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1 | /* -*-c-*- |
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2 | * |
3 | * Shamir's secret sharing |
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 sharing system ---------------------------------------* |
29 | * |
30 | * Shamir's secret-sharing system is based on polynomial interpolation modulo |
31 | * a prime number. It is `perfect' in that fewer participants than the |
32 | * threshold can derive no information about the secret by pooling their |
33 | * shares, and `ideal' in that the shares are the same size as the secret. |
34 | * |
35 | * This implementation stays close to the definition, in order to support |
36 | * other schemes for (e.g.) threshold cryptography. It is, however, rather |
37 | * slow. |
38 | */ |
39 | |
40 | #ifndef CATACOMB_SHARE_H |
41 | #define CATACOMB_SHARE_H |
42 | |
43 | #ifdef __cplusplus |
44 | extern "C" { |
45 | #endif |
46 | |
47 | /*----- Header files ------------------------------------------------------*/ |
48 | |
49 | #ifndef CATACOMB_GRAND_H |
50 | # include "grand.h" |
51 | #endif |
52 | |
53 | #ifndef CATACOMB_MP_H |
54 | # include "mp.h" |
55 | #endif |
56 | |
57 | /*----- Data structures ---------------------------------------------------*/ |
58 | |
59 | /* --- A secret sharing context --- */ |
60 | |
61 | typedef struct share_pt { |
62 | unsigned x; /* Index of this share */ |
63 | mp *y; /* Payload of this share */ |
64 | } share_pt; |
65 | |
66 | typedef struct share { |
67 | unsigned t; /* Threshold */ |
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68 | unsigned i; /* Next free slot in the vector */ |
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69 | mp *p; /* Modulus for arithmetic */ |
70 | share_pt *v; /* Vector of share information */ |
71 | } share; |
72 | |
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73 | #define SHARE_INIT(t) { t, 0, 0, 0 } |
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74 | |
75 | /*----- Functions provided ------------------------------------------------*/ |
76 | |
77 | /* --- @share_create@ --- * |
78 | * |
79 | * Arguments: @share *s@ = pointer to share context to initialize |
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80 | * @unsigned t@ = threshold for the system |
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81 | * |
82 | * Returns: --- |
83 | * |
84 | * Use: Initializes a sharing context. |
85 | */ |
86 | |
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87 | extern void share_create(share */*s*/, unsigned /*t*/); |
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88 | |
89 | /* --- @share_destroy@ --- * |
90 | * |
91 | * Arguments: @share *s@ = pointer to share context to destroy |
92 | * |
93 | * Returns: --- |
94 | * |
95 | * Use: Disposes of a sharing context. All memory is freed, all |
96 | * integers are dropped. |
97 | */ |
98 | |
99 | extern void share_destroy(share */*s*/); |
100 | |
101 | /* --- @share_mkshares@ --- * |
102 | * |
103 | * Arguments: @share *s@ = pointer to share context to fill in |
104 | * @grand *r@ = pointer to random number source |
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105 | * @mp *n@ = the secret to share |
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106 | * |
107 | * Returns: --- |
108 | * |
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109 | * Use: Initializes a sharing context to be able to create shares. |
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110 | * The context structure is expected to be mostly filled in. In |
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111 | * particular, @t@ must be initialized. If @p@ is zero, a prime |
112 | * number of appropriate size is generated automatically. If |
113 | * @v@ is zero, a vector of appropriate size is allocated. You |
114 | * should use the macro @SHARE_INIT@ or @share_create@ to |
115 | * construct sharing contexts. |
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116 | */ |
117 | |
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118 | extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/); |
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119 | |
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120 | /* --- @share_get@ --- * |
121 | * |
122 | * Arguments: @share *s@ = pointer to share conext |
123 | * @mp *d@ = destination for the share |
124 | * @unsigned x@ = share index to fetch |
125 | * |
126 | * Returns: The share, as requested. |
127 | * |
128 | * Use: Extracts a share from the system. You may extract @MPW_MAX@ |
129 | * shares, or @s->p@ shares from the system, whichever is |
130 | * smaller. Shares are indexed from 0. |
131 | */ |
132 | |
133 | extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/); |
134 | |
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135 | /* --- @share_addedp@ --- * |
136 | * |
137 | * Arguments: @share *s@ = pointer to sharing context |
138 | * @unsigned x@ = which share number to check |
139 | * |
140 | * Returns: Nonzero if share @x@ has been added already, zero if it |
141 | * hasn't. |
142 | */ |
143 | |
144 | extern int share_addedp(share */*s*/, unsigned /*x*/); |
145 | |
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146 | /* --- @share_add@ --- * |
147 | * |
148 | * Arguments: @share *s@ = pointer to sharing context |
149 | * @unsigned x@ = which share number this is |
150 | * @mp *y@ = the share value |
151 | * |
152 | * Returns: Number of shares required before recovery may be performed. |
153 | * |
154 | * Use: Adds a share to the context. The context must have been |
155 | * initialized with the correct prime @p@ and threshold @t@. |
156 | */ |
157 | |
158 | extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/); |
159 | |
160 | /* --- @share_combine@ --- * |
161 | * |
162 | * Arguments: @share *s@ = pointer to share context |
163 | * |
164 | * Returns: The secret, as a multiprecision integer. |
165 | * |
166 | * Use: Reconstructs a secret, given enough shares. |
167 | */ |
168 | |
169 | extern mp *share_combine(share */*s*/); |
170 | |
171 | /*----- That's all, folks -------------------------------------------------*/ |
172 | |
173 | #ifdef __cplusplus |
174 | } |
175 | #endif |
176 | |
177 | #endif |