Database Software

Hi everybody,
I need to provide data integrity checks on sensitive data stored within my Oracle 8i database.
The point is to create a checksum from some or even any of the fields of a table's record and store the checksum in a different table storing signatures.
The checksum must be generated using an algorithm based on encryption so that checksum re-creation should be hard to get by malicious users. I was thinking about a public/private key model: but the issues is...where to store securely those persisten keys?

The logic may resides either at the database layer (by means of triggers for example) or at the application layer (using java).

Do you have any hints about how to implement such a feature?

Thanks a lot in advance

Sergio