git.distorted.org.uk Git - u/mdw/catacomb/blob - misc/gfshare.c

   1 /* -*-c-*-
   2  *
   3  * Secret sharing over %$\gf{2^8}$%
   4  *
   5  * (c) 2000 Straylight/Edgeware
   6  */
   7
   8 /*----- Licensing notice --------------------------------------------------*
   9  *
  10  * This file is part of Catacomb.
  11  *
  12  * Catacomb is free software; you can redistribute it and/or modify
  13  * it under the terms of the GNU Library General Public License as
  14  * published by the Free Software Foundation; either version 2 of the
  15  * License, or (at your option) any later version.
  16  *
  17  * Catacomb is distributed in the hope that it will be useful,
  18  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  19  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  20  * GNU Library General Public License for more details.
  21  *
  22  * You should have received a copy of the GNU Library General Public
  23  * License along with Catacomb; if not, write to the Free
  24  * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston,
  25  * MA 02111-1307, USA.
  26  */
  27
  28 /*----- Header files ------------------------------------------------------*/
  29
  30 #include <assert.h>
  31 #include <stdarg.h>
  32 #include <stdio.h>
  33 #include <string.h>
  34
  35 #include <mLib/alloc.h>
  36 #include <mLib/bits.h>
  37
  38 #include "arena.h"
  39 #include "gfshare.h"
  40 #include "grand.h"
  41
  42 /*----- Static variables --------------------------------------------------*/
  43
  44 extern const octet gfshare_log[256], gfshare_exp[510];
  45
  46 /*----- Main code ---------------------------------------------------------*/
  47
  48 /* --- @gfshare_create@ --- *
  49  *
  50  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  51  *              @unsigned t@ = threshold for the system
  52  *              @size_t sz@ = size of the secret
  53  *
  54  * Returns:     ---
  55  *
  56  * Use:         Initializes a sharing context.
  57  */
  58
  59 void gfshare_create(gfshare *s, unsigned t, size_t sz)
  60 {
  61   s->t = t;
  62   s->i = 0;
  63   s->sz = sz;
  64   s->v = 0;
  65 }
  66
  67 /* --- @gfshare_destroy@ --- *
  68  *
  69  * Arguments:   @gfshare *s@ = pointer to share context to destroy
  70  *
  71  * Returns:     ---
  72  *
  73  * Use:         Disposes of a sharing context.  The allocations for the
  74  *              individual shares and the vector @v@ are freed; the secret is
  75  *              left alone.
  76  */
  77
  78 void gfshare_destroy(gfshare *s)
  79 {
  80   if (s->v)
  81     XS_FREE(s->v);
  82 }
  83
  84 /* --- @gfshare_mkshares@ --- *
  85  *
  86  * Arguments:   @gfshare *s@ = pointer to share context to fill in
  87  *              @grand *r@ = pointer to random number source
  88  *              @const void *buf@ = pointer to the secret to share
  89  *
  90  * Returns:     ---
  91  *
  92  * Use:         Initializes a sharing context to be able to create shares.
  93  *              The context structure is expected to be mostly filled in.  In
  94  *              particular, @t@ must be initialized.  If @v@ is zero, a
  95  *              vector of appropriate size is allocated.  You should use the
  96  *              macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
  97  *              contexts.
  98  */
  99
 100 void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
 101 {
 102   s->v = XS_ALLOC(s->sz * s->t);
 103   r->ops->fill(r, s->v, s->sz * (s->t - 1));
 104   memcpy(s->v + s->sz * (s->t - 1), buf, s->sz);
 105 }
 106
 107 /* --- @gfshare_get@ --- *
 108  *
 109  * Arguments:   @gfshare *s@ = pointer to share conext
 110  *              @unsigned x@ = share index to fetch
 111  *              @void *buf@ = pointer to output buffer
 112  *
 113  * Returns:     ---
 114  *
 115  * Use:         Extracts a share from the system.  You may extract up to 255
 116  *              shares from the system.  Shares are indexed from 0.
 117  */
 118
 119 void gfshare_get(gfshare *s, unsigned x, void *buf)
 120 {
 121   unsigned i;
 122   const octet *p = s->v;
 123   unsigned ilog = gfshare_log[x + 1];
 124
 125   /* --- Evaluate the polynomial at %$x = i + 1$% --- */
 126
 127   memcpy(buf, p, s->sz);
 128   p += s->sz;
 129
 130   for (i = 1; i < s->t; i++) {
 131     octet *q = buf;
 132     unsigned k;
 133     for (k = 0; k < s->sz; k++) {
 134       unsigned qq = *q;
 135       if (qq)
 136         qq = gfshare_exp[ilog + gfshare_log[qq]];
 137       *q++ = qq ^ *p++;
 138     }
 139   }
 140 }
 141
 142 /* --- @gfshare_addedp@ --- *
 143  *
 144  * Arguments:   @gfshare *s@ = pointer to sharing context
 145  *              @unsigned x@ = which share number to check
 146  *
 147  * Returns:     Nonzero if share @x@ has been added already, zero if it
 148  *              hasn't.
 149  */
 150
 151 int gfshare_addedp(gfshare *s, unsigned x)
 152 {
 153   unsigned i;
 154
 155   for (i = 0; i < s->i; i++) {
 156     if (GFSHARE_INDEX(s, i) == x + 1)
 157       return (1);
 158   }
 159   return (0);
 160 }
 161
 162 /* --- @gfshare_add@ --- *
 163  *
 164  * Arguments:   @gfshare *s@ = pointer to sharing context
 165  *              @unsigned x@ = which share number this is
 166  *              @const void *y@ = the share value
 167  *
 168  * Returns:     Number of shares required before recovery may be performed.
