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Announcing EncryptedCookieStore plugin for Rails 2.3

By Hongli Lai on April 13th, 2010

EncryptedCookieStore is similar to Ruby on Rails’s CookieStore (it saves session data in a cookie), but it uses encryption so that people can’t read what’s in the session data. This makes it possible to store sensitive data in the session.

EncryptedCookieStore is written for Rails 2.3. Other versions of Rails have not been tested.

Note: This is not ThinkRelevance’s EncryptedCookieStore. In the Rails 2.0 days they wrote an EncryptedCookieStore, but it seems their repository had gone defunct and their source code lost. This EncryptedCookieStore is written from scratch by Phusion.

Source code at

Installation and usage

First, install it:

./script/plugin install git://

Then edit config/initializers/session_store.rb and set your session store to EncryptedCookieStore:

ActionController::Base.session_store = EncryptedCookieStore

You need to set a few session options before EncryptedCookieStore is usable. You must set all options that CookieStore needs, plus an encryption key that EncryptedCookieStore needs. In session_store.rb:

ActionController::Base.session = {
        # CookieStore options...
        :key            => '_session',     # Name of the cookie which contains the session data.
        :secret         => 'b4589cc9...',  # A secret string used to generate the checksum for
                                           # the session data. Must be longer than 64 characters
                                           # and be completely random.

        # EncryptedCookieStore options...
        :encryption_key => 'c306779f3...', # The encryption key. See below for notes.

The encryption key must be a hexadecimal string of exactly 32 bytes. It should be entirely random, because otherwise it can make the encryption weak.

You can generate a new encryption key by running rake secret:encryption_key. This command will output a random encryption key that you can then copy and paste into your environment.rb.

Operational details

Upon generating cookie data, EncryptedCookieStore generates a new, random initialization vector for encrypting the session data. This initialization vector is then encrypted with 128-bit AES in ECB mode. The session data is first protected with an HMAC to prevent tampering. The session data, along with the HMAC, are then encrypted using 256-bit AES in CFB mode with the generated initialization vector. This encrypted session data + HMAC are then stored, along with the encrypted initialization vector, into the cookie.

Upon unmarshalling the cookie data, EncryptedCookieStore decrypts the encrypted initialization vector and use that to decrypt the encrypted session data + HMAC. The decrypted session data is then verified against the HMAC.

The reason why HMAC verification occurs after decryption instead of before decryption is because we want to be able to detect changes to the encryption key and changes to the HMAC secret key, as well as migrations from CookieStore. Verifying after decryption allows us to automatically invalidate such old session cookies.

EncryptedCookieStore is quite fast: it is able to marshal and unmarshal a simple session object 5000 times in 8.7 seconds on a MacBook Pro with a 2.4 Ghz Intel Core 2 Duo (in battery mode). This is about 0.174 ms per marshal+unmarshal action. See rake benchmark in the EncryptedCookieStore sources for details.

EncryptedCookieStore vs other session stores

EncryptedCookieStore inherits all the benefits of CookieStore:

  • It works out of the box without the need to setup a seperate data store (e.g. database table, daemon, etc).
  • It does not require any maintenance. Old, stale sessions do not need to be manually cleaned up, as is the case with PStore and ActiveRecordStore.
  • Compared to MemCacheStore, EncryptedCookieStore can “hold” an infinite number of sessions at any time.
  • It can be scaled across multiple servers without any additional setup.
  • It is fast.
  • It is more secure than CookieStore because it allows you to store sensitive data in the session.

There are of course drawbacks as well:

  • It is prone to session replay attacks. These kind of attacks are explained in the Ruby on Rails Security Guide. Therefore you should never store anything along the lines of is_admin in the session.
  • You can store at most a little less than 4 KB of data in the session because that’s the size limit of a cookie. “A little less” because EncryptedCookieStore also stores a small amount of bookkeeping data in the cookie.
  • Although encryption makes it more secure than CookieStore, there’s still a chance that a bug in EncryptedCookieStore renders it insecure. We welcome everyone to audit this code. There’s also a chance that weaknesses in AES are found in the near future which render it insecure. If you are storing *really* sensitive information in the session, e.g. social security numbers, or plans for world domination, then you should consider using ActiveRecordStore or some other server-side store.

JRuby: Illegal Key Size error

If you get this error (and your code works with MRI)…

    Illegal key size
    [...]/vendor/plugins/encrypted_cookie_store/lib/encrypted_cookie_store.rb:62:in `marshal'

…then it probably means you don’t have the “unlimited strength” policy files installed for your JVM. Download and install them. You probably have the “strong” version if they are already there.

As a workaround, you can change the cipher type from 256-bit AES to 128-bit by
inserting the following in config/initializer/session_store.rb:

EncryptedCookieStore.data_cipher_type = 'aes-128-cfb'.freeze  # was 256

Please note that after changing to 128-bit AES, EncryptedCookieStore still requires a 32 bytes hexadecimal encryption key, although only half of the key is actually used.