Greetings,
* Joe Conway (mail@joeconway.com) wrote:
> On 7/9/19 5:42 PM, Tomas Vondra wrote:
> > There are two basic ways to construct nonces - CSPRNG and sequences, and
> > then a combination of both, i.e. one part is generated from a sequence
> > and one randomly.
> >
> > FWIW not sure using OIDs as nonces directly is a good idea, as those are
> > inherently low entropy data - how often do you see databases with OIDs
> > above 1M or so? Probably not very often, and in most cases those are
> > databases where those OIDs are for OIDs and large objects, so irrelevant
> > for this purpose. I might be wrong but having a 96-bit nonce with maybe
> > just 32bits of entrophy seems suspicious.
> >
> > That does not mean we can't use the OIDs at all, but maybe hashing them
> > into a single 4B value, and then picking the remaining 8B randomly.
> > Also, we have a "natural" sequence in the database - LSNs, maybe that
> > would be a good source of nonces too?
>
> I think you missed the quoted part (upthread) from the NIST document:
>
> "There are two recommended methods for generating unpredictable IVs.
> The first method is to apply the forward cipher function, under the
> same key that is used for the encryption of the plaintext, to a
> nonce. The nonce must be a data block that is unique to each
> execution of the encryption operation. For example, the nonce may be
> a counter, as described in Appendix B, or a message number. The
> second method is to generate a random data block using a
> FIPS-approved random number generator."
>
> That first method says a counter as input produces an acceptably
> unpredictable IV as long as it is unique to each encryption operation.
> If each page is going to be an "encryption operation", so as long as our
> input nonce is unique for a given key, we should be ok. If the input
> nonce is tableoid+pagenum and the key is different per database (at
> least, hopefully different per tablespace too), we should be good to go,
> at least from what I can see.
What I think Tomas is getting at here is that we don't write a page only
once.
A nonce of tableoid+pagenum will only be unique the first time we write
out that page. Seems unlikely that we're only going to be writing these
pages once though- what we need is a nonce that's unique for *every
write* of the 8k page, isn't it? As every write of the page is going to
be encrypting something new.
With sufficient randomness, we can at least be more likely to have a
unique nonce for each 8K write. Including the LSN seems like it'd be a
possible alternative.
Thanks,
Stephen
