3 * $Id: gfshare.c,v 1.7 2001/06/16 23:42:17 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 1.7 2001/06/16 23:42:17 mdw
36 * Revision 1.6 2000/12/06 20:30:10 mdw
37 * Change secret sharing interface: present the secret at share
40 * Revision 1.5 2000/06/24 19:11:47 mdw
41 * Fix daft error in the comment for @gfshare_get@.
43 * Revision 1.4 2000/06/24 18:29:05 mdw
44 * Interface change: allow shares to be extracted from a context on demand,
45 * rather than building them all up-front.
47 * Revision 1.3 2000/06/22 18:04:13 mdw
48 * Improve secret reconstruction -- compute coefficients as needed rather
49 * than making a big array of them.
51 * Revision 1.2 2000/06/18 23:12:15 mdw
52 * Change typesetting of Galois Field names.
54 * Revision 1.1 2000/06/17 10:56:30 mdw
55 * Fast but nonstandard secret sharing system.
59 /*----- Header files ------------------------------------------------------*/
66 #include <mLib/alloc.h>
67 #include <mLib/bits.h>
71 #include "gfshare-tab.h"
74 /*----- Static variables --------------------------------------------------*/
76 static octet gflog
[] = GFSHARE_LOG
, gfexp
[] = GFSHARE_EXP
;
78 /*----- Main code ---------------------------------------------------------*/
80 /* --- @gfshare_create@ --- *
82 * Arguments: @gfshare *s@ = pointer to share context to initialize
83 * @unsigned t@ = threshold for the system
84 * @size_t sz@ = size of the secret
88 * Use: Initializes a sharing context.
91 void gfshare_create(gfshare
*s
, unsigned t
, size_t sz
)
99 /* --- @gfshare_destroy@ --- *
101 * Arguments: @gfshare *s@ = pointer to share context to destroy
105 * Use: Disposes of a sharing context. The allocations for the
106 * individual shares and the vector @v@ are freed; the secret is
110 void gfshare_destroy(gfshare
*s
)
116 /* --- @gfshare_mkshares@ --- *
118 * Arguments: @gfshare *s@ = pointer to share context to fill in
119 * @grand *r@ = pointer to random number source
120 * @const void *buf@ = pointer to the secret to share
124 * Use: Initializes a sharing context to be able to create shares.
125 * The context structure is expected to be mostly filled in. In
126 * particular, @t@ must be initialized. If @v@ is zero, a
127 * vector of appropriate size is allocated. You should use the
128 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
132 void gfshare_mkshares(gfshare
*s
, grand
*r
, const void *buf
)
134 s
->v
= XS_ALLOC(s
->sz
* s
->t
);
135 r
->ops
->fill(r
, s
->v
, s
->sz
* (s
->t
- 1));
136 memcpy(s
->v
+ s
->sz
* (s
->t
- 1), buf
, s
->sz
);
139 /* --- @gfshare_get@ --- *
141 * Arguments: @gfshare *s@ = pointer to share conext
142 * @unsigned x@ = share index to fetch
143 * @void *buf@ = pointer to output buffer
147 * Use: Extracts a share from the system. You may extract up to 255
148 * shares from the system. Shares are indexed from 0.
151 void gfshare_get(gfshare
*s
, unsigned x
, void *buf
)
154 const octet
*p
= s
->v
;
155 unsigned ilog
= gflog
[x
+ 1];
157 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
159 memcpy(buf
, p
, s
->sz
);
162 for (i
= 1; i
< s
->t
; i
++) {
165 for (k
= 0; k
< s
->sz
; k
++) {
168 qq
= gfexp
[ilog
+ gflog
[qq
]];
174 /* --- @gfshare_add@ --- *
176 * Arguments: @gfshare *s@ = pointer to sharing context
177 * @unsigned x@ = which share number this is
178 * @const void *y@ = the share value
180 * Returns: Number of shares required before recovery may be performed.
182 * Use: Adds a share to the context. The context must have been
183 * initialized with the correct threshold @t@.
186 unsigned gfshare_add(gfshare
*s
, unsigned x
, const void *y
)
190 /* --- If no vector has been allocated, create one --- */
193 s
->v
= XS_ALLOC(s
->t
* (s
->sz
+ 1));
197 assert(((void)"Share context is full", s
->i
< s
->t
));
199 /* --- Store the share in the vector --- */
201 p
= s
->v
+ s
->i
* (s
->sz
+ 1);
208 return (s
->t
- s
->i
);
211 /* --- @gfshare_combine@ --- *
213 * Arguments: @gfshare *s@ = pointer to share context
214 * @void *buf@ = pointer to output buffer for the secret
218 * Use: Reconstructs a secret, given enough shares.
221 void gfshare_combine(gfshare
*s
, void *buf
)
226 /* --- Sanity checking --- */
228 assert(((void)"Not enough shares yet", s
->i
== s
->t
));
230 /* --- Grind through the shares --- */
232 memset(buf
, 0, s
->sz
);
234 for (i
= 0; i
< s
->t
; i
++) {
236 octet
*q
= s
->v
+ i
* (s
->sz
+ 1);
237 unsigned c
= 0, ci
= 0;
239 /* --- Compute the magic coefficient --- */
242 for (j
= 0; j
< s
->t
; j
++) {
245 xj
= s
->v
[j
* (s
->sz
+ 1)];
249 ci
+= gflog
[xi
^ xj
];
257 /* --- Work out another layer of the secret --- */
260 for (j
= 0; j
< s
->sz
; j
++) {
262 *p
^= gfexp
[c
+ gflog
[*q
]];
268 /*----- Test rig ----------------------------------------------------------*/
274 static int verify(grand
*r
)
276 unsigned n
= r
->ops
->range(r
, 16) + 8;
277 unsigned t
= r
->ops
->range(r
, n
- 1) + 1;
278 unsigned len
= r
->ops
->range(r
, 32) + 1;
280 octet
*v
= xmalloc(t
* len
);
281 unsigned *p
= xmalloc(n
* sizeof(unsigned));
282 octet
*sec
= xmalloc(len
), *sbuf
= xmalloc(len
);
288 for (i
= 0; i
< n
; i
++)
290 for (i
= 0; i
< t
; i
++) {
291 unsigned long j
= r
->ops
->range(r
, n
- i
);
297 r
->ops
->fill(r
, sec
, len
);
299 gfshare_create(&s
, t
, len
);
301 gfshare_mkshares(&s
, r
, sec
);
302 for (i
= 0; i
< t
; i
++)
303 gfshare_get(&s
, p
[i
], v
+ (i
* len
));
306 gfshare_create(&s
, t
, len
);
307 for (i
= 0; i
< t
; i
++)
308 gfshare_add(&s
, p
[i
], v
+ (i
* len
));
309 gfshare_combine(&s
, sbuf
);
312 if (memcmp(sec
, sbuf
, len
) != 0) {
314 fprintf(stderr
, "\nbad recombination of shares\n");
328 grand
*r
= fibrand_create(0);
332 fputs("gfshare: ", stdout
);
333 for (i
= 0; i
< 40; i
++) {
341 fputs(" ok\n", stdout
);
343 fputs(" failed\n", stdout
);
344 return (ok ? EXIT_SUCCESS
: EXIT_FAILURE
);
349 /*----- That's all, folks -------------------------------------------------*/