Symmetric Key Algorithms and AES (Advanced Encryption Standard)

What is AES? (In Simple Words)

When DES became too weak due to its small 56-bit key and Triple DES (3DES) proved too slow in software, the US National Institute of Standards and Technology (NIST) held a global competition in 1997 to find a brand-new encryption standard.

In 2001, NIST selected the Rijndael algorithm—created by two Belgian cryptographers, Joan Daemen and Vincent Rijmen—and named it the Advanced Encryption Standard (AES).

💡 Real-Life Relevance:
Today, AES is the #1 most widely used symmetric encryption algorithm on Earth. It protects WhatsApp messages, Wi-Fi (WPA2/WPA3), HTTPS websites, UPI banking apps, and even top-secret government files!

1. Key Specifications of AES

Unlike DES (which uses a Feistel structure), AES is a Substitution-Permutation Network (SPN) that operates on a 128-bit block (16 bytes) arranged as a 4 × 4 grid of bytes called the State Matrix.

AES supports three different key lengths, and the number of rounds depends on the key size:

2. The 4 × 4 Byte “State Matrix”

AES takes 128 bits of Plain Text (which equals 128 ÷ 8 = 16 bytes: b0, b1, …, b15) and places them column by column into a 4 × 4 square table called the State Array:

State Matrix (4 × 4 Bytes):
| b0   b4   b8   b12 |
| b1   b5   b9   b13 |
| b2   b6   b10  b14 |
| b3   b7   b11  b15 |

Every round of AES modifies all 16 bytes of this 4 × 4 matrix at once!

3. The 4 Operations Inside Each AES Round

Before Round 1 starts, AES performs an initial AddRoundKey step (XORing the State Matrix with Key K0).
Then, each standard round of AES (Rounds 1 to 9 in AES-128) executes 4 distinct transformations in sequence:

1. SubBytes (Substitute Bytes — Non-Linear Substitution)

• Each of the 16 bytes in the 4 × 4 State Matrix is independently replaced by looking it up in a fixed 16 × 16 lookup table called the AES S-Box.
• For example, if a byte is 53 (in hex), the S-Box looks up Row 5, Column 3 and replaces it with ED.
• Purpose: Provides Confusion (hides the relationship between the Plain Text and Cipher Text).

2. ShiftRows (Row Permutation)

• The rows of the 4 × 4 State Matrix are cyclically shifted to the Left:
  • Row 0 (1st row): Not shifted at all (0 bytes shift).
  • Row 1 (2nd row): Shifted left by 1 byte.
  • Row 2 (3rd row): Shifted left by 2 bytes.
  • Row 3 (4th row): Shifted left by 3 bytes.
• Purpose: Spreads bytes across different columns (provides Diffusion).

3. MixColumns (Column Mixing)

• Each column of 4 bytes is multiplied by a fixed 4 × 4 mathematical matrix in finite field arithmetic (GF(2^8)).
• As a result, every output byte in a column becomes a combination of all 4 input bytes of that column.
• ⚠️ Important Exam Note: The Final Round (Round 10 in AES-128) skips the MixColumns step! It only performs SubBytes → ShiftRows → AddRoundKey.

4. AddRoundKey (Key Mixing)

• The 16 bytes (128 bits) of the State Matrix are simply XORed (⊕) with the 128-bit Round Subkey derived from the main secret key.

4. Comparison: DES vs. AES (100% Exam Favorite!)

This comparison table is one of the most frequently asked questions in DAVV BCA Unit 2:

💡 DAVV Exam Preparation Tips

5-Mark / 10-Mark Question: “Explain the AES (Advanced Encryption Standard) algorithm and its round operations. Differentiate between DES and AES.”

• Draw the AES key/round table (128-bit → 10 rounds, 192-bit → 12 rounds, 256-bit → 14 rounds).
• Draw the 4 × 4 State Matrix and the 4 Round Operations diagram (SubBytes → ShiftRows → MixColumns → AddRoundKey).
• Mention that the final round omits MixColumns.
• Write the 8-point DES vs. AES comparison table.