intital project

.gitignore

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+bin
+obj
+.vs

TotpGenerator.sln

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+
+Microsoft Visual Studio Solution File, Format Version 12.00
+# Visual Studio Version 17
+VisualStudioVersion = 17.4.33213.308
+MinimumVisualStudioVersion = 10.0.40219.1
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TotpGenerator", "TotpGenerator\TotpGenerator.csproj", "{A245C9E9-FC25-401C-9F0E-C19E74CF21E1}"
+EndProject
+Global
+	GlobalSection(SolutionConfigurationPlatforms) = preSolution
+		Debug|Any CPU = Debug|Any CPU
+		Release|Any CPU = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(ProjectConfigurationPlatforms) = postSolution
+		{A245C9E9-FC25-401C-9F0E-C19E74CF21E1}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{A245C9E9-FC25-401C-9F0E-C19E74CF21E1}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{A245C9E9-FC25-401C-9F0E-C19E74CF21E1}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{A245C9E9-FC25-401C-9F0E-C19E74CF21E1}.Release|Any CPU.Build.0 = Release|Any CPU
+	EndGlobalSection
+	GlobalSection(SolutionProperties) = preSolution
+		HideSolutionNode = FALSE
+	EndGlobalSection
+	GlobalSection(ExtensibilityGlobals) = postSolution
+		SolutionGuid = {69635BFF-0DA4-46E2-911A-254844A23EE1}
+	EndGlobalSection
+EndGlobal

TotpGenerator/TotpGenerator.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+<PropertyGroup>
+	<Description>Genarate Totp Code library.</Description>
+	<VersionPrefix>1.0.0</VersionPrefix>
+	<Authors>Farhad Zamani</Authors>
+	<TargetFrameworks>net7.0;net6.0;net5.0;netstandard2.1;netstandard2.0;</TargetFrameworks>
+	<GenerateDocumentationFile>true</GenerateDocumentationFile>
+	<AssemblyName>TotpGenerator</AssemblyName>
+	<PackageId>TotpGenerator</PackageId>
+	<PackageTags>Totp;Passwrod;Time-based one-time password;.NET;aspnetcore</PackageTags>
+	<PackageProjectUrl>https://github.com/farhadzm/totp-generator</PackageProjectUrl>
+	<RepositoryUrl>https://github.com/farhadzm/totp-generator</RepositoryUrl>
+	<RepositoryType>git</RepositoryType>
+	<PackageReadmeFile>README.md</PackageReadmeFile>
+	<PackageLicenseExpression>Apache-2.0</PackageLicenseExpression>
+</PropertyGroup>
+
+</Project>

