Otp.NET 1.4.0

dotnet add package Otp.NET --version 1.4.0
NuGet\Install-Package Otp.NET -Version 1.4.0
This command is intended to be used within the Package Manager Console in Visual Studio, as it uses the NuGet module's version of Install-Package.
<PackageReference Include="Otp.NET" Version="1.4.0" />
For projects that support PackageReference, copy this XML node into the project file to reference the package.
paket add Otp.NET --version 1.4.0
#r "nuget: Otp.NET, 1.4.0"
#r directive can be used in F# Interactive and Polyglot Notebooks. Copy this into the interactive tool or source code of the script to reference the package.
// Install Otp.NET as a Cake Addin
#addin nuget:?package=Otp.NET&version=1.4.0

// Install Otp.NET as a Cake Tool
#tool nuget:?package=Otp.NET&version=1.4.0

Otp.NET

An implementation TOTP RFC 6238 and HOTP RFC 4226 in C#.

Build status

Get it on NuGet

https://www.nuget.org/packages/Otp.NET

Install-Package Otp.NET 

or

dotnet add package Otp.NET

Documentation

TOTP (Timed One Time Password)

TOTP is an algorithm that uses a rolling window of time to calculate single use passwords. It is often used for two factor authentication. The Google Authenticator app uses TOTP to calculate one time passwords. This library implements TOTP code calculation in C#. This could be embedded in a mobile app using Mono, or used server side to simply validate codes that are provided.

Creation of a TOTP object

Use of the library is fairly straightforward. There is a class called Totp. Simply create a new instance of it and pass in the shared secret key in plaintext as a byte array.

using OtpNet;
var totp = new Totp(secretKey);

There are several options that can be used to change how the code is calculated. These are all mentioned in the RFC. These options are specified when the TOTP object is created.

Different hash algorithms can be used to calculate the code. The default is Sha1, but Sha256, and Sha512 may be used instead.

To change that behavior from the default of Sha1 simply pass in the OtpHashMode enum with the desired value into the constructor.

var totp = new Totp(secretKey, mode: OtpHashMode.Sha512);

The time step window can also be specified. The RFC recommends a window of thirty seconds. That means that a new code will be generated every thirty seconds. The step window can be changed however if required. There are not tests around this as the RFC test tables all use a 30 second window so use this feature at your own risk. Like the hash mode, pass this value into the constructor. It is an int that represents seconds.

var totp = new Totp(secretKey, step: 15); // a new code will be generated every 15 seconds

Finally the truncation level can be specified. Basically this is how many digits do you want your TOTP code to be. The tests in the RFC specify 8, but 6 has become a de-facto standard if not an actual one. For this reason the default is 6 but you can set it to something else. There aren't a lot of tests around this either so use at your own risk (other than the fact that the RFC test table uses TOTP values that are 8 digits).

var totp = new Totp(secretKey, totpSize: 8);
Code Calculation

Once you have an instance of the Totp class, you can easily calculate a code by Calling the ComputeTotp method. You need to provide the timestamp to use in the code calculation. DateTime.UtcNow is the recommended value. There is an overload that doesn't take a parameter that just uses UtcNow.

var totpCode = totp.ComputeTotp(DateTime.UtcNow);
// or use the overload that uses UtcNow
var totpCode = totp.ComputeTotp();
Remaining Time

There is a method that will tell you how much time remains in the current time step window in seconds.

var remainingTime = totp.RemainingSeconds();
// there is also an overload that lets you specify the time
var remainingSeconds = totp.RemainingSeconds(DateTime.UtcNow);
Verification

The TOTP implementation provides a mechanism for verifying TOTP codes that are passed in. There is a method called VerifyTotp with an overload that takes a specific timestamp.

public bool VerifyTotp(string totp, out long timeWindowUsed, VerificationWindow window = null);
public bool VerifyTotp(DateTime timestamp, string totp, out long timeWindowUsed, VerificationWindow window = null)

If the overload that doesn't take a timestamp is called, DateTime.UtcNow will be used as the comperand.

One Time Use

There is an output long called timeWindowUsed. This is provided so that the caller of the function can persist/check that the code has only been validated once. RFC 6238 Section 5.2 states that a code must only be accepted once. The output parameter reports the specific time window where the match occured for persistance comparison in future verification attempts.

