using System.Security.Cryptography;

namespace GB5Shared.EncryptionHelper
{
    // One-way password hashing (PBKDF2-SHA256) — distinct from PasswordEncryption.cs, which is
    // reversible AES-GCM encryption for a different purpose. This is the shared, GB5Shared-hosted
    // equivalent of DXPBLL.Auth.DXPPasswordHasher (GB5Solution/DXP/DXPBLL/Auth/DXPPasswordHasher.cs)
    // — same algorithm/parameters, promoted here so other modules (starting with Entitlement's
    // client-facing auth) can use it without taking a dependency on DXP. DXPPasswordHasher itself
    // is left untouched; it is not modified or rewired to call into this class.
    //
    // Uses .NET's built-in Rfc2898DeriveBytes (no external dependency). Self-describing stored
    // format: "{iterations}.{saltBase64}.{hashBase64}" — the iteration count travels with the hash,
    // so raising Iterations later does not invalidate passwords hashed under a lower count; they
    // keep verifying against their own stored iteration count until rehashed.
    public static class PasswordHasher
    {
        private const int SaltSize   = 16;
        private const int HashSize   = 32;
        private const int Iterations = 210_000; // OWASP-recommended minimum for PBKDF2-SHA256 (2023+)

        public static string Hash(string password)
        {
            var salt = RandomNumberGenerator.GetBytes(SaltSize);
            var hash = Rfc2898DeriveBytes.Pbkdf2(password, salt, Iterations, HashAlgorithmName.SHA256, HashSize);
            return $"{Iterations}.{Convert.ToBase64String(salt)}.{Convert.ToBase64String(hash)}";
        }

        public static bool Verify(string password, string storedHash)
        {
            var parts = storedHash.Split('.', 3);
            if (parts.Length != 3) return false;

            if (!int.TryParse(parts[0], out var iterations)) return false;

            byte[] salt, expected;
            try
            {
                salt     = Convert.FromBase64String(parts[1]);
                expected = Convert.FromBase64String(parts[2]);
            }
            catch (FormatException)
            {
                return false;
            }

            var actual = Rfc2898DeriveBytes.Pbkdf2(password, salt, iterations, HashAlgorithmName.SHA256, expected.Length);
            return CryptographicOperations.FixedTimeEquals(actual, expected);
        }
    }
}