Create a string that contains random characters

you can do this in pretty much any language. have you tried anything and need help or you're just asking us to do your homework for you?

FILE *f = popen("curl 'https://www.random.org/passwords/?num=1&len=12&format=plain&rnd=new'", "r");
char s[14];
fread(s, 1, sizeof(s), f);
s[13] = 0;

You could take inspiration from Theodore Tso's pwgen: https://github.com/tytso/pwgen

It's a Unix utility in C commonly used on Linux and FreeBSD to make truly random passwords. It's the first thing I thought of when reading this.

There are various ways to solve the first part and they're pretty generic, the only "C++-specific" part is whether you'll want to build the string step by step (adding characters) or build in one go then modify (build a prearranged string, for example of a specific size, then modify as needed).

To solve the second part we (you) also need to know the encryption algorithm. "Encrypt" is quite a generic word.

This is not doable in c++ unfortunately. Its an old case that’s only doable in f# or turbo pascal because of string handling and cryptography export rules (and possibly copy right)