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   1 /* -*-c-*-
   2  *
   3  * $Id: gfshare.c,v 1.8 2004/04/02 01:03:49 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 1.8  2004/04/02 01:03:49  mdw
  34  * Miscellaneous constification.
  35  *
  36  * Revision 1.7  2001/06/16 23:42:17  mdw
  37  * Typesetting fixes.
  38  *
  39  * Revision 1.6  2000/12/06 20:30:10  mdw
  40  * Change secret sharing interface: present the secret at share
  41  * construction time.
  42  *
  43  * Revision 1.5  2000/06/24 19:11:47  mdw
  44  * Fix daft error in the comment for @gfshare_get@.
  45  *
  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.
  49  *
  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.
  53  *
  54  * Revision 1.2  2000/06/18 23:12:15  mdw
  55  * Change typesetting of Galois Field names.
  56  *
  57  * Revision 1.1  2000/06/17 10:56:30  mdw
  58  * Fast but nonstandard secret sharing system.
  59  *
  60  */
  61
  62 /*----- Header files ------------------------------------------------------*/
  63
  64 #include <assert.h>
  65 #include <stdarg.h>
  66 #include <stdio.h>
  67 #include <string.h>
  68
  69 #include <mLib/alloc.h>
  70 #include <mLib/bits.h>
  71
  72 #include "arena.h"
  73 #include "gfshare.h"
  74 #include "gfshare-tab.h"
  75 #include "grand.h"
  76
  77 /*----- Static variables --------------------------------------------------*/
  78
  79 static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
  80
  81 /*----- Main code ---------------------------------------------------------*/
  82
  83 /* --- @gfshare_create@ --- *
  84  *
  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
  88  *
  89  * Returns:     ---
  90  *
  91  * Use:         Initializes a sharing context.
  92  */
  93
  94 void gfshare_create(gfshare *s, unsigned t, size_t sz)
  95 {
  96   s->t = t;
  97   s->i = 0;
  98   s->sz = sz;
  99   s->v = 0;
 100 }
 101
 102 /* --- @gfshare_destroy@ --- *
 103  *
 104  * Arguments:   @gfshare *s@ = pointer to share context to destroy
 105  *
 106  * Returns:     ---
 107  *
 108  * Use:         Disposes of a sharing context.  The allocations for the
 109  *              individual shares and the vector @v@ are freed; the secret is
 110  *              left alone.
 111  */
 112
 113 void gfshare_destroy(gfshare *s)
 114 {
 115   if (s->v)
 116     XS_FREE(s->v);
 117 }
 118
 119 /* --- @gfshare_mkshares@ --- *
 120  *
 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
 124  *
 125  * Returns:     ---
 126  *
 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
 132  *              contexts.
 133  */
 134
 135 void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
 136 {
 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);
 140 }
 141
 142 /* --- @gfshare_get@ --- *
 143  *
 144  * Arguments:   @gfshare *s@ = pointer to share conext
 145  *              @unsigned x@ = share index to fetch
 146  *              @void *buf@ = pointer to output buffer
 147  *
 148  * Returns:     ---
 149  *
 150  * Use:         Extracts a share from the system.  You may extract up to 255
 151  *              shares from the system.  Shares are indexed from 0.
 152  */
 153
 154 void gfshare_get(gfshare *s, unsigned x, void *buf)
 155 {
 156   unsigned i;
 157   const octet *p = s->v;
 158   unsigned ilog = gflog[x + 1];
 159
 160   /* --- Evaluate the polynomial at %$x = i + 1$% --- */
 161
 162   memcpy(buf, p, s->sz);
 163   p += s->sz;
 164
 165   for (i = 1; i < s->t; i++) {
 166     octet *q = buf;
 167     unsigned k;
 168     for (k = 0; k < s->sz; k++) {
 169       unsigned qq = *q;
 170       if (qq)
 171         qq = gfexp[ilog + gflog[qq]];
 172       *q++ = qq ^ *p++;
 173     }
 174   }
 175 }
 176
 177 /* --- @gfshare_add@ --- *
 178  *
 179  * Arguments:   @gfshare *s@ = pointer to sharing context
 180  *              @unsigned x@ = which share number this is
 181  *              @const void *y@ = the share value
 182  *
 183  * Returns:     Number of shares required before recovery may be performed.
 184  *
 185  * Use:         Adds a share to the context.  The context must have been
 186  *              initialized with the correct threshold @t@.
