c++ need help with caesar cipher decryption

[johnson roberts]

I have text file that contains username and password

userandPassword.txt

    chris ABC
    jane DEF
    john DEF
    

I am tryng to Decrypt the password using caesar cipher, with a key of 3 from the text file. which means that
if I key in username: chris and password:XYZ, it will login successfully.

however this is my password output after the decryption at this line of my codes

    for(int x = 0; x < passwordInFile.length(); x++) {
              output += caesarDecrypt(passwordInFile[x]);
    }
    cout << output << endl;

    XYZ
    XYZABC
    XYZABCABC
    

expected output of password output after the decryption

    XYZ
    ABC
    ABC
    

this is what I so far

    #include <iostream>
    #include <fstream>
    #include <sstream>
    #include <vector>
    using namespace std;
    char caesarDecrypt( char c );
    
    int main()
    {
            string line;
            string userName;
            string password;
            string output = "";
            string userNameInFile;
            string passwordInFile;
            
            ifstream readFile("userandPassword.txt");
            cout << "Enter UserName: ";
            cin >> userName;
            
            cout << "Enter Password: ";
            cin >> password;
            
            
            while (getline(readFile,line)) {
                stringstream iss(line);
                iss >> userNameInFile >> passwordInFile;
                for(int x = 0; x < passwordInFile.length(); x++) {
                    output += caesarDecrypt(passwordInFile[x]);
                }
                cout << output << endl;
                if (userName == userNameInFile && password == output) {
                    cout << "Login Successfully!"<< endl;
                }
            }
        
    }
    
    char caesarDecrypt( char c )
    {
        if( isalpha(c) )
        {
            c = toupper(c); //use upper to keep from having to use two seperate for A..Z a..z
            c = (((c+65)-3) % 26) + 65;
        }
        //if c isn't alpha, just send it back.
        return c;
    }
   

Please help me.Thanks

[Peter87]

You need to make the output string empty before handling the next password.