Let's say an application has really specific data which belongs to a user, and nobody is supposed to see it except the owner. I use MySQL database with DataMapper ORM mapper. The application is written in Ruby on Sinatra.

Application behavior:

1. User signs up for an account. Creates username and password.
2. Logs into his dashboard.
3. Some fields in specific tables must be protected.

Basically, I'm looking for auto-encryption for a model properties. Something like this:

class Transaction
  include DataMapper::Resource

  property :id, Serial
  property :value, String, :length => 1024, :encrypted => true
  ... etc ...
  belongs_to :user
end

I assume that encryption/decryption on the fly will cause performance problems, but that's ok. At least if that works - I'm fine.

Any ideas how to do this?