ApocalypseCipher Manual

📖 Introduction

This guide will teach you how to decrypt messages encoded with the Apocalypse Cipher using only pencil, paper, and a printed copy of the Vigenère table. No computers required!

The cipher combines several techniques: Caesar shifts, Vigenère substitution, and several parameters that modify how the cipher behaves.

🔧 Understanding the Parameters

Before decrypting, you need to know the settings that were used to encrypt the message. Both sender and receiver must use the same settings!

⚖️ Security Note

The sender chose different settings based on their security needs:

📊 The Vigenère Table

This is your main tool for manual decryption. Print this table or draw it on paper before starting. Each row represents a different "Caesar alphabet" (a shifted alphabet).

How to read: Find the key letter on the left edge (row), find the ciphertext letter at the top (column), then read the plaintext letter where that row and column meet.

📝 Step-by-Step Decryption Process

Follow these steps in order. Each step builds on the previous one.

1. Write Down Your Known Information

   Before touching the cipher, record everything you know:

   • The encrypted message (ciphertext)
   • The passphrase
   • Initial Shift value
   • Rounds value
   • Keyed Alphabet setting
   • Reverse Passphrase setting
   • Preserve Formatting setting
2. Prepare the Ciphertext

   Based on Preserve Formatting:

   • ON: Use the ciphertext as-is, including any spaces and punctuation
   • OFF: Remove any spaces and punctuation, keep only letters (already done by sender)
3. Write the Ciphertext and Passphrase in Rows

   Create two rows of letters. Write the ciphertext on top, and repeat the passphrase below it.

   Example

   Ciphertext: O E V X O X Passphrase: C A T C A T

   Notice how the passphrase repeats to match each letter of the ciphertext.

4. Find Each Letter on the Vigenère Table

   For each letter of ciphertext, you need to find where it appears in the table and work backwards.

   The Decryption Rule

   Look at the PASSKEY letter (left side), find the CIPHERTEXT letter in that row, then read the plaintext letter at the TOP of that column.

5. Decode Each Letter

   For each column of letters:

   • Find the passphrase letter in the left column (the row)
   • Find the ciphertext letter somewhere in that row
   • Look up to the top of that column to find the plaintext letter
6. Record Your Plaintext Letters

   Write down each decoded letter below your ciphertext row.

   Continuing the Example

   Ciphertext: O E V X O X Passphrase: C A T C A T Plaintext: M E E T M E
7. Handle Multiple Rounds

   If using more than 1 round:

   1. Work through one round of decryption
   2. Use the result as the new "ciphertext"
   3. Repeat the passphrase and decrypt again
   4. Continue until all rounds are complete

   Tip

   Work backwards! If encrypted with 3 rounds, start with round 3, then round 2, then round 1 to get the final plaintext.

🎯 Complete Worked Example

Let's walk through a complete decryption together, step by step.

How Each Letter Was Decoded:

⚙️ Handling Advanced Parameters

When additional parameters are used, modify your approach as follows.

With Initial Shift (not 0):

The Initial Shift adds a Caesar cipher layer. For decryption:

1. First, decrypt using the Vigenère table (as shown above)
2. Then, shift each resulting letter BACKWARD by the shift value
3. Example: If shift = 3, and you got "D", shift it back to "A"

With Multiple Rounds:

Work backwards through the rounds:

1. Round 3: Ciphertext 2 → Ciphertext 1
2. Round 2: Ciphertext 1 → Ciphertext 0
3. Round 1: Ciphertext 0 → Plaintext

With Keyed Alphabet:

Recreate the sender's custom alphabet:

Replace the top header of your Vigenère table with this new alphabet.

With Reverse Passphrase:

Reverse the passphrase before using it:

⚡ Quick Reference

Print this box and keep it next to your Vigenère table for fast reference.

╔════════════════════════════════════════════════════════════════╗
║                   SIMPLIFIED DECRYPTION                        ║
╠════════════════════════════════════════════════════════════════╣
║                                                                ║
║  1. WRITE: Ciphertext on top, passphrase repeated below        ║
║                                                                ║
║  2. FIND: For each pair:                                       ║
║           • Look at the left column → Passphrase letter (row)  ║
║           • Find ciphertext letter in that row                 ║
║           • Look up to the top → Plaintext letter              ║
║                                                                ║
║  3. WRITE: Plaintext letters below ciphertext                  ║
║                                                                ║
║  4. IF Rounds > 1: Repeat (work backwards through rounds)      ║
║                                                                ║
║  5. IF Shift ≠ 0: Shift each letter BACK by the shift amount   ║
║                                                                ║
╚════════════════════════════════════════════════════════════════╝

✅ Pre-Decryption Checklist

Before starting, make sure you have all the required information:

• The encrypted message (ciphertext)
• The passphrase (or word)
• Initial Shift value (if not 0, you need this)
• Rounds value (if not 1, you need this)
• Keyed Alphabet setting (if ON, you need this)
• Reverse Passphrase setting (if ON, you need this)
• Preserve Formatting setting (critical!)
• A printed copy of the Vigenère table
• Pencil and paper

🔍 Troubleshooting