main.cpp

//
//  main.cpp
//  Secure PIN Authentication sysytem
//
//  Created by AKE<3
#include <iostream>
#include <fstream>
#include <string>
#include <cctype>

using namespace std;

const string FILE_NAME = "credentials.txt";

bool isValidPIN(const string& pin);
string encryptPIN(const string& pin);
string decryptPIN(const string& encryptedPIN);
bool pinFileExists();
void setupPIN();
bool login();

int main() {
    int choice;
    cout << " Secure PIN Authentication\n";
    cout << "---------------------------\n";

    do {
        cout << "\nMenu:\n";
        cout << "1. Set up PIN\n";
        cout << "2. Login\n";
        cout << "3. Exit\n";
        cout << "Enter your choice: ";
        cin >> choice;

        if (choice == 1) {
            setupPIN();
        }
        else if (choice == 2) {
            bool success = login();

            if (success) {
                cout << "\nAccess granted. Welcome!\n";
            } else {
                cout << "\nAccess denied.\n";
            }
        }
        else if (choice == 3) {
            cout << "\nExiting program...\n";
        }
        else {
            cout << "\nInvalid choice. Please try again.\n";
        }

    } while (choice != 3);

    return 0;
}

bool isValidPIN(const string& pin) {
    if (pin.length() != 4) {
        return false;
    }

    for (char ch : pin) {
        if (!isdigit(ch)) {
            return false;
        }
    }

    return true;
}

string encryptPIN(const string& pin) {
    string encryptedPIN = "";

    for (char ch : pin) {
        int digit = ch - '0';

        int encryptedDigit = (digit + 3) % 10;

        encryptedPIN += char(encryptedDigit + '0');
    }

    return encryptedPIN;
}

string decryptPIN(const string& encryptedPIN) {
    string decryptedPIN = "";

    for (char ch : encryptedPIN) {
        int digit = ch - '0';
        
        int decryptedDigit = (digit - 3 + 10) % 10;

        decryptedPIN += char(decryptedDigit + '0');
    }

    return decryptedPIN;
}

bool pinFileExists() {
    ifstream file(FILE_NAME);
    return file.good();
}

void setupPIN() {
    string pin;

    cout << "\nCreate a 4-digit PIN: ";
    cin >> pin;

    while (!isValidPIN(pin)) {
        cout << "Invalid PIN. PIN must be exactly 4 digits.\n";
        cout << "Create a 4-digit PIN: ";
        cin >> pin;
    }

    string encryptedPIN = encryptPIN(pin);

    ofstream file(FILE_NAME);

    if (!file) {
        cout << "Error: Could not open file for writing.\n";
        return;
    }

    file << encryptedPIN;
    file.close();

    cout << "PIN setup successful.\n";
    cout << "Your PIN has been encrypted and saved.\n";
}

bool login() {
    if (!pinFileExists()) {
        cout << "\nNo PIN has been set up yet. Please set up a PIN first.\n";
        return false;
    }

    ifstream file(FILE_NAME);

    if (!file) {
        cout << "Error: Could not open credentials file.\n";
        return false;
    }

    string storedEncryptedPIN;
    file >> storedEncryptedPIN;
    file.close();

    string storedPIN = decryptPIN(storedEncryptedPIN);

    const int MAX_ATTEMPTS = 3;
    string enteredPIN;

    for (int attempt = 1; attempt <= MAX_ATTEMPTS; attempt++) {
        cout << "\nEnter your 4-digit PIN: ";
        cin >> enteredPIN;

        if (!isValidPIN(enteredPIN)) {
            cout << "Invalid input. PIN must be exactly 4 digits.\n";
        }
        else if (enteredPIN == storedPIN) {
            return true;
        }
        else {
            cout << "Incorrect PIN.\n";
        }

        cout << "Attempts remaining: " << MAX_ATTEMPTS - attempt << endl;
    }

    return false;
}