You're looking for this: http://en.wikipedia.org/wiki/Homomorphic_encryption#Fully_ho...
Practicality right now is minimal. My intuition says it is going to stay that way, but who knows.
Practicality right now is minimal. My intuition says it is going to stay that way, but who knows.
http://en.wikipedia.org/wiki/Paillier_cryptosystem
But yeah, anything more than that is currently not all that practical.