用c代码实现S-AES的加密解密过程

AES (Advanced Encryption Standard) 是一种广泛使用的块密码算法，用于对数据进行安全加密。在C语言中，使用OpenSSL库可以方便地实现AES的加密和解密操作。以下是使用OpenSSL库基本步骤：

#include <openssl/aes.h>
#include <openssl/evp.h>
#include <string.h>

// AES Key size and block size in bytes
const int key_size = AES_KEY_SIZE; // For AES-128, AES-192 or AES-256
const int block_size = AES_BLOCK_SIZE;

void encrypt(char *plaintext, char *key, char *ciphertext) {
    AES_KEY aes_key;
    AES_set_encrypt_key((unsigned char*)key, key_size * 8, &aes_key);
    
    unsigned char iv[AES_BLOCK_SIZE]; // Initialize with zeros if not using a specific IV
    memset(iv, 0, AES_BLOCK_SIZE);
    
    // Create an encryption context with the key and initialization vector
    EVP_CIPHER_CTX evp_cipher_ctx;
    EVP_CipherInit(&evp_cipher_ctx, EVP_aes_128_cbc(), NULL, NULL, iv);

    // Encrypt data
    int len = strlen(plaintext);
    ciphertext = malloc(len + AES_BLOCK_SIZE);
    if (!EVP_CipherUpdate(&evp_cipher_ctx, ciphertext, &len, (unsigned char*)plaintext, len)) {
        printf("Encryption error\n");
        free(ciphertext);
        return;
    }

    // Add padding and finalize the encryption
    ciphertext[len] = AES_BLOCK_SIZE - (len % AES_BLOCK_SIZE); // Add padding byte(s)
    if (!EVP_CipherFinal(&evp_cipher_ctx, ciphertext + len, &len)) {
        printf("Encryption error\n");
    }
    ciphertext[len] = '\0'; // Terminate the ciphertext string

    // Store any necessary information for decryption, such as IV
}

void decrypt(char *ciphertext, char *key, char *plaintext) {
    AES_KEY aes_key;
    AES_set_decrypt_key((unsigned char*)key, key_size * 8, &aes_key);
    
    unsigned char iv[AES_BLOCK_SIZE];
    // You would typically retrieve the IV from somewhere here, depending on your application

    // Reuse the same encryption context but switch to decryption mode
    EVP_CIPHER_CTX_init(&evp_cipher_ctx);
    EVP_CipherInit(&evp_cipher_ctx, EVP_aes_128_cbc(), NULL, &iv, NULL);

    int len = strlen(ciphertext);
    plaintext = malloc(len);
    if (!EVP_CipherUpdate(&evp_cipher_ctx, plaintext, &len, ciphertext, len)) {
        printf("Decryption error\n");
        free(plaintext);
        return;
    }

    // Remove padding
    int padding = ciphertext[len - 1];
    for (int i = len - 1; i >= 0 && plaintext[i] == padding; i--) {}

    plaintext[i+1] = '\0';

    if (!EVP_CipherFinal(&evp_cipher_ctx, plaintext + len, &len)) {
        printf("Decryption error\n");
    }

    free(ciphertext); // Free memory allocated for ciphertext
}