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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: gfshare.h,v 1.4 2000/06/24 18:29:05 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 1.4 2000/06/24 18:29:05 mdw |
34 | * Interface change: allow shares to be extracted from a context on demand, |
35 | * rather than building them all up-front. |
36 | * |
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37 | * Revision 1.3 2000/06/18 23:12:15 mdw |
38 | * Change typesetting of Galois Field names. |
39 | * |
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40 | * Revision 1.2 2000/06/17 11:05:27 mdw |
41 | * Add a commentary on the system. |
42 | * |
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43 | * Revision 1.1 2000/06/17 10:56:30 mdw |
44 | * Fast but nonstandard secret sharing system. |
45 | * |
46 | */ |
47 | |
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48 | /*----- Notes on the system -----------------------------------------------* |
49 | * |
50 | * This uses a variant of Shamir's secret sharing system. Shamir's original |
51 | * system used polynomials modulo a large prime. This implementation instead |
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52 | * uses the field %$\gf{2^8}$%, represented by |
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53 | * |
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54 | * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$% |
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55 | * |
56 | * and shares each byte of the secret independently. It is therefore limited |
57 | * to 255 players, although this probably isn't a serious limitation in |
58 | * practice. |
59 | * |
60 | * Share creation and reconstruction is extremely efficient. Contrast the |
61 | * performance of the straightforward implementation based on multiprecision |
62 | * arithmetic. |
63 | */ |
64 | |
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65 | #ifndef CATACOMB_GFSHARE_H |
66 | #define CATACOMB_GFSHARE_H |
67 | |
68 | #ifdef __cplusplus |
69 | extern "C" { |
70 | #endif |
71 | |
72 | /*----- Header files ------------------------------------------------------*/ |
73 | |
74 | #include <mLib/bits.h> |
75 | |
76 | #ifndef CATACOMB_GRAND_H |
77 | # include "grand.h" |
78 | #endif |
79 | |
80 | /*----- Data structures ---------------------------------------------------*/ |
81 | |
82 | /* --- A secret sharing context --- */ |
83 | |
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84 | typedef struct gfshare { |
85 | unsigned t; /* Threshold */ |
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86 | unsigned i; /* Next free slot in vector */ |
87 | size_t sz; /* Size of the secret and shares */ |
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88 | void *s; /* The secret */ |
89 | octet *v; /* Vector of share information */ |
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90 | } gfshare; |
91 | |
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92 | #define GFSHARE_INIT(t, sz) { t, 0, sz, 0, 0 } |
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93 | |
94 | /*----- Functions provided ------------------------------------------------*/ |
95 | |
96 | /* --- @gfshare_create@ --- * |
97 | * |
98 | * Arguments: @gfshare *s@ = pointer to share context to initialize |
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99 | * @unsigned t@ = threshold for the system |
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100 | * @size_t sz@ = size of the secret |
101 | * |
102 | * Returns: --- |
103 | * |
104 | * Use: Initializes a sharing context. |
105 | */ |
106 | |
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107 | extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/); |
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108 | |
109 | /* --- @gfshare_destroy@ --- * |
110 | * |
111 | * Arguments: @gfshare *s@ = pointer to share context to destroy |
112 | * |
113 | * Returns: --- |
114 | * |
115 | * Use: Disposes of a sharing context. The allocations for the |
116 | * individual shares and the vector @v@ are freed; the secret is |
117 | * left alone. |
118 | */ |
119 | |
120 | extern void gfshare_destroy(gfshare */*s*/); |
121 | |
122 | /* --- @gfshare_mkshares@ --- * |
123 | * |
124 | * Arguments: @gfshare *s@ = pointer to share context to fill in |
125 | * @grand *r@ = pointer to random number source |
126 | * |
127 | * Returns: --- |
128 | * |
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129 | * Use: Initializes a sharing context to be able to create shares. |
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130 | * The context structure is expected to be mostly filled in. In |
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131 | * particular, @t@ and @s@ must be initialized. If @v@ is zero, |
132 | * a vector of appropriate size is allocated. You should use |
133 | * the macro @GFSHARE_INIT@ or @gfshare_create@ to construct |
134 | * sharing contexts. |
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135 | */ |
136 | |
137 | extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/); |
138 | |
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139 | /* --- @gfshare_get@ --- * |
140 | * |
141 | * Arguments: @gfshare *s@ = pointer to share conext |
142 | * @unsigned x@ = share index to fetch |
143 | * @void *buf@ = pointer to output buffer |
144 | * |
145 | * Returns: The share, as requested. |
146 | * |
147 | * Use: Extracts a share from the system. You may extract up to 255 |
148 | * shares from the system. Shares are indexed from 0. |
149 | */ |
150 | |
151 | extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/); |
152 | |
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153 | /* --- @gfshare_add@ --- * |
154 | * |
155 | * Arguments: @gfshare *s@ = pointer to sharing context |
156 | * @unsigned x@ = which share number this is |
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157 | * @const void *y@ = the share value |
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158 | * |
159 | * Returns: Number of shares required before recovery may be performed. |
160 | * |
161 | * Use: Adds a share to the context. The context must have been |
162 | * initialized with the correct threshold @t@. |
163 | */ |
164 | |
165 | extern unsigned gfshare_add(gfshare */*s*/, |
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166 | unsigned /*x*/, const void */*y*/); |
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167 | |
168 | /* --- @gfshare_combine@ --- * |
169 | * |
170 | * Arguments: @gfshare *s@ = pointer to share context |
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171 | * @void *buf@ = pointer to output buffer for the secret |
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172 | * |
173 | * Returns: --- |
174 | * |
175 | * Use: Reconstructs a secret, given enough shares. |
176 | */ |
177 | |
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178 | extern void gfshare_combine(gfshare */*s*/, void */*buf*/); |
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179 | |
180 | /*----- That's all, folks -------------------------------------------------*/ |
181 | |
182 | #ifdef __cplusplus |
183 | } |
184 | #endif |
185 | |
186 | #endif |