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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: gfshare.c,v 1.5 2000/06/24 19:11:47 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 $ |
ea303d6c |
33 | * Revision 1.5 2000/06/24 19:11:47 mdw |
34 | * Fix daft error in the comment for @gfshare_get@. |
35 | * |
3d64a35c |
36 | * Revision 1.4 2000/06/24 18:29:05 mdw |
37 | * Interface change: allow shares to be extracted from a context on demand, |
38 | * rather than building them all up-front. |
39 | * |
cb06abce |
40 | * Revision 1.3 2000/06/22 18:04:13 mdw |
41 | * Improve secret reconstruction -- compute coefficients as needed rather |
42 | * than making a big array of them. |
43 | * |
4d47e157 |
44 | * Revision 1.2 2000/06/18 23:12:15 mdw |
45 | * Change typesetting of Galois Field names. |
46 | * |
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47 | * Revision 1.1 2000/06/17 10:56:30 mdw |
48 | * Fast but nonstandard secret sharing system. |
49 | * |
50 | */ |
51 | |
52 | /*----- Header files ------------------------------------------------------*/ |
53 | |
54 | #include <assert.h> |
55 | #include <stdarg.h> |
56 | #include <stdio.h> |
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57 | #include <string.h> |
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58 | |
59 | #include <mLib/alloc.h> |
60 | #include <mLib/bits.h> |
61 | |
62 | #include "arena.h" |
63 | #include "gfshare.h" |
64 | #include "gfshare-tab.h" |
65 | #include "grand.h" |
66 | |
67 | /*----- Static variables --------------------------------------------------*/ |
68 | |
69 | static octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP; |
70 | |
71 | /*----- Main code ---------------------------------------------------------*/ |
72 | |
73 | /* --- @gfshare_create@ --- * |
74 | * |
75 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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76 | * @unsigned t@ = threshold for the system |
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77 | * @size_t sz@ = size of the secret |
78 | * |
79 | * Returns: --- |
80 | * |
81 | * Use: Initializes a sharing context. |
82 | */ |
83 | |
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84 | void gfshare_create(gfshare *s, unsigned t, size_t sz) |
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85 | { |
86 | s->t = t; |
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87 | s->i = 0; |
88 | s->sz = sz; |
89 | s->s = 0; |
90 | s->v = 0; |
91 | } |
92 | |
93 | /* --- @gfshare_destroy@ --- * |
94 | * |
95 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
96 | * |
97 | * Returns: --- |
98 | * |
99 | * Use: Disposes of a sharing context. The allocations for the |
100 | * individual shares and the vector @v@ are freed; the secret is |
101 | * left alone. |
102 | */ |
103 | |
104 | void gfshare_destroy(gfshare *s) |
105 | { |
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106 | if (s->v) |
107 | XS_FREE(s->v); |
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108 | } |
109 | |
110 | /* --- @gfshare_mkshares@ --- * |
111 | * |
112 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
113 | * @grand *r@ = pointer to random number source |
114 | * |
115 | * Returns: --- |
116 | * |
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117 | * Use: Initializes a sharing context to be able to create shares. |
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118 | * The context structure is expected to be mostly filled in. In |
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119 | * particular, @t@ and @s@ must be initialized. If @v@ is zero, |
120 | * a vector of appropriate size is allocated. You should use |
121 | * the macro @GFSHARE_INIT@ or @gfshare_create@ to construct |
122 | * sharing contexts. |
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123 | */ |
124 | |
125 | void gfshare_mkshares(gfshare *s, grand *r) |
126 | { |
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127 | s->v = XS_ALLOC(s->sz * s->t); |
128 | r->ops->fill(r, s->v, s->sz * (s->t - 1)); |
129 | memcpy(s->v + s->sz * (s->t - 1), s->s, s->sz); |
130 | } |
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131 | |
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132 | /* --- @gfshare_get@ --- * |
133 | * |
134 | * Arguments: @gfshare *s@ = pointer to share conext |
135 | * @unsigned x@ = share index to fetch |
136 | * @void *buf@ = pointer to output buffer |
137 | * |
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138 | * Returns: --- |
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139 | * |
140 | * Use: Extracts a share from the system. You may extract up to 255 |
141 | * shares from the system. Shares are indexed from 0. |
142 | */ |
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143 | |
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144 | void gfshare_get(gfshare *s, unsigned x, void *buf) |
145 | { |
146 | unsigned i; |
147 | const octet *p = s->v; |
148 | unsigned ilog = gflog[x + 1]; |
149 | |
150 | /* --- Evaluate the polynomial at %$x = i + 1$% --- */ |
151 | |
152 | memcpy(buf, p, s->sz); |
153 | p += s->sz; |
154 | |
155 | for (i = 1; i < s->t; i++) { |
156 | octet *q = buf; |
157 | unsigned k; |
158 | for (k = 0; k < s->sz; k++) { |
159 | unsigned qq = *q; |
160 | if (qq) |
161 | qq = gfexp[ilog + gflog[qq]]; |
162 | *q++ = qq ^ *p++; |
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163 | } |
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164 | } |
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165 | } |
166 | |
167 | /* --- @gfshare_add@ --- * |
168 | * |
169 | * Arguments: @gfshare *s@ = pointer to sharing context |
170 | * @unsigned x@ = which share number this is |
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171 | * @const void *y@ = the share value |
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172 | * |
173 | * Returns: Number of shares required before recovery may be performed. |
174 | * |
175 | * Use: Adds a share to the context. The context must have been |
176 | * initialized with the correct threshold @t@. |
177 | */ |
178 | |
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179 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
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180 | { |
