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1 | /* -*-c-*- |
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2 | * |
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3 | * Secret sharing over %$\gf{2^8}$% |
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4 | * |
5 | * (c) 2000 Straylight/Edgeware |
6 | */ |
7 | |
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8 | /*----- Licensing notice --------------------------------------------------* |
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9 | * |
10 | * This file is part of Catacomb. |
11 | * |
12 | * Catacomb is free software; you can redistribute it and/or modify |
13 | * it under the terms of the GNU Library General Public License as |
14 | * published by the Free Software Foundation; either version 2 of the |
15 | * License, or (at your option) any later version. |
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16 | * |
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17 | * Catacomb is distributed in the hope that it will be useful, |
18 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
19 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
20 | * GNU Library General Public License for more details. |
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21 | * |
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22 | * You should have received a copy of the GNU Library General Public |
23 | * License along with Catacomb; if not, write to the Free |
24 | * Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, |
25 | * MA 02111-1307, USA. |
26 | */ |
27 | |
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28 | /*----- Notes on the system -----------------------------------------------* |
29 | * |
30 | * This uses a variant of Shamir's secret sharing system. Shamir's original |
31 | * system used polynomials modulo a large prime. This implementation instead |
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32 | * uses the field %$\gf{2^8}$%, represented by |
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33 | * |
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34 | * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$% |
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35 | * |
36 | * and shares each byte of the secret independently. It is therefore limited |
37 | * to 255 players, although this probably isn't a serious limitation in |
38 | * practice. |
39 | * |
40 | * Share creation and reconstruction is extremely efficient. Contrast the |
41 | * performance of the straightforward implementation based on multiprecision |
42 | * arithmetic. |
43 | */ |
44 | |
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45 | #ifndef CATACOMB_GFSHARE_H |
46 | #define CATACOMB_GFSHARE_H |
47 | |
48 | #ifdef __cplusplus |
49 | extern "C" { |
50 | #endif |
51 | |
52 | /*----- Header files ------------------------------------------------------*/ |
53 | |
54 | #include <mLib/bits.h> |
55 | |
56 | #ifndef CATACOMB_GRAND_H |
57 | # include "grand.h" |
58 | #endif |
59 | |
60 | /*----- Data structures ---------------------------------------------------*/ |
61 | |
62 | /* --- A secret sharing context --- */ |
63 | |
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64 | typedef struct gfshare { |
65 | unsigned t; /* Threshold */ |
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66 | unsigned i; /* Next free slot in vector */ |
67 | size_t sz; /* Size of the secret and shares */ |
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68 | octet *v; /* Vector of share information */ |
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69 | } gfshare; |
70 | |
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71 | #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 } |
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72 | |
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73 | #define GFSHARE_INDEX(s, i) ((s)->v[(i) * ((s)->sz + 1)]) |
74 | |
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75 | /*----- Functions provided ------------------------------------------------*/ |
76 | |
77 | /* --- @gfshare_create@ --- * |
78 | * |
79 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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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 | extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/); |
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89 | |
90 | /* --- @gfshare_destroy@ --- * |
91 | * |
92 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
93 | * |
94 | * Returns: --- |
95 | * |
96 | * Use: Disposes of a sharing context. The allocations for the |
97 | * individual shares and the vector @v@ are freed; the secret is |
98 | * left alone. |
99 | */ |
100 | |
101 | extern void gfshare_destroy(gfshare */*s*/); |
102 | |
103 | /* --- @gfshare_mkshares@ --- * |
104 | * |
105 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
106 | * @grand *r@ = pointer to random number source |
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107 | * @const void *buf@ = pointer to the secret to share |
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108 | * |
109 | * Returns: --- |
110 | * |
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111 | * Use: Initializes a sharing context to be able to create shares. |
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112 | * The context structure is expected to be mostly filled in. In |
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113 | * particular, @t@ must be initialized. If @v@ is zero, a |
114 | * vector of appropriate size is allocated. You should use the |
115 | * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing |
116 | * contexts. |
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117 | */ |
118 | |
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119 | extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/, |
120 | const void */*buf*/); |
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121 | |
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122 | /* --- @gfshare_get@ --- * |
123 | * |
124 | * Arguments: @gfshare *s@ = pointer to share conext |
125 | * @unsigned x@ = share index to fetch |
126 | * @void *buf@ = pointer to output buffer |
127 | * |
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128 | * Returns: --- |
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129 | * |
130 | * Use: Extracts a share from the system. You may extract up to 255 |
131 | * shares from the system. Shares are indexed from 0. |
132 | */ |
133 | |
134 | extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/); |
135 | |
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136 | /* --- @gfshare_addedp@ --- * |
137 | * |
138 | * Arguments: @gfshare *s@ = pointer to sharing context |
139 | * @unsigned x@ = which share number to check |
140 | * |
141 | * Returns: Nonzero if share @x@ has been added already, zero if it |
142 | * hasn't. |
143 | */ |
144 | |
145 | extern int gfshare_addedp(gfshare */*s*/, unsigned /*x*/); |
146 | |
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147 | /* --- @gfshare_add@ --- * |
148 | * |
149 | * Arguments: @gfshare *s@ = pointer to sharing context |
150 | * @unsigned x@ = which share number this is |
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151 | * @const void *y@ = the share value |
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152 | * |
153 | * Returns: Number of shares required before recovery may be performed. |
154 | * |
155 | * Use: Adds a share to the context. The context must have been |
156 | * initialized with the correct threshold @t@. |
157 | */ |
158 | |
159 | extern unsigned gfshare_add(gfshare */*s*/, |
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160 | unsigned /*x*/, const void */*y*/); |
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161 | |
162 | /* --- @gfshare_combine@ --- * |
163 | * |
164 | * Arguments: @gfshare *s@ = pointer to share context |
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165 | * @void *buf@ = pointer to output buffer for the secret |
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166 | * |
167 | * Returns: --- |
168 | * |
169 | * Use: Reconstructs a secret, given enough shares. |
170 | */ |
171 | |
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172 | extern void gfshare_combine(gfshare */*s*/, void */*buf*/); |
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173 | |
174 | /*----- That's all, folks -------------------------------------------------*/ |
175 | |
176 | #ifdef __cplusplus |
177 | } |
178 | #endif |
179 | |
180 | #endif |