| 1 | /* -*-c-*- |
| 2 | * |
| 3 | * $Id: gfshare.c,v 1.7 2001/06/16 23:42:17 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 1.7 2001/06/16 23:42:17 mdw |
| 34 | * Typesetting fixes. |
| 35 | * |
| 36 | * Revision 1.6 2000/12/06 20:30:10 mdw |
| 37 | * Change secret sharing interface: present the secret at share |
| 38 | * construction time. |
| 39 | * |
| 40 | * Revision 1.5 2000/06/24 19:11:47 mdw |
| 41 | * Fix daft error in the comment for @gfshare_get@. |
| 42 | * |
| 43 | * Revision 1.4 2000/06/24 18:29:05 mdw |
| 44 | * Interface change: allow shares to be extracted from a context on demand, |
| 45 | * rather than building them all up-front. |
| 46 | * |
| 47 | * Revision 1.3 2000/06/22 18:04:13 mdw |
| 48 | * Improve secret reconstruction -- compute coefficients as needed rather |
| 49 | * than making a big array of them. |
| 50 | * |
| 51 | * Revision 1.2 2000/06/18 23:12:15 mdw |
| 52 | * Change typesetting of Galois Field names. |
| 53 | * |
| 54 | * Revision 1.1 2000/06/17 10:56:30 mdw |
| 55 | * Fast but nonstandard secret sharing system. |
| 56 | * |
| 57 | */ |
| 58 | |
| 59 | /*----- Header files ------------------------------------------------------*/ |
| 60 | |
| 61 | #include <assert.h> |
| 62 | #include <stdarg.h> |
| 63 | #include <stdio.h> |
| 64 | #include <string.h> |
| 65 | |
| 66 | #include <mLib/alloc.h> |
| 67 | #include <mLib/bits.h> |
| 68 | |
| 69 | #include "arena.h" |
| 70 | #include "gfshare.h" |
| 71 | #include "gfshare-tab.h" |
| 72 | #include "grand.h" |
| 73 | |
| 74 | /*----- Static variables --------------------------------------------------*/ |
| 75 | |
| 76 | static octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP; |
| 77 | |
| 78 | /*----- Main code ---------------------------------------------------------*/ |
| 79 | |
| 80 | /* --- @gfshare_create@ --- * |
| 81 | * |
| 82 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
| 83 | * @unsigned t@ = threshold for the system |
| 84 | * @size_t sz@ = size of the secret |
| 85 | * |
| 86 | * Returns: --- |
| 87 | * |
| 88 | * Use: Initializes a sharing context. |
| 89 | */ |
| 90 | |
| 91 | void gfshare_create(gfshare *s, unsigned t, size_t sz) |
| 92 | { |
| 93 | s->t = t; |
| 94 | s->i = 0; |
| 95 | s->sz = sz; |
| 96 | s->v = 0; |
| 97 | } |
| 98 | |
| 99 | /* --- @gfshare_destroy@ --- * |
| 100 | * |
| 101 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
| 102 | * |
| 103 | * Returns: --- |
| 104 | * |
| 105 | * Use: Disposes of a sharing context. The allocations for the |
| 106 | * individual shares and the vector @v@ are freed; the secret is |
| 107 | * left alone. |
| 108 | */ |
| 109 | |
| 110 | void gfshare_destroy(gfshare *s) |
| 111 | { |
| 112 | if (s->v) |
| 113 | XS_FREE(s->v); |
| 114 | } |
| 115 | |
| 116 | /* --- @gfshare_mkshares@ --- * |
| 117 | * |
| 118 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
| 119 | * @grand *r@ = pointer to random number source |
| 120 | * @const void *buf@ = pointer to the secret to share |
| 121 | * |
| 122 | * Returns: --- |
| 123 | * |
| 124 | * Use: Initializes a sharing context to be able to create shares. |
| 125 | * The context structure is expected to be mostly filled in. In |
