TOTP: Time-Based One-Time Password Algorithm Implementation in C#

Rfc6238.cs

using System.Security.Cryptography;

namespace Rfc6238;

public enum HmacAlgo
{
    Sha1,
    Sha256,
    Sha512,
}

internal static class Extensions
{
    internal static byte[] ComputeHash(this HmacAlgo algo, byte[] key, byte[] payload)
    {
        HMAC hmac = algo switch
        {
            HmacAlgo.Sha1 => new HMACSHA1(key),
            HmacAlgo.Sha256 => new HMACSHA256(key),
            HmacAlgo.Sha512 => new HMACSHA512(key),
            _ => throw new ArgumentOutOfRangeException(nameof(algo), "Unknown HMAC algorithm"),
        };

        return hmac.ComputeHash(payload);
    }

    internal static byte[] ToBytes(this string hex)
    {
        if (hex.Length % 2 == 1)
            throw new Exception("The binary key cannot have an odd number of digits");

        byte[] arr = new byte[hex.Length >> 1];

        for (int i = 0; i < hex.Length >> 1; ++i)
            arr[i] = (byte)((GetHexVal(hex[i << 1]) << 4) + GetHexVal(hex[(i << 1) + 1]));

        return arr;
    }

    private static int GetHexVal(char hex)
    {
        // cast to int
        int val = hex;
        return val - (val < 58 ? 48 : val < 97 ? 55 : 87);
    }
}

public static class Totp
{
    public const long T0 = 0;
    public const long X = 30;

    public static string Generate(byte[] key, long timestamp, int digits, HmacAlgo crypto)
    {
        long T = (timestamp - T0) / X;

        // convert to hex and pad with 16 0s
        byte[] payload = T
            .ToString("X016")
            .ToBytes();

        byte[] hash = crypto.ComputeHash(key, payload);

        int offset = hash[^1] & 0xf;

        int binary =
            ((hash[offset] & 0x7f) << 24)
            | ((hash[offset + 1] & 0xff) << 16)
            | ((hash[offset + 2] & 0xff) << 8)
            | (hash[offset + 3] & 0xff);

        int otp = binary % (int)Math.Pow(10, digits);
        return otp.ToString($"D{digits}");
    }
}

rfc6238