ____________________ Date: __________

For the first activity, allow about 15 minutes. Work with a partner: one proposes a guess, the other looks for a contradiction. Use the optional extensions for a longer session.

1. Where is the book?

One cipher letter always represents the same plaintext letter. Spaces and punctuation are genuine boundaries retained for this exercise.

SGGA YGK ZIT WGGA GY DQHL OF ZIT LEIGGS KGGD. RG FGZ UG ZG ZIT GSR RGGK. WKOFU ZIT WGGA ZG ZIT KTQROFU KGGD QZ FGGF.

Working key: cipher letter → plaintext letter

A → ___ B → ___ C → ___ D → ___ E → ___ F → ___ G → ___ H → ___ I → ___ J → ___ K → ___ L → ___ M → ___ N → ___ O → ___ P → ___ Q → ___ R → ___ S → ___ T → ___ U → ___ V → ___ W → ___ X → ___ Y → ___ Z → ___

Clue ladder — read one at a time

  1. Circle words containing doubled letters. Underline repeated words. Count the cipher letters.
  2. The most frequent symbol need not represent E. What would make you reject that guess?
  3. The meeting time is a four-letter word with the pattern 0.1.1.0. Find a cipherword with that shape.
  4. Test THE for ZIT. Does the same mapping work everywhere?
  5. This is a library notice: try BOOK for WGGA and ROOM for KGGD.

Your plaintext:

Which two clues support your reading? What did you revise?

2. Follow a repeating key

This optional extension uses repeating-key Vigenère with A=0. Only A–Z letters are present. The message is 369 letters long. Use the online column tools or work together on paper.

EHQGPAPXYGDDZMIXWLAEPNMIYIZTMRUCRTTTXABPYDFWPLQIEEDIZTTTWINGLRKISEDTLDUCRRADXWUAWCXDDEMIQIHTNHQRVTTTYAYTZNFWPLQIEEDQPFAGPYAJRIHTTTFDEHQGPAPTCKQTAAODAYAUEHQBPSEPREMCONAIPTTTEIYTTTMGCIHTOTTTXABHSOIHEHQGZAPIZTTTZLPHNHADWAZSEHQQCIPVPOHTCTTTCIHTCTTTCEMSPRIXWLIPTTUCEHQATBDPCYGCEIXISEAISEDRZPKPCRUKPSODXPMGPTTTEWARZPUTDAZSCEODCDQKPRKRSAZVPAOAPADBPSEPREUHZNXNAADIZFFWPEHXOEZRP

  1. Find two occurrences of a three-letter group. Record their positions, counting from 1.
  2. Measure the distance and list candidate periods that divide it. Test another repeated group. Why might the clues disagree?
  3. For each candidate period, put letters at equal key positions in the same column.
  4. Try Caesar shifts on each column. Test the combined key on the complete message.

Repeated group: ______ Positions: ______ Distance: ______ Candidate periods: ______

Candidate key: ______ What verifies or contradicts it?

What could a key that repeats the same shorter keyword tell you about the true period?

3. Change an assumption

Compare two language samples in the online lesson. Record the most frequent letter and a short word in each. Does one short sample represent its whole language?

Give O four possible numerical tokens in the homophone experiment. Run it twice. Why do the observed counts change? What structure remains?

4. Exchange an original puzzle

Write a short notice, choose a substitution key, and keep the plaintext and key on a separate answer sheet. Give another pair only the ciphertext, cipher family and any intentional hints. Use the museum Workbench to check the answer. These historical puzzle methods are unsuitable for protecting private information.

Cipher Museum · ciphermuseum.com/first-codebreak.html · Original teaching exercises, not historical documents.