--- /dev/null
+/* -*-c-*-
+ *
+ * Secret sharing over %$\gf{2^8}$%
+ *
+ * (c) 2000 Straylight/Edgeware
+ */
+
+/*----- Licensing notice --------------------------------------------------*
+ *
+ * This file is part of Catacomb.
+ *
+ * Catacomb is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU Library General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * Catacomb is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public
+ * License along with Catacomb; if not, write to the Free
+ * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
+ * MA 02111-1307, USA.
+ */
+
+/*----- Header files ------------------------------------------------------*/
+
+#include <assert.h>
+#include <stdarg.h>
+#include <stdio.h>
+#include <string.h>
+
+#include <mLib/alloc.h>
+#include <mLib/bits.h>
+
+#include "arena.h"
+#include "gfshare.h"
+#include "gfshare-tab.h"
+#include "grand.h"
+
+/*----- Static variables --------------------------------------------------*/
+
+static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
+
+/*----- Main code ---------------------------------------------------------*/
+
+/* --- @gfshare_create@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to initialize
+ * @unsigned t@ = threshold for the system
+ * @size_t sz@ = size of the secret
+ *
+ * Returns: ---
+ *
+ * Use: Initializes a sharing context.
+ */
+
+void gfshare_create(gfshare *s, unsigned t, size_t sz)
+{
+ s->t = t;
+ s->i = 0;
+ s->sz = sz;
+ s->v = 0;
+}
+
+/* --- @gfshare_destroy@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to destroy
+ *
+ * Returns: ---
+ *
+ * Use: Disposes of a sharing context. The allocations for the
+ * individual shares and the vector @v@ are freed; the secret is
+ * left alone.
+ */
+
+void gfshare_destroy(gfshare *s)
+{
+ if (s->v)
+ XS_FREE(s->v);
+}
+
+/* --- @gfshare_mkshares@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context to fill in
+ * @grand *r@ = pointer to random number source
+ * @const void *buf@ = pointer to the secret to share
+ *
+ * Returns: ---
+ *
+ * Use: Initializes a sharing context to be able to create shares.
+ * The context structure is expected to be mostly filled in. In
+ * particular, @t@ must be initialized. If @v@ is zero, a
+ * vector of appropriate size is allocated. You should use the
+ * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
+ * contexts.
+ */
+
+void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
+{
+ s->v = XS_ALLOC(s->sz * s->t);
+ r->ops->fill(r, s->v, s->sz * (s->t - 1));
+ memcpy(s->v + s->sz * (s->t - 1), buf, s->sz);
+}
+
+/* --- @gfshare_get@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share conext
+ * @unsigned x@ = share index to fetch
+ * @void *buf@ = pointer to output buffer
+ *
+ * Returns: ---
+ *
+ * Use: Extracts a share from the system. You may extract up to 255
+ * shares from the system. Shares are indexed from 0.
+ */
+
+void gfshare_get(gfshare *s, unsigned x, void *buf)
+{
+ unsigned i;
+ const octet *p = s->v;
+ unsigned ilog = gflog[x + 1];
+
+ /* --- Evaluate the polynomial at %$x = i + 1$% --- */
+
+ memcpy(buf, p, s->sz);
+ p += s->sz;
+
+ for (i = 1; i < s->t; i++) {
+ octet *q = buf;
+ unsigned k;
+ for (k = 0; k < s->sz; k++) {
+ unsigned qq = *q;
+ if (qq)
+ qq = gfexp[ilog + gflog[qq]];
+ *q++ = qq ^ *p++;
+ }
+ }
+}
+
+/* --- @gfshare_addedp@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to sharing context
+ * @unsigned x@ = which share number to check
+ *
+ * Returns: Nonzero if share @x@ has been added already, zero if it
+ * hasn't.
+ */
+
+int gfshare_addedp(gfshare *s, unsigned x)
+{
+ unsigned i;
+
+ for (i = 0; i < s->i; i++) {
+ if (GFSHARE_INDEX(s, i) == x + 1)
+ return (1);
+ }
+ return (0);
+}
+
+/* --- @gfshare_add@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to sharing context
+ * @unsigned x@ = which share number this is
+ * @const void *y@ = the share value
+ *
+ * Returns: Number of shares required before recovery may be performed.
+ *
+ * Use: Adds a share to the context. The context must have been
+ * initialized with the correct threshold @t@.
