Re: storing an explicit nonce

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Stephen Frost <sfrost(at)snowman(dot)net>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)alvh(dot)no-ip(dot)org>, Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>, Tom Kincaid <tomjohnkincaid(at)gmail(dot)com>, Andres Freund <andres(at)anarazel(dot)de>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>, Masahiko Sawada <masahiko(dot)sawada(at)2ndquadrant(dot)com>
Subject: Re: storing an explicit nonce
Date: 2021-05-25 21:29:03
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On Tue, May 25, 2021 at 05:22:43PM -0400, Stephen Frost wrote:
> * Bruce Momjian (bruce(at)momjian(dot)us) wrote:
> > OK, this is good to know. I know the never-reuse rule, so it is good to
> > know it can be relaxed for certain data without causing problems in
> > other places. Should I modify my patch to do this?
> Err, to be clear, I was saying that we could exclude the hint bits
> *entirely* from what's being encrypted and I don't think that would be a
> huge issue. We still absolutely need to continue to implement a
> never-reuse rule when it comes to nonces and making sure that we don't
> encrypt different sets of data with the same key+nonce, it's just that
> if we exclude the hint bits from encryption then we don't need to worry
> about making sure to use a different nonce each time the hint bits
> change- because they're no longer relevant.

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. I do think skipping encryption of
just the hint bits is more complex, so I want to understand why if is
needed. (This is a question I eventually wanted to discuss, just like
my XXX questions.)

Bruce Momjian <bruce(at)momjian(dot)us>

If only the physical world exists, free will is an illusion.

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