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1 | /* -*-c-*- |
2 | * |
5d4fee2a |
3 | * $Id: gfshare.c,v 1.6 2000/12/06 20:30:10 mdw Exp $ |
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4 | * |
4d47e157 |
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 $ |
5d4fee2a |
33 | * Revision 1.6 2000/12/06 20:30:10 mdw |
34 | * Change secret sharing interface: present the secret at share |
35 | * construction time. |
36 | * |
ea303d6c |
37 | * Revision 1.5 2000/06/24 19:11:47 mdw |
38 | * Fix daft error in the comment for @gfshare_get@. |
39 | * |
3d64a35c |
40 | * Revision 1.4 2000/06/24 18:29:05 mdw |
41 | * Interface change: allow shares to be extracted from a context on demand, |
42 | * rather than building them all up-front. |
43 | * |
cb06abce |
44 | * Revision 1.3 2000/06/22 18:04:13 mdw |
45 | * Improve secret reconstruction -- compute coefficients as needed rather |
46 | * than making a big array of them. |
47 | * |
4d47e157 |
48 | * Revision 1.2 2000/06/18 23:12:15 mdw |
49 | * Change typesetting of Galois Field names. |
50 | * |
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51 | * Revision 1.1 2000/06/17 10:56:30 mdw |
52 | * Fast but nonstandard secret sharing system. |
53 | * |
54 | */ |
55 | |
56 | /*----- Header files ------------------------------------------------------*/ |
57 | |
58 | #include <assert.h> |
59 | #include <stdarg.h> |
60 | #include <stdio.h> |
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61 | #include <string.h> |
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62 | |
63 | #include <mLib/alloc.h> |
64 | #include <mLib/bits.h> |
65 | |
66 | #include "arena.h" |
67 | #include "gfshare.h" |
68 | #include "gfshare-tab.h" |
69 | #include "grand.h" |
70 | |
71 | /*----- Static variables --------------------------------------------------*/ |
72 | |
73 | static octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP; |
74 | |
75 | /*----- Main code ---------------------------------------------------------*/ |
76 | |
77 | /* --- @gfshare_create@ --- * |
78 | * |
79 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
3d64a35c |
80 | * @unsigned t@ = threshold for the system |
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81 | * @size_t sz@ = size of the secret |
82 | * |
83 | * Returns: --- |
84 | * |
85 | * Use: Initializes a sharing context. |
86 | */ |
87 | |
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88 | void gfshare_create(gfshare *s, unsigned t, size_t sz) |
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89 | { |
90 | s->t = t; |
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91 | s->i = 0; |
92 | s->sz = sz; |
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93 | s->v = 0; |
94 | } |
95 | |
96 | /* --- @gfshare_destroy@ --- * |
97 | * |
98 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
99 | * |
100 | * Returns: --- |
101 | * |
102 | * Use: Disposes of a sharing context. The allocations for the |
103 | * individual shares and the vector @v@ are freed; the secret is |
104 | * left alone. |
105 | */ |
106 | |
107 | void gfshare_destroy(gfshare *s) |
108 | { |
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109 | if (s->v) |
110 | XS_FREE(s->v); |
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111 | } |
112 | |
113 | /* --- @gfshare_mkshares@ --- * |
114 | * |
115 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
116 | * @grand *r@ = pointer to random number source |
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117 | * @const void *buf@ = pointer to the secret to share |
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118 | * |
119 | * Returns: --- |
120 | * |
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121 | * Use: Initializes a sharing context to be able to create shares. |
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122 | * The context structure is expected to be mostly filled in. In |
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123 | * particular, @t@ must be initialized. If @v@ is zero, a |
124 | * vector of appropriate size is allocated. You should use the |
125 | * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing |
126 | * contexts. |
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127 | */ |
