What are the key parameters of AES and why was it chosen to replace DES?

AES uses a 128-bit block size with 128, 192, or 256-bit keys over 10, 12, or 14 rounds respectively — it was chosen through an open international competition for its security, speed, and elegance.

AES parameters:

Block size: Always 128 bits (16 bytes)

Structure: Substitution-Permutation Network (SPN), NOT Feistel

Each round consists of 4 operations:

1. SubBytes — non-linear substitution via S-box (confusion)
2. ShiftRows — cyclic shift of rows (diffusion)
3. MixColumns — matrix multiplication in GF(2^8) (diffusion)
4. AddRoundKey — XOR with round key

Why AES won (2001):

• Open competition with 15 candidates, Rijndael (by Daemen & Rijmen from Belgium) won
• Extremely efficient in both hardware and software
• No known practical attacks after 25+ years
• Supported by dedicated CPU instructions (AES-NI) on modern processors

Go deeper:

• Advanced Encryption Standard (Wikipedia) — the Rijndael competition, variants, and structure.
• NIST FIPS 197 — the AES standard — the authoritative specification of AES.