Block ciphers (like AES, DES) encrypt data in fixed-size blocks (e.g., 128 bits for AES). But real-world data is usually longer or shorter than the block size, so modes of operation define how to repeatedly apply a block cipher to encrypt/decrypt data of arbitrary length securely.
Here are the most common block cipher modes:
🔒 1. ECB (Electronic Codebook Mode)
📌 How it works:
Each block is encrypted independently using the same key.
C₁ = E(K, P₁)
C₂ = E(K, P₂)
...
✅ Pros:
- Simple and fast.
- Allows parallel encryption.
❌ Cons:
- Patterns leak (identical plaintext blocks produce identical ciphertexts).
- Not semantically secure.
📘 Example:
Encrypting a BMP image with ECB will reveal the original image structure.
🔄 2. CBC (Cipher Block Chaining Mode)
📌 How it works:
Each plaintext block is XORed with the previous ciphertext block before encryption. First block uses an IV (Initialization Vector).
C₁ = E(K, P₁ ⊕ IV)
C₂ = E(K, P₂ ⊕ C₁)
...
✅ Pros:
- Hides patterns.
- Widely used and secure if IV is random.
❌ Cons:
- Slower due to chaining (no parallel encryption).
- Errors propagate (one corrupted block affects the next).
🔁 3. CFB (Cipher Feedback Mode)
📌 How it works:
Turns a block cipher into a stream cipher. Previous ciphertext block is encrypted and XORed with plaintext.
C₁ = P₁ ⊕ E(K, IV)
C₂ = P₂ ⊕ E(K, C₁)
...
✅ Pros:
- Can work with partial (non-block-size) data.
- No padding needed.
❌ Cons:
- Sequential processing (no parallelism).
- Bit errors propagate.
🔄 4. OFB (Output Feedback Mode)
📌 How it works:
Similar to CFB, but instead of using ciphertext for the next round, it uses the encrypted output.
O₁ = E(K, IV)
C₁ = P₁ ⊕ O₁
O₂ = E(K, O₁)
C₂ = P₂ ⊕ O₂
...
✅ Pros:
- Errors do not propagate.
- Preprocessing possible (parallelizable output stream).
❌ Cons:
- Sensitive to IV reuse (compromises security).
🧮 5. CTR (Counter Mode)
📌 How it works:
Encrypts a counter (nonce + counter) and XORs it with plaintext. Acts like a stream cipher.
C₁ = P₁ ⊕ E(K, CTR₁)
C₂ = P₂ ⊕ E(K, CTR₂)
...
✅ Pros:
- Fully parallelizable.
- Fast and efficient.
- Random access possible.
❌ Cons:
- Must never reuse nonce+counter with the same key.
🔁 Comparison Table:
| Mode | Parallel Encrypt | Pattern Leak | Needs IV | Stream Mode | Error Propagation |
|---|---|---|---|---|---|
| ECB | ✅ | ❌ Yes | ❌ No | ❌ No | ❌ No |
| CBC | ❌ | ✅ No | ✅ Yes | ❌ No | Affects next block |
| CFB | ❌ | ✅ No | ✅ Yes | ✅ Yes | Affects next block |
| OFB | ✅ | ✅ No | ✅ Yes | ✅ Yes | No propagation |
| CTR | ✅ | ✅ No | ✅ Yes | ✅ Yes | No propagation |