How to perform this type conversation properly? Without affecting the speed of the code

Question

This program takes a string as input , and covert it to some random string. But when the value is more that 127 , c gets some negative value assigned to it . And when I convert the c to int the int value becomes much bigger negative value.

cout << "\nEnter any string : ";
cin >> s1;

for (char& c : s1)
{
num = c;
num= (num+rand())%256;
// const char char_max = (char)(((unsigned char) char(-1)) / 2);
//c = (num & char_max)
c = num;
}
cout <<"\n"<< s1;

Answer 1

Can't you just do,

cin >> s1;
for (char& c : s1)
c = rand()%256;
cout << "\n" << s1;

but ofc, you may want to consider to not land on invisible characters (0-32).

cin >> s1;
for (char& c : s1)
c = 32+(rand()%224);
cout << "\n" << s1;

Answer 2

That's correct but That's not what I asked for. I just wanted a solution for my problem , that was just a sample program