Understanding AES and DES Encryption Algorithms: Principles and Applications

In today’s digital world, data security is more critical than ever. Encryption algorithms form the backbone of secure communication, protecting everything from personal messages to financial transactions. Among symmetric encryption standards, AES (Advanced Encryption Standard) and DES (Data Encryption Standard) stand out as foundational technologies. This article dives deep into the principles, structures, and practical applications of both AES and DES, helping you understand how they work and why AES has become the modern gold standard in encryption.

What Is AES? The Modern Standard in Symmetric Encryption

AES, or Advanced Encryption Standard, is a symmetric block cipher adopted by the U.S. government in 2001 and now used globally to secure sensitive data. Originally known as Rijndael, it was selected by the National Institute of Standards and Technology (NIST) after a five-year evaluation process to replace the aging DES algorithm.

AES operates on fixed-size blocks of 128 bits (16 bytes) and supports key lengths of 128, 192, or 256 bits, with longer keys offering higher security. It's widely used in applications ranging from secure web browsing (HTTPS) to file encryption and blockchain technology.

👉 Discover how AES encryption powers secure digital transactions today.

Key Components of AES Encryption

To fully grasp AES, it’s essential to understand its core components:

• Plaintext (P): The original unencrypted data.
• Key (K): A secret value shared between sender and receiver. In AES, the same key encrypts and decrypts.
• Encryption Function (E): Defined as C = E(K, P), where the plaintext and key produce ciphertext.
• Ciphertext (C): The encrypted output, unreadable without the correct key.
• Decryption Function (D): Reverses the process: P = D(K, C).

In real-world systems, AES keys are often exchanged securely using asymmetric encryption like RSA before switching to faster AES for bulk data transfer.

AES Encryption Process: A Step-by-Step Breakdown

AES-128—the most common variant—uses a 128-bit key and performs 10 rounds of transformation. Each round applies a series of operations that scramble the data progressively.

Core Operations in Each Round

Each round consists of four main steps (except the last round, which skips Column Mixing):

1. SubBytes (Byte Substitution)
2. ShiftRows (Row Shifting)
3. MixColumns (Column Mixing)
4. AddRoundKey (Key Addition)

These operations introduce confusion and diffusion—core principles in cryptography that make patterns unrecognizable.

SubBytes: Nonlinear Transformation via S-Box

The SubBytes step replaces each byte in the state matrix using a lookup table called the S-box. This nonlinear substitution prevents attackers from using linear approximations to crack the cipher.

For example:

• Input byte: 0x12
• High nibble (1) selects row; low nibble (2) selects column
• Output from S-box: 0xc9

This lookup ensures strong resistance against cryptanalysis.

ShiftRows: Permuting Data Across Rows

In ShiftRows, each row of the state matrix is shifted left circularly:

• Row 0: No shift
• Row 1: Shift left by 1 byte
• Row 2: Shift left by 2 bytes
• Row 3: Shift left by 3 bytes

This spreads bytes across columns, enhancing diffusion.

Decryption reverses this with right shifts.

MixColumns: Matrix Multiplication for Diffusion

MixColumns multiplies each column of the state matrix by a fixed polynomial matrix over the finite field GF(2⁸). This complex mathematical operation ensures that changing one byte affects multiple bytes in the next round.

The inverse operation is applied during decryption.

AddRoundKey: XOR with Round Key

In AddRoundKey, the state matrix is combined with a round-specific key using bitwise XOR. These round keys come from the key expansion process, which generates 44 words (176 bytes) from the original 128-bit key.

Key Expansion: Generating Round Keys

AES derives all round keys from the initial key through a process called key scheduling:

1. The 128-bit key fills a 4×4 matrix, forming four 32-bit words: W[0] to W[3].

2. Additional words W[4] to W[43] are generated recursively:

   • If i % 4 ≠ 0: W[i] = W[i−4] ⊕ W[i−1]
   • If i % 4 = 0: W[i] = W[i−4] ⊕ T(W[i−1])

Function T includes:

• RotWord: Cyclically shift bytes left
• SubWord: Apply S-box substitution
• Rcon: XOR with round constant

This ensures each round uses a unique, cryptographically strong key.

Understanding DES: The Legacy Block Cipher

Before AES, DES (Data Encryption Standard) was the dominant encryption algorithm. Introduced in the 1970s, it uses a 64-bit block size but only a 56-bit effective key length, due to 8 parity bits.

Despite its historical importance, DES is now considered insecure due to its short key length, vulnerable to brute-force attacks. However, understanding DES helps appreciate the evolution of cryptographic design.

Feistel Structure: The Backbone of DES

DES is based on the Feistel network, where data is split into two halves and processed through multiple rounds. Each round applies a function F to one half and combines it with the other using XOR.

Encryption formula:

$L_i = R_{i-1}$
$R_i = L_{i-1} \oplus F(R_{i-1}, K_i)$

Decryption works identically but uses subkeys in reverse order.

