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1 | /* -*-c-*- |
2 | * |
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3 | * $Id: share.h,v 1.3 2000/12/06 20:30:10 mdw Exp $ |
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4 | * |
5 | * Shamir's secret sharing |
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: share.h,v $ |
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33 | * Revision 1.3 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 1.2 2000/06/24 18:29:05 mdw |
38 | * Interface change: allow shares to be extracted from a context on demand, |
39 | * rather than building them all up-front. |
40 | * |
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41 | * Revision 1.1 2000/06/17 12:09:38 mdw |
42 | * Shamir's secret sharing system. |
43 | * |
44 | */ |
45 | |
46 | /*----- Notes on the sharing system ---------------------------------------* |
47 | * |
48 | * Shamir's secret-sharing system is based on polynomial interpolation modulo |
49 | * a prime number. It is `perfect' in that fewer participants than the |
50 | * threshold can derive no information about the secret by pooling their |
51 | * shares, and `ideal' in that the shares are the same size as the secret. |
52 | * |
53 | * This implementation stays close to the definition, in order to support |
54 | * other schemes for (e.g.) threshold cryptography. It is, however, rather |
55 | * slow. |
56 | */ |
57 | |
58 | #ifndef CATACOMB_SHARE_H |
59 | #define CATACOMB_SHARE_H |
60 | |
61 | #ifdef __cplusplus |
62 | extern "C" { |
63 | #endif |
64 | |
65 | /*----- Header files ------------------------------------------------------*/ |
66 | |
67 | #ifndef CATACOMB_GRAND_H |
68 | # include "grand.h" |
69 | #endif |
70 | |
71 | #ifndef CATACOMB_MP_H |
72 | # include "mp.h" |
73 | #endif |
74 | |
75 | /*----- Data structures ---------------------------------------------------*/ |
76 | |
77 | /* --- A secret sharing context --- */ |
78 | |
79 | typedef struct share_pt { |
80 | unsigned x; /* Index of this share */ |
81 | mp *y; /* Payload of this share */ |
82 | } share_pt; |
83 | |
84 | typedef struct share { |
85 | unsigned t; /* Threshold */ |
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86 | unsigned i; /* Next free slot in the vector */ |
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87 | mp *p; /* Modulus for arithmetic */ |
88 | share_pt *v; /* Vector of share information */ |
89 | } share; |
90 | |
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91 | #define SHARE_INIT(t) { t, 0, 0, 0 } |
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92 | |
93 | /*----- Functions provided ------------------------------------------------*/ |
94 | |
95 | /* --- @share_create@ --- * |
96 | * |
97 | * Arguments: @share *s@ = pointer to share context to initialize |
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98 | * @unsigned t@ = threshold for the system |
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99 | * |
100 | * Returns: --- |
101 | * |
102 | * Use: Initializes a sharing context. |
103 | */ |
104 | |
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105 | extern void share_create(share */*s*/, unsigned /*t*/); |
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106 | |
107 | /* --- @share_destroy@ --- * |
108 | * |
109 | * Arguments: @share *s@ = pointer to share context to destroy |
110 | * |
111 | * Returns: --- |
112 | * |
113 | * Use: Disposes of a sharing context. All memory is freed, all |
114 | * integers are dropped. |
115 | */ |
116 | |
117 | extern void share_destroy(share */*s*/); |
118 | |
119 | /* --- @share_mkshares@ --- * |
120 | * |
121 | * Arguments: @share *s@ = pointer to share context to fill in |
122 | * @grand *r@ = pointer to random number source |
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123 | * @mp *n@ = the secret to share |
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124 | * |
125 | * Returns: --- |
126 | * |
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127 | * Use: Initializes a sharing context to be able to create shares. |
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128 | * The context structure is expected to be mostly filled in. In |
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129 | * particular, @t@ must be initialized. If @p@ is zero, a prime |
130 | * number of appropriate size is generated automatically. If |
131 | * @v@ is zero, a vector of appropriate size is allocated. You |
132 | * should use the macro @SHARE_INIT@ or @share_create@ to |
133 | * construct sharing contexts. |
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134 | */ |
135 | |
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136 | extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/); |
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137 | |
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138 | /* --- @share_get@ --- * |
139 | * |
140 | * Arguments: @share *s@ = pointer to share conext |
141 | * @mp *d@ = destination for the share |
142 | * @unsigned x@ = share index to fetch |
143 | * |
144 | * Returns: The share, as requested. |
145 | * |
146 | * Use: Extracts a share from the system. You may extract @MPW_MAX@ |
147 | * shares, or @s->p@ shares from the system, whichever is |
148 | * smaller. Shares are indexed from 0. |
149 | */ |
150 | |
151 | extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/); |
152 | |
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153 | /* --- @share_add@ --- * |
154 | * |
155 | * Arguments: @share *s@ = pointer to sharing context |
156 | * @unsigned x@ = which share number this is |
157 | * @mp *y@ = the share value |
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 prime @p@ and threshold @t@. |
163 | */ |
164 | |
165 | extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/); |
166 | |
167 | /* --- @share_combine@ --- * |
168 | * |
169 | * Arguments: @share *s@ = pointer to share context |
170 | * |
171 | * Returns: The secret, as a multiprecision integer. |
172 | * |
173 | * Use: Reconstructs a secret, given enough shares. |
174 | */ |
175 | |
176 | extern mp *share_combine(share */*s*/); |
177 | |
178 | /*----- That's all, folks -------------------------------------------------*/ |
179 | |
180 | #ifdef __cplusplus |
181 | } |
182 | #endif |
183 | |
184 | #endif |