I'm writing a small Win32 Console app that manipulates the contents of binary files. It accepts the filename as a command line parameter and writes to a file named filename.scramble

This is the first time I'm dynamically allocating memory for C strings (which I'm using since the fstream functions use const char*).

The code is as follows. The comments should help you understand what I'm trying to do. Please tell me whether I'm doing anything unsafe with pointers or whether I'm allocating memory wrong or any unsafe practice you see regarding dynamically allocating and concatenating C strings in C++.

#include<iostream>
#include<fstream>
using namespace std;

int main(int argc, char *argv[])
{
    //Program needs at least one extra argument
    //If less, print 'usage'
    if (argc < 2) 
        cout<<"usage: "<< argv[0] <<" <filename>\n";

    else 
    {
        //Assuming argv[1] is a file name 
        fstream fin (argv[1],ios::binary|ios::in);
        
        if (!fin.is_open())
            cout<<"Could not open file\n";
        else 
        {
            //Calculate size of file
            fin.seekg(0,ios::end);
            unsigned long long size = fin.tellg();
            //Find out how many left over characters
            //if using blocks of 16 characters
            int lastset=size%16;
            //Change size to number of blocks
            size /= 16;

            //File name must be <original filename>.scramble
            //Allocate space
            char* name = new char[sizeof(argv[1])+8];

            //Concatenate <original filename>
            strcat_s(name,sizeof(argv[1]),argv[1]);
            //Concatenate ".scramble"
            strcat_s(name,sizeof(argv[1])+8,".scramble");

            fstream fout (name,ios::binary|ios::out);
            //Buffer
            char memblock[16];
            fin.seekg(0,ios::beg);

            //Read from fin and write to fout in blocks of 16 characters
            for(unsigned long long i=0;i<size;++i)
            {
                fin.read(memblock,16);
                /*
                *
                * Manipulation on memblock to be done here
                *
                */
                fout.write(memblock,16);
            }
            
            //No manipulation for last n bytes, n<16
            fin.read(memblock,lastset);
            fout.write(memblock,lastset);

            fin.close();
            fout.close();

            delete[] name;
        }
    }
    return 0;
}