3 * Secret sharing over %$\gf{2^8}$%
5 * (c) 2000 Straylight/Edgeware
8 /*----- Licensing notice --------------------------------------------------*
10 * This file is part of Catacomb.
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.
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.
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,
28 /*----- Notes on the system -----------------------------------------------*
30 * This uses a variant of Shamir's secret sharing system. Shamir's original
31 * system used polynomials modulo a large prime. This implementation instead
32 * uses the field %$\gf{2^8}$%, represented by
34 * %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
36 * and shares each byte of the secret independently. It is therefore limited
37 * to 255 players, although this probably isn't a serious limitation in
40 * Share creation and reconstruction is extremely efficient. Contrast the
41 * performance of the straightforward implementation based on multiprecision
45 #ifndef CATACOMB_GFSHARE_H
46 #define CATACOMB_GFSHARE_H
52 /*----- Header files ------------------------------------------------------*/
54 #include <mLib/bits.h>
56 #ifndef CATACOMB_GRAND_H
60 /*----- Data structures ---------------------------------------------------*/
62 /* --- A secret sharing context --- */
64 typedef struct gfshare {
65 unsigned t; /* Threshold */
66 unsigned i; /* Next free slot in vector */
67 size_t sz; /* Size of the secret and shares */
68 octet *v; /* Vector of share information */
71 #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 }
73 #define GFSHARE_INDEX(s, i) ((s)->v[(i) * ((s)->sz + 1)])
75 /*----- Functions provided ------------------------------------------------*/
77 /* --- @gfshare_create@ --- *
79 * Arguments: @gfshare *s@ = pointer to share context to initialize
80 * @unsigned t@ = threshold for the system
81 * @size_t sz@ = size of the secret
85 * Use: Initializes a sharing context.
88 extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/);
90 /* --- @gfshare_destroy@ --- *
92 * Arguments: @gfshare *s@ = pointer to share context to destroy
96 * Use: Disposes of a sharing context. The allocations for the
97 * individual shares and the vector @v@ are freed; the secret is
101 extern void gfshare_destroy(gfshare */*s*/);
103 /* --- @gfshare_mkshares@ --- *
105 * Arguments: @gfshare *s@ = pointer to share context to fill in
106 * @grand *r@ = pointer to random number source
107 * @const void *buf@ = pointer to the secret to share
111 * Use: Initializes a sharing context to be able to create shares.
112 * The context structure is expected to be mostly filled in. In
113 * particular, @t@ must be initialized. If @v@ is zero, a
114 * vector of appropriate size is allocated. You should use the
115 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
119 extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/,
120 const void */*buf*/);
122 /* --- @gfshare_get@ --- *
124 * Arguments: @gfshare *s@ = pointer to share conext
125 * @unsigned x@ = share index to fetch
126 * @void *buf@ = pointer to output buffer
130 * Use: Extracts a share from the system. You may extract up to 255
131 * shares from the system. Shares are indexed from 0.
134 extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/);
136 /* --- @gfshare_addedp@ --- *
138 * Arguments: @gfshare *s@ = pointer to sharing context
139 * @unsigned x@ = which share number to check
141 * Returns: Nonzero if share @x@ has been added already, zero if it
145 extern int gfshare_addedp(gfshare */*s*/, unsigned /*x*/);
147 /* --- @gfshare_add@ --- *
149 * Arguments: @gfshare *s@ = pointer to sharing context
150 * @unsigned x@ = which share number this is
151 * @const void *y@ = the share value
153 * Returns: Number of shares required before recovery may be performed.
155 * Use: Adds a share to the context. The context must have been
156 * initialized with the correct threshold @t@.
159 extern unsigned gfshare_add(gfshare */*s*/,
160 unsigned /*x*/, const void */*y*/);
162 /* --- @gfshare_combine@ --- *
164 * Arguments: @gfshare *s@ = pointer to share context
165 * @void *buf@ = pointer to output buffer for the secret
169 * Use: Reconstructs a secret, given enough shares.
172 extern void gfshare_combine(gfshare */*s*/, void */*buf*/);
174 /*----- That's all, folks -------------------------------------------------*/