Secure Terminal Authentication System

A robust C-based console application simulating a secure employee authentication terminal. This project demonstrates best practices for safe input handling, overflow protection, and buffer management in C without relying on standard, unsafe functions like scanf or gets.

📖 Overview

This program simulates a security checkpoint where employees enter their names and IDs. The system checks these credentials against an internal database and grants access based on a minimum security clearance level.

The primary focus of this codebase is defensive programming. It implements custom input functions to rigorously validate user input, ensuring the program is resistant to common vulnerabilities such as integer overflows and buffer overruns.

✨ Key Features

• Custom Safe Input Parsing (my_scanf):

• Manually parses unsigned integers character by character.

• Overflow Protection: Mathematically verifies that adding the next digit will not exceed UINT_MAX before processing.

• Automatic input flushing on invalid data detection.

• Secure String Handling (get_string_safe):

• Reads string input with strict bounds checking to prevent buffer overflows.

• Input Validation: Automatically rejects names containing numeric characters.

• Handles whitespace and tabs gracefully.

• Mock Database: Internal structure array representing an employee database with Security Clearance Levels.

• Access Control Logic: Compares user clearance against MIN_REQUIRED_LEVEL (Default: 3).

🚀 Getting Started

Prerequisites

• A C compiler (GCC recommended).

Compilation

Open your terminal and run the following command:

gcc main.c -o auth_system

Running the Application

./auth_system

💻 Usage Example

1. Successful Login (Access Granted)

Alice has Clearance Level 5 (Required: 3).

--- SECURE TERMINAL ACTIVE ---
Enter Name: 
Alice Gabriel
Enter Employee ID: 
1001
Checking Database for User credentials
MATCH FOUND
CHECKING ACCESS ENTRY

--- AUTHENTICATING: Alice Gabriel ---
[ACCESS GRANTED] Welcome, Officer Alice Gabriel.

2. Successful Login (Access Denied)

Bob has Clearance Level 1 (Required: 3).

--- SECURE TERMINAL ACTIVE ---
Enter Name: 
Bob Chen
Enter Employee ID: 
1002
Checking Database for User credentials
MATCH FOUND
CHECKING ACCESS ENTRY

--- AUTHENTICATING: Bob Chen ---
[ACCESS DENIED] Clearance Level 3 required.

3. Overflow/Error Handling

Demonstrating the robust input validation.

--- SECURE TERMINAL ACTIVE ---
Enter Name: 
Alice123
Invalid Username
[CRITICAL ERROR] Invalid Input format

⚙️ Technical Details

Integer Overflow Logic

The standard scanf("%u", &var) does not inherently protect against entering a number larger than the variable can hold (wrapping around). This program uses a custom algorithm:

By rearranging the overflow check (total * 10 + c) <= UINT_MAX, we ensure the calculation itself never causes an overflow during the check.

Database Schema

The system simulates a database using the following structure:

struct Employee {
    char name[50];      // 50 char limit
    short access_lvl;   // 1-10 scale
    unsigned int id;    // Unique ID
};

Default Users:

Name	ID	Clearance	Result
Alice Gabriel	1001	5	Granted
Bob Chen	1002	1	Denied
Charlie Grey	1003	7	Granted

🛡️ License

MIT LICENSE - This project is open-source and available for educational purposes.