It is up to the consumer of this library to ensure that only one match for a given time step window is actually accepted. This library will only go so far as to determine that there was a valid code provided given the current time and the key, not that it was truly used one time as this library has no persistence.

Expanded time Window

RFC 6238 Section 5.2 defines the recommended conditions for accepting a TOTP validation code. The exact text in the RFC is "We RECOMMEND that at most one time step is allowed as the network delay."

The VerifyTotp method takes an optional VerificationWindow parameter. This parameter allows you to define the window of steps that are considered acceptable. The actual step where the match was found will be reported in the aforementioned output parameter.

The default is that no delay will be accepted and the code must match the current code in order to be considered a match. Simply omitting the optional parameter will cause this default behavior.

If a time delay is required, a VerificationWindow object can be provided that describes the acceptable range of values to check.

To allow a delay as per the recommendation of the RFC (one time step delay) create a verification window object as follows

var window = new VerificationWindow(previous:1, future:1);

This means that the current step, and 1 step prior to the current will be allowed in the match. If you wanted to accept 5 steps backward (not recommended in the RFC) then you would change the previous parameter to 5.

There is also a parameter called future that allows you to match future codes. This might be useful if the client is ahead of the server by just enough that the code provided in slightly ahead.

Ideally the times should match and every effort should be taken to ensure that the client and server times are in sync.

It is not recommended to provide any value other than the default (current frame only) or the RFC recommendation of this and one frame prior as well as one frame ahead (see below)

var window = new VerificationWindow(previous:1, future:1);

In order to make using the RFC recommendation easier, there is a constant that contains a verification window object that complies with the RFC recommendation.

VerificationWindow.RfcSpecifiedNetworkDelay

This can be used as follows

totp.VerifyTotp(totpCode, out timeWindowUsed, VerificationWindow.RfcSpecifiedNetworkDelay);
Time compensation

In an ideal world both the client and the server's system time are correct to the second with NIST or other authoritative time standards. This would ensure that the generated code is always correct. If at all possible, sync the system time as closely as with NIST.

There are cases where this simply isn't possible. Perhaps you are writing an app to generate codes for use with a server who's time is significantly off. You can't control the erroneous time of the server. You could set your system clock to match but then your time would be off significantly which isn't the desired result. There is a class called TimeCorrection that helps with these cases.

A time correction object creates an offset that can be used to correct (at least for the purposes of this calculation) the time relative to the incorrect system time.

It is created as follows

var correction = new TimeCorrection(correctTime);

Where the correct time parameter is a DateTime object that represents the current correct (at least for the purposes of verification) UTC time. For this to work there needs to be some way to get an instance of the current acceptable time. This could be done with an NTP (NTP with NIST is coming soon in this library) or looking for a Date response header from an HTTP request or some other way.

Once this instance is created it can be used for a long time as it always will use the current system time as a base to apply the correction factor. The object is threadsafe and thus can be used by multiple threads or web requests simultaneously.

There is an overload that takes both the correct time and the reference time to use as well. This can be used in cases where UTC time isn't used.

The Totp class constructor can take a TimeCorrection object that will be applied to all time calculations and verifications.

var totp = new Totp(secretKey, timeCorrection: correction);

HOTP (HMAC-based One Time Password)

In addition to TOTP, this library implements HOTP (counter based) code calculation in C#.

Creation of an HOTP object
using OtpNet;
var hotp = new Hotp(secretKey);

There are several options that can be used to change how the code is calculated. These are all mentioned in the RFC. These options are specified when the HOTP object is created.

Different hash algorithms can be used to calculate the code. The default is Sha1, but Sha256, and Sha512 may be used instead.

To change that behavior from the default of Sha1 simply pass in the OtpHashMode enum with the desired value into the constructor.

var hotp = new Hotp(secretKey, mode: OtpHashMode.Sha512);

Finally the truncation level can be specified. Basically this is how many digits do you want your HOTP code to be. The tests in the RFC specify 8, but 6 has become a de-facto standard if not an actual one. For this reason the default is 6 but you can set it to something else. There aren't a lot of tests around this either so use at your own risk (other than the fact that the RFC test table uses HOTP values that are 8 digits).

var hotp = new Hotp(secretKey, hotpSize: 8);
Verification

The HOTP implementation provides a mechanism for verifying HOTP codes that are passed in. There is a method called VerifyHotp with an overload that takes a counter value.

