3 * $Id: gfshare.c,v 1.8 2004/04/02 01:03:49 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 1.8 2004/04/02 01:03:49 mdw
34 * Miscellaneous constification.
36 * Revision 1.7 2001/06/16 23:42:17 mdw
39 * Revision 1.6 2000/12/06 20:30:10 mdw
40 * Change secret sharing interface: present the secret at share
43 * Revision 1.5 2000/06/24 19:11:47 mdw
44 * Fix daft error in the comment for @gfshare_get@.
46 * Revision 1.4 2000/06/24 18:29:05 mdw
47 * Interface change: allow shares to be extracted from a context on demand,
48 * rather than building them all up-front.
50 * Revision 1.3 2000/06/22 18:04:13 mdw
51 * Improve secret reconstruction -- compute coefficients as needed rather
52 * than making a big array of them.
54 * Revision 1.2 2000/06/18 23:12:15 mdw
55 * Change typesetting of Galois Field names.
57 * Revision 1.1 2000/06/17 10:56:30 mdw
58 * Fast but nonstandard secret sharing system.
62 /*----- Header files ------------------------------------------------------*/
69 #include <mLib/alloc.h>
70 #include <mLib/bits.h>
74 #include "gfshare-tab.h"
77 /*----- Static variables --------------------------------------------------*/
79 static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
81 /*----- Main code ---------------------------------------------------------*/
83 /* --- @gfshare_create@ --- *
85 * Arguments: @gfshare *s@ = pointer to share context to initialize
86 * @unsigned t@ = threshold for the system
87 * @size_t sz@ = size of the secret
91 * Use: Initializes a sharing context.
94 void gfshare_create(gfshare *s, unsigned t, size_t sz)
102 /* --- @gfshare_destroy@ --- *
104 * Arguments: @gfshare *s@ = pointer to share context to destroy
108 * Use: Disposes of a sharing context. The allocations for the
109 * individual shares and the vector @v@ are freed; the secret is
113 void gfshare_destroy(gfshare *s)
119 /* --- @gfshare_mkshares@ --- *
121 * Arguments: @gfshare *s@ = pointer to share context to fill in
122 * @grand *r@ = pointer to random number source
123 * @const void *buf@ = pointer to the secret to share
127 * Use: Initializes a sharing context to be able to create shares.
128 * The context structure is expected to be mostly filled in. In
129 * particular, @t@ must be initialized. If @v@ is zero, a
130 * vector of appropriate size is allocated. You should use the
131 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
135 void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
137 s->v = XS_ALLOC(s->sz * s->t);
138 r->ops->fill(r, s->v, s->sz * (s->t - 1));
139 memcpy(s->v + s->sz * (s->t - 1), buf, s->sz);
142 /* --- @gfshare_get@ --- *
144 * Arguments: @gfshare *s@ = pointer to share conext
145 * @unsigned x@ = share index to fetch
146 * @void *buf@ = pointer to output buffer
150 * Use: Extracts a share from the system. You may extract up to 255
151 * shares from the system. Shares are indexed from 0.
154 void gfshare_get(gfshare *s, unsigned x, void *buf)
157 const octet *p = s->v;
158 unsigned ilog = gflog[x + 1];
160 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
162 memcpy(buf, p, s->sz);
165 for (i = 1; i < s->t; i++) {
168 for (k = 0; k < s->sz; k++) {
171 qq = gfexp[ilog + gflog[qq]];
177 /* --- @gfshare_add@ --- *
179 * Arguments: @gfshare *s@ = pointer to sharing context
180 * @unsigned x@ = which share number this is
181 * @const void *y@ = the share value
183 * Returns: Number of shares required before recovery may be performed.
185 * Use: Adds a share to the context. The context must have been
186 * initialized with the correct threshold @t@.
189 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
193 /* --- If no vector has been allocated, create one --- */
196 s->v = XS_ALLOC(s->t * (s->sz + 1));
200 assert(((void)"Share context is full", s->i < s->t));
202 /* --- Store the share in the vector --- */
204 p = s->v + s->i * (s->sz + 1);
211 return (s->t - s->i);
214 /* --- @gfshare_combine@ --- *
216 * Arguments: @gfshare *s@ = pointer to share context
217 * @void *buf@ = pointer to output buffer for the secret
221 * Use: Reconstructs a secret, given enough shares.
224 void gfshare_combine(gfshare *s, void *buf)
229 /* --- Sanity checking --- */
231 assert(((void)"Not enough shares yet", s->i == s->t));
233 /* --- Grind through the shares --- */
235 memset(buf, 0, s->sz);
237 for (i = 0; i < s->t; i++) {
239 octet *q = s->v + i * (s->sz + 1);
240 unsigned c = 0, ci = 0;
242 /* --- Compute the magic coefficient --- */
245 for (j = 0; j < s->t; j++) {
248 xj = s->v[j * (s->sz + 1)];
252 ci += gflog[xi ^ xj];
260 /* --- Work out another layer of the secret --- */
263 for (j = 0; j < s->sz; j++) {
265 *p ^= gfexp[c + gflog[*q]];
271 /*----- Test rig ----------------------------------------------------------*/
277 static int verify(grand *r)
279 unsigned n = r->ops->range(r, 16) + 8;
280 unsigned t = r->ops->range(r, n - 1) + 1;
281 unsigned len = r->ops->range(r, 32) + 1;
283 octet *v = xmalloc(t * len);
284 unsigned *p = xmalloc(n * sizeof(unsigned));
285 octet *sec = xmalloc(len), *sbuf = xmalloc(len);
291 for (i = 0; i < n; i++)
293 for (i = 0; i < t; i++) {
294 unsigned long j = r->ops->range(r, n - i);
300 r->ops->fill(r, sec, len);
302 gfshare_create(&s, t, len);
304 gfshare_mkshares(&s, r, sec);
305 for (i = 0; i < t; i++)
306 gfshare_get(&s, p[i], v + (i * len));
309 gfshare_create(&s, t, len);
310 for (i = 0; i < t; i++)
311 gfshare_add(&s, p[i], v + (i * len));
312 gfshare_combine(&s, sbuf);
315 if (memcmp(sec, sbuf, len) != 0) {
317 fprintf(stderr, "\nbad recombination of shares\n");
331 grand *r = fibrand_create(0);
335 fputs("gfshare: ", stdout);
336 for (i = 0; i < 40; i++) {
344 fputs(" ok\n", stdout);
346 fputs(" failed\n", stdout);
347 return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
352 /*----- That's all, folks -------------------------------------------------*/