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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: gfshare.h,v 1.6 2000/12/06 20:30:10 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 1.6 2000/12/06 20:30:10 mdw |
34 | * Change secret sharing interface: present the secret at share |
35 | * construction time. |
36 | * |
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37 | * Revision 1.5 2000/06/24 19:11:47 mdw |
38 | * Fix daft error in the comment for @gfshare_get@. |
39 | * |
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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 | * |
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44 | * Revision 1.3 2000/06/18 23:12:15 mdw |
45 | * Change typesetting of Galois Field names. |
46 | * |
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47 | * Revision 1.2 2000/06/17 11:05:27 mdw |
48 | * Add a commentary on the system. |
49 | * |
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50 | * Revision 1.1 2000/06/17 10:56:30 mdw |
51 | * Fast but nonstandard secret sharing system. |
52 | * |
53 | */ |
54 | |
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55 | /*----- Notes on the system -----------------------------------------------* |
56 | * |
57 | * This uses a variant of Shamir's secret sharing system. Shamir's original |
58 | * system used polynomials modulo a large prime. This implementation instead |
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59 | * uses the field %$\gf{2^8}$%, represented by |
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60 | * |
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61 | * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$% |
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62 | * |
63 | * and shares each byte of the secret independently. It is therefore limited |
64 | * to 255 players, although this probably isn't a serious limitation in |
65 | * practice. |
66 | * |
67 | * Share creation and reconstruction is extremely efficient. Contrast the |
68 | * performance of the straightforward implementation based on multiprecision |
69 | * arithmetic. |
70 | */ |
71 | |
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72 | #ifndef CATACOMB_GFSHARE_H |
73 | #define CATACOMB_GFSHARE_H |
74 | |
75 | #ifdef __cplusplus |
76 | extern "C" { |
77 | #endif |
78 | |
79 | /*----- Header files ------------------------------------------------------*/ |
80 | |
81 | #include <mLib/bits.h> |
82 | |
83 | #ifndef CATACOMB_GRAND_H |
84 | # include "grand.h" |
85 | #endif |
86 | |
87 | /*----- Data structures ---------------------------------------------------*/ |
88 | |
89 | /* --- A secret sharing context --- */ |
90 | |
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91 | typedef struct gfshare { |
92 | unsigned t; /* Threshold */ |
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93 | unsigned i; /* Next free slot in vector */ |
94 | size_t sz; /* Size of the secret and shares */ |
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95 | octet *v; /* Vector of share information */ |
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96 | } gfshare; |
97 | |
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98 | #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 } |
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99 | |
100 | /*----- Functions provided ------------------------------------------------*/ |
101 | |
102 | /* --- @gfshare_create@ --- * |
103 | * |
104 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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105 | * @unsigned t@ = threshold for the system |
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106 | * @size_t sz@ = size of the secret |
107 | * |
108 | * Returns: --- |
109 | * |
110 | * Use: Initializes a sharing context. |
111 | */ |
112 | |
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113 | extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/); |
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114 | |
115 | /* --- @gfshare_destroy@ --- * |
116 | * |
117 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
118 | * |
119 | * Returns: --- |
120 | * |
121 | * Use: Disposes of a sharing context. The allocations for the |
122 | * individual shares and the vector @v@ are freed; the secret is |
123 | * left alone. |
124 | */ |
125 | |
126 | extern void gfshare_destroy(gfshare */*s*/); |
127 | |
128 | /* --- @gfshare_mkshares@ --- * |
129 | * |
130 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
131 | * @grand *r@ = pointer to random number source |
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132 | * @const void *buf@ = pointer to the secret to share |
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133 | * |
134 | * Returns: --- |
135 | * |
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136 | * Use: Initializes a sharing context to be able to create shares. |
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137 | * The context structure is expected to be mostly filled in. In |
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138 | * particular, @t@ must be initialized. If @v@ is zero, a |
139 | * vector of appropriate size is allocated. You should use the |
140 | * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing |
141 | * contexts. |
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142 | */ |
143 | |
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144 | extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/, |
145 | const void */*buf*/); |
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146 | |
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147 | /* --- @gfshare_get@ --- * |
148 | * |
149 | * Arguments: @gfshare *s@ = pointer to share conext |
150 | * @unsigned x@ = share index to fetch |
151 | * @void *buf@ = pointer to output buffer |
152 | * |
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153 | * Returns: --- |
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154 | * |
155 | * Use: Extracts a share from the system. You may extract up to 255 |
156 | * shares from the system. Shares are indexed from 0. |
157 | */ |
158 | |
159 | extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/); |
160 | |
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161 | /* --- @gfshare_add@ --- * |
162 | * |
163 | * Arguments: @gfshare *s@ = pointer to sharing context |
164 | * @unsigned x@ = which share number this is |
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165 | * @const void *y@ = the share value |
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166 | * |
167 | * Returns: Number of shares required before recovery may be performed. |
168 | * |
169 | * Use: Adds a share to the context. The context must have been |
170 | * initialized with the correct threshold @t@. |
171 | */ |
172 | |
173 | extern unsigned gfshare_add(gfshare */*s*/, |
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174 | unsigned /*x*/, const void */*y*/); |
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175 | |
176 | /* --- @gfshare_combine@ --- * |
177 | * |
178 | * Arguments: @gfshare *s@ = pointer to share context |
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179 | * @void *buf@ = pointer to output buffer for the secret |
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180 | * |
181 | * Returns: --- |
182 | * |
183 | * Use: Reconstructs a secret, given enough shares. |
184 | */ |
185 | |
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186 | extern void gfshare_combine(gfshare */*s*/, void */*buf*/); |
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187 | |
188 | /*----- That's all, folks -------------------------------------------------*/ |
189 | |
190 | #ifdef __cplusplus |
191 | } |
192 | #endif |
193 | |
194 | #endif |