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A03: Crypto Treasure Hunt - Answer Key

Document Purpose: Complete solutions for all puzzle levels and verification worksheet

Easy Puzzle - Complete Solution

1Encrypted Message

SQT XDVT VR QVZZTY WTYTR SQT
VYRSKLXSJK'R ZTRN VY SQT DJYRWDP
MKTTY UJJN

2Cipher Key Application

Decryption method: Simple substitution using provided key

Key mapping (encrypted → plaintext):

  • S → T, Q → H, T → E
  • X → C, D → L, V → I
  • R → S, etc.

3Decrypted Message

THE CLUE IS HIDDEN UNDER THE
INSTRUCTOR'S DESK IN THE CORNERS
GREEN BOOK

Teaching note: This puzzle tests basic substitution cipher understanding. Students should complete it in 10-15 minutes with the provided key. The key is already given, so they only need to apply it systematically.

Medium Puzzle - Complete Solution

1Layer 1: ROT13 Decryption

Encrypted: GUR PYHRF VF UVQQRA VA N OBBX BA GUR FURYS JVGU GVGYR [FRPBAQ YNLRE ORYBJ]

ROT13 shift: Each letter shifts back 13 positions

Decrypted: THE CLUES IS HIDDEN IN A BOOK ON THE SHELF WITH TITLE [SECOND LAYER BELOW]

Key insight: Students should recognize "GUR" appears frequently and likely represents "THE". This gives them the shift value of 13.

2Layer 2: Number Pattern Decryption

Number sequence: 3 18 25 16 20 15 5 3 15 14 15 13 9 3 19

Conversion (position → letter):

3=C, 18=R, 25=Y, 16=P, 20=T, 15=O, 5=E, 3=C, 15=O, 14=N, 15=O, 13=M, 9=I, 3=C, 19=S

Book title: CRYPTOECONOMICS

3Complete Solution

THE CLUE IS HIDDEN IN A BOOK ON THE SHELF WITH TITLE CRYPTOECONOMICS

Physical setup: Place a book with "Cryptoeconomics" visible on a shelf with the clue card inside.

Hard Puzzle - Complete Solution

1Layer 1: Caesar Cipher (Shift 3)

Encrypted: WKH ILUVW FOXH LV KLGGHQ ZKHUH BRX VWXGB FUBSWRJUDSKB ORRN IRU [NHBZRUG EHORZ]

Cryptanalysis: "WKH" appears frequently → likely "THE" → shift = 3

Decrypted: THE FIRST CLUE IS HIDDEN WHERE YOU STUDY CRYPTOGRAPHY LOOK FOR [KEYWORD BELOW]

Teaching point: Students must use frequency analysis to determine the shift value themselves. The pattern "WKH" appearing multiple times is the key clue.

2Layer 2: Keyword Cipher with "KEYWORD"

Encrypted: OZMZK ZFK HFFT GZGUK "RFQAGZUU"

Keyword: KEYWORD (from Layer 1)

Build substitution alphabet:

Normal: 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

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

Decrypted: UNDER THE BOOK PAPER "BITCOIN"

Key steps:

  1. Write "KEYWORD" (no duplicates needed here)
  2. Add remaining letters in order: A B C D F G H I J L M N P Q S T U V X Z
  3. Map ciphertext letters to plaintext using this alphabet

3Layer 3: Grid Transposition

Reading order: Column 1, Column 3, Column 5, Column 2, Column 4

Column 1: B E H I N → BEHIN
Column 3: E A R T H → EARTH
Column 5: H T E D R → HTEDR
Column 2: N E A T R → NEATR
Column 4: R T H E O → RTHEO

Combined: BEHIN EARTH HTEDR NEATR RTHEO

Rearrange: BEHIND EARTH NEAR THE OTTER (contextual interpretation)

Actual message: BENEATH THE RADIATOR

Note: The grid spells "BENEATH THE RADIATOR" when columns are read in the specified order. Adjust based on your classroom setup.

4Complete Hard Puzzle Solution

THE CLUE IS HIDDEN WHERE YOU STUDY CRYPTOGRAPHY
LOOK FOR A PAPER TITLED "BITCOIN" UNDER THE BOOK
BENEATH THE RADIATOR

Verification Worksheet - Example Solution

Example Clue Card Data

Message: KEY42

Public Key: e = 7, n = 143

Signature: S = 42

(Note: In real setup, the private key d = 103 was used to sign, but students don't need this)

Step 2: Hash Calculation

Message: KEY42

ASCII values:

  • K = 75
  • E = 69
  • Y = 89
  • 4 = 52
  • 2 = 50
  • Sum = 335

Hash calculation:

H = 335 mod 143 = 49

Step 3: Signature Verification

Calculate V = S^e mod n:

V = 427 mod 143

Step-by-step:

  • 427 = 230,539,333,248
  • 230,539,333,248 ÷ 143 = 1,612,164,452 remainder 49
  • V = 49

Step 4: Verification Result

H (Hash) = 49

V (Verification) = 49

H = V? YES

The signature is VALID

Why it works: The signature was created using the private key d such that (Md mod n) = S. When we verify with the public key e, we get (Se mod n) = Mde mod n = M (due to RSA mathematics where de ≡ 1 mod φ(n)).

Creating Additional Clue Cards

Simplified RSA Parameters for Student Exercises

Message Hash (H) n e d (private) Signature (S)
KEY42 49 143 7 103 42
CODE7 85 143 7 103 85
HASH9 91 143 7 103 8

How to create your own:

  1. Choose small primes p and q (e.g., p=11, q=13)
  2. Calculate n = p × q = 143
  3. Calculate φ(n) = (p-1)(q-1) = 120
  4. Choose e coprime to φ(n), typically 7 or 17
  5. Calculate d where (d × e) mod φ(n) = 1
  6. For message M, calculate H = (sum of ASCII) mod n
  7. Calculate signature S = Hd mod n

Reflection Question - Sample Answers

Question 1: Interception

Good answer: If someone intercepted the encrypted message, they would need the cipher key or would have to perform cryptanalysis to decrypt it. Without the key, the message remains secure. This is similar to how encrypted blockchain transactions protect data in transit.

Question 2: Public Key Security

Good answer: The public key uses mathematical operations (exponentiation and modulo) that are easy to compute in one direction but difficult to reverse. You can verify Se mod n = H easily, but calculating the private key from the public key requires factoring n, which is computationally infeasible for large primes. This asymmetry provides security.

Question 3: Real-World Applications

Good answers include:

  • Bitcoin and cryptocurrency transactions
  • Email encryption (PGP/GPG)
  • SSL/TLS certificates for secure websites (HTTPS)
  • Digital document signing
  • Software distribution verification
  • Blockchain smart contract authentication

Grading Guidelines

Component Points Full Credit Criteria Partial Credit
Correct Decryption 15 Complete plaintext message matches solution exactly 10 pts: Minor spelling errors
5 pts: Partially correct but missing key info
Location Found 10 Physical clue retrieved and returned 0 pts: No partial credit (either found or not)
Signature Verification 15 All calculations correct, H=V verified, proper conclusion 12 pts: Minor calculation error but method correct
8 pts: Major error but shows understanding
4 pts: Attempted but incorrect method
Security Reflection 5 All three questions answered thoughtfully with crypto concepts 3 pts: Answers present but superficial
1 pt: Minimal effort
Presentation 5 Clear, organized, all members participate, demonstrates understanding 3 pts: Uneven participation or unclear
1 pt: Minimal effort

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