Encrypted secrete messages in C++

How to send Encrypted secrete messages in C++ and Decrypt the same message?

#include <iostream>

#include<string>

using namespace std;

int main()

{

    string message;

    int i,a,word;

    string letters ={"ABCDEFGHIKLMNOPQRSTVXYZabcdefghijklmnopqrstuvwxyz"};

    string key=     {"sub£rmatoglyphicAXPYRIQW\CMSWBFG@1029384756!&()="};

    cout<<"Enter the message: ";

    getline(cin,message);

    string encrypted_message;

    string decrypted;

    for(char c:message){

    size_t position=letters.find(c);

    if (position !=string::npos)

    {

    char new_char = key.at(position);

    encrypted_message+=new_char;

    }

    else

    {

    encrypted_message+=c;

    }

    for(char c:key)

    size_t position=key.find(c);

    if(position!=string::npos)

    {

    char new_char=letters.at(position);

    decrypted +=new_char;

    }

    else

    {

    decrypted+=c;

    }

    }

    cout<<"\nEncrypted message:  "<<encrypted_message<<endl;

    cout<<"\nDecrypted message: "<<decrypted<<endl;

    cout<<endl;

    return 0;

}