 187  */
 188
 189 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
 190 {
 191   octet *p;
 192
 193   /* --- If no vector has been allocated, create one --- */
 194
 195   if (!s->v) {
 196     s->v = XS_ALLOC(s->t * (s->sz + 1));
 197     s->i = 0;
 198   }
 199
 200   assert(((void)"Share context is full", s->i < s->t));
 201
 202   /* --- Store the share in the vector --- */
 203
 204   p = s->v + s->i * (s->sz + 1);
 205   *p++ = x + 1;
 206   memcpy(p, y, s->sz);
 207   s->i++;
 208
 209   /* --- Done --- */
 210
 211   return (s->t - s->i);
 212 }
 213
 214 /* --- @gfshare_combine@ --- *
 215  *
 216  * Arguments:   @gfshare *s@ = pointer to share context
 217  *              @void *buf@ = pointer to output buffer for the secret
 218  *
 219  * Returns:     ---
 220  *
 221  * Use:         Reconstructs a secret, given enough shares.
 222  */
 223
 224 void gfshare_combine(gfshare *s, void *buf)
 225 {
 226   unsigned i, j;
 227   unsigned xi, xj;
 228
 229   /* --- Sanity checking --- */
 230
 231   assert(((void)"Not enough shares yet", s->i == s->t));
 232
 233   /* --- Grind through the shares --- */
 234
 235   memset(buf, 0, s->sz);
 236
 237   for (i = 0; i < s->t; i++) {
 238     octet *p = buf;
 239     octet *q = s->v + i * (s->sz + 1);
 240     unsigned c = 0, ci = 0;
 241
 242     /* --- Compute the magic coefficient --- */
 243
 244     xi = *q++;
 245     for (j = 0; j < s->t; j++) {
 246       if (i == j)
 247         continue;
 248       xj = s->v[j * (s->sz + 1)];
 249       c += gflog[xj];
 250       if (c >= 0xff)
 251         c -= 0xff;
 252       ci += gflog[xi ^ xj];
 253       if (ci >= 0xff)
 254         ci -= 0xff;
 255     }
 256     if (ci > c)
 257       c += 0xff;
 258     c -= ci;
 259
 260     /* --- Work out another layer of the secret --- */
 261
 262     p = buf;
 263     for (j = 0; j < s->sz; j++) {
 264       if (*q)
 265         *p ^= gfexp[c + gflog[*q]];
 266       p++, q++;
 267     }
 268   }
 269 }
 270
 271 /*----- Test rig ----------------------------------------------------------*/
 272
 273 #ifdef TEST_RIG
 274
 275 #include "fibrand.h"
 276
 277 static int verify(grand *r)
 278 {
 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;
 282
 283   octet *v = xmalloc(t * len);
 284   unsigned *p = xmalloc(n * sizeof(unsigned));
 285   octet *sec = xmalloc(len), *sbuf = xmalloc(len);
 286   gfshare s;
 287   unsigned i;
 288
 289   int ok = 1;
 290
 291   for (i = 0; i < n; i++)
 292     p[i] = i;
 293   for (i = 0; i < t; i++) {
 294     unsigned long j = r->ops->range(r, n - i);
 295     unsigned x = p[i];
 296     p[i] = p[i + j];
 297     p[i + j] = x;
 298   }
 299
 300   r->ops->fill(r, sec, len);
 301
 302   gfshare_create(&s, t, len);
 303
 304   gfshare_mkshares(&s, r, sec);
 305   for (i = 0; i < t; i++)
 306     gfshare_get(&s, p[i], v + (i * len));
 307   gfshare_destroy(&s);
 308
 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);
 313   gfshare_destroy(&s);
 314
 315   if (memcmp(sec, sbuf, len) != 0) {
 316     ok = 0;
 317     fprintf(stderr, "\nbad recombination of shares\n");
 318   };
 319
 320   xfree(sec);
 321   xfree(sbuf);
 322
 323   xfree(v);
 324   xfree(p);
 325
 326   return (ok);
 327 }
 328
 329 int main(void)
 330 {
 331   grand *r = fibrand_create(0);
 332   unsigned i;
 333   int ok = 1;
 334
 335   fputs("gfshare: ", stdout);
 336   for (i = 0; i < 40; i++) {
 337     if (!verify(r))
 338       ok = 0;
 339     fputc('.', stdout);
 340     fflush(stdout);
 341   }
 342
 343   if (ok)
 344     fputs(" ok\n", stdout);
 345   else
 346     fputs(" failed\n", stdout);
 347   return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 348 }
 349
 350 #endif
 351
 352 /*----- That's all, folks -------------------------------------------------*/