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Webauthn and the future of 2FA

Although this library will continue to be maintained, if you're implementing a 2FA solution today, you should take a look at Webauthn. It doesn't involve shared secrets and it's supported by most modern browsers and operating systems.

Ruby resources for Webauthn

The Ruby One Time Password Library

Build Status Gem Version Documentation License

A ruby library for generating and validating one time passwords (HOTP & TOTP) according to RFC 4226 and RFC 6238.

ROTP is compatible with Google Authenticator available for Android and iPhone and any other TOTP based implementations.

Many websites use this for multi-factor authentication, such as GMail, Facebook, Amazon EC2, WordPress, and Salesforce. You can find a more complete list here.


  • OpenSSL
  • Ruby 2.3 or higher

Breaking changes

Breaking changes in >= 6.0

  • Dropping support for Ruby <2.3

Breaking changes in >= 5.0

  • ROTP::Base32.random_base32 is now ROTP::Base32.random and the argument has changed from secret string length to byte length to allow for more precision. There is an alias to allow for random_base32 for the time being.
  • Cleaned up the Base32 implementation to match Google Authenticator's version.

Breaking changes in >= 4.0

  • Simplified API
    • verify now takes options for drift and after,padding is no longer an option
    • verify returns a timestamp if true, nil if false
  • Dropping support for Ruby < 2.0
  • Docs for 3.x can be found here


gem install rotp

Library Usage

Time based OTP's

totp ="base32secret3232", issuer: "My Service") # => "492039"

# OTP verified for current time - returns timestamp of the current interval
# period.
totp.verify("492039") # => 1474590700

sleep 30

# OTP fails to verify - returns nil
totp.verify("492039") # => nil

Counter based OTP's

hotp ="base32secretkey3232") # => "786922" # => "595254" # => "259769"

# OTP verified with a counter
hotp.verify("259769", 1401) # => 1401
hotp.verify("259769", 1402) # => nil

Preventing reuse of Time based OTP's

By keeping track of the last time a user's OTP was verified, we can prevent token reuse during the interval window (default 30 seconds)

The following is an example of this in action:

user = User.find(someUserID)
totp = # => "492039"

# Let's take a look at the last time the user authenticated with an OTP
user.last_otp_at # => 1432703530

# Verify the OTP
last_otp_at = totp.verify("492039", after: user.last_otp_at) #=> 1472145760
# ROTP returns the timestamp(int) of the current period

# Store this on the user's account
user.update(last_otp_at: last_otp_at)

# Someone attempts to reuse the OTP inside the 30s window
last_otp_at = totp.verify("492039", after: user.last_otp_at) #=> nil
# It fails to verify because we are still in the same 30s interval window

Verifying a Time based OTP with drift

Some users may enter a code just after it has expired. By adding 'drift' you can allow for a recently expired token to remain valid.

totp ="base32secret3232")
now = #2016-09-23 00:30:00 UTC # => "250939"

# OTP verified for current time along with 15 seconds earlier
# ie. User enters a code just after it expired
totp.verify("250939", drift_behind: 15, at: now + 35) # => 1474590600
# User waits too long. Fails to validate previous OTP
totp.verify("250939", drift_behind: 15, at: now + 45) # => nil

Generating a Base32 Secret key

ROTP::Base32.random  # returns a 160 bit (32 character) base32 secret. Compatible with Google Authenticator

Note: The Base32 format conforms to RFC 4648 Base32

Generating QR Codes for provisioning mobile apps

Provisioning URI's generated by ROTP are compatible with most One Time Password applications, including Google Authenticator.

totp ="base32secret3232", issuer: "My Service")
totp.provisioning_uri("") # => 'otpauth://totp/'

hotp ="base32secret3232", issuer: "My Service")
hotp.provisioning_uri("", 0) # => 'otpauth://hotp/'

This can then be rendered as a QR Code which the user can scan using their mobile phone and the appropriate application.

Working example

Scan the following barcode with your phone, using Google Authenticator

QR Code for OTP

Now run the following and compare the output

require 'rubygems'
require 'rotp'
p "Current OTP: #{}"


bundle install
bundle exec rspec

Testing with Docker

In order to make it easier to test against different ruby version, ROTP comes with a set of Dockerfiles for each version that we test against in Travis

docker build -f Dockerfile-2.6 -t rotp_2.6 .
docker run --rm -v $(pwd):/usr/src/app rotp_2.6

Alternately, you may use docker-compose to run all the tests:

docker-compose up

Executable Usage

The rotp rubygem includes CLI version to help with testing and debugging

# Try this to get an overview of the commands
rotp --help

# Examples
rotp --secret p4ssword                       # Generates a time-based one-time password
rotp --hmac --secret p4ssword --counter 42   # Generates a counter-based one-time password


Have a look at the contributors graph on Github.


MIT Copyright (C) 2019 by Mark Percival, see LICENSE for details.

Other implementations

A list can be found at Wikipedia.