Caesar cipher implementation in C++20

caesar_ids.cpp

#include <iostream>
#include <fstream>
#include <string>

// The number of places to shift the letters in the Caesar cipher
const int SHIFT = 3;

int main() {
    std::ofstream out_file("user_ids.txt");

    for (int i = 0; i < 1000000000; ++i) {
        std::string user_id = std::to_string(i) + std::to_string(rand());

        std::string encrypted_user_id;
        for (char c : user_id) {
            encrypted_user_id += (c + SHIFT) % 26;
        }
        out_file << encrypted_user_id << std::endl;
    }

    std ::cout << "Done!" << std::endl;
    std::cout << "Took: " << (clock() / (double) CLOCKS_PER_SEC) << " seconds" << std::endl;
    out_file.close();
    return 0;
}