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   1 /* -*-c-*-
   2  *
   3  * $Id: gfshare.c,v 1.4 2000/06/24 18:29:05 mdw Exp $
   4  *
   5  * Secret sharing over %$\gf(2^8)$%
   6  *
   7  * (c) 2000 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  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.
  18  *
  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.
  23  *
  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,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: gfshare.c,v $
  33  * Revision 1.4  2000/06/24 18:29:05  mdw
  34  * Interface change: allow shares to be extracted from a context on demand,
  35  * rather than building them all up-front.
  36  *
  37  * Revision 1.3  2000/06/22 18:04:13  mdw
  38  * Improve secret reconstruction -- compute coefficients as needed rather
  39  * than making a big array of them.
  40  *
  41  * Revision 1.2  2000/06/18 23:12:15  mdw
  42  * Change typesetting of Galois Field names.
  43  *
  44  * Revision 1.1  2000/06/17 10:56:30  mdw
  45  * Fast but nonstandard secret sharing system.
  46  *
  47  */
  48
  49 /*----- Header files ------------------------------------------------------*/
  50
  51 #include <assert.h>
  52 #include <stdarg.h>
  53 #include <stdio.h>
  54 #include <string.h>
  55
  56 #include <mLib/alloc.h>
  57 #include <mLib/bits.h>
  58
  59 #include "arena.h"
  60 #include "gfshare.h"
  61 #include "gfshare-tab.h"
  62 #include "grand.h"
  63
  64 /*----- Static variables --------------------------------------------------*/
  65
  66 static octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
  67
  68 /*----- Main code ---------------------------------------------------------*/
  69
  70 /* --- @gfshare_create@ --- *
  71  *
  72  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  73  *              @unsigned t@ = threshold for the system
  74  *              @size_t sz@ = size of the secret
  75  *
  76  * Returns:     ---
  77  *
  78  * Use:         Initializes a sharing context.
  79  */
  80
  81 void gfshare_create(gfshare *s, unsigned t, size_t sz)
  82 {
  83   s->t = t;
  84   s->i = 0;
  85   s->sz = sz;
  86   s->s = 0;
  87   s->v = 0;
  88 }
  89
  90 /* --- @gfshare_destroy@ --- *
  91  *
  92  * Arguments:   @gfshare *s@ = pointer to share context to destroy
  93  *
  94  * Returns:     ---
  95  *
  96  * Use:         Disposes of a sharing context.  The allocations for the
  97  *              individual shares and the vector @v@ are freed; the secret is
  98  *              left alone.
  99  */
 100
 101 void gfshare_destroy(gfshare *s)
 102 {
 103   if (s->v)
 104     XS_FREE(s->v);
 105 }
 106
 107 /* --- @gfshare_mkshares@ --- *
 108  *
 109  * Arguments:   @gfshare *s@ = pointer to share context to fill in
 110  *              @grand *r@ = pointer to random number source
 111  *
 112  * Returns:     ---
 113  *
 114  * Use:         Initializes a sharing context to be able to create shares.
 115  *              The context structure is expected to be mostly filled in.  In
 116  *              particular, @t@ and @s@ must be initialized.  If @v@ is zero,
 117  *              a vector of appropriate size is allocated.  You should use
 118  *              the macro @GFSHARE_INIT@ or @gfshare_create@ to construct
 119  *              sharing contexts.
 120  */
 121
 122 void gfshare_mkshares(gfshare *s, grand *r)
 123 {
 124   s->v = XS_ALLOC(s->sz * s->t);
 125   r->ops->fill(r, s->v, s->sz * (s->t - 1));
 126   memcpy(s->v + s->sz * (s->t - 1), s->s, s->sz);
 127 }
 128
 129 /* --- @gfshare_get@ --- *
 130  *
 131  * Arguments:   @gfshare *s@ = pointer to share conext
 132  *              @unsigned x@ = share index to fetch
 133  *              @void *buf@ = pointer to output buffer
 134  *
 135  * Returns:     The share, as requested.
 136  *
 137  * Use:         Extracts a share from the system.  You may extract up to 255
 138  *              shares from the system.  Shares are indexed from 0.
 139  */
 140
 141 void gfshare_get(gfshare *s, unsigned x, void *buf)
 142 {
 143   unsigned i;
 144   const octet *p = s->v;
 145   unsigned ilog = gflog[x + 1];
 146
 147   /* --- Evaluate the polynomial at %$x = i + 1$% --- */
 148
 149   memcpy(buf, p, s->sz);
 150   p += s->sz;
 151
 152   for (i = 1; i < s->t; i++) {
 153     octet *q = buf;
 154     unsigned k;
 155     for (k = 0; k < s->sz; k++) {
 156       unsigned qq = *q;
 157       if (qq)
 158         qq = gfexp[ilog + gflog[qq]];
 159       *q++ = qq ^ *p++;
 160     }
 161   }
 162 }
 163
 164 /* --- @gfshare_add@ --- *
 165  *
 166  * Arguments:   @gfshare *s@ = pointer to sharing context
 167  *              @unsigned x@ = which share number this is
 168  *              @const void *y@ = the share value
 169  *
 170  * Returns:     Number of shares required before recovery may be performed.
