app.py

from flask import Flask, render_template, request, redirect, url_for, session
import os

app = Flask(__name__)
app.secret_key = "super_secret_chat_key"

MESSAGES_FILE = "messages.txt"
USERS_FILE = "users.txt"

# Encryption helper: 5-letter Caeser-style shift cipher using maketrans
ALPHABET = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
SHIFTED  = "fghijklmnopqrstuvwxyzabcDEFG...0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ" # Standard 5-character rotation mapping

# Build translation tables for shift = +5
NORMAL_CHARS = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
SHIFTED_CHARS = "fghijklmnopqrstuvwxyzabcdeFGHIJKLMNOPQRSTUVWXYZABCDE5678901234"

ENCRYPT_TRANS = str.maketrans(NORMAL_CHARS, SHIFTED_CHARS)
DECRYPT_TRANS = str.maketrans(SHIFTED_CHARS, NORMAL_CHARS)

def encrypt(text):
    return text.translate(ENCRYPT_TRANS)

def decrypt(text):
    return text.translate(DECRYPT_TRANS)

# Ensure data storage files exist
for file_path in [MESSAGES_FILE, USERS_FILE]:
    if not os.path.exists(file_path):
        with open(file_path, "w", encoding="utf-8") as f:
            pass

def user_exists(username):
    if not os.path.exists(USERS_FILE):
        return False
    with open(USERS_FILE, "r", encoding="utf-8") as f:
        for line in f:
            line = line.strip()
            if line:
                decrypted_line = decrypt(line)
                if ":" in decrypted_line:
                    stored_user, _ = decrypted_line.split(":", 1)
                    if stored_user == username:
                        return True
    return False

def save_user(username, password):
    if not user_exists(username):
        user_line = f"{username}:{password}"
        encrypted_line = encrypt(user_line)
        with open(USERS_FILE, "a", encoding="utf-8") as f:
            f.write(encrypted_line + "\n")
        return True
    return False

def check_credentials(username, password):
    if not os.path.exists(USERS_FILE):
        return False
    
    with open(USERS_FILE, "r", encoding="utf-8") as f:
        for line in f:
            line = line.strip()
            if line:
                decrypted_line = decrypt(line)
                if ":" in decrypted_line:
                    stored_user, stored_pass = decrypted_line.split(":", 1)
                    if stored_user == username and stored_pass == password:
                        return True
    return False

def save_message(username, message):
    with open(MESSAGES_FILE, "a", encoding="utf-8") as f:
        f.write(f"{username}: {message}\n")

def load_messages():
    if os.path.exists(MESSAGES_FILE):
        with open(MESSAGES_FILE, "r", encoding="utf-8") as f:
            return [line.strip() for line in f.readlines() if line.strip()]
    return []

@app.route("/", methods=["GET", "POST"])
def home():
    if "username" not in session:
        return redirect(url_for("login"))

    if request.method == "POST":
        user_message = request.form.get("message", "").strip()
        if user_message:
            save_message(session["username"], user_message)
        return redirect(url_for("home"))

    messages_list = load_messages()
    return render_template("index.html", messages=messages_list, user=session["username"])

@app.route("/login", methods=["GET", "POST"])
def login():
    error = None
    if request.method == "POST":
        username = request.form.get("username", "").strip()
        password = request.form.get("password", "").strip()

        if check_credentials(username, password):
            session["username"] = username
            return redirect(url_for("home"))
        else:
            error = "Invalid username or password"

    return render_template("login.html", error=error)

@app.route("/signup", methods=["GET", "POST"])
@app.route("/signup", methods=["GET", "POST"])
def signup():
    error = None
    if request.method == "POST":
        username = request.form.get("username", "").strip()
        password = request.form.get("password", "").strip()

        if username and password:
            if user_exists(username):
                error = "Username already exists. Please choose another."
            else:
                save_user(username, password)
                session["username"] = username
                return redirect(url_for("home"))

    return render_template("signup.html", error=error)

@app.route("/logout")
def logout():
    session.pop("username", None)
    return redirect(url_for("login"))

if __name__ == "__main__":
    app.run(debug=True)