🔒 The Safe Deposit Box

AES Cipher Layer Explained

In 5 minutes: Understanding password-based AES encryption
Prerequisite: None

🎯 The Simple Story

Remember those safe deposit boxes at the bank?

You go to the bank:
1. Get a numbered box
2. Put your treasure inside
3. Lock it with a 4-digit code
4. Only YOU know the code!

Anyone tries to open the box:
- They need YOUR 4-digit code
- They have 9999 possibilities
- If they try all and it's empty... STILL can't get in!

AES Cipher Layer is like that safe deposit box:

Your message "I love you":
↓
AES encrypts with password "secr3t!"
↓
Becomes "x7k!m#4L$d"
↓
Like putting it in a lockbox with code: secr3t!

🧠 Mental Model

Hold this picture in your head:

AES Cipher Layer = Online Password Manager

Password Manager:
┌─────────────────────────┐
│ 💾 Password: "p@ssw0rd"     │
│                             │
│   ↓ Stores secrets           │
│      Securely!              │
│                             │
│  🔑 Encrypted with:          │
│     - AES-GCM (algorithm)   │
│     - Password derived key   │
│     - Random salt each time   │
│     - IV never reused        │
│     - GPU/ASIC resistant    │
└─────────────────────────┘

Retrieving secrets:
└─ Password + Salt → Key → Decrypt

Think of it like:

🏛️ Bank Safe Deposit Box

You set a 4-digit code
Only you know it
Anyone else tries... 9999 possibilities!
Same concept, but with AES-GCM encryption!

📊 How AES Encryption Works

The Process

What Each Step Does

1. Add Password:

User enters: "my-secret-password"
Manager provides: password

2. Generate Salt + IV:

Salt: Random bytes (8 bytes) - "a7x!mL#4"
IV: Random bytes (12 bytes)  - "k7$x!m#4L$d"
Purpose: Make each encryption unique!

3. Scrypt Derive Key:

Salt + Password → Scrypt (CPU/GPU/ASIC resistant) → Encryption Key
N=32768, r=8, p=1 = SLOW for attackers!

4. AES-GCM Encrypt:

Plaintext + Key + IV → Ciphertext + Auth Tag
Ciphertext: "x7k!m#4L$d"
Auth Tag: Verifies integrity (no tampering!)

🎭 The Story: Alice's Online Diary

Alice wants to store her secret diary online.

Without AES encryption:

Alice's Diary:
┌────────────────────────────┐
│ Today I found $1000!            │
│ I'm so happy!                 │
├────────────────────────────┤
│ Stored on cloud storage       │
│ No encryption!                 │
└────────────────────────────┘

Six months later:
Eve steals Alice's account
Eve reads: "Today I found $1000!"
Alice's money gone!

With AES encryption (Cipher Layer):

Alice's Diary:
┌────────────────────────────┐
│ Today I found $1000!            │
│ I'm so happy!                 │
├────────────────────────────┤
│ BEFORE storing:               │
│ Alice AES encrypts with          │
│ Password: "my-diary!pass"      │
│                               │
│ Stored:                        │
│ "x7k!m#4L$d" (encrypted)     │
└────────────────────────────┘

Six months later:
Eve steals Alice's account
Eve reads: "x7k!m#4L$d" (encrypted)
Eve tries all passwords:
- "password"? → Fails!
- "123456"? → Fails!
- "diary!pass"? → Fails!

Alice: "My diary's safe! She can't read it!"

🔑 Passwords + Scrypt

Why Not Just Use the Password Directly?

Problem with direct password:

Weak Password: "password123"
❌ Easy to guess
❌ Common attack vector
❌ Compromised password lists

Result: Attacker tries "password123" → SUCCESS! → Broken!

Solution: Scrypt Key Derivation

Scrypt transforms password:
Password: "diary!pass"
       ↓
   Scrypt (SLOW on purpose!)
       ↓
Encryption Key: 32 bytes (random-looking)
       ↓
Result:
- Attacker tries "diary!pass"
- Computer tries to derive key...
- Takes YEARS (N=32768 iterations)
- Eve gives up!

Scrypt makes it computationally expensive to derive the key from the password:

Normal computers: Takes milliseconds
GPUs/ASICs: Still takes TOO LONG to be practical

⚡ Features of AES-GCM

1. Authenticated Encryption

AES-GCM doesn't just encrypt; it also signs!

