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1 | /* -*-c-*- |
2 | * |
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3 | * $Id$ |
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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 | |
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30 | /*----- Header files ------------------------------------------------------*/ |
31 | |
32 | #include <assert.h> |
33 | #include <stdarg.h> |
34 | #include <stdio.h> |
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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 | |
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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 | { |
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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 |
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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 |
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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)); |
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107 | memcpy(s->v + s->sz * (s->t - 1), buf, s->sz); |
3d64a35c |
108 | } |
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109 | |
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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 | * |
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116 | * Returns: --- |
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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 | } |
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142 | } |
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143 | } |
144 | |
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145 | /* --- @gfshare_addedp@ --- * |
146 | * |
147 | * Arguments: @gfshare *s@ = pointer to sharing context |
148 | * @unsigned x@ = which share number to check |
149 | * |
150 | * Returns: Nonzero if share @x@ has been added already, zero if it |
151 | * hasn't. |
152 | */ |
153 | |
154 | int gfshare_addedp(gfshare *s, unsigned x) |
155 | { |
156 | unsigned i; |
157 | |
158 | for (i = 0; i < s->i; i++) { |
159 | if (GFSHARE_INDEX(s, i) == x + 1) |
160 | return (1); |
161 | } |
162 | return (0); |
163 | } |
164 | |
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165 | /* --- @gfshare_add@ --- * |
166 | * |
167 | * Arguments: @gfshare *s@ = pointer to sharing context |
168 | * @unsigned x@ = which share number this is |
3d64a35c |
169 | * @const void *y@ = the share value |
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170 | * |
171 | * Returns: Number of shares required before recovery may be performed. |
172 | * |
173 | * Use: Adds a share to the context. The context must have been |
174 | * initialized with the correct threshold @t@. |
175 | */ |
176 | |
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177 | unsigned gfshare_add(gfshare *s, unsigned x, const void *y) |
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178 | { |
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179 | octet *p; |
180 | |
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181 | assert(((void)"Share context is full", s->i < s->t)); |
182 | assert(((void)"Share already present", !gfshare_addedp(s, x))); |
183 | |
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184 | /* --- If no vector has been allocated, create one --- */ |
185 | |
186 | if (!s->v) { |
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187 | s->v = XS_ALLOC(s->t * (s->sz + 1)); |
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188 | s->i = 0; |
189 | } |
190 | |
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191 | /* --- Store the share in the vector --- */ |
192 | |
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193 | p = &GFSHARE_INDEX(s, s->i); |
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194 | *p++ = x + 1; |
195 | memcpy(p, y, s->sz); |
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196 | s->i++; |
197 | |
198 | /* --- Done --- */ |
199 | |
200 | return (s->t - s->i); |
201 | } |
202 | |
203 | /* --- @gfshare_combine@ --- * |
204 | * |
205 | * Arguments: @gfshare *s@ = pointer to share context |
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206 | * @void *buf@ = pointer to output buffer for the secret |
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207 | * |
208 | * Returns: --- |
209 | * |
210 | * Use: Reconstructs a secret, given enough shares. |
211 | */ |
212 | |
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213 | void gfshare_combine(gfshare *s, void *buf) |
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214 | { |
215 | unsigned i, j; |
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216 | unsigned xi, xj; |
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217 | |
218 | /* --- Sanity checking --- */ |
219 | |
220 | assert(((void)"Not enough shares yet", s->i == s->t)); |
221 | |
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222 | /* --- Grind through the shares --- */ |
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223 | |
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224 | memset(buf, 0, s->sz); |
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225 | |
226 | for (i = 0; i < s->t; i++) { |
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227 | octet *p = buf; |
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228 | octet *q = &GFSHARE_INDEX(s, i); |
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229 | unsigned c = 0, ci = 0; |
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230 | |
231 | /* --- Compute the magic coefficient --- */ |
232 | |
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233 | xi = *q++; |
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234 | for (j = 0; j < s->t; j++) { |
235 | if (i == j) |
236 | continue; |
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237 | xj = GFSHARE_INDEX(s, j); |
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238 | c += gflog[xj]; |
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239 | if (c >= 0xff) |
240 | c -= 0xff; |
3d64a35c |
241 | ci += gflog[xi ^ xj]; |
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242 | if (ci >= 0xff) |
243 | ci -= 0xff; |
244 | } |
245 | if (ci > c) |
246 | c += 0xff; |
247 | c -= ci; |
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248 | |
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249 | /* --- Work out another layer of the secret --- */ |
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250 | |
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251 | p = buf; |
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252 | for (j = 0; j < s->sz; j++) { |
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253 | if (*q) |
254 | *p ^= gfexp[c + gflog[*q]]; |
255 | p++, q++; |
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256 | } |
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257 | } |
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258 | } |
259 | |
260 | /*----- Test rig ----------------------------------------------------------*/ |
261 | |
262 | #ifdef TEST_RIG |
263 | |
264 | #include "fibrand.h" |
265 | |
266 | static int verify(grand *r) |
267 | { |
268 | unsigned n = r->ops->range(r, 16) + 8; |
269 | unsigned t = r->ops->range(r, n - 1) + 1; |
270 | unsigned len = r->ops->range(r, 32) + 1; |
271 | |
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272 | octet *v = xmalloc(t * len); |
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273 | unsigned *p = xmalloc(n * sizeof(unsigned)); |
274 | octet *sec = xmalloc(len), *sbuf = xmalloc(len); |
275 | gfshare s; |
276 | unsigned i; |
277 | |
278 | int ok = 1; |
279 | |
280 | for (i = 0; i < n; i++) |
281 | p[i] = i; |
282 | for (i = 0; i < t; i++) { |
283 | unsigned long j = r->ops->range(r, n - i); |
284 | unsigned x = p[i]; |
285 | p[i] = p[i + j]; |
286 | p[i + j] = x; |
287 | } |
288 | |
289 | r->ops->fill(r, sec, len); |
290 | |
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291 | gfshare_create(&s, t, len); |
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292 | |
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293 | gfshare_mkshares(&s, r, sec); |
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294 | for (i = 0; i < t; i++) |
295 | gfshare_get(&s, p[i], v + (i * len)); |
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296 | gfshare_destroy(&s); |
297 | |
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298 | gfshare_create(&s, t, len); |
299 | for (i = 0; i < t; i++) |
300 | gfshare_add(&s, p[i], v + (i * len)); |
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301 | gfshare_combine(&s, sbuf); |
302 | gfshare_destroy(&s); |
303 | |
304 | if (memcmp(sec, sbuf, len) != 0) { |
305 | ok = 0; |
306 | fprintf(stderr, "\nbad recombination of shares\n"); |
307 | }; |
308 | |
309 | xfree(sec); |
310 | xfree(sbuf); |
311 | |
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312 | xfree(v); |
313 | xfree(p); |
314 | |
315 | return (ok); |
316 | } |
317 | |
318 | int main(void) |
319 | { |
320 | grand *r = fibrand_create(0); |
321 | unsigned i; |
322 | int ok = 1; |
323 | |
324 | fputs("gfshare: ", stdout); |
325 | for (i = 0; i < 40; i++) { |
326 | if (!verify(r)) |
327 | ok = 0; |
328 | fputc('.', stdout); |
329 | fflush(stdout); |
330 | } |
331 | |
332 | if (ok) |
333 | fputs(" ok\n", stdout); |
334 | else |
335 | fputs(" failed\n", stdout); |
336 | return (ok ? EXIT_SUCCESS : EXIT_FAILURE); |
337 | } |
338 | |
339 | #endif |
340 | |
341 | /*----- That's all, folks -------------------------------------------------*/ |