X-Git-Url: https://git.distorted.org.uk/u/mdw/catacomb/blobdiff_plain/6c1035f55a83af8083af17432f2b4eb5afdb39dd..31cb4e2ef91d4ab2a6cc2d5fea5ac790be8f3801:/gfshare.c

diff --git a/gfshare.c b/gfshare.c
index cdc00d2..a0b102d 100644
--- a/gfshare.c
+++ b/gfshare.c
@@ -1,8 +1,8 @@
 /* -*-c-*-
  *
- * $Id: gfshare.c,v 1.1 2000/06/17 10:56:30 mdw Exp $
+ * $Id: gfshare.c,v 1.3 2000/06/22 18:04:13 mdw Exp $
  *
- * Secret sharing over %$gf(2^8)$%
+ * Secret sharing over %$\gf(2^8)$%
  *
  * (c) 2000 Straylight/Edgeware
  */
@@ -30,6 +30,13 @@
 /*----- Revision history --------------------------------------------------* 
  *
  * $Log: gfshare.c,v $
+ * Revision 1.3  2000/06/22 18:04:13  mdw
+ * Improve secret reconstruction -- compute coefficients as needed rather
+ * than making a big array of them.
+ *
+ * Revision 1.2  2000/06/18 23:12:15  mdw
+ * Change typesetting of Galois Field names.
+ *
  * Revision 1.1  2000/06/17 10:56:30  mdw
  * Fast but nonstandard secret sharing system.
  *
@@ -40,6 +47,7 @@
 #include <assert.h>
 #include <stdarg.h>
 #include <stdio.h>
+#include <string.h>
 
 #include <mLib/alloc.h>
 #include <mLib/bits.h>
@@ -219,18 +227,20 @@ unsigned gfshare_add(gfshare *s, unsigned x, const octet *y)
 void gfshare_combine(gfshare *s, octet *buf)
 {
   unsigned i, j;
-  octet *v;
 
   /* --- Sanity checking --- */
 
   assert(((void)"Not enough shares yet", s->i == s->t));
 
-  /* --- Precomputation of coefficients --- */
+  /* --- Grind through the shares --- */
 
-  v = XS_ALLOC(s->t);
+  memset(buf, 0, s->sz);
 
   for (i = 0; i < s->t; i++) {
     unsigned c = 0, ci = 0;
+
+    /* --- Compute the magic coefficient --- */
+
     for (j = 0; j < s->t; j++) {
       if (i == j)
 	continue;
@@ -244,21 +254,14 @@ void gfshare_combine(gfshare *s, octet *buf)
     if (ci > c)
       c += 0xff;
     c -= ci;
-    v[i] = c;
-  }
 
-  /* --- Grind through the shares --- */
+    /* --- Work out another layer of the secret --- */
 
-  for (i = 0; i < s->sz; i++) {
-    unsigned x = 0;
-    for (j = 0; j < s->t; j++) {
-      if (s->v[j].y[i])
-	x ^= gfexp[v[j] + gflog[s->v[j].y[i]]];
+    for (j = 0; j < s->sz; j++) {
+      if (s->v[i].y[j])
+	buf[j] ^= gfexp[c + gflog[s->v[i].y[j]]];
     }
-    buf[i] = x;
   }
-
-  XS_FREE(v);
 }
 
 /*----- Test rig ----------------------------------------------------------*/