C++ encryption

#include <iostream>
//#include <math.h>
#include <string>
#include <fstream>

using namespace std;

string encrypt(char str[],short key);
string decrypt(char str[],short key);

int main(int argc, char **argv)
{
    /*char str[150];
    cout << "enter string to encrypt: ";
    cin >> str;
    encrypt(str,8);
    cout << "\nencrypted: " << str;
    decrypt(str,8);
    cout << "\n\ndecrypted: " << str; */

    char filename[85];
    char NewFilename[85];
    string str;
    string a1;
    cout << "enter file to encrypt: ";
    cin >> filename;
    ifstream ifile(filename);
    if(!ifile | !ifile.good())
    {
        cout << "error, invalid file.\n\n";
        main(1,'\0');
    }
    cout << "enter new filename: ";
    cin >> NewFilename;
    ofstream ofile(NewFilename);
    if(!ofile)
    {
        cout << "error, file failed to create.\n\n";
        main(1,'\0');
    }
    int num;
    for(int i = 0; !ifile.eof(); i++)
    {
        getline(ifile,a1);
        str.append(a1);
    }
//*******************************//
    char abc[str.size() +1];
    for(int i = 0; i <= str.size(); i++)
        abc[i] = str[i];
//convert 'str' to a char array
//*******************************//
    encrypt(abc,8);
    cout <<"\n\n\n"<< abc<<"\n\n\n";
    ofile << abc;
    return 0;
}

string encrypt(char str[],short key) //encrypt function
{
    int a = 256-key;
    for(int i =  0; i < sizeof(str); i++)
    {
        if(str[i] < a)
        {
            for(int b = 0; b < key; b++)
                (int)str[i]++;
        }
    }
    return str;
}

string decrypt(char str[],short key) //decrypt function
{
    for(int i = 0; i < sizeof(str); i++)
    {
        int a = 256-key;
        if(str[i] < a)
        {
            for(int b = 0; b < key; b++)
                (int)str[i]--;
        }
    }
    return str;
}