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