foo.cc: In function ‘int main()’:
foo.cc:20:23: warning: comparison of integer expressions of different signedness: ‘int’ and ‘std::__cxx11::basic_string<char>::size_type’ {aka ‘long unsigned int’} [-Wsign-compare]
   20 |     for (int i = 0; i < plaintext.length(); i++) {
      |                     ~~^~~~~~~~~~~~~~~~~~~~