math/scaf.c: Fix conditional subtractions in `scaf_reduce'.

So that they actually subtract the right thing.  Obvious blunder.  The
big surprise is that none of the literally thousands of Ed25519 tests
which have hammered on that code caught it.  (Found during development
of Ed448, coming later.)

diff --git a/math/scaf.c b/math/scaf.c
index 778e5e3432bd7f28ec87e8edbd2fd2cecdbc232d..84ef3de812264e08a16816f7f4218232141962d2 100644 (file)
--- a/math/scaf.c
+++ b/math/scaf.c
@@ -269,7 +269,7 @@ void scaf_reduce(scaf_piece *z, const scaf_dblpiece *x,
       u += z[j] + (l[j] ^ m);
       t[j] = u&m; u >>= piecewd;
     }
-    for (j = 0, u = -u; j < npiece; j++) z[i] = (t[i]&u) | (z[i]&~u);
+    for (j = 0, u = -u; j < npiece; j++) z[j] = (t[j]&u) | (z[j]&~u);
   }
 }