+ */
+
+unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
+{
+ octet *p;
+
+ assert(((void)"Share context is full", s->i < s->t));
+ assert(((void)"Share already present", !gfshare_addedp(s, x)));
+
+ /* --- If no vector has been allocated, create one --- */
+
+ if (!s->v) {
+ s->v = XS_ALLOC(s->t * (s->sz + 1));
+ s->i = 0;
+ }
+
+ /* --- Store the share in the vector --- */
+
+ p = &GFSHARE_INDEX(s, s->i);
+ *p++ = x + 1;
+ memcpy(p, y, s->sz);
+ s->i++;
+
+ /* --- Done --- */
+
+ return (s->t - s->i);
+}
+
+/* --- @gfshare_combine@ --- *
+ *
+ * Arguments: @gfshare *s@ = pointer to share context
+ * @void *buf@ = pointer to output buffer for the secret
+ *
+ * Returns: ---
+ *
+ * Use: Reconstructs a secret, given enough shares.
+ */
+
+void gfshare_combine(gfshare *s, void *buf)
+{
+ unsigned i, j;
+ unsigned xi, xj;
+
+ /* --- Sanity checking --- */
+
+ assert(((void)"Not enough shares yet", s->i == s->t));
+
+ /* --- Grind through the shares --- */
+
+ memset(buf, 0, s->sz);
+
+ for (i = 0; i < s->t; i++) {
+ octet *p = buf;
+ octet *q = &GFSHARE_INDEX(s, i);
+ unsigned c = 0, ci = 0;
+
+ /* --- Compute the magic coefficient --- */
+
+ xi = *q++;
+ for (j = 0; j < s->t; j++) {
+ if (i == j)
+ continue;
+ xj = GFSHARE_INDEX(s, j);
+ c += gflog[xj];
+ if (c >= 0xff)
+ c -= 0xff;
+ ci += gflog[xi ^ xj];
+ if (ci >= 0xff)
+ ci -= 0xff;
+ }
+ if (ci > c)
+ c += 0xff;
+ c -= ci;
+
+ /* --- Work out another layer of the secret --- */
+
+ p = buf;
+ for (j = 0; j < s->sz; j++) {
+ if (*q)
+ *p ^= gfexp[c + gflog[*q]];
+ p++, q++;
+ }
+ }
+}
+
+/*----- Test rig ----------------------------------------------------------*/
+
+#ifdef TEST_RIG
+
+#include "fibrand.h"
+
+static int verify(grand *r)
+{
+ unsigned n = r->ops->range(r, 16) + 8;
+ unsigned t = r->ops->range(r, n - 1) + 1;
+ unsigned len = r->ops->range(r, 32) + 1;
+
+ octet *v = xmalloc(t * len);
+ unsigned *p = xmalloc(n * sizeof(unsigned));
+ octet *sec = xmalloc(len), *sbuf = xmalloc(len);
+ gfshare s;
+ unsigned i;
+
+ int ok = 1;
+
+ for (i = 0; i < n; i++)
+ p[i] = i;
+ for (i = 0; i < t; i++) {
+ unsigned long j = r->ops->range(r, n - i);
+ unsigned x = p[i];
+ p[i] = p[i + j];
+ p[i + j] = x;
+ }
+
+ r->ops->fill(r, sec, len);
+
+ gfshare_create(&s, t, len);
+
+ gfshare_mkshares(&s, r, sec);
+ for (i = 0; i < t; i++)
+ gfshare_get(&s, p[i], v + (i * len));
+ gfshare_destroy(&s);
+
+ gfshare_create(&s, t, len);
+ for (i = 0; i < t; i++)
+ gfshare_add(&s, p[i], v + (i * len));
+ gfshare_combine(&s, sbuf);
+ gfshare_destroy(&s);
+
+ if (memcmp(sec, sbuf, len) != 0) {
+ ok = 0;
+ fprintf(stderr, "\nbad recombination of shares\n");
+ };
+
+ xfree(sec);
+ xfree(sbuf);
+
+ xfree(v);
+ xfree(p);
+
+ return (ok);
+}
+
+int main(void)
+{
+ grand *r = fibrand_create(0);
+ unsigned i;
+ int ok = 1;
+
+ fputs("gfshare: ", stdout);
+ for (i = 0; i < 40; i++) {
+ if (!verify(r))
+ ok = 0;
+ fputc('.', stdout);
+ fflush(stdout);
+ }
+
+ if (ok)
+ fputs(" ok\n", stdout);
+ else
+ fputs(" failed\n", stdout);
+ return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
+}
+
+#endif
+
+/*----- That's all, folks -------------------------------------------------*/
projects / u / mdw / catacomb / blobdiff