Horton

Re: storing an explicit nonce

Antonin Houska <ah@cybertec.at>

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From: Antonin Houska <ah@cybertec.at>
To: Bruce Momjian <bruce@momjian.us>
Cc: Andres Freund <andres@anarazel.de>, Stephen Frost <sfrost@snowman.net>, Robert Haas <robertmhaas@gmail.com>, Alvaro Herrera <alvherre@alvh.no-ip.org>, Masahiko Sawada <sawada.mshk@gmail.com>, Tom Kincaid <tomjohnkincaid@gmail.com>, Amit Kapila <amit.kapila16@gmail.com>, Thomas Munro <thomas.munro@gmail.com>, PostgreSQL-development <pgsql-hackers@postgresql.org>, Masahiko Sawada <masahiko.sawada@2ndquadrant.com>
Date: 2021-05-26T05:14:47Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. Rethink method for assigning OIDs to the template0 and postgres DBs.

  2. pg_upgrade: Preserve database OIDs.

  3. pg_upgrade: Preserve relfilenodes and tablespace OIDs.

  4. Fix for new Boolean node

  5. Improve error handling of HMAC computations

  6. Add macro RelationIsPermanent() to report relation permanence

  7. Enhance nbtree index tuple deletion.

Bruce Momjian <bruce@momjian.us> wrote:

> On Tue, May 25, 2021 at 04:48:21PM -0700, Andres Freund wrote:
> > Hi,
> > 
> > On 2021-05-25 17:29:03 -0400, Bruce Momjian wrote:
> > > So, let me ask --- I thought CTR basically took an encrypted stream of
> > > bits and XOR'ed them with the data.  If that is true, then why are
> > > changing hint bits a problem?  We already can see some of the bit stream
> > > by knowing some bytes of the page.
> > 
> > A *single* reuse of the nonce in CTR reveals nearly all of the
> > plaintext. As you say, the data is XORed with the key stream. Reusing
> > the nonce means that you reuse the key stream. Which in turn allows you
> > to do:
> >   (data ^ stream) ^ (data' ^ stream)
> > which can be simplified to
> >   (data ^ data')
> > thereby leaking all of data except the difference between data and
> > data'. That's why it's so crucial to ensure that stream *always* differs
> > between two rounds of encrypting "related" data.
> > 
> > We can't just "hope" that data doesn't change and use CTR.
> 
> My point was about whether we need to change the nonce, and hence
> WAL-log full page images if we change hint bits.  If we don't and
> reencrypt the page with the same nonce, don't we only expose the hint
> bits?  I was not suggesting we avoid changing the nonce in non-hint-bit
> cases.
> 
> I don't understand your computation above.  You decrypt the page into
> shared buffers, you change a hint bit, and rewrite the page.  You are
> re-XOR'ing the buffer copy with the same key and nonce.  Doesn't that
> only change the hint bits in the new write?

The way I view things is that the CTR mode encrypts each individual bit,
independent from any other bit on the page. For non-hint bits data=data', so
(data ^ data') is always zero, regardless the actual values of the data. So I
agree with you that by reusing the nonce we only expose the hint bits.

-- 
Antonin Houska
Web: https://www.cybertec-postgresql.com