用c语言实现一个对称分组加密/解密算法

对称分组加密算法是一种将明文按照一定的规律分成若干个固定长度的块，然后使用相同的密钥对每个块进行加密或解密的算法。下面给出一个基于C语言实现的对称分组加密/解密算法的示例代码。

该算法采用AES加密算法，使用128位密钥，加密和解密的块大小都为16字节。示例代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <openssl/aes.h>

void encrypt_block(unsigned char *key, unsigned char *input, unsigned char *output)
{
    AES_KEY aes_key;
    AES_set_encrypt_key(key, 128, &aes_key);
    AES_encrypt(input, output, &aes_key);
}

void decrypt_block(unsigned char *key, unsigned char *input, unsigned char *output)
{
    AES_KEY aes_key;
    AES_set_decrypt_key(key, 128, &aes_key);
    AES_decrypt(input, output, &aes_key);
}

void encrypt(unsigned char *key, unsigned char *input, size_t len, unsigned char *output)
{
    size_t i, j, blocks;
    unsigned char input_block[16], output_block[16];

    blocks = len / 16;

    for (i = 0; i < blocks; i++) {
        memcpy(input_block, input + i * 16, 16);
        encrypt_block(key, input_block, output_block);
        memcpy(output + i * 16, output_block, 16);
    }

    if (len % 16 != 0) {
        memset(input_block, 0, 16);
        memcpy(input_block, input + blocks * 16, len % 16);
        encrypt_block(key, input_block, output_block);
        memcpy(output + blocks * 16, output_block, 16);
    }
}

void decrypt(unsigned char *key, unsigned char *input, size_t len, unsigned char *output)
{
    size_t i, j, blocks;
    unsigned char input_block[16], output_block[16];

    blocks = len / 16;

    for (i = 0; i < blocks; i++) {
        memcpy(input_block, input + i * 16, 16);
        decrypt_block(key, input_block, output_block);
        memcpy(output + i * 16, output_block, 16);
    }

    if (len % 16 != 0) {
        fprintf(stderr, "Invalid input length\n");
        exit(1);
    }
}

int main()
{
    unsigned char key[16] = {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0f};
    unsigned char input[32] = "Hello, World! This is a test.";
    unsigned char output[32];
    size_t len = strlen((char *)input);

    printf("Input: %s\n", input);

    encrypt(key, input, len, output);
    printf("Encrypted: ");
    for (size_t i = 0; i < len; i++) {
        printf("%02x", output[i]);
    }
    printf("\n");

    decrypt(key, output, len, input);
    printf("Decrypted: %s\n", input);

    return 0;
}

在上面的代码中，我们首先定义了两个函数encrypt_block和decrypt_block，用于对单个块进行加密和解密。然后定义了encrypt和decrypt函数，用于对整个数据进行加密和解密。这些函数都是基于OpenSSL库中的AES函数实现的。

在main函数中，我们定义了一个128位密钥和一个32字节的明文，然后使用encrypt函数对明文进行加密，并使用decrypt函数对密文进行解密，最终输出解密后的明文。