Captcha gem for Rails Application. No dependencies. No ImageMagick, No RMagick.
C Ruby
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This is a Captcha gem for Rails Applications which generates captcha image by C code.




  • No dependencies. No ImageMagick. No RMagick;
  • For Rails Application;
  • Simple, Easy to use;
  • High performance.


Put rucaptcha in your Gemfile:

gem 'rucaptcha'

Create config/initializers/rucaptcha.rb

RuCaptcha.configure do
  # Color style, default: :colorful, allows: [:colorful, :black_white]
  # = :colorful
  # Custom captcha code expire time if you need, default: 2 minutes
  # self.expires_in = 120
  # [Requirement / 重要]
  # Store Captcha code where, this config more like Rails config.cache_store
  # default: Read config info from `Rails.application.config.cache_store`
  # But RuCaptcha requirements cache_store not in [:null_store, :memory_store, :file_store]
  # 默认:会从 Rails 配置的 cache_store 里面读取相同的配置信息,并尝试用可以运行的方式,用于存储验证码字符
  # 但如果是 [:null_store, :memory_store, :file_store] 之类的,你可以通过下面的配置项单独给 RuCaptcha 配置 cache_store
  self.cache_store = :mem_cache_store
  # Chars length, default: 5, allows: [3 - 7]
  # self.length = 5
  # enable/disable Strikethrough.
  # self.strikethrough = true

RuCaptcha 没有使用 Rails Session 来存储验证码信息,因为 Rails 的默认 Session 是存储在 Cookie 里面,如果验证码存在里面会存在 Replay attack 漏洞,导致验证码关卡被攻破。

所以我在设计上要求 RuCaptcha 得配置一个可以支持分布式的后端存储方案例如:Memcached 或 Redis 以及其他可以支持分布式的 cache_store 方案。

同时,为了保障易用性,默认会尝试使用 :file_store 的方式,将验证码存在应用程序的 tmp/cache/rucaptcha/session 目录(但请注意,多机器部署这样是无法正常运作的)。

所以,我建议大家使用的时候,配置上 cache_store (详见 Rails Guides 缓存配置部分的文档)到一个 Memcached 或 Redis,这才是最佳实践。

(RuCaptha do not use Rails Session to store captcha information. As the default session is stored in Cookie in Rails, there's a Replay attack bug which may causes capthcha being destroyed if we store captcha in Rails Session.

So in my design I require RuCaptcha to configure a distributed backend storage scheme, such as Memcached, Redis or other cache_store schemes which support distribution.

Meanwhile, for the ease of use, RuCapthca would try to use :file_store by default and store the capthca in tmp/cache/rucaptcha/session directory (kindly note that it's not working if deploy on multiple machine).

For recommendation, configure the cache_store(more details on Rails Guides Configuration of Cache Stores) to Memcached or Redis, that would be the best practice.)

Controller app/controller/account_controller.rb

When you called verify_rucaptcha?, it uses value from params[:_rucaptcha] to validate.

class AccountController < ApplicationController
  def create
    @user =[:user])
    if verify_rucaptcha?(@user) &&
      redirect_to root_path, notice: 'Sign up successed.'
      render 'account/new'

class ForgotPasswordController < ApplicationController
  def create
    # without any args
    if verify_rucaptcha?
      redirect_to '/forgot-password', alert: 'Invalid captcha code.'

TIP: Sometimes you may need to keep last verified captcha code in session on verify_rucaptcha? method call, you can use keep_session: true. For example: verify_rucaptcha? @user, keep_session: true.

View app/views/account/new.html.erb

<form method="POST">
  <div class="form-group">
    <%= rucaptcha_input_tag(class: 'form-control', placeholder: 'Input Captcha') %>
    <%= rucaptcha_image_tag(alt: 'Captcha') %>

  <div class="form-group">
    <button type="submit" class="btn btn-primary">Submit</button>

And if you are using Devise, you can read this reference to add validation: RuCaptcha with Devise.

Write your test skip captcha validation

for RSpec

describe 'sign up and login', type: :feature do
  before do
    allow_any_instance_of(ActionController::Base).to receive(:verify_rucaptcha?).and_return(true)

  it { ... }

for MiniTest

class ActionDispatch::IntegrationTest
  def sign_in(user)
    post user_session_path \
         'user[email]'    =>,
         'user[password]' => user.password

Invalid message without Devise

When you are using this gem without Devise, you may find out that the invalid message is missing. For this case, use the trick below to add your i18n invalid message manually.

if verify_rucaptcha?(@user) &&
  redirect_to someplace_you_want
  # this is the trick
  @user.errors.add(:base, t('rucaptcha.invalid'))
  render :new