3 * $Id: gfshare.c,v 1.6 2000/12/06 20:30:10 mdw Exp $
5 * Secret sharing over %$\gf(2^8)$%
7 * (c) 2000 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
14 * Catacomb is free software; you can redistribute it and/or modify
15 * it under the terms of the GNU Library General Public License as
16 * published by the Free Software Foundation; either version 2 of the
17 * License, or (at your option) any later version.
19 * Catacomb is distributed in the hope that it will be useful,
20 * but WITHOUT ANY WARRANTY; without even the implied warranty of
21 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
22 * GNU Library General Public License for more details.
24 * You should have received a copy of the GNU Library General Public
25 * License along with Catacomb; if not, write to the Free
26 * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
30 /*----- Revision history --------------------------------------------------*
33 * Revision 1.6 2000/12/06 20:30:10 mdw
34 * Change secret sharing interface: present the secret at share
37 * Revision 1.5 2000/06/24 19:11:47 mdw
38 * Fix daft error in the comment for @gfshare_get@.
40 * Revision 1.4 2000/06/24 18:29:05 mdw
41 * Interface change: allow shares to be extracted from a context on demand,
42 * rather than building them all up-front.
44 * Revision 1.3 2000/06/22 18:04:13 mdw
45 * Improve secret reconstruction -- compute coefficients as needed rather
46 * than making a big array of them.
48 * Revision 1.2 2000/06/18 23:12:15 mdw
49 * Change typesetting of Galois Field names.
51 * Revision 1.1 2000/06/17 10:56:30 mdw
52 * Fast but nonstandard secret sharing system.
56 /*----- Header files ------------------------------------------------------*/
63 #include <mLib/alloc.h>
64 #include <mLib/bits.h>
68 #include "gfshare-tab.h"
71 /*----- Static variables --------------------------------------------------*/
73 static octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
75 /*----- Main code ---------------------------------------------------------*/
77 /* --- @gfshare_create@ --- *
79 * Arguments: @gfshare *s@ = pointer to share context to initialize
80 * @unsigned t@ = threshold for the system
81 * @size_t sz@ = size of the secret
85 * Use: Initializes a sharing context.
88 void gfshare_create(gfshare *s, unsigned t, size_t sz)
96 /* --- @gfshare_destroy@ --- *
98 * Arguments: @gfshare *s@ = pointer to share context to destroy
102 * Use: Disposes of a sharing context. The allocations for the
103 * individual shares and the vector @v@ are freed; the secret is
107 void gfshare_destroy(gfshare *s)
113 /* --- @gfshare_mkshares@ --- *
115 * Arguments: @gfshare *s@ = pointer to share context to fill in
116 * @grand *r@ = pointer to random number source
117 * @const void *buf@ = pointer to the secret to share
121 * Use: Initializes a sharing context to be able to create shares.
122 * The context structure is expected to be mostly filled in. In
123 * particular, @t@ must be initialized. If @v@ is zero, a
124 * vector of appropriate size is allocated. You should use the
125 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
129 void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
131 s->v = XS_ALLOC(s->sz * s->t);
132 r->ops->fill(r, s->v, s->sz * (s->t - 1));
133 memcpy(s->v + s->sz * (s->t - 1), buf, s->sz);
136 /* --- @gfshare_get@ --- *
138 * Arguments: @gfshare *s@ = pointer to share conext
139 * @unsigned x@ = share index to fetch
140 * @void *buf@ = pointer to output buffer
144 * Use: Extracts a share from the system. You may extract up to 255
145 * shares from the system. Shares are indexed from 0.
148 void gfshare_get(gfshare *s, unsigned x, void *buf)
151 const octet *p = s->v;
152 unsigned ilog = gflog[x + 1];
154 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
156 memcpy(buf, p, s->sz);
159 for (i = 1; i < s->t; i++) {
162 for (k = 0; k < s->sz; k++) {
165 qq = gfexp[ilog + gflog[qq]];
171 /* --- @gfshare_add@ --- *
173 * Arguments: @gfshare *s@ = pointer to sharing context
174 * @unsigned x@ = which share number this is
175 * @const void *y@ = the share value
177 * Returns: Number of shares required before recovery may be performed.
179 * Use: Adds a share to the context. The context must have been
180 * initialized with the correct threshold @t@.
183 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
187 /* --- If no vector has been allocated, create one --- */
190 s->v = XS_ALLOC(s->t * (s->sz + 1));
194 assert(((void)"Share context is full", s->i < s->t));
196 /* --- Store the share in the vector --- */
198 p = s->v + s->i * (s->sz + 1);
205 return (s->t - s->i);
208 /* --- @gfshare_combine@ --- *
210 * Arguments: @gfshare *s@ = pointer to share context
211 * @void *buf@ = pointer to output buffer for the secret
215 * Use: Reconstructs a secret, given enough shares.
218 void gfshare_combine(gfshare *s, void *buf)
223 /* --- Sanity checking --- */
225 assert(((void)"Not enough shares yet", s->i == s->t));
227 /* --- Grind through the shares --- */
229 memset(buf, 0, s->sz);
231 for (i = 0; i < s->t; i++) {
233 octet *q = s->v + i * (s->sz + 1);
234 unsigned c = 0, ci = 0;
236 /* --- Compute the magic coefficient --- */
239 for (j = 0; j < s->t; j++) {
242 xj = s->v[j * (s->sz + 1)];
246 ci += gflog[xi ^ xj];
254 /* --- Work out another layer of the secret --- */
257 for (j = 0; j < s->sz; j++) {
259 *p ^= gfexp[c + gflog[*q]];
265 /*----- Test rig ----------------------------------------------------------*/
271 static int verify(grand *r)
273 unsigned n = r->ops->range(r, 16) + 8;
274 unsigned t = r->ops->range(r, n - 1) + 1;
275 unsigned len = r->ops->range(r, 32) + 1;
277 octet *v = xmalloc(t * len);
278 unsigned *p = xmalloc(n * sizeof(unsigned));
279 octet *sec = xmalloc(len), *sbuf = xmalloc(len);
285 for (i = 0; i < n; i++)
287 for (i = 0; i < t; i++) {
288 unsigned long j = r->ops->range(r, n - i);
294 r->ops->fill(r, sec, len);
296 gfshare_create(&s, t, len);
298 gfshare_mkshares(&s, r, sec);
299 for (i = 0; i < t; i++)
300 gfshare_get(&s, p[i], v + (i * len));
303 gfshare_create(&s, t, len);
304 for (i = 0; i < t; i++)
305 gfshare_add(&s, p[i], v + (i * len));
306 gfshare_combine(&s, sbuf);
309 if (memcmp(sec, sbuf, len) != 0) {
311 fprintf(stderr, "\nbad recombination of shares\n");
325 grand *r = fibrand_create(0);
329 fputs("gfshare: ", stdout);
330 for (i = 0; i < 40; i++) {
338 fputs(" ok\n", stdout);
340 fputs(" failed\n", stdout);
341 return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
346 /*----- That's all, folks -------------------------------------------------*/