AES is a symmetric key encryption algorithm that has replaced DES due to its higher security and efficiency.

1. Block Cipher Structure

AES encrypts fixed-size 128-bit blocks of data.
Unlike DES (which uses a Feistel structure), AES uses a substitution-permutation network (SPN).

2. Key Length Variants

AES supports three different key sizes, each defining the number of encryption rounds:

AES-128: 128-bit key, 10 rounds
AES-192: 192-bit key, 12 rounds
AES-256: 256-bit key, 14 rounds

3. Security & Resistance to Attacks

Stronger than DES and 3DES, resistant to brute-force attacks.
Resistant to differential and linear cryptanalysis, which were used to attack DES.
Not vulnerable to practical attacks except for side-channel attacks (e.g., timing attacks).

4. Encryption Process (Rounds Structure)

Each round consists of four main operations:

SubBytes: Byte substitution using an S-Box (nonlinear transformation).
ShiftRows: Row-wise shifting to enhance diffusion.
MixColumns: Column-wise mixing to introduce confusion (not used in the last round).
AddRoundKey: XOR operation with a round key derived from the main key.

5. Key Expansion (Key Schedule)

Uses Rijndael key schedule to generate multiple round keys from the original key.
Round keys are derived through byte substitution (S-Box), XOR operations, and shifts.

6. Speed & Efficiency

Faster than DES because it encrypts in 128-bit blocks instead of 64-bit.
Optimized for both hardware and software implementations.
Widely used in embedded systems, IoT, and cloud security due to its efficiency.

7. Common Applications

TLS/SSL (secure web communication)
Disk encryption (BitLocker, VeraCrypt)
Wi-Fi security (WPA2, WPA3)
Blockchain encryption (protecting private keys)
File and database encryption

8. Future Considerations

AES is currently secure against classical attacks, but quantum computing could pose future risks.
Post-quantum cryptography (PQC) research is ongoing to find alternatives.

AES remains the gold standard for symmetric encryption, providing high security, efficiency, and adaptability in modern cryptographic applications. 🚀

9. Weaknesses of AES

📌 Summary of AES Vulnerabilities

Vulnerability	Description	Mitigation
Brute-Force Attack	AES-128 weakened by quantum computing	Use AES-256
Key Schedule Weakness	Structural patterns in AES-256 key schedule	Use random keys, apply KMS (Key Management System)
Side-Channel Attacks	Timing, power analysis, and electromagnetic attacks	Maintain constant execution time, strengthen hardware security
Chosen-Plaintext Attack	Vulnerable to certain plaintext pattern analysis	Use random IV, implement AES-GCM
Related-Key Attack	Weakness in AES-192/AES-256 key schedule	Use fully random keys
Quantum Computing Threat	Grover’s algorithm weakens AES-128 security	Use AES-256, research Post-Quantum Cryptography (PQC)
Poor Implementation	Timing attacks successfully exploited in OpenSSL	Use secure libraries, apply security patches

AES remains one of the most secure encryption algorithms, but its implementation and key management must be carefully handled to mitigate potential vulnerabilities. 🚀

Symmetric key algorithm - (2) AES

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