TotpGenerator/TotpService.cs

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+using System;
+using System.Text;
+using System.Security.Cryptography;
+using System.Diagnostics;
+using System.Net;
+
+namespace TotpGenerator
+{
+    /// <summary>
+    /// A class for generate and validate totp code based on time
+    /// </summary>
+    public class TotpService
+    {
+        private static readonly Encoding _encoding = new UTF8Encoding(false, true);
+
+        private static readonly TimeSpan _timeStep = TimeSpan.FromMinutes(1);
+#if NETSTANDARD2_0
+        private static readonly DateTime _unixEpoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
+        private static readonly RandomNumberGenerator _rng = RandomNumberGenerator.Create();
+#endif
+
+        /// <summary>
+        /// Genarating totp code based on securityStampToken, modifier and time. 
+        /// </summary>
+        /// <param name="securityStampToken"></param>
+        /// <param name="modifier"></param>
+        /// <returns></returns>
+        /// <exception cref="ArgumentNullException"></exception>
+        public static int GenerateCode(string securityStampToken, string modifier)
+        {
+            if (securityStampToken == null)
+            {
+                throw new ArgumentNullException(nameof(securityStampToken));
+            }
+
+            byte[] securityTokenBytes = GetBytes(securityStampToken);
+
+            var currentTimeStep = GetCurrentTimeStepNumber();
+
+            using (var hashAlgorithm = new HMACSHA1(securityTokenBytes))
+            {
+                return ComputeTotp(hashAlgorithm, currentTimeStep, modifier);
+            }
+        }
+
+        /// <summary>
+        /// Validating totp code based on securityStampToken, code, modifier, expirationInMinutes
+        /// </summary>
+        /// <param name="securityStampToken"></param>
+        /// <param name="code"></param>
+        /// <param name="modifier"></param>
+        /// <param name="expirationInMinutes"></param>
+        /// <returns></returns>
+        /// <exception cref="ArgumentNullException"></exception>
+        public static bool ValidateCode(
+            string securityStampToken,
+            int code,
+            string modifier,
+            int expirationInMinutes)
+        {
+            if (securityStampToken == null)
+            {
+                throw new ArgumentNullException(nameof(securityStampToken));
+            }
+
+            byte[] securityTokenBytes = GetBytes(securityStampToken);
+
+            using (var hashAlgorithm = new HMACSHA1(securityTokenBytes))
+            {
+                for (var i = -Math.Abs(expirationInMinutes); i <= 1; i++)
+                {
+                    var currentTimeStep = GetNextTimeStepNumber(i);
+
+                    var computedTotp = ComputeTotp(hashAlgorithm, (ulong)((long)currentTimeStep), modifier);
+                    if (computedTotp == code)
+                    {
+                        return true;
+                    }
+                }
+            }
+
+            // No match
+            return false;
+        }
+
+        private static int ComputeTotp(HashAlgorithm hashAlgorithm, ulong timestepNumber, string modifier)
+        {
+            // # of 0's = length of pin
+            const int Mod = 1000000;
+
+            // See https://tools.ietf.org/html/rfc4226
+            // We can add an optional modifier
+            var timestepAsBytes = BitConverter.GetBytes(IPAddress.HostToNetworkOrder((long)timestepNumber));
+            var hash = hashAlgorithm.ComputeHash(ApplyModifier(timestepAsBytes, modifier));
+
+            // Generate DT string
+            var offset = hash[hash.Length - 1] & 0xf;
+            Debug.Assert(offset + 4 < hash.Length);
+            var binaryCode = (hash[offset] & 0x7f) << 24
+                             | (hash[offset + 1] & 0xff) << 16
+                             | (hash[offset + 2] & 0xff) << 8
+                             | (hash[offset + 3] & 0xff);
+
+            return binaryCode % Mod;
+        }
+
+        private static byte[] ApplyModifier(byte[] input, string modifier)
+        {
+            if (String.IsNullOrEmpty(modifier))
+            {
+                return input;
+            }
+
+            var modifierBytes = GetBytes(modifier);
+            var combined = new byte[checked(input.Length + modifierBytes.Length)];
+            Buffer.BlockCopy(input, 0, combined, 0, input.Length);
+            Buffer.BlockCopy(modifierBytes, 0, combined, input.Length, modifierBytes.Length);
+            return combined;
+        }
+
+        // More info: https://tools.ietf.org/html/rfc6238#section-4
+        private static ulong GetCurrentTimeStepNumber()
+        {
+#if NETSTANDARD2_0
+            var delta = DateTime.UtcNow - _unixEpoch;
+#else
+            var delta = DateTimeOffset.UtcNow - DateTimeOffset.UnixEpoch;
+#endif
+            return (ulong)(delta.Ticks / _timeStep.Ticks);
+        }
+
+        private static ulong GetNextTimeStepNumber(int minutes)
+        {
+#if NETSTANDARD2_0
+            var delta = DateTime.UtcNow - _unixEpoch;
+#else
+            var delta = DateTimeOffset.UtcNow.AddMinutes(minutes) - DateTimeOffset.UnixEpoch;
+#endif
+            return (ulong)(delta.Ticks / _timeStep.Ticks);
+        }
+
+        private static byte[] GetBytes(string securityToken)
+        {
+            return _encoding.GetBytes(securityToken);
+        }
+    }
+}