Decode Challenge Packet Worksheet

Instructions

• In your groups, decode each of the ciphered excerpts below.
• For each excerpt:
  1. Identify the cipher technique and key (shift number or keyword).
  2. Decode the message into plain English.
  3. Describe any patterns, letter‐frequency clues, or strategies you used.

Excerpt 1: Caesar Cipher (Shift +5)

Cipher text:

NYBFX YMJ GJXY TK YNRJX, NYBFX YMJ BTWXY TK YNRJX.

1. Plaintext:
   
   
   
   
   
   
   
2. Patterns/Strategies:
   
   
   
   

Excerpt 2: Vigenère Cipher (Key: MATH)

Cipher text:

MLE ATAM NXIMAQRL PE NHA SOEK

1. Plaintext:
   
   
   
   
   
   
   
2. Patterns/Strategies:
   
   
   
   

Excerpt 3: Simple Substitution (Atbash)

Cipher text:

GSV LMOB DZB GL SZEV Z UIRVMW RH GL YV LMV.

1. Plaintext:
   
   
   
   
   
   
   
2. Patterns/Strategies:
   
   
   
   

History Cipher Relay Activity Guide

In this relay, students rotate through three stations that highlight historical uses of ciphers. They spend 3 minutes at each station before rotating. Your role as the teacher is to prompt groups, monitor their strategies, and keep time.

Station 1: Roman Caesar Dispatch

Historical Context: Julius Caesar (58–51 BC) secured military orders using a simple shift cipher.

Materials:

• Card with cipher text: “YMNX NX F YJXYT” (Shift +5)
• Caesar shift wheel or alphabet chart

Instructions for Students:

1. Recall that Caesar’s messages were shifted forward; apply a reverse shift of 5.
2. Decode each letter and write the plaintext.
3. Note any letter‐frequency patterns you observed.

Station 2: Renaissance Vigenère Letter

Historical Context: In the 16th century, the Vigenère cipher encrypted diplomatic correspondence across Europe.

Materials:

• Envelope containing cipher text: “LXFOPV EF RNHR”
• Printed Vigenère square chart
• Key card labeled “LEO”

Instructions for Students:

1. Align the keyword “LEO” with the cipher text using the Vigenère square.
2. Decode letter by letter, recording the plaintext.
3. Discuss how repeating the key adds complexity compared to a fixed shift.

Station 3: Atbash – Ancient Hebrew Correspondence

Historical Context: The Atbash substitution (dating to at least the 6th century BC) appears in Hebrew scriptures to obscure names and places.

Materials:

• Scroll fragment with cipher text: “GSRH RH Z ORPV”
• Reversed alphabet chart (A↔Z, B↔Y, C↔X…)

Instructions for Students:

1. Match each cipher letter to its reversed‐alphabet counterpart.
2. Translate the result into English.
3. Reflect on how a one‐to‐one substitution differs from shifting methods.

Teacher Tips:

• Rotate groups with a timer set to 3 minutes.
• Encourage students to jot down questions or strategies on a shared board.
• After the relay, reconvene for the Strategy Share Discussion Prompts.

Strategy Share Discussion

Time: 5 minutes
Goal: Reflect on your decoding strategies, challenges faced, and the connections between math, history, and literacy.

Discussion Guidelines

• One representative from each group shares briefly (30–45 seconds).
• Listen actively and build on others’ ideas.
• Use evidence from your decoding work to support your points.

1. Strategy Spotlight

Prompt: Which decoding technique or clue (e.g., frequency analysis, keyword alignment, shift patterns) was most effective for your group, and why?
Follow-Ups:

• How did you recognize which cipher type you were working with?
• What small “aha” moments guided your approach?

2. Challenge Check-In

Prompt: What was the biggest obstacle you encountered during the Decode Challenge or Relay stations?
Follow-Ups:

• Did you try more than one strategy before finding success?
• How did group discussion help you overcome the hurdle?

3. Historical & Mathematical Connections

Prompt: How did the historical context (e.g., Caesar’s military use, Renaissance diplomacy) or the underlying math (shift values, modular arithmetic) deepen your understanding of these ciphers?
Follow-Ups:

• Can you think of other real-world scenarios where these concepts apply?
• How does knowing the history influence your approach to problem-solving?

4. Beyond the Cipher

Prompt: Which problem-solving skills (pattern-finding, collaboration, strategy adjustment) did you practice today, and how can you apply them in other classes or everyday situations?
Follow-Ups:

• In what other subjects might you use frequency analysis or systematic trial-and-error?
• How can reflecting on your process improve your future work?

After all groups have shared, summarize common themes on the board and highlight one or two surprising insights before moving to the Review and Assessment phase.

Decoded Excerpts Answer Key

Excerpt 1: Caesar Cipher (Shift +5)

Cipher Text:
NYBFX YMJ GJXY TK YNRJX, NYBFX YMJ BTWXY TK YNRJX.

Plaintext:
IT WAS THE BEST OF TIMES, IT WAS THE WORST OF TIMES.

Step-by-Step Decoding:

1. Identify a Caesar shift of +5 (so to decrypt, shift each letter back 5 positions).
2. Convert letters to their alphabetical indices (A = 0…Z = 25), subtract 5, then wrap around mod 26.
3. Example:
   • N (13) → 13 − 5 = 8 → I
   • Y (24) → 24 − 5 = 19 → T
   • B (1) → 1 − 5 = −4 → 22 → W
     …forming “ITWAS.”
4. Continue across the full sentence to recover the complete phrase.

Strategies & Clues:

• Noticed repeated blocks “NYBFX YMJ … TK YNRJX.”
• Recognized pattern as a famous opening line from literature.
• Used letter‐frequency (high occurrence of Y/M/J) to confirm shift amount.

Excerpt 2: Vigenère Cipher (Key: MATH)

Cipher Text:
MLE ATAM NXIMAQRL PE NHA SOEK

Plaintext:
ALL THAT GLITTERS IS NOT GOLD

Step-by-Step Decoding:

1. Write the keyword “MATH” repeatedly under the cipher text:

   MLE ATAM NXIMAQRL PE NHA SOEK
   MATH MATH MATHMATH MA TH MAT

2. For each position, compute:
   Plain = (Cipher index − Key index) mod 26.

3. Sample calculations:

   • M(12) − M(12) = 0 → A
   • L(11) − A(0) = 11 → L
   • E(4) − T(19) = −15 → 11 → L
     …giving “ALL.”

4. Continue to decode “ATAM” → THAT, “NXIMAQRL” → GLITTERS, etc.

Strategies & Clues:

• Spotted repeating key of length 4 by looking at recurring letter shifts.
• Used a Vigenère square or numeric substitution to manage wrap-around.

Excerpt 3: Simple Substitution (Atbash)

Cipher Text:
GSV LMOB DZB GL SZEV Z UIRVMW RH GL YV LMV.

Plaintext:
THE ONLY WAY TO HAVE A FRIEND IS TO BE ONE.

Step-by-Step Decoding:

1. Recall Atbash maps A↔Z, B↔Y, C↔X, …, M↔N.
2. Apply this one-to-one substitution to each letter:
   • G → T, S → H, V → E → “THE”
   • L → O, M → N, O → L, B → Y → “ONLY”
   • Continue for each word.

Strategies & Clues:

• The reversed-alphabet hint pointed directly to Atbash.
• One-to-one mapping meant no shifting pattern—every letter had a unique partner.