using System.Globalization;
using System.Security.Cryptography;
using System.Text;

namespace Sunfish.Auth;

/// <summary>
/// Hashes de contrasena con PBKDF2-SHA256: "pbkdf2-sha256$iteraciones$sal$hash" (Base64).
/// La contrasena nunca se guarda en claro.
/// </summary>
public static class Passwords
{
    public const int DefaultIterations = 100_000;
    private const string Prefix = "pbkdf2-sha256";
    private const int SaltBytes = 16;
    private const int HashBytes = 32;

    public static string Hash(string password, int iterations = DefaultIterations)
    {
        byte[] salt = RandomNumberGenerator.GetBytes(SaltBytes);
        byte[] hash = Rfc2898DeriveBytes.Pbkdf2(Encoding.UTF8.GetBytes(password), salt, iterations, HashAlgorithmName.SHA256, HashBytes);
        return string.Join('$', Prefix, iterations.ToString(CultureInfo.InvariantCulture),
            Convert.ToBase64String(salt), Convert.ToBase64String(hash));
    }

    public static bool Verify(string password, string stored)
    {
        var parts = stored.Split('$');
        if (parts.Length != 4 || parts[0] != Prefix ||
            !int.TryParse(parts[1], NumberStyles.None, CultureInfo.InvariantCulture, out int iterations) || iterations <= 0)
            return false;
        byte[] salt, expected;
        try
        {
            salt = Convert.FromBase64String(parts[2]);
            expected = Convert.FromBase64String(parts[3]);
        }
        catch (FormatException)
        {
            return false;
        }
        byte[] actual = Rfc2898DeriveBytes.Pbkdf2(Encoding.UTF8.GetBytes(password), salt, iterations, HashAlgorithmName.SHA256, expected.Length);
        return CryptographicOperations.FixedTimeEquals(actual, expected);
    }
}