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1 | /* -*-c-*- |
2 | * |
b817bfc6 |
3 | * $Id: gfshare.c,v 1.9 2004/04/08 01:36:15 mdw Exp $ |
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4 | * |
eaa515d8 |
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 | |
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30 | /*----- Header files ------------------------------------------------------*/ |
31 | |
32 | #include <assert.h> |
33 | #include <stdarg.h> |
34 | #include <stdio.h> |
cb06abce |
35 | #include <string.h> |
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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 | |
4e66da02 |
47 | static const octet gflog[] = GFSHARE_LOG, gfexp[] = GFSHARE_EXP; |
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48 | |
49 | /*----- Main code ---------------------------------------------------------*/ |
50 | |
51 | /* --- @gfshare_create@ --- * |
52 | * |
53 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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54 | * @unsigned t@ = threshold for the system |
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55 | * @size_t sz@ = size of the secret |
56 | * |
57 | * Returns: --- |
58 | * |
59 | * Use: Initializes a sharing context. |
60 | */ |
61 | |
3d64a35c |
62 | void gfshare_create(gfshare *s, unsigned t, size_t sz) |
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63 | { |
64 | s->t = t; |
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65 | s->i = 0; |
66 | s->sz = sz; |
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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 | { |
3d64a35c |
83 | if (s->v) |
84 | XS_FREE(s->v); |
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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 |
5d4fee2a |
91 | * @const void *buf@ = pointer to the secret to share |
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92 | * |
93 | * Returns: --- |
94 | * |
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95 | * Use: Initializes a sharing context to be able to create shares. |
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96 | * The context structure is expected to be mostly filled in. In |
5d4fee2a |
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. |
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101 | */ |
102 | |
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103 | void gfshare_mkshares(gfshare *s, grand *r, const void *buf) |
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104 | { |
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105 | s->v = XS_ALLOC(s->sz * s->t); |
106 | r->ops->fill(r, s->v, s->sz * (s->t - 1)); |
5d4fee2a |
107 | memcpy(s->v + s->sz * (s->t - 1), buf, s->sz); |
3d64a35c |
108 | } |
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109 | |
3d64a35c |
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 | * |
ea303d6c |
116 | * Returns: --- |
3d64a35c |
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 | */ |
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121 | |
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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++; |
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141 | } |
6c1035f5 |
142 | } |
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143 | } |
144 | |
145 | /* --- @gfshare_add@ --- * |
146 | * |
147 | * Arguments: @gfshare *s@ = pointer to sharing context |
148 | * @unsigned x@ = which share number this is |
3d64a35c |
149 | * @const void *y@ = the share value |
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150 | * |
151 | * Returns: Number of shares required before recovery may be performed. |
152 | * |
153 | * Use: Adds a share to the context. The context must have been |
154 | * initialized with the correct threshold @t@. |
155 | */ |
156 | |
3d64a35c |
157 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
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158 | { |
3d64a35c |
159 | octet *p; |
160 | |
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161 | /* --- If no vector has been allocated, create one --- */ |
162 | |
163 | if (!s->v) { |
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164 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
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165 | s->i = 0; |
166 | } |
167 | |
168 | assert(((void)"Share context is full", s->i < s->t)); |
169 | |
170 | /* --- Store the share in the vector --- */ |
171 | |
3d64a35c |
172 | p = s->v + s->i * (s->sz + 1); |
173 | *p++ = x + 1; |
174 | memcpy(p, y, s->sz); |
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175 | s->i++; |
176 | |
177 | /* --- Done --- */ |
178 | |
179 | return (s->t - s->i); |
180 | } |
181 | |
182 | /* --- @gfshare_combine@ --- * |
183 | * |
184 | * Arguments: @gfshare *s@ = pointer to share context |
3d64a35c |
185 | * @void *buf@ = pointer to output buffer for the secret |
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186 | * |
187 | * Returns: --- |
188 | * |
