From: Mark Wooding <mdw@distorted.org.uk>
Date: Fri, 28 Aug 2020 23:25:56 +0000 (+0100)
Subject: rand/rand.c (rand_gate): Evolve r->ibits in a more sensible manner.
X-Git-Url: https://git.distorted.org.uk/~mdw/catacomb/commitdiff_plain/5f0438e6324f770ec8e1f53eadceea0857e4792d

rand/rand.c (rand_gate): Evolve r->ibits in a more sensible manner.

It's not really clear what this code was trying to do.  Write i and o
for the initial values of r->ibits and r->obits, respectively, i' and 'o
for their respective final values, and O for RAND_OBITS.  In the case
that i + o <= O, we update i' = 0 and o' = i + o, maintaining the
invariant that i' + o' = i + o.  But if i + o > O, then we set o' = O and
i' = (i + o) - i = o, which seems nonsensical.  In particular, in the
case that i = 1 and o = O, it apparently magics O - 1 bits of entropy
from nowhere.

Modify the code so that it at least maintains the sum of the entropy
counters in either branch.  I'm not sure this is actually correct, but
it seems like a defensible position.
---

diff --git a/rand/rand.c b/rand/rand.c
index e2211d54..01e6422a 100644
--- a/rand/rand.c
+++ b/rand/rand.c
@@ -342,7 +342,7 @@ void rand_gate(rand_pool *r)
   r->o = RAND_SECSZ;
   r->obits += r->ibits;
   if (r->obits > RAND_OBITS) {
-    r->ibits = r->obits - r->ibits;
+    r->ibits = r->obits - RAND_OBITS;
     r->obits = RAND_OBITS;
   } else
     r->ibits = 0;