3 * $Id: gfshare.h,v 1.6 2000/12/06 20:30:10 mdw Exp $
5 * Secret sharing over %$\gf{2^8}$%
7 * (c) 2000 Straylight/Edgeware
10 /*----- Licensing notice --------------------------------------------------*
12 * This file is part of Catacomb.
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.
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.
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,
30 /*----- Revision history --------------------------------------------------*
33 * Revision 1.6 2000/12/06 20:30:10 mdw
34 * Change secret sharing interface: present the secret at share
37 * Revision 1.5 2000/06/24 19:11:47 mdw
38 * Fix daft error in the comment for @gfshare_get@.
40 * Revision 1.4 2000/06/24 18:29:05 mdw
41 * Interface change: allow shares to be extracted from a context on demand,
42 * rather than building them all up-front.
44 * Revision 1.3 2000/06/18 23:12:15 mdw
45 * Change typesetting of Galois Field names.
47 * Revision 1.2 2000/06/17 11:05:27 mdw
48 * Add a commentary on the system.
50 * Revision 1.1 2000/06/17 10:56:30 mdw
51 * Fast but nonstandard secret sharing system.
55 /*----- Notes on the system -----------------------------------------------*
57 * This uses a variant of Shamir's secret sharing system. Shamir's original
58 * system used polynomials modulo a large prime. This implementation instead
59 * uses the field %$\gf{2^8}$%, represented by
61 * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
63 * and shares each byte of the secret independently. It is therefore limited
64 * to 255 players, although this probably isn't a serious limitation in
67 * Share creation and reconstruction is extremely efficient. Contrast the
68 * performance of the straightforward implementation based on multiprecision
72 #ifndef CATACOMB_GFSHARE_H
73 #define CATACOMB_GFSHARE_H
79 /*----- Header files ------------------------------------------------------*/
81 #include <mLib/bits.h>
83 #ifndef CATACOMB_GRAND_H
87 /*----- Data structures ---------------------------------------------------*/
89 /* --- A secret sharing context --- */
91 typedef struct gfshare
{
92 unsigned t
; /* Threshold */
93 unsigned i
; /* Next free slot in vector */
94 size_t sz
; /* Size of the secret and shares */
95 octet
*v
; /* Vector of share information */
98 #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 }
100 /*----- Functions provided ------------------------------------------------*/
102 /* --- @gfshare_create@ --- *
104 * Arguments: @gfshare *s@ = pointer to share context to initialize
105 * @unsigned t@ = threshold for the system
106 * @size_t sz@ = size of the secret
110 * Use: Initializes a sharing context.
113 extern void gfshare_create(gfshare */
*s*/
, unsigned /*t*/, size_t /*sz*/);
115 /* --- @gfshare_destroy@ --- *
117 * Arguments: @gfshare *s@ = pointer to share context to destroy
121 * Use: Disposes of a sharing context. The allocations for the
122 * individual shares and the vector @v@ are freed; the secret is
126 extern void gfshare_destroy(gfshare */
*s*/
);
128 /* --- @gfshare_mkshares@ --- *
130 * Arguments: @gfshare *s@ = pointer to share context to fill in
131 * @grand *r@ = pointer to random number source
132 * @const void *buf@ = pointer to the secret to share
136 * Use: Initializes a sharing context to be able to create shares.
137 * The context structure is expected to be mostly filled in. In
138 * particular, @t@ must be initialized. If @v@ is zero, a
139 * vector of appropriate size is allocated. You should use the
140 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
144 extern void gfshare_mkshares(gfshare */
*s*/
, grand */
*r*/
,
145 const void */
*buf*/
);
147 /* --- @gfshare_get@ --- *
149 * Arguments: @gfshare *s@ = pointer to share conext
150 * @unsigned x@ = share index to fetch
151 * @void *buf@ = pointer to output buffer
155 * Use: Extracts a share from the system. You may extract up to 255
156 * shares from the system. Shares are indexed from 0.
159 extern void gfshare_get(gfshare */
*s*/
, unsigned /*x*/, void */
*buf*/
);
161 /* --- @gfshare_add@ --- *
163 * Arguments: @gfshare *s@ = pointer to sharing context
164 * @unsigned x@ = which share number this is
165 * @const void *y@ = the share value
167 * Returns: Number of shares required before recovery may be performed.
169 * Use: Adds a share to the context. The context must have been
170 * initialized with the correct threshold @t@.
173 extern unsigned gfshare_add(gfshare */
*s*/
,
174 unsigned /*x*/, const void */
*y*/
);
176 /* --- @gfshare_combine@ --- *
178 * Arguments: @gfshare *s@ = pointer to share context
179 * @void *buf@ = pointer to output buffer for the secret
183 * Use: Reconstructs a secret, given enough shares.
186 extern void gfshare_combine(gfshare */
*s*/
, void */
*buf*/
);
188 /*----- That's all, folks -------------------------------------------------*/