Jetty/Tomcat encrypted cookie-based session storage?

Ruby on Rails has supported signed cookie-based sessions for quite some time, with a few encrypted implementations springing up since then. Python and PHP also have implementations.

Does such a beast exist for the Java servlet containers Jetty or Tomcat?

We've received significant performance gains over RDBMS-based sessions with the PHP implementation in our clustered environment, and I'd be interested in trying something similar with one of our Java applications (which currently uses Jetty 7).

I'm aware of other ways to achieve this goal (memcached, synchronized in-memory caches) but I believe that for our particular needs the limitations of this storage method (sessions finalization before output, in-efficient storage after the 4K cookie size limit, reliance on an ultra-secret server-side key) are outweighed by the simpler deployment environment for this particular application.

If an implementation doesn't exist, does anybody have any ideas why it wouldn't? (e.g. Java sessions are typically larger than 4K, and so aren't as amenable to this storage method)