Quick check of 6-digit totp code

totp.cs

public static bool Verify(string digits, string secret)
{
    if (string.IsNullOrEmpty(digits) || string.IsNullOrEmpty(secret))
        return false;

    long iterations = DateTimeOffset.Now.ToUnixTimeSeconds() / 30L;
    byte[] secretBytes = Base32ToBytes(secret);
    if (Verify(digits, secretBytes, iterations))
        return true;
    else
        return Verify(digits, secretBytes, iterations - 1);
}

private static bool Verify(string digits, byte[] secret, long iterations)
{
    byte[] buffer = BitConverter.GetBytes(iterations);
    Array.Reverse(buffer);

    using (HMACSHA1 sha1 = new HMACSHA1(secret))
    {
        byte[] hash = sha1.ComputeHash(buffer);
        byte[] result = new byte[4];
        Array.Copy(hash, hash[hash.Length - 1] & 0x0F, result, 0, 4);
        result[0] = (byte)(result[0] & 0b01111111);
        Array.Reverse(result);
        int resultDigits = BitConverter.ToInt32(result) % 1000000; // Take last 6 digits

        return $"{resultDigits:d6}" == digits;
    }
}

public static byte[] Base32ToBytes(string base32)
{
    const string charset = "ABCDEFGHIJKLMNOPQRSTUVWXYZ234567";

    if (base32 is null)
        throw new ArgumentNullException(nameof(base32));

    if (base32 == string.Empty)
        return new byte[0];

    base32 = base32.TrimEnd('=').ToUpper();
    int count = base32.Length * 5 / 8;
    byte[] result = new byte[count];

    byte current = 0;
    int bitsRemaining = 8;
    int i = 0;

    foreach (char c in base32)
    {
        int val = charset.IndexOf(c);
        if (bitsRemaining > 5)
        {
            int mask = val << (bitsRemaining - 5);
            current = (byte)(current | mask);
            bitsRemaining -= 5;
        }
        else
        {
            int mask = val >> (5 - bitsRemaining);
            current = (byte)(current | mask);
            result[i++] = current;
            current = (byte)(val << (3 + bitsRemaining));
            bitsRemaining += 3;
        }
    }

    if (i != count)
        result[i] = current;

    return result;
}