Greetings,
* Joe Conway (mail@joeconway.com) wrote:
> On 7/9/19 7:28 PM, Stephen Frost wrote:
> > * 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.
>
> Hmm, good point. I'm not entirely sure it would be required if the two
> page versions don't exist at the same time, but I guess backups mean
> that it would, so yeah.
Uh, or an attacker got a copy of the page and then just waited a few
minutes for a new version to be written and then grabbed that...
Definitely not limited to just concerns about the fact that other
versions would exist in backups too.
Thanks,
Stephen
