TOTP 介绍及基于C#的简单实现

Intro

TOTP 是基于时间的一次性密码生成算法，它由 RFC 6238 定义。和基于事件的一次性密码生成算法不同 HOTP，TOTP 是基于时间的，它和 HOTP 具有如下关系：

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：客户端和服务端的共享密钥，不同的客户端的密钥各不相同。
• HOTP：该算法请参考 RFC，也可参考 理解 HMAC-Based One-Time Password Algorithm

TOTP 算法是基于 HOTP 的，对于 HOTP 算法来说，HOTP 的输入一致时始终输出相同的值，而 TOTP 是基于时间来算出来的一个值，可以在一段时间内（官方推荐是30s）保证这个值是固定以实现，在一段时间内始终是同一个值，以此来达到基于时间的一次性密码生成算法，使用下来整体还不错，有个小问题，如果需要实现一个密码只能验证一次需要自己在业务逻辑里实现，只能自己实现，TOTP 只负责生成和验证。

C# 实现 TOTP

实现代码

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 parameterif (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/rfc4226var 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-4private 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算法，输出4位var secretKey = "12345678901234567890";var output = otp.Compute(secretKey);Console.WriteLine($"output: {output}");Thread.Sleep(1000 * 30);var verifyResult = otp.Verify(secretKey, output); // 使用默认的验证方式，30s内有效Console.WriteLine($"Verify result: {verifyResult}");verifyResult = otp.Verify(secretKey, output, TimeSpan.FromSeconds(60)); // 指定可容忍的时间差，60s内有效Console.WriteLine($"Verify result: {verifyResult}");

输出示例：

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