Block Ciphers

Block ciphers encrypt data in fixed-size blocks (e.g., 128 bits) using a symmetric key, making them efficient and secure. AES, a widely used block cipher, protects everything from files to internet traffic. This article explains their mechanics, AES, modes, and uses.

How They Work

Block ciphers process plaintext in blocks:

• Encryption: \( C_i = E(P_i, K) \), where \( P_i \) is a plaintext block, \( K \) is the key.
• Decryption: \( P_i = D(C_i, K) \).

They use rounds of substitution and permutation for confusion and diffusion.

AES Overview

Advanced Encryption Standard (AES):

• Key Sizes: 128, 192, or 256 bits.
• Block Size: 128 bits.
• Rounds: 10, 12, or 14 (based on key size).

Steps: Key expansion, SubBytes, ShiftRows, MixColumns, AddRoundKey.

Operation Modes

• ECB (Electronic Codebook): Each block encrypted independently (weak).
• CBC (Cipher Block Chaining): XORs previous ciphertext with next plaintext.
• GCM: Adds authentication (used in TLS).

Modes enhance security and flexibility.

Applications

Block ciphers secure:

• Storage: Disk encryption (e.g., BitLocker).
• Networks: VPNs, HTTPS (AES in TLS).
• Devices: Secure boot in hardware.

They’re ubiquitous in modern security.