已知一段密文32字节，同时提供加密的源代码如下。要求根据加密源码得到解密代码，从而获得password明文。
CipherText：
6B562E2D3E7B6C61636078616C666C62
#include <stdio.h>
void reverseBits(unsigned char* password) {
int i, j;
unsigned char temp;
for (i = 0; i < 16; i++) {
temp = 0;
for (j = 0; j < 8; j++) {
temp |= ((password[i] >> j) & 1) << (7 - j);
}
password[i] = temp;
}
}
void swapPositions(unsigned char* password) {
int i;
unsigned char temp[16];
int positions[16] =
{
13, 4, 0, 5,
2, 12, 11, 8,
10, 6, 1, 9,
3, 15, 7, 14
};
for (i = 0; i < 16; i++) {
temp[positions[i]] = password[i];
}
for (i = 0; i < 16; i++) {
password[i] = temp[i];
}
}
void leftShiftBytes(unsigned char* password) {
for (int i = 0; i < 16; i++) {
password[i] = password[i] << 3 | password[i] >> 5;
}
}
void xorWithKeys(unsigned char* password, unsigned int round) {
int i;
for (i = 0; i < 16; i++) {
password[i] ^= (unsigned char)(0x78 * round & 0xFF);
}
}
void encryptPassword(unsigned char* password) {
int i;
unsigned int round;
for (round = 0; round < 16; round++) {
reverseBits(password);
swapPositions(password);
leftShiftBytes(password);
xorWithKeys(password, round);
}
}
int main() {
unsigned char password[17] = "1234567890";
printf("加密前的口令为：\n");
for (int i = 0; i < 16; i++) {
printf("%02X ", password[i]);
}
encryptPassword(password);
printf("加密后的口令为：\n");
for (int i = 0; i < 16; i++) {
printf("%02X ", password[i]);
}
printf("\n");
return 0;
}
分析：思路比较直接，解密是加密的逆过程来一遍就可以了。
解密代码如下：
#include <stdio.h>
void reverseBits_dec(unsigned char* password) {
int i, j;
unsigned char temp;
for (i = 0; i < 16; i++) {
temp = 0;
for (j = 0; j < 8; j++) {
temp |= ((password[i] >> (7 - j)) & 1) << j;
}
password[i] = temp;
}
}
void swapPositions_dec(unsigned char* password) {
int i;
unsigned char temp[16];
int positions[16] =
{
13, 4, 0, 5,
2, 12, 11, 8,
10, 6, 1, 9,
3, 15, 7, 14
};
for (i = 0; i < 16; i++) {
temp[i] = password[positions[i]];
}
for (i = 0; i < 16; i++) {
password[i] = temp[i];
}
}
void leftShiftBytes_dec(unsigned char* password) {
for (int i = 0; i < 16; i++) {
password[i] = password[i] >> 3 | password[i] << 5;
}
}
void xorWithKeys_dec(unsigned char* password, unsigned int round) {
int i;
for (i = 0; i < 16; i++) {
password[i] ^= (unsigned char)(0x78 * round & 0xFF);
}
}
void encryptPassword_dec(unsigned char* password) {
int i;
unsigned int round;
for (round = 16; round >= 1; round--) {
xorWithKeys_dec(password, round-1); //处理函数不变，修改参数即可
leftShiftBytes_dec(password); //加密是循环左移3位，解密则循环右3位
swapPositions_dec(password); //position矩阵不变，index<-->value
reverseBits_dec(password); //位逆转做逆向处理即可
}
}
int main() {
unsigned char password[17] = {0x6B,0x56,0x2E,0x2D,0x3E,0x7B,0x6C,0x61,0x63,0x60,0x78,0x61,0x6C,0x66,0x6C,0x62};
printf("加密后的口令为：\n");
for (int i = 0; i < 16; i++) {
printf("%02X ", password[i]);
}
printf("\n");
encryptPassword_dec(password);
printf("加密前的口令为：\n");
for (int i = 0; i < 16; i++) {
printf("%c", password[i]);
}
printf("\n");
return 0;
}
运行结果如下：
----------------------------------------------------
加密后的口令为：
6B 56 2E 2D 3E 7B 6C 61 63 60 78 61 6C 66 6C 62
加密前的口令为：
pdksidicndjh%^&6
-----------------------------------------------------