 171  *
 172  * Use:         Adds a share to the context.  The context must have been
 173  *              initialized with the correct threshold @t@.
 174  */
 175
 176 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
 177 {
 178   octet *p;
 179
 180   /* --- If no vector has been allocated, create one --- */
 181
 182   if (!s->v) {
 183     s->v = XS_ALLOC(s->t * (s->sz + 1));
 184     s->i = 0;
 185   }
 186
 187   assert(((void)"Share context is full", s->i < s->t));
 188
 189   /* --- Store the share in the vector --- */
 190
 191   p = s->v + s->i * (s->sz + 1);
 192   *p++ = x + 1;
 193   memcpy(p, y, s->sz);
 194   s->i++;
 195
 196   /* --- Done --- */
 197
 198   return (s->t - s->i);
 199 }
 200
 201 /* --- @gfshare_combine@ --- *
 202  *
 203  * Arguments:   @gfshare *s@ = pointer to share context
 204  *              @void *buf@ = pointer to output buffer for the secret
 205  *
 206  * Returns:     ---
 207  *
 208  * Use:         Reconstructs a secret, given enough shares.
 209  */
 210
 211 void gfshare_combine(gfshare *s, void *buf)
 212 {
 213   unsigned i, j;
 214   unsigned xi, xj;
 215
 216   /* --- Sanity checking --- */
 217
 218   assert(((void)"Not enough shares yet", s->i == s->t));
 219
 220   /* --- Grind through the shares --- */
 221
 222   memset(buf, 0, s->sz);
 223
 224   for (i = 0; i < s->t; i++) {
 225     octet *p = buf;
 226     octet *q = s->v + i * (s->sz + 1);
 227     unsigned c = 0, ci = 0;
 228
 229     /* --- Compute the magic coefficient --- */
 230
 231     xi = *q++;
 232     for (j = 0; j < s->t; j++) {
 233       if (i == j)
 234         continue;
 235       xj = s->v[j * (s->sz + 1)];
 236       c += gflog[xj];
 237       if (c >= 0xff)
 238         c -= 0xff;
 239       ci += gflog[xi ^ xj];
 240       if (ci >= 0xff)
 241         ci -= 0xff;
 242     }
 243     if (ci > c)
 244       c += 0xff;
 245     c -= ci;
 246
 247     /* --- Work out another layer of the secret --- */
 248
 249     p = buf;
 250     for (j = 0; j < s->sz; j++) {
 251       if (*q)
 252         *p ^= gfexp[c + gflog[*q]];
 253       p++, q++;
 254     }
 255   }
 256 }
 257
 258 /*----- Test rig ----------------------------------------------------------*/
 259
 260 #ifdef TEST_RIG
 261
 262 #include "fibrand.h"
 263
 264 static int verify(grand *r)
 265 {
 266   unsigned n = r->ops->range(r, 16) + 8;
 267   unsigned t = r->ops->range(r, n - 1) + 1;
 268   unsigned len = r->ops->range(r, 32) + 1;
 269
 270   octet *v = xmalloc(t * len);
 271   unsigned *p = xmalloc(n * sizeof(unsigned));
 272   octet *sec = xmalloc(len), *sbuf = xmalloc(len);
 273   gfshare s;
 274   unsigned i;
 275
 276   int ok = 1;
 277
 278   for (i = 0; i < n; i++)
 279     p[i] = i;
 280   for (i = 0; i < t; i++) {
 281     unsigned long j = r->ops->range(r, n - i);
 282     unsigned x = p[i];
 283     p[i] = p[i + j];
 284     p[i + j] = x;
 285   }
 286
 287   r->ops->fill(r, sec, len);
 288
 289   gfshare_create(&s, t, len);
 290   s.s = sec;
 291
 292   gfshare_mkshares(&s, r);
 293   for (i = 0; i < t; i++)
 294     gfshare_get(&s, p[i], v + (i * len));
 295   gfshare_destroy(&s);
 296
 297   gfshare_create(&s, t, len);
 298   for (i = 0; i < t; i++)
 299     gfshare_add(&s, p[i], v + (i * len));
 300   gfshare_combine(&s, sbuf);
 301   gfshare_destroy(&s);
 302
 303   if (memcmp(sec, sbuf, len) != 0) {
 304     ok = 0;
 305     fprintf(stderr, "\nbad recombination of shares\n");
 306   };
 307
 308   xfree(sec);
 309   xfree(sbuf);
 310
 311   xfree(v);
 312   xfree(p);
 313
 314   return (ok);
 315 }
 316
 317 int main(void)
 318 {
 319   grand *r = fibrand_create(0);
 320   unsigned i;
 321   int ok = 1;
 322
 323   fputs("gfshare: ", stdout);
 324   for (i = 0; i < 40; i++) {
 325     if (!verify(r))
 326       ok = 0;
 327     fputc('.', stdout);
 328     fflush(stdout);
 329   }
 330
 331   if (ok)
 332     fputs(" ok\n", stdout);
 333   else
 334     fputs(" failed\n", stdout);
 335   return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 336 }
 337
 338 #endif
 339
 340 /*----- That's all, folks -------------------------------------------------*/