public bool VerifyHotp(string totp, long counter);

OTP Uri

You can use the OtpUri class to generate OTP style uris in the "Key Uri Format" as defined here: https://github.com/google/google-authenticator/wiki/Key-Uri-Format

var uriString = new OtpUri(OtpType.Totp, "JBSWY3DPEHPK3PXP", "alice@google.com", "ACME Co").ToString();
// uriString is otpauth://totp/ACME%20Co:alice%40google.com?secret=JBSWY3DPEHPK3PXP&issuer=ACME%20Co&algorithm=SHA1&digits=6&period=30

Base32 Encoding

Also included is a Base32 helper.

var key = KeyGeneration.GenerateRandomKey(20);

var base32String = Base32Encoding.ToString(key);
var base32Bytes = Base32Encoding.ToBytes(base32String);

var otp = new Totp(base32Bytes);

Credits

This project is originally based on the OtpSharp library. OtpSharp was written by Devin Martin.

Product Compatible and additional computed target framework versions.
.NET net5.0 is compatible.  net5.0-windows was computed.  net6.0 is compatible.  net6.0-android was computed.  net6.0-ios was computed.  net6.0-maccatalyst was computed.  net6.0-macos was computed.  net6.0-tvos was computed.  net6.0-windows was computed.  net7.0 is compatible.  net7.0-android was computed.  net7.0-ios was computed.  net7.0-maccatalyst was computed.  net7.0-macos was computed.  net7.0-tvos was computed.  net7.0-windows was computed.  net8.0 is compatible.  net8.0-android was computed.  net8.0-browser was computed.  net8.0-ios was computed.  net8.0-maccatalyst was computed.  net8.0-macos was computed.  net8.0-tvos was computed.  net8.0-windows was computed. 
.NET Core netcoreapp2.0 was computed.  netcoreapp2.1 was computed.  netcoreapp2.2 was computed.  netcoreapp3.0 was computed.  netcoreapp3.1 was computed. 
.NET Standard netstandard2.0 is compatible.  netstandard2.1 was computed. 
.NET Framework net461 is compatible.  net462 was computed.  net463 was computed.  net47 was computed.  net471 was computed.  net472 was computed.  net48 was computed.  net481 was computed. 
MonoAndroid monoandroid was computed. 
MonoMac monomac was computed. 
MonoTouch monotouch was computed. 
Tizen tizen40 was computed.  tizen60 was computed. 
Xamarin.iOS xamarinios was computed. 
Xamarin.Mac xamarinmac was computed. 
Xamarin.TVOS xamarintvos was computed. 
Xamarin.WatchOS xamarinwatchos was computed. 
Compatible target framework(s)
Included target framework(s) (in package)
Learn more about Target Frameworks and .NET Standard.
  • .NETFramework 4.6.1

    • No dependencies.
  • .NETStandard 2.0

    • No dependencies.
  • net5.0

    • No dependencies.
  • net6.0

    • No dependencies.
  • net7.0

    • No dependencies.
  • net8.0

    • No dependencies.

NuGet packages (72)

Showing the top 5 NuGet packages that depend on Otp.NET:

Package Downloads
Dynamics365.UIAutomation.Api

The purpose of this library is to provide Dynamics customers the ability to facilitate automated UI testing for their projects.

Jakar.Extensions

Extensions to aid in development.

hexasync.common

Package Description

HealthCheck.WebUI.MFA.TOTP

Shortcuts for adding TOTP multi-factor authentication support to HealtCheck login.

Fsel.Core

Fsel Core Package

GitHub repositories (8)

Showing the top 5 popular GitHub repositories that depend on Otp.NET:

Repository Stars
bitwarden/server
Bitwarden infrastructure/backend (API, database, Docker, etc).
duplicati/duplicati
Store securely encrypted backups in the cloud!
ProtonVPN/win-app
Official ProtonVPN Windows app
engindemirog/nArchitecture
microsoft/EasyRepro
Automated UI testing API for Dynamics 365
Version Downloads Last updated
1.4.0 162,729 4/12/2024
1.3.0 2,511,631 12/23/2022
1.2.2 6,344,274 11/7/2019
1.2.1 1,507,800 10/31/2018
1.2.0 291,424 1/5/2018
1.1.0 19,391 8/17/2017
1.0.1 44,206 1/29/2017
1.0.0 3,480 1/13/2017