| 126 | * particular, @t@ must be initialized. If @v@ is zero, a |
| 127 | * vector of appropriate size is allocated. You should use the |
| 128 | * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing |
| 129 | * contexts. |
| 130 | */ |
| 131 | |
| 132 | void gfshare_mkshares(gfshare *s, grand *r, const void *buf) |
| 133 | { |
| 134 | s->v = XS_ALLOC(s->sz * s->t); |
| 135 | r->ops->fill(r, s->v, s->sz * (s->t - 1)); |
| 136 | memcpy(s->v + s->sz * (s->t - 1), buf, s->sz); |
| 137 | } |
| 138 | |
| 139 | /* --- @gfshare_get@ --- * |
| 140 | * |
| 141 | * Arguments: @gfshare *s@ = pointer to share conext |
| 142 | * @unsigned x@ = share index to fetch |
| 143 | * @void *buf@ = pointer to output buffer |
| 144 | * |
| 145 | * Returns: --- |
| 146 | * |
| 147 | * Use: Extracts a share from the system. You may extract up to 255 |
| 148 | * shares from the system. Shares are indexed from 0. |
| 149 | */ |
| 150 | |
| 151 | void gfshare_get(gfshare *s, unsigned x, void *buf) |
| 152 | { |
| 153 | unsigned i; |
| 154 | const octet *p = s->v; |
| 155 | unsigned ilog = gflog[x + 1]; |
| 156 | |
| 157 | /* --- Evaluate the polynomial at %$x = i + 1$% --- */ |
| 158 | |
| 159 | memcpy(buf, p, s->sz); |
| 160 | p += s->sz; |
| 161 | |
| 162 | for (i = 1; i < s->t; i++) { |
| 163 | octet *q = buf; |
| 164 | unsigned k; |
| 165 | for (k = 0; k < s->sz; k++) { |
| 166 | unsigned qq = *q; |
| 167 | if (qq) |
| 168 | qq = gfexp[ilog + gflog[qq]]; |
| 169 | *q++ = qq ^ *p++; |
| 170 | } |
| 171 | } |
| 172 | } |
| 173 | |
| 174 | /* --- @gfshare_add@ --- * |
| 175 | * |
| 176 | * Arguments: @gfshare *s@ = pointer to sharing context |
| 177 | * @unsigned x@ = which share number this is |
| 178 | * @const void *y@ = the share value |
| 179 | * |
| 180 | * Returns: Number of shares required before recovery may be performed. |
| 181 | * |
| 182 | * Use: Adds a share to the context. The context must have been |
| 183 | * initialized with the correct threshold @t@. |
| 184 | */ |
| 185 | |
| 186 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
| 187 | { |
| 188 | octet *p; |
| 189 | |
| 190 | /* --- If no vector has been allocated, create one --- */ |
| 191 | |
| 192 | if (!s->v) { |
| 193 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
| 194 | s->i = 0; |
| 195 | } |
| 196 | |
| 197 | assert(((void)"Share context is full", s->i < s->t)); |
| 198 | |
| 199 | /* --- Store the share in the vector --- */ |
| 200 | |
| 201 | p = s->v + s->i * (s->sz + 1); |
| 202 | *p++ = x + 1; |
| 203 | memcpy(p, y, s->sz); |
| 204 | s->i++; |
| 205 | |
| 206 | /* --- Done --- */ |
| 207 | |
| 208 | return (s->t - s->i); |
| 209 | } |
| 210 | |
| 211 | /* --- @gfshare_combine@ --- * |
| 212 | * |
| 213 | * Arguments: @gfshare *s@ = pointer to share context |
| 214 | * @void *buf@ = pointer to output buffer for the secret |
| 215 | * |
| 216 | * Returns: --- |
| 217 | * |
| 218 | * Use: Reconstructs a secret, given enough shares. |
| 219 | */ |
| 220 | |
| 221 | void gfshare_combine(gfshare *s, void *buf) |
| 222 | { |
| 223 | unsigned i, j; |
| 224 | unsigned xi, xj; |
| 225 | |
| 226 | /* --- Sanity checking --- */ |
| 227 | |
| 228 | assert(((void)"Not enough shares yet", s->i == s->t)); |
