Overview of AES
The Advanced Encryption Standard (AES), also known as Rijndael cipher, is a symmetric block cipher adopted by the U.S. federal government. Developed to replace the aging DES (Data Encryption Standard), AES became effective on May 26, 2002, after a five-year selection process by the National Institute of Standards and Technology (NIST).
Key Characteristics:
- Block size: Fixed at 128 bits (16 bytes)
- Key lengths: 128-bit, 192-bit, or 256-bit options
- Rounds: 10 (AES-128), 12 (AES-192), or 14 (AES-256) encryption rounds
- Operation: Byte-oriented substitution-permutation network
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AES Encryption Components
Core Elements:
- Plaintext (P): Unencrypted input data
- Secret Key (K): Symmetric key for both encryption/decryption
- Ciphertext (C): Encrypted output data
- Encryption Function (E): C = E(K,P)
- Decryption Function (D): P = D(K,C)
Practical Implementation:
In real-world applications, RSA typically encrypts the AES key for secure transmission, while AES handles bulk data encryption.
AES Encryption Standards
| Variant | Key Length | Block Size | Rounds |
|---|---|---|---|
| AES-128 | 128-bit | 128-bit | 10 |
| AES-192 | 192-bit | 128-bit | 12 |
| AES-256 | 256-bit | 128-bit | 14 |
AES Algorithm Workflow (AES-128)
1. State Matrix Initialization
- Divides 128-bit input into 4×4 byte matrix
- Arranged in column-major order
2. Key Expansion
- 128-bit key → 44-word expanded key schedule
- First 4 words: Original key
- Subsequent 40 words: Derived via Rijndael's key schedule
Encryption Rounds (10 iterations):
- AddRoundKey: XOR state with round key
- SubBytes: Nonlinear byte substitution via S-box
- ShiftRows: Cyclic row shifts
- MixColumns: Matrix multiplication in GF(2⁸)
Final round omits MixColumns
Detailed Round Operations
Byte Substitution (SubBytes)
- Uses predefined 16×16 S-box
- Each byte replaced via: row = high nibble, column = low nibble
Row Shifting (ShiftRows)
- Row 0: Unchanged
- Row 1: 1-byte left shift
- Row 2: 2-byte left shift
- Row 3: 3-byte left shift
Column Mixing (MixColumns)
- Matrix multiplication in Galois Field
Each column transformed via:
s'0,c = (02 • s0,c) ⊕ (03 • s1,c) ⊕ s2,c ⊕ s3,c s'1,c = s0,c ⊕ (02 • s1,c) ⊕ (03 • s2,c) ⊕ s3,c s'2,c = s0,c ⊕ s1,c ⊕ (02 • s2,c) ⊕ (03 • s3,c) s'3,c = (03 • s0,c) ⊕ s1,c ⊕ s2,c ⊕ (02 • s3,c)
Key Addition (AddRoundKey)
- Simple XOR between state and round key
DES Encryption Standard
Overview
The Data Encryption Standard (DES) features:
- 64-bit blocks (8 bytes)
- 56-bit effective key length (64-bit with parity)
- 16-round Feistel structure
DES Structure
- Initial Permutation (IP)
- 16 Rounds of Feistel Network
- Final Permutation (IP⁻¹)
Round Function Components:
- Expansion: 32-bit → 48-bit via E-box
- Key Mixing: XOR with 48-bit subkey
- Substitution: 8 S-boxes (6-bit → 4-bit)
- Permutation: 32-bit P-box
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Block Cipher Modes of Operation
1. ECB (Electronic Codebook)
- Simplest mode: Each block encrypted independently
- Vulnerability: Reveals data patterns
2. CBC (Cipher Block Chaining)
- XORs plaintext with previous ciphertext
- Requires Initialization Vector (IV)
- Prevents pattern leakage
3. CFB (Cipher Feedback)
- Turns block cipher into stream cipher
- Self-synchronizing
4. OFB (Output Feedback)
- Stream cipher mode with error propagation
- Requires synchronization
5. CTR (Counter Mode)
- Parallelizable
- Random access capability
- Uses nonce + counter
Padding Schemes
PKCS#5/PKCS#7 Padding
Appends N bytes each of value N where:
N = block_size - (data_length % block_size)Example for 8-byte block:
- 3-byte data → 5 padding bytes (0x05)
- 8-byte data → 8 padding bytes (0x08)
FAQ
Q: Why does AES have different key lengths?
A: The 128/192/256-bit options provide tradeoffs between security and performance. Longer keys increase resistance to brute-force attacks but require more computation.
Q: How does CBC mode improve security over ECB?
A: CBC introduces chaining where each block's encryption depends on all previous blocks, hiding identical plaintext patterns that ECB reveals.
Q: What's the practical difference between DES and AES?
A: AES offers larger block sizes (128-bit vs 64-bit), longer key options, and more efficient software implementation while being mathematically stronger against modern cryptanalysis.
Q: When should I use CTR mode?
A: CTR is ideal when you need parallel encryption/decryption or random access to encrypted data, such as in disk encryption or network protocols.