C语言在安全领域的应用

```c
#include <stdio.h>
#include <string.h>
#include <openssl/aes.h>
#define AES_BLOCK_SIZE 16
void aes_encrypt(unsigned char *plaintext, int plaintext_len, unsigned char *key, unsigned char *ciphertext) {
    AES_KEY aesKey;
    AES_set_encrypt_key(key, 128, &aesKey);
    
    int num_blocks = plaintext_len / AES_BLOCK_SIZE;
    int left_bytes = plaintext_len % AES_BLOCK_SIZE;
    
    unsigned char *buf = (unsigned char *)malloc(plaintext_len + AES_BLOCK_SIZE);
    memset(buf, 0, plaintext_len + AES_BLOCK_SIZE);
    memcpy(buf, plaintext, plaintext_len);
    
    for (int i = 0; i < num_blocks; i++) {
        AES_encrypt(buf + i * AES_BLOCK_SIZE, ciphertext + i * AES_BLOCK_SIZE, &aesKey);
    }
    
    if (left_bytes > 0) {
        for (int i = 0; i < left_bytes; i++) {
            ciphertext[num_blocks * AES_BLOCK_SIZE + i] = buf[num_blocks * AES_BLOCK_SIZE + i];
        }
    }
    
    free(buf);
}
void aes_decrypt(unsigned char *ciphertext, int ciphertext_len, unsigned char *key, unsigned char *plaintext) {
    AES_KEY aesKey;
    AES_set_decrypt_key(key, 128, &aesKey);
    
    int num_blocks = ciphertext_len / AES_BLOCK_SIZE;
    int left_bytes = ciphertext_len % AES_BLOCK_SIZE;
    
    unsigned char *buf = (unsigned char *)malloc(ciphertext_len + AES_BLOCK_SIZE);
    memset(buf, 0, ciphertext_len + AES_BLOCK_SIZE);
    memcpy(buf, ciphertext, ciphertext_len);
    
    for (int i = 0; i < num_blocks; i++) {
        AES_decrypt(buf + i * AES_BLOCK_SIZE, plaintext + i * AES_BLOCK_SIZE, &aesKey);
    }
    
    if (left_bytes > 0) {
        for (int i = 0; i < left_bytes; i++) {
            plaintext[num_blocks *AES_BLOCK_SIZE + i] = buf[num_blocks * AES_BLOCK_SIZE + i];
        }
    }
    
    plaintext[plaintext_len] = '\0'; // Ensure null-terminated string for safety
    free(buf);
}
int main() {
    unsigned char key[] = "0123456789abcdef"; // 128-bit AES key
    unsigned char plaintext[] = "This is a secret message";
    unsigned char ciphertext[128]; // Must be large enough to hold encrypted data (padded)
    unsigned char decryptedtext[128]; // Must be large enough to hold decrypted data
    
    int plaintext_len = strlen((char *)plaintext);
    int ciphertext_len = (plaintext_len + AES_BLOCK_SIZE) / AES_BLOCK_SIZE * AES_BLOCK_SIZE; // Padding required
    
    printf("Original message: %s\n", plaintext);
    
    aes_encrypt(plaintext, plaintext_len, key, ciphertext);
    printf("Encrypted message: ");
    for (int i = 0; i < ciphertext_len; i++) {
        printf("%02x", ciphertext[i]);
    }
    printf("\n");
    
    aes_decrypt(ciphertext, ciphertext_len, key, decryptedtext);
    printf("Decrypted message: %s\n", decryptedtext);
    
    return 0;
}
```