 169  *
 170  * Use:         Adds a share to the context.  The context must have been
 171  *              initialized with the correct threshold @t@.
 172  */
 173
 174 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
 175 {
 176   octet *p;
 177
 178   assert(((void)"Share context is full", s->i < s->t));
 179   assert(((void)"Share already present", !gfshare_addedp(s, x)));
 180
 181   /* --- If no vector has been allocated, create one --- */
 182
 183   if (!s->v) {
 184     s->v = XS_ALLOC(s->t * (s->sz + 1));
 185     s->i = 0;
 186   }
 187
 188   /* --- Store the share in the vector --- */
 189
 190   p = &GFSHARE_INDEX(s, s->i);
 191   *p++ = x + 1;
 192   memcpy(p, y, s->sz);
 193   s->i++;
 194
 195   /* --- Done --- */
 196
 197   return (s->t - s->i);
 198 }
 199
 200 /* --- @gfshare_combine@ --- *
 201  *
 202  * Arguments:   @gfshare *s@ = pointer to share context
 203  *              @void *buf@ = pointer to output buffer for the secret
 204  *
 205  * Returns:     ---
 206  *
 207  * Use:         Reconstructs a secret, given enough shares.
 208  */
 209
 210 void gfshare_combine(gfshare *s, void *buf)
 211 {
 212   unsigned i, j;
 213   unsigned xi, xj;
 214
 215   /* --- Sanity checking --- */
 216
 217   assert(((void)"Not enough shares yet", s->i == s->t));
 218
 219   /* --- Grind through the shares --- */
 220
 221   memset(buf, 0, s->sz);
 222
 223   for (i = 0; i < s->t; i++) {
 224     octet *p = buf;
 225     octet *q = &GFSHARE_INDEX(s, i);
 226     unsigned c = 0, ci = 0;
 227
 228     /* --- Compute the magic coefficient --- */
 229
 230     xi = *q++;
 231     for (j = 0; j < s->t; j++) {
 232       if (i == j)
 233         continue;
 234       xj = GFSHARE_INDEX(s, j);
 235       c += gfshare_log[xj];
 236       if (c >= 0xff)
 237         c -= 0xff;
 238       ci += gfshare_log[xi ^ xj];
 239       if (ci >= 0xff)
 240         ci -= 0xff;
 241     }
 242     if (ci > c)
 243       c += 0xff;
 244     c -= ci;
 245
 246     /* --- Work out another layer of the secret --- */
 247
 248     p = buf;
 249     for (j = 0; j < s->sz; j++) {
 250       if (*q)
 251         *p ^= gfshare_exp[c + gfshare_log[*q]];
 252       p++, q++;
 253     }
 254   }
 255 }
 256
 257 /*----- Test rig ----------------------------------------------------------*/
 258
 259 #ifdef TEST_RIG
 260
 261 #include "fibrand.h"
 262
 263 static int verify(grand *r)
 264 {
 265   unsigned n = r->ops->range(r, 16) + 8;
 266   unsigned t = r->ops->range(r, n - 1) + 1;
 267   unsigned len = r->ops->range(r, 32) + 1;
 268
 269   octet *v = xmalloc(t * len);
 270   unsigned *p = xmalloc(n * sizeof(unsigned));
 271   octet *sec = xmalloc(len), *sbuf = xmalloc(len);
 272   gfshare s;
 273   unsigned i;
 274
 275   int ok = 1;
 276
 277   for (i = 0; i < n; i++)
 278     p[i] = i;
 279   for (i = 0; i < t; i++) {
 280     unsigned long j = r->ops->range(r, n - i);
 281     unsigned x = p[i];
 282     p[i] = p[i + j];
 283     p[i + j] = x;
 284   }
 285
 286   r->ops->fill(r, sec, len);
 287
 288   gfshare_create(&s, t, len);
 289
 290   gfshare_mkshares(&s, r, sec);
 291   for (i = 0; i < t; i++)
 292     gfshare_get(&s, p[i], v + (i * len));
 293   gfshare_destroy(&s);
 294
 295   gfshare_create(&s, t, len);
 296   for (i = 0; i < t; i++)
 297     gfshare_add(&s, p[i], v + (i * len));
 298   gfshare_combine(&s, sbuf);
 299   gfshare_destroy(&s);
 300
 301   if (memcmp(sec, sbuf, len) != 0) {
 302     ok = 0;
 303     fprintf(stderr, "\nbad recombination of shares\n");
 304   };
 305
 306   xfree(sec);
 307   xfree(sbuf);
 308
 309   xfree(v);
 310   xfree(p);
 311
 312   return (ok);
 313 }
 314
 315 int main(void)
 316 {
 317   grand *r = fibrand_create(0);
 318   unsigned i;
 319   int ok = 1;
 320
 321   fputs("gfshare: ", stdout);
 322   for (i = 0; i < 40; i++) {
 323     if (!verify(r))
 324       ok = 0;
 325     fputc('.', stdout);
 326     fflush(stdout);
 327   }
 328
 329   if (ok)
 330     fputs(" ok\n", stdout);
 331   else
 332     fputs(" failed\n", stdout);
 333   return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 334 }
 335
 336 #endif
 337
 338 /*----- That's all, folks -------------------------------------------------*/