I am not sure what your concern ab ou it uniqueness is: encrypti on has to be invertible, therefore if each non encrypted integer is unique, it means that each

I am not sure what your concern ab ou it uniqueness is: encrypti on has to be invertible, therefore if each non encrypted integer is unique, it means that each encrypted representation is also unique. w.r.t encryption cost, I think AES is very cheap on modern hardware compared to the cost of SQL or the overhead of writing your app in an interpreted language.