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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: gfshare.h,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.h,v $ |
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33 | * Revision 1.5 2000/06/24 19:11:47 mdw |
34 | * Fix daft error in the comment for @gfshare_get@. |
35 | * |
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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 | * |
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40 | * Revision 1.3 2000/06/18 23:12:15 mdw |
41 | * Change typesetting of Galois Field names. |
42 | * |
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43 | * Revision 1.2 2000/06/17 11:05:27 mdw |
44 | * Add a commentary on the system. |
45 | * |
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46 | * Revision 1.1 2000/06/17 10:56:30 mdw |
47 | * Fast but nonstandard secret sharing system. |
48 | * |
49 | */ |
50 | |
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51 | /*----- Notes on the system -----------------------------------------------* |
52 | * |
53 | * This uses a variant of Shamir's secret sharing system. Shamir's original |
54 | * system used polynomials modulo a large prime. This implementation instead |
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55 | * uses the field %$\gf{2^8}$%, represented by |
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56 | * |
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57 | * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$% |
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58 | * |
59 | * and shares each byte of the secret independently. It is therefore limited |
60 | * to 255 players, although this probably isn't a serious limitation in |
61 | * practice. |
62 | * |
63 | * Share creation and reconstruction is extremely efficient. Contrast the |
64 | * performance of the straightforward implementation based on multiprecision |
65 | * arithmetic. |
66 | */ |
67 | |
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68 | #ifndef CATACOMB_GFSHARE_H |
69 | #define CATACOMB_GFSHARE_H |
70 | |
71 | #ifdef __cplusplus |
72 | extern "C" { |
73 | #endif |
74 | |
75 | /*----- Header files ------------------------------------------------------*/ |
76 | |
77 | #include <mLib/bits.h> |
78 | |
79 | #ifndef CATACOMB_GRAND_H |
80 | # include "grand.h" |
81 | #endif |
82 | |
83 | /*----- Data structures ---------------------------------------------------*/ |
84 | |
85 | /* --- A secret sharing context --- */ |
86 | |
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87 | typedef struct gfshare { |
88 | unsigned t; /* Threshold */ |
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89 | unsigned i; /* Next free slot in vector */ |
90 | size_t sz; /* Size of the secret and shares */ |
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91 | void *s; /* The secret */ |
92 | octet *v; /* Vector of share information */ |
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93 | } gfshare; |
94 | |
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95 | #define GFSHARE_INIT(t, sz) { t, 0, sz, 0, 0 } |
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96 | |
97 | /*----- Functions provided ------------------------------------------------*/ |
98 | |
99 | /* --- @gfshare_create@ --- * |
100 | * |
101 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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102 | * @unsigned t@ = threshold for the system |
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103 | * @size_t sz@ = size of the secret |
104 | * |
105 | * Returns: --- |
106 | * |
107 | * Use: Initializes a sharing context. |
108 | */ |
109 | |
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110 | extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/); |
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111 | |
112 | /* --- @gfshare_destroy@ --- * |
113 | * |
114 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
115 | * |
116 | * Returns: --- |
117 | * |
118 | * Use: Disposes of a sharing context. The allocations for the |
119 | * individual shares and the vector @v@ are freed; the secret is |
120 | * left alone. |
121 | */ |
122 | |
123 | extern void gfshare_destroy(gfshare */*s*/); |
124 | |
125 | /* --- @gfshare_mkshares@ --- * |
126 | * |
127 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
128 | * @grand *r@ = pointer to random number source |
129 | * |
130 | * Returns: --- |
131 | * |
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132 | * Use: Initializes a sharing context to be able to create shares. |
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133 | * The context structure is expected to be mostly filled in. In |
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134 | * particular, @t@ and @s@ must be initialized. If @v@ is zero, |
135 | * a vector of appropriate size is allocated. You should use |
136 | * the macro @GFSHARE_INIT@ or @gfshare_create@ to construct |
137 | * sharing contexts. |
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138 | */ |
139 | |
140 | extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/); |
141 | |
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142 | /* --- @gfshare_get@ --- * |
143 | * |
144 | * Arguments: @gfshare *s@ = pointer to share conext |
145 | * @unsigned x@ = share index to fetch |
146 | * @void *buf@ = pointer to output buffer |
147 | * |
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148 | * Returns: --- |
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149 | * |
150 | * Use: Extracts a share from the system. You may extract up to 255 |
151 | * shares from the system. Shares are indexed from 0. |
152 | */ |
153 | |
154 | extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/); |
155 | |
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156 | /* --- @gfshare_add@ --- * |
157 | * |
158 | * Arguments: @gfshare *s@ = pointer to sharing context |
159 | * @unsigned x@ = which share number this is |
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160 | * @const void *y@ = the share value |
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161 | * |
162 | * Returns: Number of shares required before recovery may be performed. |
163 | * |
164 | * Use: Adds a share to the context. The context must have been |
165 | * initialized with the correct threshold @t@. |
166 | */ |
167 | |
168 | extern unsigned gfshare_add(gfshare */*s*/, |
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169 | unsigned /*x*/, const void */*y*/); |
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170 | |
171 | /* --- @gfshare_combine@ --- * |
172 | * |
173 | * Arguments: @gfshare *s@ = pointer to share context |
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174 | * @void *buf@ = pointer to output buffer for the secret |
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175 | * |
176 | * Returns: --- |
177 | * |
178 | * Use: Reconstructs a secret, given enough shares. |
179 | */ |
180 | |
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181 | extern void gfshare_combine(gfshare */*s*/, void */*buf*/); |
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182 | |
183 | /*----- That's all, folks -------------------------------------------------*/ |
184 | |
185 | #ifdef __cplusplus |
186 | } |
187 | #endif |
188 | |
189 | #endif |