EncryptedCookieStore

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.

Installation and usage

First, add EncryptedCookieStore to your Gemfile

gem 'encrypted_cookie_store'

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 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.
}

Operational details

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

Upon unmarshalling the cookie data, EncryptedCookieStore decrypts and decompresses the encrypted session data. The decrypted session data is then verified against the HMAC. It is also verified that the timestamp isn't too old, too prevent replay attacks.

EncryptedCookieStore also changes how CookieStore sets the cookie. If the session has not changed, and the timestamp is less than 5 minutes old, it will not send the cookie to the browser.

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. EncryptedCookieStore does improve on CookieStore in reducing the amount of time allowed for a replay attack to the :expire_after value, instead of forever, but is still weaker than a server side session with an accompanying cookie that allows re-establishment of a session, but not replay of the session contents.
• 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.