128 | |
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129 | void gfshare_mkshares(gfshare *s, grand *r, const void *buf) |
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130 | { |
3d64a35c |
131 | s->v = XS_ALLOC(s->sz * s->t); |
132 | r->ops->fill(r, s->v, s->sz * (s->t - 1)); |
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133 | memcpy(s->v + s->sz * (s->t - 1), buf, s->sz); |
3d64a35c |
134 | } |
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135 | |
3d64a35c |
136 | /* --- @gfshare_get@ --- * |
137 | * |
138 | * Arguments: @gfshare *s@ = pointer to share conext |
139 | * @unsigned x@ = share index to fetch |
140 | * @void *buf@ = pointer to output buffer |
141 | * |
ea303d6c |
142 | * Returns: --- |
3d64a35c |
143 | * |
144 | * Use: Extracts a share from the system. You may extract up to 255 |
145 | * shares from the system. Shares are indexed from 0. |
146 | */ |
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147 | |
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148 | void gfshare_get(gfshare *s, unsigned x, void *buf) |
149 | { |
150 | unsigned i; |
151 | const octet *p = s->v; |
152 | unsigned ilog = gflog[x + 1]; |
153 | |
154 | /* --- Evaluate the polynomial at %$x = i + 1$% --- */ |
155 | |
156 | memcpy(buf, p, s->sz); |
157 | p += s->sz; |
158 | |
159 | for (i = 1; i < s->t; i++) { |
160 | octet *q = buf; |
161 | unsigned k; |
162 | for (k = 0; k < s->sz; k++) { |
163 | unsigned qq = *q; |
164 | if (qq) |
165 | qq = gfexp[ilog + gflog[qq]]; |
166 | *q++ = qq ^ *p++; |
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167 | } |
6c1035f5 |
168 | } |
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169 | } |
170 | |
171 | /* --- @gfshare_add@ --- * |
172 | * |
173 | * Arguments: @gfshare *s@ = pointer to sharing context |
174 | * @unsigned x@ = which share number this is |
3d64a35c |
175 | * @const void *y@ = the share value |
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176 | * |
177 | * Returns: Number of shares required before recovery may be performed. |
178 | * |
179 | * Use: Adds a share to the context. The context must have been |
180 | * initialized with the correct threshold @t@. |
181 | */ |
182 | |
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183 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
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184 | { |
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185 | octet *p; |
186 | |
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187 | /* --- If no vector has been allocated, create one --- */ |
188 | |
189 | if (!s->v) { |
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190 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
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191 | s->i = 0; |
192 | } |
193 | |
194 | assert(((void)"Share context is full", s->i < s->t)); |
195 | |
196 | /* --- Store the share in the vector --- */ |
197 | |
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198 | p = s->v + s->i * (s->sz + 1); |
199 | *p++ = x + 1; |
200 | memcpy(p, y, s->sz); |
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201 | s->i++; |
202 | |
203 | /* --- Done --- */ |
204 | |
205 | return (s->t - s->i); |
206 | } |
207 | |
208 | /* --- @gfshare_combine@ --- * |
209 | * |
210 | * Arguments: @gfshare *s@ = pointer to share context |
3d64a35c |
211 | * @void *buf@ = pointer to output buffer for the secret |
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212 | * |
213 | * Returns: --- |
214 | * |
215 | * Use: Reconstructs a secret, given enough shares. |
216 | */ |
217 | |
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218 | void gfshare_combine(gfshare *s, void *buf) |
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219 | { |
220 | unsigned i, j; |
3d64a35c |
221 | unsigned xi, xj; |
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222 | |
223 | /* --- Sanity checking --- */ |
224 | |
225 | assert(((void)"Not enough shares yet", s->i == s->t)); |
226 | |
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227 | /* --- Grind through the shares --- */ |
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228 | |
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229 | memset(buf, 0, s->sz); |