| 229 | |
| 230 | /* --- Grind through the shares --- */ |
| 231 | |
| 232 | memset(buf, 0, s->sz); |
| 233 | |
| 234 | for (i = 0; i < s->t; i++) { |
| 235 | octet *p = buf; |
| 236 | octet *q = s->v + i * (s->sz + 1); |
| 237 | unsigned c = 0, ci = 0; |
| 238 | |
| 239 | /* --- Compute the magic coefficient --- */ |
| 240 | |
| 241 | xi = *q++; |
| 242 | for (j = 0; j < s->t; j++) { |
| 243 | if (i == j) |
| 244 | continue; |
| 245 | xj = s->v[j * (s->sz + 1)]; |
| 246 | c += gflog[xj]; |
| 247 | if (c >= 0xff) |
| 248 | c -= 0xff; |
| 249 | ci += gflog[xi ^ xj]; |
| 250 | if (ci >= 0xff) |
| 251 | ci -= 0xff; |
| 252 | } |
| 253 | if (ci > c) |
| 254 | c += 0xff; |
| 255 | c -= ci; |
| 256 | |
| 257 | /* --- Work out another layer of the secret --- */ |
| 258 | |
| 259 | p = buf; |
| 260 | for (j = 0; j < s->sz; j++) { |
| 261 | if (*q) |
| 262 | *p ^= gfexp[c + gflog[*q]]; |
| 263 | p++, q++; |
| 264 | } |
| 265 | } |
| 266 | } |
| 267 | |
| 268 | /*----- Test rig ----------------------------------------------------------*/ |
| 269 | |
| 270 | #ifdef TEST_RIG |
| 271 | |
| 272 | #include "fibrand.h" |
| 273 | |
| 274 | static int verify(grand *r) |
| 275 | { |
| 276 | unsigned n = r->ops->range(r, 16) + 8; |
| 277 | unsigned t = r->ops->range(r, n - 1) + 1; |
| 278 | unsigned len = r->ops->range(r, 32) + 1; |
| 279 | |
| 280 | octet *v = xmalloc(t * len); |
| 281 | unsigned *p = xmalloc(n * sizeof(unsigned)); |
| 282 | octet *sec = xmalloc(len), *sbuf = xmalloc(len); |
| 283 | gfshare s; |
| 284 | unsigned i; |
| 285 | |
| 286 | int ok = 1; |
| 287 | |
| 288 | for (i = 0; i < n; i++) |
| 289 | p[i] = i; |
| 290 | for (i = 0; i < t; i++) { |
| 291 | unsigned long j = r->ops->range(r, n - i); |
| 292 | unsigned x = p[i]; |
| 293 | p[i] = p[i + j]; |
| 294 | p[i + j] = x; |
| 295 | } |
| 296 | |
| 297 | r->ops->fill(r, sec, len); |
| 298 | |
| 299 | gfshare_create(&s, t, len); |
| 300 | |
| 301 | gfshare_mkshares(&s, r, sec); |
| 302 | for (i = 0; i < t; i++) |
| 303 | gfshare_get(&s, p[i], v + (i * len)); |
| 304 | gfshare_destroy(&s); |
| 305 | |
| 306 | gfshare_create(&s, t, len); |
| 307 | for (i = 0; i < t; i++) |
| 308 | gfshare_add(&s, p[i], v + (i * len)); |
| 309 | gfshare_combine(&s, sbuf); |
| 310 | gfshare_destroy(&s); |
| 311 | |
| 312 | if (memcmp(sec, sbuf, len) != 0) { |
| 313 | ok = 0; |
| 314 | fprintf(stderr, "\nbad recombination of shares\n"); |
| 315 | }; |
| 316 | |
| 317 | xfree(sec); |
| 318 | xfree(sbuf); |
| 319 | |
| 320 | xfree(v); |
| 321 | xfree(p); |
| 322 | |
| 323 | return (ok); |
| 324 | } |
| 325 | |
| 326 | int main(void) |
| 327 | { |
| 328 | grand *r = fibrand_create(0); |
| 329 | unsigned i; |
| 330 | int ok = 1; |
| 331 | |
| 332 | fputs("gfshare: ", stdout); |
| 333 | for (i = 0; i < 40; i++) { |
| 334 | if (!verify(r)) |
| 335 | ok = 0; |
| 336 | fputc('.', stdout); |
| 337 | fflush(stdout); |
| 338 | } |
| 339 | |
| 340 | if (ok) |
| 341 | fputs(" ok\n", stdout); |
| 342 | else |
| 343 | fputs(" failed\n", stdout); |
| 344 | return (ok ? EXIT_SUCCESS : EXIT_FAILURE); |
| 345 | } |
| 346 | |
| 347 | #endif |
| 348 | |
| 349 | /*----- That's all, folks -------------------------------------------------*/ |