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1 | /* -*-c-*- |
2 | * |
4e66da02 |
3 | * $Id: gfshare.c,v 1.8 2004/04/02 01:03:49 mdw Exp $ |
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4 | * |
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5 | * Secret sharing over %$\gf{2^8}$% |
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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 $ |
4e66da02 |
33 | * Revision 1.8 2004/04/02 01:03:49 mdw |
34 | * Miscellaneous constification. |
35 | * |
eaa515d8 |
36 | * Revision 1.7 2001/06/16 23:42:17 mdw |
37 | * Typesetting fixes. |
38 | * |
5d4fee2a |
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 | * |
ea303d6c |
43 | * Revision 1.5 2000/06/24 19:11:47 mdw |
44 | * Fix daft error in the comment for @gfshare_get@. |
45 | * |
3d64a35c |
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 | * |
cb06abce |
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 | * |
4d47e157 |
54 | * Revision 1.2 2000/06/18 23:12:15 mdw |
55 | * Change typesetting of Galois Field names. |
56 | * |
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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> |
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67 | #include <string.h> |
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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 | |
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79 | static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP; |
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80 | |
81 | /*----- Main code ---------------------------------------------------------*/ |
82 | |
83 | /* --- @gfshare_create@ --- * |
84 | * |
85 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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86 | * @unsigned t@ = threshold for the system |
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87 | * @size_t sz@ = size of the secret |
88 | * |
89 | * Returns: --- |
90 | * |
91 | * Use: Initializes a sharing context. |
92 | */ |
93 | |
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94 | void gfshare_create(gfshare *s, unsigned t, size_t sz) |
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95 | { |
96 | s->t = t; |
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97 | s->i = 0; |
98 | s->sz = sz; |
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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 | { |
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115 | if (s->v) |
116 | XS_FREE(s->v); |
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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 |
5d4fee2a |
123 | * @const void *buf@ = pointer to the secret to share |
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124 | * |
125 | * Returns: --- |
126 | * |
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127 | * Use: Initializes a sharing context to be able to create shares. |
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128 | * The context structure is expected to be mostly filled in. In |
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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. |
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133 | */ |
134 | |
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135 | void gfshare_mkshares(gfshare *s, grand *r, const void *buf) |
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136 | { |
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137 | s->v = XS_ALLOC(s->sz * s->t); |
138 | r->ops->fill(r, s->v, s->sz * (s->t - 1)); |
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139 | memcpy(s->v + s->sz * (s->t - 1), buf, s->sz); |
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140 | } |
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141 | |
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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 | * |
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148 | * Returns: --- |
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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 | */ |
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153 | |
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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++; |
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173 | } |
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174 | } |
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175 | } |
176 | |
177 | /* --- @gfshare_add@ --- * |
178 | * |
179 | * Arguments: @gfshare *s@ = pointer to sharing context |
180 | * @unsigned x@ = which share number this is |
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181 | * @const void *y@ = the share value |
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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 | |
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189 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
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190 | { |
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191 | octet *p; |
192 | |
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193 | /* --- If no vector has been allocated, create one --- */ |
194 | |
195 | if (!s->v) { |
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196 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
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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 | |
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204 | p = s->v + s->i * (s->sz + 1); |
205 | *p++ = x + 1; |
206 | memcpy(p, y, s->sz); |
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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 |
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217 | * @void *buf@ = pointer to output buffer for the secret |
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218 | * |
219 | * Returns: --- |
220 | * |
221 | * Use: Reconstructs a secret, given enough shares. |
222 | */ |
223 | |
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224 | void gfshare_combine(gfshare *s, void *buf) |
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225 | { |
226 | unsigned i, j; |
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227 | unsigned xi, xj; |
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228 | |
229 | /* --- Sanity checking --- */ |
230 | |
231 | assert(((void)"Not enough shares yet", s->i == s->t)); |
232 | |
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233 | /* --- Grind through the shares --- */ |
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234 | |
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235 | memset(buf, 0, s->sz); |
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236 | |
237 | for (i = 0; i < s->t; i++) { |
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238 | octet *p = buf; |
239 | octet *q = s->v + i * (s->sz + 1); |
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240 | unsigned c = 0, ci = 0; |
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241 | |
242 | /* --- Compute the magic coefficient --- */ |
243 | |
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244 | xi = *q++; |
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245 | for (j = 0; j < s->t; j++) { |
246 | if (i == j) |
247 | continue; |
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248 | xj = s->v[j * (s->sz + 1)]; |
249 | c += gflog[xj]; |
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250 | if (c >= 0xff) |
251 | c -= 0xff; |
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252 | ci += gflog[xi ^ xj]; |
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253 | if (ci >= 0xff) |
254 | ci -= 0xff; |
255 | } |
256 | if (ci > c) |
257 | c += 0xff; |
258 | c -= ci; |
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259 | |
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260 | /* --- Work out another layer of the secret --- */ |
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261 | |
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262 | p = buf; |
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263 | for (j = 0; j < s->sz; j++) { |
3d64a35c |
264 | if (*q) |
265 | *p ^= gfexp[c + gflog[*q]]; |
266 | p++, q++; |
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267 | } |
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268 | } |
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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 | |
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283 | octet *v = xmalloc(t * len); |
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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 | |
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302 | gfshare_create(&s, t, len); |
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303 | |
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304 | gfshare_mkshares(&s, r, sec); |
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305 | for (i = 0; i < t; i++) |
306 | gfshare_get(&s, p[i], v + (i * len)); |
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307 | gfshare_destroy(&s); |
308 | |
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309 | gfshare_create(&s, t, len); |
310 | for (i = 0; i < t; i++) |
311 | gfshare_add(&s, p[i], v + (i * len)); |
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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 | |
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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 -------------------------------------------------*/ |