shoc-backend/SeaHaven.Services/Helpers/PasswordResetCodeSecrets.cs

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

namespace SeaHaven.Services.Helpers
{
    /// <summary>
    /// Generation and hashing for emailed password reset codes. The raw code exists
    /// only in memory and in the email sent to the account holder. Hashes are keyed
    /// with a server-side key, so a copy of the database alone cannot be used to
    /// brute-force the six-digit codes offline.
    /// </summary>
    public static class PasswordResetCodeSecrets
    {
        private static readonly byte[] KeyInfo = Encoding.UTF8.GetBytes("password-reset-code-v1");

        /// <summary>
        /// Derives the code-hashing key from an existing server secret with HKDF, so no
        /// new secret is needed and the derived key is useless for anything else.
        /// </summary>
        public static byte[] DeriveKey(string serverSecret)
        {
            ArgumentException.ThrowIfNullOrEmpty(serverSecret);
            return HKDF.DeriveKey(HashAlgorithmName.SHA256, Encoding.UTF8.GetBytes(serverSecret), 32, Array.Empty<byte>(), KeyInfo);
        }

        public static string NewCode()
        {
            return RandomNumberGenerator.GetInt32(0, 1_000_000).ToString("D6", CultureInfo.InvariantCulture);
        }

        public static string NewSalt()
        {
            return Convert.ToHexString(RandomNumberGenerator.GetBytes(16)).ToLowerInvariant();
        }

        /// <summary>A value shaped like a hash that no code can match.</summary>
        public static string NewUnmatchableHash()
        {
            return Convert.ToHexString(RandomNumberGenerator.GetBytes(32)).ToLowerInvariant();
        }

        public static string Hash(byte[] key, string salt, string code)
        {
            ArgumentNullException.ThrowIfNull(key);
            ArgumentNullException.ThrowIfNull(salt);
            ArgumentNullException.ThrowIfNull(code);
            return Convert.ToHexString(HMACSHA256.HashData(key, Encoding.UTF8.GetBytes(salt + ":" + code))).ToLowerInvariant();
        }

        public static bool Matches(byte[] key, string salt, string candidate, string expectedHash)
        {
            if (string.IsNullOrEmpty(salt) || string.IsNullOrEmpty(expectedHash))
                return false;

            var actual = Encoding.ASCII.GetBytes(Hash(key, salt, candidate.Trim()));
            var expected = Encoding.ASCII.GetBytes(expectedHash);
            return CryptographicOperations.FixedTimeEquals(actual, expected);
        }
    }
}