I have made this RSA encryption program but it crashes when it is finished decrypting the line. What is wrong and how can I fix it?

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

bool IsPrime(int p);

int main()
{
    int p, q;
    int n, phi;
    int e, d;
    long long int c, dc, m;
    long long int newdc, dc2;
    string msg1, msg = "Hallo world!";

    do
    {
        cout<<"Enter two large prime numbers (seperated by a space): ";
        cin>>p>>q;
    }while(!(IsPrime(p) && IsPrime(q)));

    cout<<"Computing phi..."<<endl;
    phi = (q - 1) * (p - 1);
    cout<<"\tphi: "<<phi<<endl;

    cout<<"Enter a number ";
    cin>>e;

    cout<<"Computing d..."<<endl;
    d = (e % phi);
    cout<<"\td: "<<d<<endl;

    cout<<"Computing n..."<<endl;
    n = p * q;
    cout<<"\tn: "<<n<<endl;

    cout<<"Original line:\n";

    // Print the ASCII values of the message
    for(int i = 0; i < msg.size(); i++)
    {
        // Show the character it is based on.
        cout<<static_cast<int>(msg[i]) << "(" << msg[i] << ") " ;
    }

    cout<<"\nEncrypted line:\n";

    // Show the encrypted bytes
    for (int i = 0; i < msg.size(); i++)
    {
        c = msg[i] ^ e % d;
        dc = c ^ d % n;
        cout<<dc<<"("<<static_cast<char>(dc)<<") ";
        newdc = dc;
        msg1[i] = newdc;
    }

    cout<<"\nDecrypted line:\n";

    for(int i = 0; i < msg.size(); i++)
    {
        m = msg1[i] ^ e % d;
        dc2 = m ^ e % n;
        cout<<dc2<<"("<<static_cast<char>(dc2)<<") ";
    }


    cin.get();
}

bool IsPrime(int p)
{
    if(p <= 1)
        return false;
    else if(p == 2)
        return true;
    else if(p % 2 == 0)
        return false;
    else
    {
        bool prime = true;
        int divisor = 3;
        double num_p = static_cast<double>(p);
        int UpperLimitp = static_cast<int>(sqrt(num_p));

        while(divisor <= UpperLimitp )
        {
            if(p % divisor == 0)
                prime = false;
            divisor += 2;
        }
        return prime;
    }
}

Also in the encrypting and decrypting loop:

what will it do if i change this:

    c = msg[i] ^ e % d;

To this:

    c = msg[i];

Will it make my encryption brakable?
Also I heard thay RSA is suppose to be a certain ammount of bits eg(1024, 2048, 3082, 4096)... How do I achieve this?

What input causes it to crash?

Right now your crypto is very easy to break as it's not RSA. As I said in your other thread, ^ is not "to the power of".

As Moschops said: ^ is not a "to the power of" - it is a bit-wise xor operation. Use the pow() function (or one of its siblings such as powf() or powl()) instead.

Ok, now i have run into a nother error:

for (int i = 0; i < msg.size(); i++)
    {
        c = msg[i];
        dc = pow(c, d) % n;
        cout<<dc<<"("<<static_cast<char>(dc)<<") ";
        newdc = dc;
        msg1[i] = newdc;
    }

    cout<<"\nDecrypted line:\n";

    for(int i = 0; i < msg.size(); i++)
    {
        m = msg1[i];
        dc = pow(m, e) % n;
        cout<<dc2<<"("<<static_cast<char>(dc2)<<") ";
    }

it gives this error:

C:\Users\User\Desktop\Software\C++\RSA Encryption\main.cpp|54|error: invalid operands of types 'double' and 'int' to binary 'operator%'|
C:\Users\User\Desktop\Software\C++\RSA Encryption\main.cpp|65|error: invalid operands of types 'double' and 'int' to binary 'operator%'|

And my program crashes right after decrypting the line.

You are using the % operator with a double on the left (pow(c, d)) and an int on the right (n). The % operator works on only int values.

And my program crashes right after decrypting the line.

With what input? What do you type in that leads to the crash?