Order preserving encryption of numeric data

[Anindya Mozumdar <anindya.lugbang@gmail.com>]

Hi,
    Dont know whether this is an appropriate question to ask in this
list, but someone experienced in dealing with such a situation could
help.

    I would like an algorithm which will encrypt/decrypt numeric data
using a key, such that after encryption their orders will be
preserved,i.e, if crypt(a) denotes the encrypted form of a, and a < b,
then crypt(a) < crypt(b). ( Note that this assumes that the algorithm
will produce crypted data which can actually be ordered ). Of course,
something like a -> ma + n, where m and n are constants would work,
but I would like something more nontrivial.

    The order preserving property is necessary as the crypted data
will be stored in a database, and I would like to issue database
queries on them. ( The other solution is to retrieve all values from
the database, decrypt them, and find the appropriate subset, but that
would be too expensive ).

Thanks.
Anindya.