[sqlite] What happens to unused space?

[Tim Browse]

Hi,

Before I go diving into the SQLite source code, can anyone tell me what 
happens to unused space? (i.e. from deleted data in the db)

Specifically, I'm interested in whether it gets over-written by zeroes, 
or something.  I'm storing information in a database (in a blob field), 
which the user can then decide to encrypt, and then the plaintext blob 
is set to NULL, and the ciphertext blob (i.e. a different column) is set 
to contain the encrypted data.

So what I'm asking is, in this scenario, is it possible that sometimes I 
could load the sqlite db into a hex editor, and see the original 
unencrypted data? (i.e. the old deleted data from the plaintext blob)

If so, a workaround is obviously to set the original blob data to 
contain a block of zeroes (of the same length), and *then* set it to 
NULL.  But then it's also within the realms of possibility (not wishing 
to criticise anyone's code; only guessing) that SQLite might reallocate 
a block for the blob even if it's the same length as the old data, in 
which case my cunning plan wouldn't work either.

So in summary, when I delete a blob, I want it to stay deleted, even 
from prying eyes with a hex editor - what's the best way to do this in 
the SQLite model?

NB. To pre-empt various invigorating arguments, I know about the various 
'encrypt the whole db' solutions that are available, but I don't need 
them; I have my own encryption framework. I just want to know how to 
*really* delete data from the DB (without calling VACUUM after every 
update).

Tim

PS. Couldn't see anything obvious about this in the docs from a quick 
search, but if anyone has a link that explains all this, then apologies, 
and let me at it! :)