Rails 4: See Cookie

Table of Contents

I wondered how the Rails cookie structure is and checked it.

Environment

Rails 4.2.5
Ruby 2.0.0p481

Code to Decode Cookie Value

First, I checked the cookie value. Launch rails console, bundle exec rails c, and execute the following code.

cookie = 'your cookie value'
key    = 'your secret key base'
  

cookie = CGI::unescape(cookie)
  
# Default values for Rails 4 apps
key_iter_num = 1000
key_size     = 64
## Rails.application.config.action_dispatch.encrypted_cookie_salt
salt         = "encrypted cookie"         
## Rails.application.config.action_dispatch.encrypted_signed_cookie_salt
signed_salt  = "signed encrypted cookie"  

key_generator = ActiveSupport::KeyGenerator.new(
  key, iterations: key_iter_num)
secret        = key_generator.generate_key(salt)
sign_secret   = key_generator.generate_key(signed_salt)

encryptor     = ActiveSupport::MessageEncryptor.new(
  secret, sign_secret,
  serializer: ActiveSupport::MessageEncryptor::NullSerializer)
encryptor.decrypt_and_verify(cookie)

cookie = 'your cookie value'

key = 'your secret key base'

cookie = CGI::unescape(cookie)

# Default values for Rails 4 apps

key_iter_num = 1000

key_size = 64

## Rails.application.config.action_dispatch.encrypted_cookie_salt

salt = "encrypted cookie"

## Rails.application.config.action_dispatch.encrypted_signed_cookie_salt

signed_salt = "signed encrypted cookie"

key_generator = ActiveSupport::KeyGenerator.new(

key, iterations: key_iter_num)

secret = key_generator.generate_key(salt)

sign_secret = key_generator.generate_key(signed_salt)

encryptor = ActiveSupport::MessageEncryptor.new(

secret, sign_secret,

serializer: ActiveSupport::MessageEncryptor::NullSerializer)

encryptor.decrypt_and_verify(cookie)

Set cookie value to the variable cookie, and set secret_key_base value to the variable key.

cookie value

If you use Chrome browser (Version 47.0.2526.106), you can see the cookie value with developer tools. Open Developer Tools and choose resource view and select cookie that you want to see on left side tree.

Cookie is name and value pair. The name of the cookie generated by Rails 4 is like _xxx_session as default. You can specify it as Rails.application.config.session_store, which is written in config/initializers/session_store.rb usually.

secret_key_base value

Normally, it is written in config/secrets.yml.

salt and secret

In the above code, I set values defined as config to salt and secret. It is “encrypted cookie” and “signed encrypted cookie” as default.

You can see that values by checking cookies variable in controller.

Example of Cookie Value

Rails のテスト用アプリケーションでクッキーの値を取得してみました。テスト用アプリケーションでは devise gem を使用しており、クッキーを取得したときユーザID 2 のユーザでログインしていました。

{
  "session_id": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
  "warden.user.key": [[2], "xxxxxxxxxxxxxxxxxxxxxxxxxx"],
  "_csrf_token": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
}

{

"session_id": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",

"warden.user.key": [[2], "xxxxxxxxxxxxxxxxxxxxxxxxxx"],

"_csrf_token": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

}

デコードの処理内容

クッキーデコード時の処理内容を確認します。

CGI.unescape でクッキーに入っている値を URLデコードします。

キージェネレータを作成します。キージェネレータはシークレットキーの値とイテレーション回数をインスタンス変数にもつオブジェクトで、初期化処理は次のようになっています。

# File activesupport/lib/active_support/key_generator.rb, line 10
def initialize(secret, options = {})
  @secret = secret
  # The default iterations are higher than required for our key derivation uses
  # on the off chance someone uses this for password storage
  @iterations = options[:iterations] || 2**16
end

# File activesupport/lib/active_support/key_generator.rb, line 10

def initialize(secret, options = {})

@secret = secret

# The default iterations are higher than required for our key derivation uses

# on the off chance someone uses this for password storage

@iterations = options[:iterations] || 2**16

end

デコード時にiterationsに 1000 を代入しているのは、Rails::Application class で次のように1000 が使用されているからです。