Encryption:
Plaintext → "x7k!m#4L$d" (ciphertext)
Auth Tag: "7x!mL#4$dK9" (signature)

Decryption:
1. Verify auth tag first!
2. If matches → Decrypt
3. If doesn't match → REJECT! (tampered with!)

2. Unique IV Every Time

Message 1: "Hello!" → IV "a7x!mL#" → Encrypt
Message 2: "Hello!" → IV "k7$!m#4" → Encrypt (different IV!)
Message 3: "Hello!" → IV "L9$dK7x!"   → Encrypt (different!)

Never reuse IV! Otherwise:
- If IV reused with same key → patterns emerge
- Attacker learns about your plaintext!

3. Salt Makes It Unique

Even with same password, each encryption is different:

Encryption #1:
Password: "password"
Salt: "a7x!mL#4"
→ Different encryption!

Encryption #2:
Password: "password"  
Salt: "k7$!m#4"
→ Different encryption again!

Result:
- Eve doesn't know salt (encrypted in metadata)
- Eve doesn't know IV (encrypted in metadata)
- Eve can't recreate encryption!

🎮 Try It Yourself

Question 1: If Alice uses AES cipher layer with password "secret123", what does Eve need to decrypt?

Show Answer

Eve needs:

The ciphertext Bob receives
The password "secret123" (or brute force)
The salt (embedded in the encrypted metadata)
The IV (embedded in the encrypted metadata)
Wait for scrypt to derive the key (SLOOOOW!)

Eve's problem: Even if she has password, she needs the salt + IV, and scrypt takes too long to be practical!

Question 2: Why does AES-GCM use authenticated encryption?

Answer: So you can detect if someone tampered with the ciphertext while in transit. If Eve modifies even one bit of the encrypted data, the authentication tag won't match when Bob tries to decrypt, and Bob rejects the message as tampered with.

Question 3: What happens if someone reuses the same IV with the same encryption key?

Answer: This can leak information about your plaintext! If you encrypt the same message twice with the same IV, attacker might notice the ciphertexts are identical and learn something about your messages. Also, some attacks (like related-key attacks) can break encryption when keys are reused. Always use a random IV for each encryption!

💡 Real-World Applications

Password Managers

LastPass, 1Password, Bitwarden:
- Store your passwords online
- Use AES-GCM encryption
- You provide YOUR password → Encrypted blob
- They can't see your password
- They can't decrypt your vault!

Attackers:
- Steal encrypted blob
- Without YOUR password → Can't decrypt!

Secure Cloud Storage

Google Drive, Dropbox, iCloud:
- Files you upload are encrypted
- AES-GCM encryption
- Without your account → Can't access
- Even Google can't read your files!

Encrypted Messaging

WhatsApp (optional), Signal (optional):
- Message keys protected
- AES encryption + DH handoff
- Even if server compromised → Messages still encrypted
- Forward secrecy: Old messages deleted after use

✅ Quick Check

**Can you explain AES encryption to a 5-year-old?

Try saying this out loud:

"AES is like a magic box that changes your message into scrambled gibberish. You use a special code number (password) to lock it. Only someone with the code number can open the box and get your message back. It's like a safe deposit box at the bank with a combination lock that only you know!"

🎓 Key Takeaways

✅ AES Cipher Layer = Safe deposit box from bank
✅ Password → Scrypt → Key = Making passwords hard to guess
✅ AES-GCM = Encryption + signature (authenticated)
✅ Unique IV = Never reuse (security risk)
✅ Salt makes it unique = Same password, different encryption
✅ GPU/ASIC resistant = Scrypt slows attackers

Now you understand AES cipher layer. Next: The key exchange method (DH layer)!

AES Cipher Layer Explained​

🎯 The Simple Story​

🧠 Mental Model​

📊 How AES Encryption Works​

The Process​

What Each Step Does​

🎭 The Story: Alice's Online Diary​

Without AES encryption:​

With AES encryption (Cipher Layer):​

🔑 Passwords + Scrypt​

Why Not Just Use the Password Directly?​

⚡ Features of AES-GCM​

1. Authenticated Encryption​

2. Unique IV Every Time​

3. Salt Makes It Unique​

🎮 Try It Yourself​

💡 Real-World Applications​

Password Managers​

Secure Cloud Storage​

Encrypted Messaging​

✅ Quick Check​

🎓 Key Takeaways​