I believe that Klaus is trying to tell you that if you create an index on an encrypted column, you are storing the encrypted data in plaintext within the index. An attacker can access the secure data via the index instead of the column.
I suppose you could partially mitigate this by putting the index in a separate table space and putting that table space on a virtual disk device that is encrypted.
But if that meets your needs why not just put the whole table on that encrypted table space and leave the column unencrypted inside Pg?
Steve