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230 | |
231 | for (i = 0; i < s->t; i++) { |
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232 | octet *p = buf; |
233 | octet *q = s->v + i * (s->sz + 1); |
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234 | unsigned c = 0, ci = 0; |
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235 | |
236 | /* --- Compute the magic coefficient --- */ |
237 | |
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238 | xi = *q++; |
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239 | for (j = 0; j < s->t; j++) { |
240 | if (i == j) |
241 | continue; |
3d64a35c |
242 | xj = s->v[j * (s->sz + 1)]; |
243 | c += gflog[xj]; |
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244 | if (c >= 0xff) |
245 | c -= 0xff; |
3d64a35c |
246 | ci += gflog[xi ^ xj]; |
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247 | if (ci >= 0xff) |
248 | ci -= 0xff; |
249 | } |
250 | if (ci > c) |
251 | c += 0xff; |
252 | c -= ci; |
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253 | |
cb06abce |
254 | /* --- Work out another layer of the secret --- */ |
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255 | |
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256 | p = buf; |
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257 | for (j = 0; j < s->sz; j++) { |
3d64a35c |
258 | if (*q) |
259 | *p ^= gfexp[c + gflog[*q]]; |
260 | p++, q++; |
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261 | } |
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262 | } |
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263 | } |
264 | |
265 | /*----- Test rig ----------------------------------------------------------*/ |
266 | |
267 | #ifdef TEST_RIG |
268 | |
269 | #include "fibrand.h" |
270 | |
271 | static int verify(grand *r) |
272 | { |
273 | unsigned n = r->ops->range(r, 16) + 8; |
274 | unsigned t = r->ops->range(r, n - 1) + 1; |
275 | unsigned len = r->ops->range(r, 32) + 1; |
276 | |
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277 | octet *v = xmalloc(t * len); |
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278 | unsigned *p = xmalloc(n * sizeof(unsigned)); |
279 | octet *sec = xmalloc(len), *sbuf = xmalloc(len); |
280 | gfshare s; |
281 | unsigned i; |
282 | |
283 | int ok = 1; |
284 | |
285 | for (i = 0; i < n; i++) |
286 | p[i] = i; |
287 | for (i = 0; i < t; i++) { |
288 | unsigned long j = r->ops->range(r, n - i); |
289 | unsigned x = p[i]; |
290 | p[i] = p[i + j]; |
291 | p[i + j] = x; |
292 | } |
293 | |
294 | r->ops->fill(r, sec, len); |
295 | |
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296 | gfshare_create(&s, t, len); |
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297 | |
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298 | gfshare_mkshares(&s, r, sec); |
3d64a35c |
299 | for (i = 0; i < t; i++) |
300 | gfshare_get(&s, p[i], v + (i * len)); |
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301 | gfshare_destroy(&s); |
302 | |
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303 | gfshare_create(&s, t, len); |
304 | for (i = 0; i < t; i++) |
305 | gfshare_add(&s, p[i], v + (i * len)); |
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306 | gfshare_combine(&s, sbuf); |
307 | gfshare_destroy(&s); |
308 | |
309 | if (memcmp(sec, sbuf, len) != 0) { |
310 | ok = 0; |
311 | fprintf(stderr, "\nbad recombination of shares\n"); |
312 | }; |
313 | |
314 | xfree(sec); |
315 | xfree(sbuf); |
316 | |
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317 | xfree(v); |
318 | xfree(p); |
319 | |
320 | return (ok); |
321 | } |
322 | |
323 | int main(void) |
324 | { |
325 | grand *r = fibrand_create(0); |
326 | unsigned i; |
327 | int ok = 1; |
328 | |
329 | fputs("gfshare: ", stdout); |
330 | for (i = 0; i < 40; i++) { |
331 | if (!verify(r)) |
332 | ok = 0; |
333 | fputc('.', stdout); |
334 | fflush(stdout); |
335 | } |
336 | |
337 | if (ok) |
338 | fputs(" ok\n", stdout); |
339 | else |
340 | fputs(" failed\n", stdout); |
341 | return (ok ? EXIT_SUCCESS : EXIT_FAILURE); |
342 | } |
343 | |
344 | #endif |
345 | |
346 | /*----- That's all, folks -------------------------------------------------*/ |