Support for binary types? Normal Marshal classes?

We have some information we need to encrypt in our dynamodb store.... It appears there are no `attr` methods for declaring objects that will be of type `B` in Dynamo.

I'm trying to build an appropriate marshaler (sic), but I'm also a bit stymied by the unfamiliar abstract interface (compared to say, Ruby's Marshal class, AR's [serialization support](http://ruby-journal.com/how-to-write-custom-serializer-for-activerecord-number-serialize/), or Data Mapper's [dm-types](https://github.com/datamapper/dm-types)) for these in this library. In what way does `type_cast` imply "read from database representation"? And why is it always called, even when _writing_ to the database representation?

If it has to work this way, what class can my marshaler assume the raw data is in when read?