TOTP 介紹及基於C#的簡單實現

Intro

TOTP 是基於時間的一次性密碼生成演算法，它由 RFC 6238 定義。和基於事件的一次性密碼生成演算法不同 HOTP，TOTP 是基於時間的，它和 HOTP 具有如下關係：

TOTP = HOTP(K, T)
HOTP(K,C) = Truncate(HMAC-SHA-1(K,C))

其中：

• T：T = (Current Unix time - T0) / X， T0 = 0，X = 30
• K：客戶端和服務端的共用密鑰，不同的客戶端的密鑰各不相同。
• HOTP：該演算法請參考 RFC，也可參考 理解 HMAC-Based One-Time Password Algorithm

TOTP 演算法是基於 HOTP 的，對於 HOTP 演算法來說，HOTP 的輸入一致時始終輸出相同的值，而 TOTP 是基於時間來算出來的一個值，可以在一段時間內（官方推薦是30s）保證這個值是固定以實現，在一段時間內始終是同一個值，以此來達到基於時間的一次性密碼生成演算法，使用下來整體還不錯，有個小問題，如果需要實現一個密碼只能驗證一次需要自己在業務邏輯里實現，只能自己實現，TOTP 只負責生成和驗證。

C# 實現 TOTP

實現代碼

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

namespace WeihanLi.Totp
{
    public class Totp
    {
        private readonly OtpHashAlgorithm _hashAlgorithm;
        private readonly int _codeSize;

        public Totp() : this(OtpHashAlgorithm.SHA1, 6)
        {
        }

        public Totp(OtpHashAlgorithm otpHashAlgorithm, int codeSize)
        {
            _hashAlgorithm = otpHashAlgorithm;

            // valid input parameter
            if (codeSize <= 0 || codeSize > 10)
            {
                throw new ArgumentOutOfRangeException(nameof(codeSize), codeSize, "length must between 1 and 9");
            }
            _codeSize = codeSize;
        }

        private static readonly Encoding Encoding = new UTF8Encoding(false, true);

        public virtual string Compute(string securityToken) => Compute(Encoding.GetBytes(securityToken));

        public virtual string Compute(byte[] securityToken) => Compute(securityToken, GetCurrentTimeStepNumber());

        private string Compute(byte[] securityToken, long counter)
        {
            HMAC hmac;
            switch (_hashAlgorithm)
            {
                case OtpHashAlgorithm.SHA1:
                    hmac = new HMACSHA1(securityToken);
                    break;

                case OtpHashAlgorithm.SHA256:
                    hmac = new HMACSHA256(securityToken);
                    break;

                case OtpHashAlgorithm.SHA512:
                    hmac = new HMACSHA512(securityToken);
                    break;

                default:
                    throw new ArgumentOutOfRangeException(nameof(_hashAlgorithm), _hashAlgorithm, null);
            }

            using (hmac)
            {
                var stepBytes = BitConverter.GetBytes(counter);
                if (BitConverter.IsLittleEndian)
                {
                    Array.Reverse(stepBytes); // need BigEndian
                }
                // See https://tools.ietf.org/html/rfc4226
                var hashResult = hmac.ComputeHash(stepBytes);

                var offset = hashResult[hashResult.Length - 1] & 0xf;
                var p = "";
                for (var i = 0; i < 4; i++)
                {
                    p += hashResult[offset + i].ToString("X2");
                }
                var num = Convert.ToInt64(p, 16) & 0x7FFFFFFF;

                //var binaryCode = (hashResult[offset] & 0x7f) << 24
                //                 | (hashResult[offset + 1] & 0xff) << 16
                //                 | (hashResult[offset + 2] & 0xff) << 8
                //                 | (hashResult[offset + 3] & 0xff);

                return (num % (int)Math.Pow(10, _codeSize)).ToString();
            }
        }

        public virtual bool Verify(string securityToken, string code) => Verify(Encoding.GetBytes(securityToken), code);

        public virtual bool Verify(string securityToken, string code, TimeSpan timeToleration) => Verify(Encoding.GetBytes(securityToken), code, timeToleration);

        public virtual bool Verify(byte[] securityToken, string code) => Verify(securityToken, code, TimeSpan.Zero);

        public virtual bool Verify(byte[] securityToken, string code, TimeSpan timeToleration)
        {
            var futureStep = (int)(timeToleration.TotalSeconds / 30);
            var step = GetCurrentTimeStepNumber();
            for (int i = -futureStep; i <= futureStep; i++)
            {
                if (step + i < 0)
                {
                    continue;
                }
                var totp = Compute(securityToken, step + i);
                if (totp == code)
                {
                    return true;
                }
            }
            return false;
        }

        private static readonly DateTime _unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

        /// <summary>
        /// timestep
        /// 30s(Recommend)
        /// </summary>
        private static readonly long _timeStepTicks = TimeSpan.TicksPerSecond * 30;

        // More info: https://tools.ietf.org/html/rfc6238#section-4
        private static long GetCurrentTimeStepNumber()
        {
            var delta = DateTime.UtcNow - _unixEpoch;
            return delta.Ticks / _timeStepTicks;
        }
    }
}

使用方式：

    var otp = new Totp(OtpHashAlgorithm.SHA1, 4); // 使用 SHA1演算法，輸出4位
    var secretKey = "12345678901234567890";
    var output = otp.Compute(secretKey);
    Console.WriteLine($"output: {output}");
    Thread.Sleep(1000 * 30);
    var verifyResult = otp.Verify(secretKey, output); // 使用預設的驗證方式，30s內有效
    Console.WriteLine($"Verify result: {verifyResult}");
    verifyResult = otp.Verify(secretKey, output, TimeSpan.FromSeconds(60)); // 指定可容忍的時間差，60s內有效
    Console.WriteLine($"Verify result: {verifyResult}");

輸出示例：

Reference

• https://tools.ietf.org/html/rfc4226
• https://tools.ietf.org/html/rfc6238
• http://wsfdl.com/algorithm/2016/04/05/%E7%90%86%E8%A7%A3HOTP.html
• http://wsfdl.com/algorithm/2016/04/14/%E7%90%86%E8%A7%A3TOTP.html
• https://www.cnblogs.com/voipman/p/6216328.html