This experiment teaches you how to recover both plaintext and the secret shift key in a Shift Cipher (Caesar Cipher) using cryptanalysis. Follow this single workflow from PART I to PART IV.

STEP 1: Understand the Challenge (PART I)

In PART I - Decryption Challenge:

• Read the ciphertext shown in the challenge box.
• Your objective is to recover both:
  • the plaintext
  • the shift key
• Use Next Ciphertext to cycle through different challenges.
• Each challenge has a different plaintext-key pair.

Example challenge:

• Ciphertext: haahjr ha khdu
• Expected recovery: attack at dawn with shift 7

STEP 2: Analyze in the Workspace (PART II)

Use PART II - Analysis Workspace to infer possible keys before final submission.

A) Frequency Analysis

Click Frequency Analysis to generate:

• Letter distribution table
• Observation fields:
  • Most frequent ciphertext letter
  • Estimated corresponding plaintext letter
  • Possible shift value
  • Remarks

How to use it:

• Compare dominant ciphertext letters with common English frequencies.
• Treat the suggested shift as a hypothesis, not guaranteed truth.
• Validate the hypothesis using Test All Shifts or PART III.

B) Test All Shifts

Click Test All Shifts to perform brute-force decryption for all 26 keys.

• The full list of keys 0-25 is displayed.
• A most probable candidate is automatically highlighted.
• A summary is included:
  • Total Keys Tested
  • Most Probable Key
  • Reason

How to use it:

• Scan all rows, not only the highlighted one.
• Look for meaningful English words and coherent sentence structure.
• Cross-check the likely key in PART III.

C) Rough Work and Clear

• Use the workspace text area to record assumptions, key trials, and interim findings.
• Use Clear to reset only rough work content when needed.

STEP 3: Verify in Simulator (PART III)

Use PART III - Shift Cipher Simulator to test hypotheses.

For encryption check:

• Enter candidate plaintext.
• Select candidate key.
• Click Encrypt and compare against PART I ciphertext.

For decryption check:

• Enter ciphertext in the simulator ciphertext box.
• Select candidate key.
• Click Decrypt and inspect readability.

Purpose:

• Confirm whether your inferred key produces consistent encryption/decryption behavior.

STEP 4: Submit Solution (PART IV)

When you are confident:

• Enter the recovered plaintext.
• Select the recovered key.
• Click Check my answer.

Feedback behavior:

• If correct, the simulation reports:
  • Recovered Key
  • Recovered Plaintext
  • Method Used
  • Keys Tested
  • Conclusion
• If incorrect, the simulation provides guidance (without revealing the final answer), including prompts to:
  • review frequency analysis
  • compare Test All Shifts outputs
  • verify selected key using PART III

STEP 5: Learn from Verification

After a correct submission:

• PART III is auto-populated to demonstrate verification.
• You can observe that the recovered plaintext and key regenerate the challenge ciphertext.
• Use Next Ciphertext to repeat the same methodology on a new challenge.

Example Walkthrough

Given ciphertext: KRZ DUH BRX

1. Run Test All Shifts.
2. Observe candidate outputs:
   • k=0: krz duh brx
   • k=1: jqy ctg aqw
   • k=2: ipx bsf zpv
   • k=3: how are you (meaningful English)
3. Verify in PART III:
   • plaintext: how are you
   • shift: 3
   • encrypt result should match KRZ DUH BRX
4. Submit in PART IV:
   • plaintext: how are you
   • key: 3

Tips for Effective Cryptanalysis

• Start with PART II before trying random keys.
• Use frequency analysis for initial hypothesis generation.
• Use Test All Shifts for exhaustive comparison.
• Use PART III to verify key-plaintext consistency.
• Use PART IV feedback to iterate methodically.