How can I edit a value stored at an address through C++?

I need to be able to edit two regions of memory that are pointed to by two addresses stored in the ECX & EAX registers.

I'm doing this by placing a Code Cave in the clients original Executable that jumps to my Prepare_Encrypt() function.

My general idea is that Prepare_Encrypt() would get the addresses and should somehow be able to use them to create a pointer to the data in C++.

Once the pointer has been created I would create a For Loop that would preform an XOR operation on each character value in the pointer.

I'm new to using inline assembly so I would really like to get this working the way I want.

I figured creating a char* and using a MOV operation would work, but it doesn't. Does anyone have any ideas of how to get this to work?

Here is my current Prepare_Encrypt() function and my current Encrypt() function.

char* data_1;
char* data_2;

void Encrypt()
{
    for(var i = 0; i < sizeof(data_1); i++)
    {
        data_1[i] ^= 200;
    }

    for(var i = 0; i < sizeof(data_2); i++)
    {
        data_2[i] ^= 125;
    }
}

_declspec(naked) void Prepare_Encrypt()
{
    // Save program state.
    __asm
    {
        MOV ECX, ESI
        MOV DWORD PTR SS : [ESP+0x1C], EAX
        PUSHAD
        PUSHFD

        // Get Current Data From Memory
        MOV data_1, ECX
        MOV data_2, EAX
    }

    Encrypt();

    // Restore the program state and return to original code.
    __asm
    {
        POPFD
        POPAD
        JMP SendJMPTo
    }
}