Postgresql + digital signature

I'm working in a project which is using postgres (great database!, I love it)

We're in a stage where I need to implement a mechanism to prevent the data modification.

I'm thinking on 'Digital Signatures' (maybe RSA) in each row. If there's a modification, the signature doesn't verify.


However before start I need your help to know:

Is there in postgres something functionality like this?
Does any know if there's something similar another database system?


thanks in advance!



Luis Alberto Pérez Paz