On 7/10/19 2:45 AM, Masahiko Sawada wrote: > On Wed, Jul 10, 2019 at 11:06 AM Stephen Frost <sfr...@snowman.net> wrote: >> >> Greetings, >> >> * Ryan Lambert (r...@rustprooflabs.com) wrote: >> > > 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. >> > >> > Agreed. I know little of the inner details about the LSN but what I read >> > in [1] sounds encouraging in addition to tableoid + pagenum. >> > >> > [1] https://www.postgresql.org/docs/current/datatype-pg-lsn.html >> >> Yes, but it's still something that we'd have to store somewhere- the >> actual LSN of the page is going to be in the 8K block. > > Can we use CBC-ESSIV[1] or XTS[2] instead? IIUC with these modes we > can use table oid and page number for IV or tweak and we don't need to > change them each time to encrypt pages. > > [1] > https://en.wikipedia.org/wiki/Disk_encryption_theory#Encrypted_salt-sector_initialization_vector_.28ESSIV.29 > [2] > https://en.wikipedia.org/wiki/Disk_encryption_theory#XEX-based_tweaked-codebook_mode_with_ciphertext_stealing_(XTS)
From what I can tell [1] is morally equivalent to the NIST method and does nothing to change the fact that the input nonce needs to be unique for each encryption operation. I have not had time to review [2] yet... While it would be very tempting to convince ourselves that a unique input nonce is not a requirement, I think we are better off being conservative unless we find some extremely clear guidance that allows us to draw that conclusion. Joe -- Crunchy Data - http://crunchydata.com PostgreSQL Support for Secure Enterprises Consulting, Training, & Open Source Development
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