🌊 The Waterfall

How Cascading Cipher Works

In 10 minutes: Understanding encryption and decryption flows
Prerequisite: Cipher layers explained

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🎯 The Simple Story

Imagine a waterfall flowing downstream.

Encryption (flowing down):

Waterfall (Plaintext):
┌──────────────────────┐
│ "Alice sends money 123"  │ ← Starts here!
└──────────────────────┘
    ↓ Falls to...
Rock 1 (Layer 1: AES):
    └─ Scrambles into "Xj7k!$#m"
       ↓ Falls to...
Rock 2 (Layer 2: DH):
    └─ Scrambles into "$!x9mL#k7x"
       ↓ Falls to...
Rock 3 (Layer 3: Signal):
    └─ Scrambles into "9m#k!x$7z..."
       ↓ Falls to...
Pool (Ciphertext):
    └─ Completely scrambled!

Decryption (climbing back):

Bob starts at the bottom (Ciphertext) and climbs back up:
Pool → Rock 3⁻¹ → Rock 2⁻¹ → Rock 1⁻¹ → Waterfall top

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🧠 Mental Model

Think of it like an assembly line:

Plaintext enters assembly line:

Station 1 (Layer 1: AES):
  Encrypt: "Hello!" → "Xy7$z#4"
  Output → Station 2

Station 2 (Layer 2: DH):
  Encrypt: "Xy7$z#4" → "$9x!mL#k7"
  Output → Station 3

Station 3 (Layer 3: Signal):
  Encrypt: "$9x!mL#k7" → "k7$!x#mL9"
  Output → Final

Final result: Ciphertext encrypted with all 3 layers!

Decryption enters station 3 first and works backward!

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📊 See The Flow

Encryption Flow

Decryption Flow

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🎭 The Story: The Envelope Chain

Alice wants to send "I love you" to Bob using cascading cipher.

Encryption (Alice's side):

Step 1: Alice writes the message
Message: "I love you"

Step 2: Alice passes through Layer 1 (AES)
  - Alice AES-encrypts: "I love you"
  - Result: "7x#k$4mL"
  - Alice puts result in Envelope 1

Step 3: Alice passes Envelope 1 through Layer 2 (DH)
  - Alice takes "7x#k$4mL" from Envelope 1
  - Alice DH-encrypts the envelope
  - Result: "$9x!mL#k7"
  - Alice puts result in Envelope 2

Step 4: Alice passes Envelope 2 through Layer 3 (Signal)
  - Alice takes "$9x!mL#k7" from Envelope 2
  - Alice Signal-encrypts the envelope
  - Result: "k7$!x#mL9"
  - Alice puts result in Envelope 3

Step 5: Alice sends Envelope 3 to Bob
  - Eve sees: "k7$!x#mL9"
  - Eve can't decrypt without going through layers

Decryption (Bob's side):

Step 1: Bob receives Envelope 3
  - Contains: "k7$!x#mL9"

Step 2: Bob goes through Layer 3⁻¹ (Signal) first
  - Bob Signal-decrypts: "k7$!x#mL9"
  - Result: "$9x!mL#k7"
  - Bob removes Signal layer!

Step 3: Bob goes through Layer 2⁻¹ (DH)
  - Bob DH-decrypts: "$9x!mL#k7"
  - Result: "7x#k$4mL"
  - Bob removes DH layer!

Step 4: Bob goes through Layer 1⁻¹ (AES)
  - Bob AES-decrypts: "7x#4mL"
  - Result: "I love you"
  - Bob removes AES layer!

Step 5: Bob reads: "I love you!"

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🔄 Forward and Reverse

Encryption: Downstream

Direction: ↓ (down/forward)

Plaintext
  ↓
Layer 1 (first to encrypt)
  ↓
Layer 2 (second)
  ↓
Layer 3 (third)
  ↓
Layer 4 (fourth)
  ↓
...
  ↓
Final Ciphertext

Decryption: Upstream

Direction: ↑ (up/backward)

Final Ciphertext
  ↓
Layer 4⁻¹ (last to decrypt)
  ↓
Layer 3⁻¹ (third from last)
  ↓
Layer 2⁻¹ (second from last)
  ↓
Layer 1⁻¹ (last)
  ↓
Original Plaintext

Key principle:

• Encrypted in order: Layer 1, 2, 3, 4...
• Decrypted in reverse: Layer 4⁻¹, 3⁰1, 2⁻¹, 1⁻¹

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🎮 Try It Yourself

Question 1: If plaintext is "hello" and we have 2 layers (AES + DH), what's the encryption order?

Show Answer

Encryption order: AES first, then DH

1. Plaintext "hello"
2. AES encrypts → "x7k!m#4"
3. DH encrypts → "$k!mx#7"
4. Final ciphertext: "$k!mx#7"

Decryption order: DH first (reverse), then AES

1. Ciphertext "$k!mx#7"
2. DH⁻¹ decrypt → "x7k!m#4"
3. AES⁻¹ decrypt → "hello"
4. Original plaintext recovered!

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Question 2: If you have layers A, B, C, D and E, which order do you decrypt in?

Show Answer

Encrypted order: A → B → C → D → E

Decrypted order (reverse!): E⁻¹ → D⁻¹ → C⁻¹ → B⁻¹ → A⁻¹

Always decrypt from OUTSIDE to INSIDE (last in, first out) just like Russian nesting dolls!

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Question 3: Can you decrypt with the wrong order? Why or why not?

No! You cannot decrypt with the wrong order. Each layer's output is the input to the next layer during encryption. If you try to decrypt Layer 2 first when Layer 3 encrypted last, Layer 2 doesn't recognize the data!

Example:

• Layer 1: "hello" → "x7k!m"
• Layer 2: "x7k!m" → "$k!mx#"
• Layer 3: "k!mx#" → "k7x!m#"

If you try to decrypt Layer 2 first on final "k7x!m#", Layer 2 expects "x7k!m" (AES output), but gets "k7x!m" (from Layer 3) → FAILS!

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💡 Why This Matters

Dependency Chain

Each layer's output depends on the previous layer's input:

Chain: Input 1 → Layer 1 → Output 1 → Input 2 → Layer 2 → Output 2 → ...

Layer 1's Output 2 becomes Layer 2's Input 2
Layer 2's Output 3 becomes Layer 3's Input 3

If Layer 3 decrypts first:
- It expects Input 3 (from Layer 2's output)
- But got Final Ciphertext (from Layer 4's output)
- Doesn't make sense → Fails!

Order Matters

Just like Russian nesting dolls:

• You must open from OUTSIDE (biggest) to INSIDE (smallest)
• You cannot start with the smallest doll!
• You must remove biggest doll first, then next, etc.

Same with cascading cipher:

• You must decrypt from OUTSIDE (last layer) to INSIDE (first layer)
• You cannot decrypt inner layers first!
• Remove outer layers first, then work your way in!

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✅ Quick Check

Can you explain the waterfall metaphor to a 5-year-old?

Try saying this out loud:

"Think of a waterfall flowing from the top of a mountain down to a pool at the bottom. As the water falls, it passes over 5 rocks. Each rock makes the water go through a different tunnel. When Bob wants to read your message, he crawls back up the mountain from the bottom, going through all 5 tunnels in reverse order to find the original message at the top!"

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🎓 Key Takeaways

✅ Encryption flows downstream (first layer first)
✅ Decryption flows upstream (reverse order)
✅ Layer outputs become next layer inputs
✅ Order matters - cannot decrypt in wrong order
✅ Russian nesting dolls - open from outside in!

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Now you understand how cascading works. Next: What each cipher layer does!