On 7/9/19 7:28 PM, Stephen Frost wrote: > Greetings, > > * Joe Conway (m...@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. > 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. Joe -- Crunchy Data - http://crunchydata.com PostgreSQL Support for Secure Enterprises Consulting, Training, & Open Source Development
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