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

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