DES Encryption Steps

DES performs 16 rounds of processing:

1. Initial Permutation (IP)

The 64-bit plaintext undergoes a fixed bit rearrangement via the IP table. While not cryptographically strong, it helps obscure input patterns.

2. Round Function F(R, K)

Each round applies four operations:

• Expansion (E-box): Expands 32 bits to 48 for key mixing
• XOR with Subkey: Combines expanded data with a 48-bit round key
• S-box Substitution: Eight S-boxes reduce 48 bits back to 32
• P-box Permutation: Rearranges bits for diffusion

The Role of S-Boxes

S-boxes are the only nonlinear component in DES. Each takes a 6-bit input:

• First and sixth bits select one of four rows
• Middle four bits select a column
• Output is a 4-bit value from that cell

This design resists differential and linear cryptanalysis when properly implemented.

3. Final Inverse Permutation (IP⁻¹)

After 16 rounds, the final output undergoes IP⁻¹ to restore bit order, producing the ciphertext.

Common Block Cipher Modes of Operation

Block ciphers like AES and DES operate on fixed-size blocks. But real-world data varies in length—so how do we handle messages longer than one block?

Enter modes of operation, which define how multiple blocks are encrypted securely.

ECB – Electronic Codebook Mode

Each block is encrypted independently with the same key.

✅ Simple
❌ Exposes patterns: identical plaintext → identical ciphertext
🚫 Not recommended for structured data

Ideal only for encrypting small, random values like keys.

CBC – Cipher Block Chaining Mode

Each plaintext block is XORed with the previous ciphertext block before encryption. Requires an Initialization Vector (IV) for the first block.

✅ Hides patterns
✅ Widely supported
❌ No parallel encryption; error propagates

Used in older TLS versions and disk encryption.

👉 See how modern platforms use CBC and other modes for secure data transfer.

CFB – Cipher Feedback Mode

Turns a block cipher into a stream cipher. Encrypts an IV, then XORs output with plaintext to produce ciphertext.

✅ Supports streaming
✅ Self-synchronizing
❌ Sensitive to transmission errors

OFB – Output Feedback Mode

Generates keystream independently of plaintext/ciphertext. Similar to CFB but feedback comes from cipher output.

✅ No error propagation
✅ Suitable for noisy channels
❌ Keystream reuse is catastrophic

CTR – Counter Mode

Uses a counter + nonce instead of chaining. Each block encrypts a unique counter value, then XORs with plaintext.

✅ Highly parallelizable
✅ Random access to encrypted blocks
✅ Efficient and secure

Widely used in high-performance systems like databases and networks.

GCM – Galois/Counter Mode

Combines CTR encryption with GHASH authentication for authenticated encryption (AEAD).

✅ Provides confidentiality + integrity
✅ Hardware-accelerated on modern CPUs
✅ Used in TLS 1.2+ and secure APIs

AES-GCM is now the preferred mode for most applications.

Padding Schemes for Block Ciphers

When data doesn't align with block size (e.g., 8 bytes for DES, 16 for AES), padding fills the gap.

PKCS#7 Padding (Includes PKCS#5)

Adds N bytes of value N to reach full block size:

PKCS#5 is just PKCS#7 with fixed 8-byte blocks—so they’re interchangeable when block size is 8.

Always remove padding after decryption using the last byte’s value.

Frequently Asked Questions (FAQ)

Q1: What is the main difference between AES and DES?

AES uses a larger block size (128 bits vs. 64), supports longer keys (up to 256 bits), and is resistant to modern attacks. DES’s 56-bit key makes it obsolete for sensitive data.

Q2: Why is ECB mode considered insecure?

ECB encrypts identical plaintext blocks into identical ciphertext blocks, revealing patterns in images or structured data—making it easy to infer content without decryption.

Q3: Can AES be broken?

No practical attacks exist against full-round AES when properly implemented. Brute-force would take billions of years even with supercomputers.

Q4: How are AES keys exchanged securely?

Typically via asymmetric encryption like RSA or ECDH during handshake protocols (e.g., TLS). Once exchanged, AES handles fast bulk encryption.

Q5: What does “GCM” mean in AES-GCM?

Galois/Counter Mode combines CTR encryption with Galois field hashing for authenticated encryption—ensuring both privacy and message integrity.

Q6: Is DES still used anywhere?

Rarely. Some legacy systems may still use it, but triple DES (3DES) was its stopgap successor until AES adoption became widespread.

Final Thoughts: Why AES Dominates Modern Cryptography

While DES laid the groundwork for modern block ciphers, AES has emerged as the de facto global standard due to its efficiency, scalability, and resilience against known attacks. With support for multiple key sizes and advanced modes like GCM, it secures everything from mobile apps to blockchain transactions.

Understanding these algorithms isn’t just academic—it empowers developers, cybersecurity professionals, and users to make informed decisions about data protection.

👉 Learn how cutting-edge platforms implement AES for maximum security and performance.