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Active Model OTP Gem Version Reviewed by Hound


ActiveModel::Otp makes adding Two Factor Authentication (TFA) to a model simple. Let's see what's required to get AMo::Otp working in our Application, using Rails 5.0 (AMo::Otp is also compatible with Rails 4.x versions). We're going to use a User model and try to add options provided by ActiveModel::Otp. Inspired by AM::SecurePassword


  • ROTP 6.2.0 or higher
  • Ruby 2.3 or greater


Add this line to your application's Gemfile:

gem 'active_model_otp'

And then execute:

$ bundle

Or install it yourself as follows:

$ gem install active_model_otp

Setting your Model

We're going to add a field to our User Model, so each user can have an otp secret key. The next step is to run the migration generator in order to add the secret key field.

rails g migration AddOtpSecretKeyToUsers otp_secret_key:string
      invoke  active_record
      create    db/migrate/20130707010931_add_otp_secret_key_to_users.rb

We’ll then need to run rake db:migrate to update the users table in the database. The next step is to update the model code. We need to use has_one_time_password to make it use TFA.

class User < ApplicationRecord

Note: If you're adding this to an existing user model you'll need to generate otp_secret_key with a migration like:

User.find_each { |user| user.update_attribute(:otp_secret_key, User.otp_random_secret) }

To use a custom column to store the secret key field you can use the column_name option. It is also possible to generate codes with a specified length.

class User < ApplicationRecord
  has_one_time_password column_name: :my_otp_secret_column, length: 4


The has_one_time_password statement provides to the model some useful methods in order to implement our TFA system. AMo:Otp generates one time passwords according to TOTP RFC 6238 and the HOTP RFC 4226. This is compatible with Google Authenticator apps available for Android and iPhone, and now in use on GMail.

The otp_secret_key is saved automatically when an object is created,

user = User.create(email: "")
 => "jt3gdd2qm6su5iqh"

Note: You can fork the applications for iPhone & Android and customize them

Getting current code (e.g. to send via SMS)

user.otp_code # => '186522'
sleep 30
user.otp_code # => '850738'

# Override current time
user.otp_code(time: + 3600) # => '317438'

Authenticating using a code

user.authenticate_otp('186522') # => true
sleep 30 # let's wait 30 secs
user.authenticate_otp('186522') # => false

Authenticating using a slightly old code

user.authenticate_otp('186522') # => true
sleep 30 # lets wait again
user.authenticate_otp('186522', drift: 60) # => true

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). It is useful with SMS, that is commonly used in combination with drift to extend the life of the code.

rails g migration AddLastOtpAtToUsers last_otp_at:integer
      invoke  active_record
      create    db/migrate/20220407010931_add_last_otp_at_to_users.rb
class User < ApplicationRecord
  has_one_time_password after_column_name: :last_otp_at
user.authenticate_otp('186522') # => true
user.authenticate_otp('186522') # => false

Counter based OTP

An additional counter field is required in our User Model

rails g migration AddCounterForOtpToUsers otp_counter:integer
      invoke  active_record
      create    db/migrate/20130707010931_add_counter_for_otp_to_users.rb

Set default value for otp_counter to 0.

change_column :users, :otp_counter, :integer, default: 0

In addition set the counter flag option to true

class User < ApplicationRecord
  has_one_time_password counter_based: true

And for a custom counter column

class User < ApplicationRecord
  has_one_time_password counter_based: true, counter_column_name: :my_otp_secret_counter_column

Authentication is done the same. You can manually adjust the counter for your usage or set auto_increment on success to true.

user.authenticate_otp('186522') # => true
user.authenticate_otp('186522', auto_increment: true) # => true
user.authenticate_otp('186522') # => false
user.otp_counter -= 1
user.authenticate_otp('186522') # => true

When retrieving an otp_code you can also pass the auto_increment option.

user.otp_code # => '186522'
user.otp_code # => '186522'
user.otp_code(auto_increment: true) # => '768273'
user.otp_code(auto_increment: true) # => '002811'
user.otp_code # => '002811'

Backup codes

We're going to add a field to our User Model, so each user can have an otp backup codes. The next step is to run the migration generator in order to add the backup codes field.

rails g migration AddOtpBackupCodesToUsers otp_backup_codes:text
      invoke  active_record
      create    db/migrate/20210126030834_add_otp_backup_codes_to_users.rb

You can change backup codes column name by option backup_codes_column_name:

class User < ApplicationRecord
  has_one_time_password backup_codes_column_name: 'secret_codes'

Then use array type in schema or serialize attribute in model as Array (depending on used db type). Or even consider to use some libs like (lockbox)[] with type array.

After that user can use one of automatically generated backup codes for authentication using same method authenticate_otp.

By default it generates 12 backup codes. You can change it by option backup_codes_count:

class User < ApplicationRecord
  has_one_time_password backup_codes_count: 6

By default each backup code can be reused an infinite number of times. You can change it with option one_time_backup_codes:

class User < ApplicationRecord
  has_one_time_password one_time_backup_codes: true
user.authenticate_otp('186522') # => true
user.authenticate_otp('186522') # => false

Google Authenticator Compatible

The library works with the Google Authenticator iPhone and Android app, and also includes the ability to generate provisioning URI's to use with the QR Code scanner built into the app.

# Use your user's email address to generate the provisioning_url
user.provisioning_uri # => 'otpauth://totp/'

# Use a custom field to generate the provisioning_url
user.provisioning_uri("hello") # => 'otpauth://totp/hello?secret=2z6hxkdwi3uvrnpn'

# You can customize the generated url, by passing a hash of Options
# `:issuer` lets you set the Issuer name in Google Authenticator, so it doesn't show as a blank entry.
user.provisioning_uri(nil, issuer: 'MYAPP') #=> 'otpauth://totp/'

This can then be rendered as a QR Code which can be scanned and added to the users list of OTP credentials.

Setting up a customer interval

If you define a custom interval for TOTP codes, just as has_one_time_password interval: 10 (for example), remember to include the interval also in provisioning_uri method. If not defined, the default value is 30 seconds (according to ROTP gem:

class User < ApplicationRecord
  has_one_time_password interval: 10 # the interval value is in seconds

user =
user.provisioning_uri("hello", interval: 10) # => 'otpauth://totp/hello?secret=2z6hxkdwi3uvrnpn&period=10'

# This code snippet generates OTP codes that expires every 10 seconds.

Note: Only some authenticator apps are compatible with custom period of tokens, for more details check these links:

So, be careful and aware when using custom intervals/periods for your TOTP codes beyond the default 30 seconds :)

Working example

Scan the following barcode with your phone, using Google Authenticator


Now run the following and compare the output

require "active_model_otp"

class User
  extend ActiveModel::Callbacks
  include ActiveModel::Validations
  include ActiveModel::OneTimePassword

  define_model_callbacks :create
  attr_accessor :otp_secret_key, :email


user = = ''
user.otp_secret_key = "2z6hxkdwi3uvrnpn"
puts "Current code #{user.otp_code}"

Note: otp_secret_key must be generated using RFC 3548 base32 key strings (for compatilibity with google authenticator)

Useful Examples


  1. Fork it
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create new Pull Request


Adds methods to set and authenticate against one time passwords (Two-Factor Authentication). Inspired in AM::SecurePassword







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