189 | * Use: Reconstructs a secret, given enough shares. |
190 | */ |
191 | |
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192 | void gfshare_combine(gfshare *s, void *buf) |
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193 | { |
194 | unsigned i, j; |
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195 | unsigned xi, xj; |
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196 | |
197 | /* --- Sanity checking --- */ |
198 | |
199 | assert(((void)"Not enough shares yet", s->i == s->t)); |
200 | |
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201 | /* --- Grind through the shares --- */ |
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202 | |
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203 | memset(buf, 0, s->sz); |
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204 | |
205 | for (i = 0; i < s->t; i++) { |
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206 | octet *p = buf; |
207 | octet *q = s->v + i * (s->sz + 1); |
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208 | unsigned c = 0, ci = 0; |
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209 | |
210 | /* --- Compute the magic coefficient --- */ |
211 | |
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212 | xi = *q++; |
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213 | for (j = 0; j < s->t; j++) { |
214 | if (i == j) |
215 | continue; |
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216 | xj = s->v[j * (s->sz + 1)]; |
217 | c += gflog[xj]; |
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218 | if (c >= 0xff) |
219 | c -= 0xff; |
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220 | ci += gflog[xi ^ xj]; |
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221 | if (ci >= 0xff) |
222 | ci -= 0xff; |
223 | } |
224 | if (ci > c) |
225 | c += 0xff; |
226 | c -= ci; |
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227 | |
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228 | /* --- Work out another layer of the secret --- */ |
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229 | |
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230 | p = buf; |
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231 | for (j = 0; j < s->sz; j++) { |
3d64a35c |
232 | if (*q) |
233 | *p ^= gfexp[c + gflog[*q]]; |
234 | p++, q++; |
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235 | } |
6c1035f5 |
236 | } |
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237 | } |
238 | |
239 | /*----- Test rig ----------------------------------------------------------*/ |
240 | |
241 | #ifdef TEST_RIG |
242 | |
243 | #include "fibrand.h" |
244 | |
245 | static int verify(grand *r) |
246 | { |
247 | unsigned n = r->ops->range(r, 16) + 8; |
248 | unsigned t = r->ops->range(r, n - 1) + 1; |
249 | unsigned len = r->ops->range(r, 32) + 1; |
250 | |
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251 | octet *v = xmalloc(t * len); |
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252 | unsigned *p = xmalloc(n * sizeof(unsigned)); |
253 | octet *sec = xmalloc(len), *sbuf = xmalloc(len); |
254 | gfshare s; |
255 | unsigned i; |
256 | |
257 | int ok = 1; |
258 | |
259 | for (i = 0; i < n; i++) |
260 | p[i] = i; |
261 | for (i = 0; i < t; i++) { |
262 | unsigned long j = r->ops->range(r, n - i); |
263 | unsigned x = p[i]; |
264 | p[i] = p[i + j]; |
265 | p[i + j] = x; |
266 | } |
267 | |
268 | r->ops->fill(r, sec, len); |
269 | |
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270 | gfshare_create(&s, t, len); |
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271 | |
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272 | gfshare_mkshares(&s, r, sec); |
3d64a35c |
273 | for (i = 0; i < t; i++) |
274 | gfshare_get(&s, p[i], v + (i * len)); |
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275 | gfshare_destroy(&s); |
276 | |
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277 | gfshare_create(&s, t, len); |
278 | for (i = 0; i < t; i++) |
279 | gfshare_add(&s, p[i], v + (i * len)); |
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280 | gfshare_combine(&s, sbuf); |
281 | gfshare_destroy(&s); |
282 | |
283 | if (memcmp(sec, sbuf, len) != 0) { |
284 | ok = 0; |
285 | fprintf(stderr, "\nbad recombination of shares\n"); |
286 | }; |
287 | |
288 | xfree(sec); |
289 | xfree(sbuf); |
290 | |
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291 | xfree(v); |
292 | xfree(p); |
293 | |
294 | return (ok); |
295 | } |
296 | |
297 | int main(void) |
298 | { |
299 | grand *r = fibrand_create(0); |
300 | unsigned i; |
301 | int ok = 1; |
302 | |
303 | fputs("gfshare: ", stdout); |
304 | for (i = 0; i < 40; i++) { |
305 | if (!verify(r)) |
306 | ok = 0; |
307 | fputc('.', stdout); |
308 | fflush(stdout); |
309 | } |
310 | |
311 | if (ok) |
312 | fputs(" ok\n", stdout); |
313 | else |
314 | fputs(" failed\n", stdout); |
315 | return (ok ? EXIT_SUCCESS : EXIT_FAILURE); |
316 | } |
317 | |
318 | #endif |
319 | |
320 | /*----- That's all, folks -------------------------------------------------*/ |