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 "gfshare-tab.h"
  41 #include "grand.h"
  42
  43 /*----- Static variables --------------------------------------------------*/
  44
  45 static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP;
  46
  47 /*----- Main code ---------------------------------------------------------*/
  48
  49 /* --- @gfshare_create@ --- *
  50  *
  51  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  52  *              @unsigned t@ = threshold for the system
  53  *              @size_t sz@ = size of the secret
  54  *
  55  * Returns:     ---
  56  *
  57  * Use:         Initializes a sharing context.
  58  */
  59
  60 void gfshare_create(gfshare *s, unsigned t, size_t sz)
  61 {
  62   s->t = t;
  63   s->i = 0;
  64   s->sz = sz;
  65   s->v = 0;
  66 }
  67
  68 /* --- @gfshare_destroy@ --- *
  69  *
  70  * Arguments:   @gfshare *s@ = pointer to share context to destroy
  71  *
  72  * Returns:     ---
  73  *
  74  * Use:         Disposes of a sharing context.  The allocations for the
  75  *              individual shares and the vector @v@ are freed; the secret is
  76  *              left alone.
  77  */
  78
  79 void gfshare_destroy(gfshare *s)
  80 {
  81   if (s->v)
  82     XS_FREE(s->v);
  83 }
  84
  85 /* --- @gfshare_mkshares@ --- *
  86  *
  87  * Arguments:   @gfshare *s@ = pointer to share context to fill in
  88  *              @grand *r@ = pointer to random number source
  89  *              @const void *buf@ = pointer to the secret to share
  90  *
  91  * Returns:     ---
  92  *
  93  * Use:         Initializes a sharing context to be able to create shares.
  94  *              The context structure is expected to be mostly filled in.  In
  95  *              particular, @t@ must be initialized.  If @v@ is zero, a
  96  *              vector of appropriate size is allocated.  You should use the
  97  *              macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
  98  *              contexts.
  99  */
 100
 101 void gfshare_mkshares(gfshare *s, grand *r, const void *buf)
 102 {
 103   s->v = XS_ALLOC(s->sz * s->t);
 104   r->ops->fill(r, s->v, s->sz * (s->t - 1));
 105   memcpy(s->v + s->sz * (s->t - 1), buf, s->sz);
 106 }
 107
 108 /* --- @gfshare_get@ --- *
 109  *
 110  * Arguments:   @gfshare *s@ = pointer to share conext
 111  *              @unsigned x@ = share index to fetch
 112  *              @void *buf@ = pointer to output buffer
 113  *
 114  * Returns:     ---
 115  *
 116  * Use:         Extracts a share from the system.  You may extract up to 255
 117  *              shares from the system.  Shares are indexed from 0.
 118  */
 119
 120 void gfshare_get(gfshare *s, unsigned x, void *buf)
 121 {
 122   unsigned i;
 123   const octet *p = s->v;
 124   unsigned ilog = gflog[x + 1];
 125
 126   /* --- Evaluate the polynomial at %$x = i + 1$% --- */
 127
 128   memcpy(buf, p, s->sz);
 129   p += s->sz;
 130
 131   for (i = 1; i < s->t; i++) {
 132     octet *q = buf;
 133     unsigned k;
 134     for (k = 0; k < s->sz; k++) {
 135       unsigned qq = *q;
 136       if (qq)
 137         qq = gfexp[ilog + gflog[qq]];
 138       *q++ = qq ^ *p++;
 139     }
 140   }
 141 }
 142
 143 /* --- @gfshare_addedp@ --- *
 144  *
 145  * Arguments:   @gfshare *s@ = pointer to sharing context
 146  *              @unsigned x@ = which share number to check
 147  *
 148  * Returns:     Nonzero if share @x@ has been added already, zero if it
 149  *              hasn't.
 150  */
 151
 152 int gfshare_addedp(gfshare *s, unsigned x)
 153 {
 154   unsigned i;
 155
 156   for (i = 0; i < s->i; i++) {
 157     if (GFSHARE_INDEX(s, i) == x + 1)
 158       return (1);
 159   }
 160   return (0);
 161 }
 162
 163 /* --- @gfshare_add@ --- *
 164  *
 165  * Arguments:   @gfshare *s@ = pointer to sharing context
 166  *              @unsigned x@ = which share number this is
 167  *              @const void *y@ = the share value
 168  *
 169  * Returns:     Number of shares required before recovery may be performed.
 170  *
 171  * Use:         Adds a share to the context.  The context must have been
 172  *              initialized with the correct threshold @t@.
 173  */
 174
 175 unsigned gfshare_add(gfshare *s, unsigned x, const void *y)
 176 {
 177   octet *p;
 178
 179   assert(((void)"Share context is full", s->i < s->t));
 180   assert(((void)"Share already present", !gfshare_addedp(s, x)));
 181
 182   /* --- If no vector has been allocated, create one --- */
 183
 184   if (!s->v) {
 185     s->v = XS_ALLOC(s->t * (s->sz + 1));
 186     s->i = 0;
 187   }
 188
 189   /* --- Store the share in the vector --- */
 190
 191   p = &GFSHARE_INDEX(s, s->i);
 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 = &GFSHARE_INDEX(s, i);
 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 = GFSHARE_INDEX(s, j);
 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
 291   gfshare_mkshares(&s, r, sec);
 292   for (i = 0; i < t; i++)
 293     gfshare_get(&s, p[i], v + (i * len));
 294   gfshare_destroy(&s);
 295
 296   gfshare_create(&s, t, len);
 297   for (i = 0; i < t; i++)
 298     gfshare_add(&s, p[i], v + (i * len));
 299   gfshare_combine(&s, sbuf);
 300   gfshare_destroy(&s);
 301
 302   if (memcmp(sec, sbuf, len) != 0) {
 303     ok = 0;
 304     fprintf(stderr, "\nbad recombination of shares\n");
 305   };
 306
 307   xfree(sec);
 308   xfree(sbuf);
 309
 310   xfree(v);
 311   xfree(p);
 312
 313   return (ok);
 314 }
 315
 316 int main(void)
 317 {
 318   grand *r = fibrand_create(0);
 319   unsigned i;
 320   int ok = 1;
 321
 322   fputs("gfshare: ", stdout);
 323   for (i = 0; i < 40; i++) {
 324     if (!verify(r))
 325       ok = 0;
 326     fputc('.', stdout);
 327     fflush(stdout);
 328   }
 329
 330   if (ok)
 331     fputs(" ok\n", stdout);
 332   else
 333     fputs(" failed\n", stdout);
 334   return (ok ? EXIT_SUCCESS : EXIT_FAILURE);
 335 }
 336
 337 #endif
 338
 339 /*----- That's all, folks -------------------------------------------------*/