// All rights reserved, pituach.dev

using System;

using System.Linq;

using System.Security.Cryptography;

using Microsoft.AspNetCore.Cryptography.KeyDerivation;

​

public class Program

{

    // Gets a securely randomized salt for us.

    public static byte[] RandomSalt()

    {

        // We securly randomize a long enough salt, 16 bytes should suffice.

        return RandomNumberGenerator.GetBytes(16);

    }

​

    // Computes the hash for the given password.

    public static byte[] ComputeHash(string password, byte[] salt, int iterations)

    {

        // We compute the hash with the given variables, we take 36 bytes from the result, this should suffice.

        return KeyDerivation.Pbkdf2(password, salt, KeyDerivationPrf.HMACSHA512, iterations, 36);

    }

    // Slow compares the two given arrays.

    public static bool SlowEquals(byte[] a, byte[] b)

    {

        uint diff = (uint)a.Length ^ (uint)b.Length;

        for (int i = 0; i < a.Length && i < b.Length; i++)

            diff |= (uint)(a[i] ^ b[i]);

        return diff == 0;

    }

    // Whether the given password is hashed correctly to the given saved hash.

    public static bool ComparePassword(string password, byte[] salt, int iterations, byte[] savedHash)

    {

        // Compute the hash for the given password.

        var computedHash        = ComputeHash(password, salt, iterations);

​

        // Check that the given password is exactly the stored password.

        if (SlowEquals(savedHash, computedHash))

        {

            return true;

        }

​

        // Check if the given password has an extra char at the start.

        computedHash            = ComputeHash(password.Substring(1), salt, iterations);

​

        if (SlowEquals(savedHash, computedHash))

        {

            return true;

        }

​

        // Check if the given password has an extra char at the end.

        computedHash            = ComputeHash(password.Substring(0, password.Length - 1), salt, iterations);

​

        if (SlowEquals(savedHash, computedHash))

        {

            return true;

        }

​

        // If the first letter is upper case, check if the given password with first letter inverted case is the stored password.

        if (char.IsUpper(password[0]))

        {

            computedHash        = ComputeHash(char.ToLower(password[0]) + password.Substring(1), salt, iterations);

​

            if (SlowEquals(savedHash, computedHash))

            {

                return true;

            }

        }

​

        // Check if the password has inverted case (e.g. using Caps-Lock).

        var invertCasePassword  = new string(password.Select(c => char.IsLetter(c) ? (char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c)) : c).ToArray());

        computedHash            = ComputeHash(invertCasePassword, salt, iterations);

​

        if (SlowEquals(savedHash, computedHash))

        {

            return true;

        }

​

        // If no check succeeded, the password is incorrect.

        return false;

    }

​

    public static void Main()

    {

        // Original password should be received from the client, either when setting a password or authenticating the user (comparing the password with the stored hash).

        string originalPassword = "P@ssw0rd1";

​

        // Salt should be calculated once when password is set, and saved to the DB. When authenticating the user - the salt should be pulled from the DB for the user.

        byte[] salt = RandomSalt();

​

        // Iterations should be a program constant/parameter and should be high enough while still being useful - in PBKDF2, at least 200000 (as of 2023).

        int iterations = 200000;

​

        // Now we compute the hash for the given password. When setting the password - we will store the computed hash, with the salt and iterations in the DB.

        byte[] computedHash = ComputeHash(originalPassword, salt, iterations);

        // We will also test a test password.

        string testPassword1    = "1dr0wss@P";

        string testPassword2    = "P@ssw0rd11";

        string testPassword3    = "PP@ssw0rd1";

        string testPassword4    = "p@SSW0RD1";

        string testPassword5    = "P@ssw0rd5";

        string testPassword6    = "P@ssw0rd123";

​

        Console.WriteLine("Checking password {0}, is it valid? {1}", originalPassword, ComparePassword(originalPassword, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword1, ComparePassword(testPassword1, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword2, ComparePassword(testPassword2, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword3, ComparePassword(testPassword3, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword4, ComparePassword(testPassword4, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword5, ComparePassword(testPassword5, salt, iterations, computedHash));

        Console.WriteLine("Checking password {0}, is it valid? {1}", testPassword6, ComparePassword(testPassword6, salt, iterations, computedHash));

    }

}