Implementing AES-128 CTR Mode in C: A Step-by-Step Guide

Implementing AES-128 CTR Mode in C

This guide walks through implementing AES-128 CTR mode in C, covering block operations, key expansion, and encryption/decryption. The process involves 11 round keys and 16-byte block manipulations.

Why This Matters

AES-128 is a standardized symmetric encryption algorithm, but its security relies on precise implementation. Errors in memory management (e.g., forgetting to free allocated rows/columns) or incorrect sbox/mixColumns logic can break encryption entirely. A single misstep in the 11-round key schedule could compromise 128-bit security, leading to data breaches in systems relying on this implementation.

Key Insights

• “128-bit CTR mode requires 11 round keys derived from the cipher key using key expansion.”
• “Sbox substitution and mixColumns operations use precomputed Galois Field tables for efficiency.”
• “Dynamic memory allocation (malloc/free) is critical for row/column operations in 4x4 blocks.”

Working Example

void printBlock(uint8_t* block) {
    for (int i = 0; i < 16; i++) {
        printf("%02X ", block[i]);
        if ((i + 1) % 4 == 0) printf("\n");
    }
}

uint8_t* getRow(uint8_t* block, int nthRow) {
    uint8_t* row = malloc(4);
    memcpy(row, block + nthRow * 4, 4);
    return row;
}

void rotateLeftRow(uint8_t* row) {
    uint8_t temp = row[0];
    memmove(row, row + 1, 3);
    row[3] = temp;
}

Practical Applications

• Use Case: Secure messaging apps using AES-CTR for real-time encryption.
• Pitfall: Forgetting to free dynamically allocated rows/columns causes memory leaks and instability.

References:

• https://dev.to/denizbasgoren/aes-algorithm-for-beginners-114f

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