C语言通过位移、异或计算对字节数组加密

最近在工作中做数据安全模块，有些非常巧妙的位移、异或计算加解密使用的非常好，下面记录一下。

1、加密代码 

参数中 plaintext为明文、plaintext_size=64、ciphertext为密文、ciphertext_size=128

int fake_rsa_encrypt(
	const unsigned char* plaintext,
	unsigned int plaintext_size,
	unsigned char *ciphertext,
	size_t *ciphertext_size)
{
	struct timeval time;
	gettimeofday(&time, NULL);
	// microsecond has 1 000 000
	// Assuming you did not need quite that accuracy
	// Also do not assume the system clock has that accuracy.
	srand((time.tv_sec * 1000) + (time.tv_usec / 1000));
	if ((plaintext_size < 128) && (!(plaintext_size & 3))) {
		int i, j;
		for (i = 0; i < (int)(128 - plaintext_size); i += 4) {
			long rdm = random();
			ciphertext[i + 0] = (rdm >> 24) & 0xff;
			ciphertext[i + 1] = (rdm >> 16) & 0xff;
			ciphertext[i + 2] = (rdm >> 8) & 0xff;
			ciphertext[i + 3] = (rdm)& 0xff;
		}
		for (i = 128 - plaintext_size, j = 0; i < 128; ++i, ++j) {
			ciphertext[i] = plaintext[j] ^ ciphertext[j];
		}
	}
	else {
		return -1;
	}
	*ciphertext_size = 128;
	return 0;
}

注意：!(plaintext_size & 3) 的意思是判断是否非0，参考以下文章，传送门：&运算（位运算）

上述代码中有一段 

struct timeval time;
gettimeofday(&time, NULL);
// microsecond has 1 000 000
// Assuming you did not need quite that accuracy
// Also do not assume the system clock has that accuracy.
srand((time.tv_sec * 1000) + (time.tv_usec / 1000));

这个代码的含义是将当前时间设置为一个随机种子，然后通过以下代码获取随机数

    long rdm = random();

 上面这两个方法是配合使用的，需要先用一个独特的数设置种子，然后再获取随机数。

2、解密代码 

参数中 ciphertext为密文、plaintext为明文、plaintext_size=64

int fake_rsa_decrypt(
	const unsigned char* ciphertext,
	unsigned char *plaintext,
	size_t *plaintext_size)
{
	*plaintext_size = 64;
	if (((*plaintext_size) < 128) && (!((*plaintext_size) & 3))) {
		int i;
		for (i = 0; i < (int)(*plaintext_size); i++) {
			plaintext[i] = ciphertext[128 - (*plaintext_size) + i] ^ ciphertext[i];
		}
	}
	else {
		return -1;
	}
	return 0;
}

精妙之处就是 ciphertext 是128位的密文数组，他的前64位保存的是随机数数组，后64位保存的是异或计算后的数组，用后64位和前64位进行异或计算就能够得到加密之前传入的明文数组了，非常巧妙吧。