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181 | octet *p; |
182 | |
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183 | /* --- If no vector has been allocated, create one --- */ |
184 | |
185 | if (!s->v) { |
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186 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
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187 | s->i = 0; |
188 | } |
189 | |
190 | assert(((void)"Share context is full", s->i < s->t)); |
191 | |
192 | /* --- Store the share in the vector --- */ |
193 | |
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194 | p = s->v + s->i * (s->sz + 1); |
195 | *p++ = x + 1; |
196 | memcpy(p, y, s->sz); |
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197 | s->i++; |
198 | |
199 | /* --- Done --- */ |
200 | |
201 | return (s->t - s->i); |
202 | } |
203 | |
204 | /* --- @gfshare_combine@ --- * |
205 | * |
206 | * Arguments: @gfshare *s@ = pointer to share context |
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207 | * @void *buf@ = pointer to output buffer for the secret |
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208 | * |
209 | * Returns: --- |
210 | * |
211 | * Use: Reconstructs a secret, given enough shares. |
212 | */ |
213 | |
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214 | void gfshare_combine(gfshare *s, void *buf) |
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215 | { |
216 | unsigned i, j; |
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217 | unsigned xi, xj; |
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218 | |
219 | /* --- Sanity checking --- */ |
220 | |
221 | assert(((void)"Not enough shares yet", s->i == s->t)); |
222 | |
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223 | /* --- Grind through the shares --- */ |
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224 | |
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225 | memset(buf, 0, s->sz); |
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226 | |
227 | for (i = 0; i < s->t; i++) { |
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228 | octet *p = buf; |
229 | octet *q = s->v + i * (s->sz + 1); |
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230 | unsigned c = 0, ci = 0; |
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231 | |
232 | /* --- Compute the magic coefficient --- */ |
233 | |
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234 | xi = *q++; |
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235 | for (j = 0; j < s->t; j++) { |
236 | if (i == j) |
237 | continue; |
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238 | xj = s->v[j * (s->sz + 1)]; |
239 | c += gflog[xj]; |
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240 | if (c >= 0xff) |
241 | c -= 0xff; |
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242 | ci += gflog[xi ^ xj]; |
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243 | if (ci >= 0xff) |
244 | ci -= 0xff; |
245 | } |
246 | if (ci > c) |
247 | c += 0xff; |
248 | c -= ci; |
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249 | |
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250 | /* --- Work out another layer of the secret --- */ |
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251 | |
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252 | p = buf; |
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253 | for (j = 0; j < s->sz; j++) { |
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254 | if (*q) |
255 | *p ^= gfexp[c + gflog[*q]]; |
256 | p++, q++; |
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257 | } |
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258 | } |
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259 | } |
260 | |
261 | /*----- Test rig ----------------------------------------------------------*/ |
262 | |
263 | #ifdef TEST_RIG |
264 | |
265 | #include "fibrand.h" |
266 | |
267 | static int verify(grand *r) |
268 | { |
269 | unsigned n = r->ops->range(r, 16) + 8; |
270 | unsigned t = r->ops->range(r, n - 1) + 1; |
271 | unsigned len = r->ops->range(r, 32) + 1; |
272 | |
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273 | octet *v = xmalloc(t * len); |
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274 | unsigned *p = xmalloc(n * sizeof(unsigned)); |
275 | octet *sec = xmalloc(len), *sbuf = xmalloc(len); |
276 | gfshare s; |
277 | unsigned i; |
278 | |
279 | int ok = 1; |
280 | |
281 | for (i = 0; i < n; i++) |
282 | p[i] = i; |
283 | for (i = 0; i < t; i++) { |
284 | unsigned long j = r->ops->range(r, n - i); |
285 | unsigned x = p[i]; |
286 | p[i] = p[i + j]; |
287 | p[i + j] = x; |
288 | } |
289 | |
290 | r->ops->fill(r, sec, len); |
291 | |
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292 | gfshare_create(&s, t, len); |
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293 | s.s = sec; |
294 | |
295 | gfshare_mkshares(&s, r); |
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296 | for (i = 0; i < t; i++) |
297 | gfshare_get(&s, p[i], v + (i * len)); |
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298 | gfshare_destroy(&s); |
299 | |
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300 | gfshare_create(&s, t, len); |
301 | for (i = 0; i < t; i++) |
302 | gfshare_add(&s, p[i], v + (i * len)); |
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303 | gfshare_combine(&s, sbuf); |
304 | gfshare_destroy(&s); |
305 | |
306 | if (memcmp(sec, sbuf, len) != 0) { |
307 | ok = 0; |
308 | fprintf(stderr, "\nbad recombination of shares\n"); |
309 | }; |
310 | |
311 | xfree(sec); |
312 | xfree(sbuf); |
313 | |
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314 | xfree(v); |
315 | xfree(p); |
316 | |
317 | return (ok); |
318 | } |
319 | |
320 | int main(void) |
321 | { |
322 | grand *r = fibrand_create(0); |
323 | unsigned i; |
324 | int ok = 1; |
325 | |
326 | fputs("gfshare: ", stdout); |
327 | for (i = 0; i < 40; i++) { |
328 | if (!verify(r)) |
329 | ok = 0; |
330 | fputc('.', stdout); |
331 | fflush(stdout); |
332 | } |
333 | |
334 | if (ok) |
335 | fputs(" ok\n", stdout); |
336 | else |
337 | fputs(" failed\n", stdout); |
338 | return (ok ? EXIT_SUCCESS : EXIT_FAILURE); |
339 | } |
340 | |
341 | #endif |
342 | |
343 | /*----- That's all, folks -------------------------------------------------*/ |