mobile/src/Core/Services/TotpService.cs

using System;
using System.Threading.Tasks;
using Bit.Core.Abstractions;
using Bit.Core.Enums;
using Bit.Core.Utilities;

namespace Bit.Core.Services
{
    public class TotpService : ITotpService
    {
        private const string SteamChars = "23456789BCDFGHJKMNPQRTVWXY";

        private readonly ICryptoFunctionService _cryptoFunctionService;

        public TotpService(
            ICryptoFunctionService cryptoFunctionService)
        {
            _cryptoFunctionService = cryptoFunctionService;
        }

        public async Task<string> GetCodeAsync(string key)
        {
            if (string.IsNullOrWhiteSpace(key))
            {
                return null;
            }
            var period = Constants.TotpDefaultTimer;
            var alg = CryptoHashAlgorithm.Sha1;
            var digits = 6;
            var keyB32 = key;

            var isOtpAuth = key?.ToLowerInvariant().StartsWith("otpauth://") ?? false;
            var isSteamAuth = key?.ToLowerInvariant().StartsWith("steam://") ?? false;
            if (isOtpAuth)
            {
                var otpData = new OtpData(key.ToLowerInvariant());
                if (otpData.Digits > 0)
                {
                    digits = Math.Min(otpData.Digits.Value, 10);
                }
                if (otpData.Period.HasValue)
                {
                    period = otpData.Period.Value;
                }
                if (otpData.Secret != null)
                {
                    keyB32 = otpData.Secret;
                }
                if (otpData.Algorithm.HasValue)
                {
                    alg = otpData.Algorithm.Value;
                }
            }
            else if (isSteamAuth)
            {
                digits = 5;
                keyB32 = key.Substring(8);
            }

            var keyBytes = Base32.FromBase32(keyB32);
            if (keyBytes == null || keyBytes.Length == 0)
            {
                return null;
            }
            var now = CoreHelpers.EpocUtcNow() / 1000;
            var time = now / period;
            var timeBytes = BitConverter.GetBytes(time);
            if (BitConverter.IsLittleEndian)
            {
                Array.Reverse(timeBytes, 0, timeBytes.Length);
            }

            var hash = await _cryptoFunctionService.HmacAsync(timeBytes, keyBytes, alg);
            if (hash.Length == 0)
            {
                return null;
            }

            var offset = (hash[hash.Length - 1] & 0xf);
            var binary = ((hash[offset] & 0x7f) << 24) | ((hash[offset + 1] & 0xff) << 16) |
                ((hash[offset + 2] & 0xff) << 8) | (hash[offset + 3] & 0xff);

            string otp = string.Empty;
            if (isSteamAuth)
            {
                var fullCode = binary & 0x7fffffff;
                for (var i = 0; i < digits; i++)
                {
                    otp += SteamChars[fullCode % SteamChars.Length];
                    fullCode = (int)Math.Truncate(fullCode / (double)SteamChars.Length);
                }
            }
            else
            {
                var rawOtp = binary % (int)Math.Pow(10, digits);
                otp = rawOtp.ToString().PadLeft(digits, '0');
            }
            return otp;
        }

        public int GetTimeInterval(string key)
        {
            var period = Constants.TotpDefaultTimer;
            if (key != null && key.ToLowerInvariant().StartsWith("otpauth://"))
            {
                var qsParams = CoreHelpers.GetQueryParams(key);
                if (qsParams.ContainsKey("period") && qsParams["period"] != null &&
                    int.TryParse(qsParams["period"].Trim(), out var periodParam) && periodParam > 0)
                {
                    period = periodParam;
                }
            }
            return period;
        }
    }
}