    # Returns the application's KeyGenerator
    def key_generator
      # number of iterations selected based on consultation with the google security
      # team. Details at https://github.com/rails/rails/pull/6952#issuecomment-7661220
      @caching_key_generator ||=
        if secrets.secret_key_base
          unless secrets.secret_key_base.kind_of?(String)
            raise ArgumentError, "`secret_key_base` for #{Rails.env} environment must be a type of String, change this value in `config/secrets.yml`"
          end
          key_generator = ActiveSupport::KeyGenerator.new(secrets.secret_key_base, iterations: 1000)
          ActiveSupport::CachingKeyGenerator.new(key_generator)
        else
          ActiveSupport::LegacyKeyGenerator.new(secrets.secret_token)
        end
    end

# Returns the application's KeyGenerator

def key_generator

# number of iterations selected based on consultation with the google security

# team. Details at https://github.com/rails/rails/pull/6952#issuecomment-7661220

@caching_key_generator ||=

if secrets.secret_key_base

unless secrets.secret_key_base.kind_of?(String)

raise ArgumentError, "`secret_key_base` for #{Rails.env} environment must be a type of String, change this value in `config/secrets.yml`"

end

key_generator = ActiveSupport::KeyGenerator.new(secrets.secret_key_base, iterations: 1000)

ActiveSupport::CachingKeyGenerator.new(key_generator)

else

ActiveSupport::LegacyKeyGenerator.new(secrets.secret_token)

end

Generate key from salt. generate_key method is defined as below.

# File activesupport/lib/active_support/key_generator.rb, line 20
def generate_key(salt, key_size=64)
  OpenSSL::PKCS5.pbkdf2_hmac_sha1(@secret, salt, @iterations, key_size)
end

# File activesupport/lib/active_support/key_generator.rb, line 20

def generate_key(salt, key_size=64)

OpenSSL::PKCS5.pbkdf2_hmac_sha1(@secret, salt, @iterations, key_size)

end

OpenSSL::PKCS5.pbkdf2_hmac_sha1 generates key for encryption by hash method sha1. In this case, iterates 1000 times.

Generate sign_secret like above.

Crete MessageEncryptor object. Initializer of MessageEncryptor is defined as below.

# File activesupport/lib/active_support/message_encryptor.rb, line 45
def initialize(secret, *signature_key_or_options)
  options = signature_key_or_options.extract_options!
  sign_secret = signature_key_or_options.first
  @secret = secret
  @sign_secret = sign_secret
  @cipher = options[:cipher] || 'aes-256-cbc'
  @verifier = MessageVerifier.new(@sign_secret || @secret, digest: options[:digest] || 'SHA1', serializer: NullSerializer)
  @serializer = options[:serializer] || Marshal
end

# File activesupport/lib/active_support/message_encryptor.rb, line 45

def initialize(secret, *signature_key_or_options)

options = signature_key_or_options.extract_options!

sign_secret = signature_key_or_options.first

@secret = secret

@sign_secret = sign_secret

@cipher = options[:cipher] || 'aes-256-cbc'

@verifier = MessageVerifier.new(@sign_secret || @secret, digest: options[:digest] || 'SHA1', serializer: NullSerializer)

@serializer = options[:serializer] || Marshal

end

options[:chiper]は設定されないまま実行されますから、 AES-256-CBC が暗号化方式として設定されます。 AES は共通鍵暗号で、ここで作成したMessageEncryptorオブジェクトはencryptとdecryptが可能です。

verify and decrypt. verify を行うのは padding attack を防ぐためだそうです。 It doesn’t decrypt simply, so incorrect secret value trigger exception and you can’t get decrypted value.

Generate Cookie Value

前節で書いたのと同じ要領で MessageEncryptor のオブジェクトを作成して encrypt_and_sign を実行すれば cookie の値を作成できます。 encrypt_and_signメソッドは実行する度に結果が変わります。

value = '{"session_id":"12345","key_1":"value_1","key_2":"value_2"}'

cookie_value = encryptor.encrypt_and_sign(value)

encryptor.decrypt_and_verify(
  encryptor.encrypt_and_sign(value)) == value
# => true

value = '{"session_id":"12345","key_1":"value_1","key_2":"value_2"}'

cookie_value = encryptor.encrypt_and_sign(value)

encryptor.decrypt_and_verify(

encryptor.encrypt_and_sign(value)) == value

# => true

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