Encrypting application-level data at rest

In a previous post I described how I keep sensitive config data such as API keys secret for a public app.

What about data that is kept per-user, such as a password or API key for each user or other similar record in your databases?

For that I use the attr_encrypted gem. There is a great post on how to do this; if you're using Figaro as recommended in my earlier post, that gives you an easy place to store the symmetric-encryption key(s) used by attr_encrypted, so instead of

attr_encrypted :user, :key => ENV["USERKEY"]

you would instead say

attr_encrypted :user, :key => Figaro.env.USERKEY

Beware of attr_encrypted and serialize

Be aware that if you are using serialize to store (say) a hash or array in an ActiveRecord text field, there appear to be some weird interactions if you try to use attr_encrypted on that field. What I have found to work is this:

Before:

class User < ActiveRecord::Base
  serialize :api_keys, Hash
end

User.new.api_keys  # =>  {}

After:

class User < ActiveRecord::Base
  attr_encrypted :api_keys, :key => Figaro.env.SECRET, \

     :marshal => true

end

User.new.api_keys  # => nil

That last option instructs attr_encrypted to marshal the resulting data structure before encrypting, and unmarshal after reading from the database and decrypting. However, whereas using serialize gives newly-created attributes a default value of a new instance of the serialized type (in this case, that would be the empty hash), this is not true with attr_encrypted. To remedy this, if your app relies on the serialized value always being non-nil, I'd advise using an after_initialize block to enforce the invariant that the attribute's default value is always an instance of the serialized class:

class User < ActiveRecord::Base

  attr_encrypted :api_keys, :key => Figaro.env.SECRET, \

    :marshal => true

  def ensure_is_hash ; self.api_keys ||= {} ; end

  after_initialize :ensure_is_hash

  private          :ensure_is_hash

  # ...

end

User.new.api_keys  # =>  {}

Et voila, with the exception of the after_initialize callback, your encrypted-at-rest attributes will now behave the same way as regular unencrypted attributes.