Adding and removing byte from a file in a loop

Question

I'm a security researcher and am wondering how I could remove the added bytes from a file. This function below adds a byte every 4 bytes. But I'd like to know how I could remove those bytes as well. Need a function that will remove these bytes in the order they were added.

Travis

int AddByte(BYTE* bIn, BYTE* bOut, DWORD dwSize, int inc_every)
{
    int increased = 0;

    for (int i = 0; i < dwSize; i++)
    {
        *(bOut + increased) = *(bIn + i);
        increased++;

        if (i % inc_every == 0)
            increased += 1;
    }

    return increased;
}

Unfortunately you can't just remove a single byte from a file. You need to "shift" every byte after it to overwrite that byte. Looks like your file is memory mapped in which case one option is to use memmove. — kaylum
Do the same thing in reverse. Copy each byte to another array, skipping every 5th byte. — Weather Vane
What have you tried? Requests for code are OT. You can use something similar to what you have. — Weather Vane

Elzaidir Elzaidir · Accepted Answer · 2022-07-08T01:10:27

int RemoveByte(BYTE* bIn, BYTE* bOut, DWORD dwSize, int inc_every)
{
    int increased = 0;

    for (int i = 0; i < dwSize; i++)
    {

        if ((i - 1) % inc_every != 0)
        {
             *(bOut + increased) = *(bIn + i);
            increased++;
        }
    }

    return increased;
}

Applying this function after AddByte() would give back the original string. Your original function has an offset of 1 (it will skip the second byte), so this function includes the same offset

Adding and removing byte from a file in a loop

1 Answers