PoC program to generate Steam TOTP codes. Mostly copy-paste from https://github.com/winauth/winauth Requires the BouncyCastle nuget package.

totp.cs

using System;
using System.Text;
using Org.BouncyCastle.Crypto.Digests;
using Org.BouncyCastle.Crypto.Macs;
using Org.BouncyCastle.Crypto.Parameters;

namespace ConsoleApp1
{
    public static class Program
    {
        private const string Secret = "Put your secret here.";
        private const int CodeDigits = 5;
        private const int Period = 30;
        private const long ServerTimeDiff = 0;
        private static readonly byte[] SecretBytes = Convert.FromBase64String(Secret);
        private static readonly char[] SteamChars =
        {
            '2', '3', '4', '5', '6', '7', '8', '9',
            'B', 'C', 'D', 'F', 'G', 'H', 'J', 'K',
            'M', 'N', 'P', 'Q', 'R', 'T', 'V', 'W',
            'X', 'Y'
        };

        private static long CurrentTime =>
            Convert.ToInt64((DateTime.UtcNow - new DateTime(1970, 1, 1)).TotalMilliseconds);

        private static long ServerTime => CurrentTime - ServerTimeDiff;

        private static long CodeInterval => (CurrentTime + ServerTimeDiff) / (Period * 1000);

        public static void Main()
        {
            var hmac = new HMac(new Sha1Digest());

            hmac.Init(new KeyParameter(SecretBytes));

            var codeIntervalArray = BitConverter.GetBytes(CodeInterval);

            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(codeIntervalArray);
            }

            hmac.BlockUpdate(codeIntervalArray, 0, codeIntervalArray.Length);

            var mac = new byte[hmac.GetMacSize()];

            hmac.DoFinal(mac, 0);

            var start = mac[19] & 0x0f;
            var bytes = new byte[4];

            Array.Copy(mac, start, bytes, 0, 4);

            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(bytes);
            }

            var fullCode = BitConverter.ToUInt32(bytes, 0) & 0x7fffffff;
            var code = new StringBuilder();

            for (var i = 0; i < CodeDigits; i++)
            {
                code.Append(SteamChars[fullCode % SteamChars.Length]);

                fullCode /= (uint) SteamChars.Length;
            }

            Console.WriteLine(code);
        }
    }
}