C/C++ Encrypt/Decrypt Functions & Memory At Runtime

// Our testfunction that we are going to encrypt
void TestFunction()
{
	MessageBox(0, L"Hello From Testfunction!", L"Test", 0);
}
// Marks the end of testfunction()
void FunctionStub() { return; }

void XorBlock(DWORD dwStartAddress, DWORD dwSize)
{
    __asm
	{
        push eax
        push ecx
        mov ecx, dwStartAddress          // Move Start Address to ECX
        add ecx, dwSize                  // Add the size of the function to ECX
        mov eax, dwStartAddress          // Copy the Start Address to EAX

        crypt_loop:                         // Start of the loop
            xor byte ptr ds:[eax], 0x4D     // XOR The current byte with 0x4D
            inc eax                         // Increment EAX with dwStartAddress++
            cmp eax,ecx                     // Check if every byte is XORed
        jl crypt_loop;                      // Else jump back to the start label

        pop ecx // pop ECX from stack
        pop eax // pop EAX from stack
    }
}

DWORD GetFuncSize(DWORD* Function, DWORD* StubFunction)
{
	DWORD dwFunctionSize = 0, dwOldProtect;
	DWORD *fnA = NULL, *fnB = NULL;

	fnA = (DWORD *)Function;
	fnB = (DWORD *)StubFunction;
	dwFunctionSize = (fnB - fnA);
	VirtualProtect(fnA, dwFunctionSize, PAGE_EXECUTE_READWRITE, &dwOldProtect); // Need to modify our privileges to the memory
	return dwFunctionSize;
}

void HexDumpMemory(void *addr, int len)
{
	int i;
	unsigned char buff[17];
	unsigned char *pc = (unsigned char*)addr;

	// Process every byte in the data.
	for (i = 0; i < len; i++) {
		// Multiple of 16 means new line (with line offset).

		if ((i % 16) == 0) {
			// Just don't print ASCII for the zeroth line.
			if (i != 0)
				printf("  %s\n", buff);

			// Output the offset.
			printf("  %04X ", i);
		}

		// Now the hex code for the specific character.
		printf(" %02X", pc[i]);
		// And store a printable ASCII character for later.
		if ((pc[i] < 0x20) || (pc[i] > 0x7e)) {
			buff[i % 16] = '.';
		}
		else {
			buff[i % 16] = pc[i];
		}

		buff[(i % 16) + 1] = '\0';
	}

	// Pad out last line if not exactly 16 characters.
	while ((i % 16) != 0) {
		printf("   ");
		i++;
	}

	// And print the final ASCII bit.
	printf("  %s\n", buff);
}

int main()
{
	DWORD dwFuncSize = GetFuncSize((DWORD*)&TestFunction, (DWORD*)&FunctionStub);
	
	printf("\n\nEncrypted:\n");
	XorBlock((DWORD)&TestFunction, dwFuncSize); // XOR encrypt the function
	HexDumpMemory(&TestFunction, dwFuncSize);

	//TestFunction(); // If you try to run the encrypted function you will get Access Violation Exception.

	printf("\n\nDecrypted:\n");
	XorBlock((DWORD)&TestFunction, dwFuncSize); // XOR decrypt the function   
	HexDumpMemory(&TestFunction, dwFuncSize);

	TestFunction(); // Fine here

	getchar();
    return 0;
}