Advanced Encryption Standard (AES)

The Advanced Encryption Standard (AES) is an encryption standard established in 2001 by the National Institute of Standards and Technology (NIST) of the USA. It is implemented worldwide in both hardware and software to encrypt sensitive data. AES finds wide usage while transmitting data over computer networks, particularly in wireless networks.

Features of AES

• Block cipher architecture − AES is a subset of the Rijndael block cipher that operates on fixed-size data blocks.

• DES successor − It is stronger and faster than Data Encryption Standard (DES), providing enhanced security.

• Symmetric key encryption − Uses the same key for both encryption and decryption operations.

• 128-bit block size − Operates on 128-bit (16 bytes) data blocks arranged in a 4×4 matrix.

• Variable key lengths − Supports cipher keys of 128, 192, or 256 bits for different security levels.

• Byte-oriented operations − All computations are performed on bytes rather than individual bits.

Operation of AES

The encryption process of AES is based upon substitution and permutation operations in an iterative manner. The 16 bytes of data are arranged in a 4×4 matrix. AES performs rounds of substitution-permutation operations, with each round using a different cipher key calculated from the original AES key.

The number of rounds depends upon the key size:

• 128-bit cipher key − 10 rounds

• 192-bit cipher key − 12 rounds

• 256-bit cipher key − 14 rounds

AES Encryption Stages

The AES encryption algorithm consists of the following stages:

• Key Expansion − Round keys are calculated from the cipher key using Rijndael's key schedule algorithm.

• Initial Round (Pre-Transformation) − Comprises only AddRoundKey operation, where XOR is performed between each data byte and the corresponding round key byte.

• Main Rounds (1 to N-1) − Four sub-processes are performed:

  • SubBytes − Non-linear substitution using an S-box lookup table

  • ShiftRows − Cyclical left shift of the last three rows by different offsets

  • MixColumns − Matrix multiplication to mix data within columns

  • AddRoundKey − XOR operation with the round key

• Final Round (Round N) − Contains SubBytes, ShiftRows, and AddRoundKey (MixColumns is omitted).

Security and Applications

AES provides robust security through its large key space and complex round operations. It is widely used in:

• Network security − VPNs, wireless protocols (WPA2/WPA3), and secure communications

• File encryption − Protecting sensitive documents and databases

• Government applications − Approved for classified information up to SECRET level

Conclusion

AES is a highly secure symmetric encryption standard that operates on 128-bit blocks using variable key lengths. Its iterative round-based structure with substitution and permutation operations provides strong cryptographic protection for sensitive data across various applications.