chiark / gitweb /
utils/split-pieces (QfConvert): Construct an instance of the right class.
[catacomb] / misc / share.h
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71dfe576 1/* -*-c-*-
71dfe576 2 *
3 * Shamir's secret sharing
4 *
5 * (c) 2000 Straylight/Edgeware
6 */
7
45c0fd36 8/*----- Licensing notice --------------------------------------------------*
71dfe576 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.
45c0fd36 16 *
71dfe576 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.
45c0fd36 21 *
71dfe576 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
71dfe576 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
61typedef struct share_pt {
62 unsigned x; /* Index of this share */
63 mp *y; /* Payload of this share */
64} share_pt;
65
66typedef struct share {
67 unsigned t; /* Threshold */
71dfe576 68 unsigned i; /* Next free slot in the vector */
71dfe576 69 mp *p; /* Modulus for arithmetic */
70 share_pt *v; /* Vector of share information */
71} share;
72
5d4fee2a 73#define SHARE_INIT(t) { t, 0, 0, 0 }
71dfe576 74
75/*----- Functions provided ------------------------------------------------*/
76
77/* --- @share_create@ --- *
78 *
79 * Arguments: @share *s@ = pointer to share context to initialize
3d64a35c 80 * @unsigned t@ = threshold for the system
71dfe576 81 *
82 * Returns: ---
83 *
84 * Use: Initializes a sharing context.
85 */
86
3d64a35c 87extern void share_create(share */*s*/, unsigned /*t*/);
71dfe576 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
99extern 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
5d4fee2a 105 * @mp *n@ = the secret to share
71dfe576 106 *
107 * Returns: ---
108 *
3d64a35c 109 * Use: Initializes a sharing context to be able to create shares.
71dfe576 110 * The context structure is expected to be mostly filled in. In
5d4fee2a 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.
71dfe576 116 */
117
5d4fee2a 118extern void share_mkshares(share */*s*/, grand */*r*/, mp */*n*/);
71dfe576 119
3d64a35c 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
133extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/);
134
aec42286 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
144extern int share_addedp(share */*s*/, unsigned /*x*/);
145
71dfe576 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
158extern 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
169extern mp *share_combine(share */*s*/);
170
171/*----- That's all, folks -------------------------------------------------*/
172
173#ifdef __cplusplus
174 }
175#endif
176
177#endif