C++ columnar transposition encrypt

#include <iostream>
#include <algorithm>
#include <string>
#include <map>
#include <random>
using namespace std;

std::random_device dev;
std::mt19937 rng(dev());
const std::string nulls = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
std::uniform_int_distribution<std::mt19937::result_type> dist_nulls(0, nulls.length()-1);

std::string key = "DBECA";
std::map<int, int> key_order;

    void set_permutation_order(){
        for(int i = 0; i < key.length(); ++i)
            key_order[key[i]] = i;
    }

    void normalize(std::string& str){
        str.erase(std::remove_if(str.begin(), str.end(), ::isspace), str.end());
        std::transform(str.begin(), str.end(), str.begin(), ::toupper);
    }

    std::string columnar_transpos_encrypt(std::string msg){

        normalize(msg);
        //std::cout << msg << '\n';
        int cols = key.length();
        int rows = msg.length()/cols;

        if(msg.length() % cols)
            ++rows;


        char** matrix = new char*[rows];
        for(int i = 0; i < rows; ++i)
            matrix[i] = new char[cols];


        int ind{0};
        for(int row = 0; row < rows; ++row){
            for(int col = 0; col < cols; ++col){
                if(ind >= msg.length())
                    matrix[row][col] = nulls[dist_nulls(rng)];
                else
                    matrix[row][col] = msg[ind++];
            }
        }

        //std::cout << rows << " " << cols << '\n';
        std::string encrypted{};
        int col_order{};
        for(auto it = key_order.begin(); it != key_order.end(); ++it){
            col_order = it->second;
            for(int row = 0; row < rows; ++row){
                encrypted += matrix[row][col_order];
            }
        }
        /*
        for debug purpose

        for(int row = 0; row < rows; ++row){
            for(int col = 0; col < cols; ++col){
                std::cout << matrix[row][col] << ' ';
            }
            std::cout << '\n';
        }
        */

        for(int i = 0; i < rows; ++i)
            delete[] matrix[i];
        delete[] matrix;

        return encrypted;

    }


int main()
{
    std::string ms = "Which wristwatches are Swiss wristwatches";
    set_permutation_order();
    std::string res = columnar_transpos_encrypt(ms);
    std::cout << res;

    return 0;
}