Whenever you send a message, pay online, or backup files to the cloud, encryption is operating behind the scenes to keep your information safe using AES cryptography. In the centre of this secure digital experience is the Advanced Encryption Standard (AES).
AES was officially approved by NIST (National Institute of Standards and Technology) in 2001 to replace the ageing DES algorithm, a new standard for the entire world’s encryption. This article details the explanation of AES in a simple format suitable for interview preparation.
Before learning about Advanced Encryption Standard (AES), its classification must be understood first symmetric encryption.
Symmetric vs. Asymmetric Encryption
Symmetric Encryption
Symmetric encryption depends on a single shared secret key for both encrypting and decrypting data. AES is one of the most commonly used examples of this method.
Asymmetric Encryption
In Asymmetric encryption, there is a pair of keys, a public and a private, with one key encrypting the data, and another key decrypting the data. RSA is an example of this method.
Block Cipher Basics
AES is an algorithm type of Block Cipher which encrypts the data in a fixed-size block (128 bits). Unlike stream ciphers that handle data bit by bit, block ciphers like AES process chunks of information, offering stronger and more structured security.
Key Size in AES
Advanced Encryption Standard (AES) is supported with three key lengths
- AES-128: 128-bit key
- AES-192: 192-bit key
- AES-256: 256-bit key
Most generally, the longer the encryption key, the harder it is to crack. Take AES-128, for example, cracking it by brute force with today’s computing power would take billions of years, making it practically unbreakable.
Importance of Key Management
Although AES is very good in itself, the issue of how to handle encryption keys is something that remains a major challenge. Ensuring how keys are stored, shared and handled during their entire lifecycle forms a major factor in the security of any encrypted system.
Creation of Round Keys
AES uses a process called key expansion, or key scheduling, to generate a series of round keys from the original key. This makes sure that each round of encryption or decryption uses a different key derived from the main one.
The AES Encryption: A Step-by-Step Guide
Advanced Encryption Standard (AES Cryptography) encryption transforms plaintext into ciphertext through a series of mathematical operations known as rounds. The number of rounds in AES encryption changes based on the length of the key used in the process.
- 10 rounds for AES-128
- 12 rounds for AES-192
- 14 rounds for AES-256
High-Level Overview
Each round of AES involves four main steps
- SubBytes
- ShiftRows
- MixColumns
- AddRoundKey
SubBytes
This is a substitution step.
- Every byte in the 4×4 matrix is substituted using an S-box, which acts like a lookup table.
- No byte is replaced by itself or its complement.
- This introduces non-linearity and confusion.
ShiftRows
This is a permutation step.
Each row of the matrix is moved to the left.
- Row 1: No shift
- Row 2: Shift left by 1
- Row 3: Shift left by 2
- Row 4: Shift left by 3
Before
[ b0 | b1 | b2 | b3 ]
[ b4 | b5 | b6 | b7 ]
[ b8 | b9 | b10 | b11 ]
[ b12 | b13 | b14 | b15 ]
After
[ b0 | b1 | b2 | b3 ]
[ b5 | b6 | b7 | b4 ]
[ b10 | b11 | b8 | b9 ]
[ b15 | b12 | b13 | b14 ]
Mix Columns
Another permutation step.
- Each column is multiplied by a fixed matrix.
- This spreads byte influence across the matrix.
Example transformation
[ c0 ] [ 2 3 1 1 ] [ b0 ]
[ c1 ] = [ 1 2 3 1 ] x [ b1 ]
[ c2 ] [ 1 1 2 3 ] [ b2 ]
[ c3 ] [ 3 1 1 2 ] [ b3 ]
(Note: This step is not performed in the final round.)
Add Round Key
- The matrix is then combined with the round key through an XOR operation.
- This is where the actual encryption happens by mixing in the key.
The AES process proceeds for all the rounds. The final round of AES skips the MixColumns step.
AES Decryption
AES decryption works by reversing the steps of the encryption process. Each block (128 bits) passes through 10, 12, or 14 rounds depending on the key size.
Each round of decryption goes through these steps
- Add round key
- Inverse MixColumns
- ShiftRows
- Inverse SubByte
AddRoundKey
XORs the state with the round key.
Inverse MixColumns
Matrix multiplication is performed using a constant matrix that differs from the one used in encryption.
Example
[ b0 ] [ 14 11 13 9 ] [ c0 ]
[ b1 ] = [ 9 14 11 13 ] x [ c1 ]
[ b2 ] [ 13 9 14 11 ] [ c2 ]
[ b3 ] [ 11 13 9 14 ] [ c3 ]
Inverse ShiftRows
The row shifts are reversed by moving them to the right.
Inverse SubBytes
The bytes are replaced using the inverse S-box, which undoes the SubBytes transformation.
These reversed steps collectively restore the original plaintext from the ciphertext.
AES Key Expansion (Key Schedule)
AES generates a different round key for each stage using a key scheduling algorithm.
Key Expansion Process
Start with the initial cipher key.
For each new round key
- Apply byte rotation (RotWord).
- Substitute bytes using the S-box (SubWord).
- Add a constant using Rcon.
- XOR with previous key data.
This ensures cryptographic uniqueness for every round.
Why AES cryptography is Highly Trusted
- Resistant to cryptographic attacks
- Supports strong key lengths
- Efficient in both hardware and software
Real-World Applications
- HTTPS & TLS
- VPNs & Network Encryption
- Disk and File Encryption
- Wi-Fi Security (WPA2/WPA3)
AES in Interview Preparation
Frequently Asked Questions in the interview
- What is AES, and how does it work?
- How does symmetric encryption differ from asymmetric encryption?
- What are AES key lengths and their associated rounds?
- Explain each AES step in simple terms.
- Where is AES used in practice?
Conclusion
AES is a basic element in current data protection. From its initial design of having symmetric constructs to its transformational processes, this building block has become a permanent structure in securing communication in different platforms and industries. Its popularity and strong architecture demonstrate its efficiency and resistance to modern risks.
Whether you are learning about cybersecurity, getting yourself ready for technical interviews or working with modern AES cryptography technologies, knowledge of AES is beneficial for the long term, professionally. It is not only with technical understanding but also with awareness regarding the importance of data protection in a growing digital environment – the more one knows about how AES works, the better.