3 * $Id: gfshare.c,v 1.3 2000/06/22 18:04:13 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 1.3 2000/06/22 18:04:13 mdw
34 * Improve secret reconstruction -- compute coefficients as needed rather
35 * than making a big array of them.
37 * Revision 1.2 2000/06/18 23:12:15 mdw
38 * Change typesetting of Galois Field names.
40 * Revision 1.1 2000/06/17 10:56:30 mdw
41 * Fast but nonstandard secret sharing system.
45 /*----- Header files ------------------------------------------------------*/
52 #include <mLib/alloc.h>
53 #include <mLib/bits.h>
57 #include "gfshare-tab.h"
60 /*----- Static variables --------------------------------------------------*/
62 static octet gflog
[] = GFSHARE_LOG
, gfexp
[] = GFSHARE_EXP
;
64 /*----- Main code ---------------------------------------------------------*/
66 /* --- @gfshare_create@ --- *
68 * Arguments: @gfshare *s@ = pointer to share context to initialize
69 * @unsigned t, n@ = threshold parameters for the system
70 * @size_t sz@ = size of the secret
74 * Use: Initializes a sharing context.
77 void gfshare_create(gfshare
*s
, unsigned t
, unsigned n
, size_t sz
)
87 /* --- @gfshare_destroy@ --- *
89 * Arguments: @gfshare *s@ = pointer to share context to destroy
93 * Use: Disposes of a sharing context. The allocations for the
94 * individual shares and the vector @v@ are freed; the secret is
98 void gfshare_destroy(gfshare
*s
)
103 /* --- Dispose of the share vector --- */
112 for (i
= 0; i
< n
; i
++) {
120 /* --- @gfshare_mkshares@ --- *
122 * Arguments: @gfshare *s@ = pointer to share context to fill in
123 * @grand *r@ = pointer to random number source
127 * Use: Generates @c->n@ secret shares, such that any @c->t@ of them
128 * may be used to recover the secret.
130 * The context structure is expected to be mostly filled in. In
131 * particular, @t@, @n@, @ssz@ and @s@ must be initialized. If
132 * @v@ is zero, a vector of appropriate size is allocated. You
133 * should use the macro @GFSHARE_INIT@ or @gfshare_create@ to
134 * construct sharing contexts.
137 void gfshare_mkshares(gfshare
*s
, grand
*r
)
142 /* --- Construct the coefficients --- */
144 v
= XS_ALLOC(s
->sz
* s
->t
);
145 r
->ops
->fill(r
, v
, s
->sz
* (s
->t
- 1));
146 memcpy(v
+ s
->sz
* (s
->t
- 1), s
->s
, s
->sz
);
149 /* --- Construct the shares --- */
152 s
->v
= xmalloc(s
->n
* sizeof(gfshare_pt
));
154 for (i
= 0; i
< s
->n
; i
++) {
157 unsigned ilog
= gflog
[i
+ 1];
159 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
161 s
->v
[i
].y
= XS_ALLOC(s
->sz
);
162 memcpy(s
->v
[i
].y
, p
, s
->sz
);
164 for (j
= 1; j
< s
->t
; j
++) {
165 octet
*q
= s
->v
[i
].y
;
167 for (k
= 0; k
< s
->sz
; k
++) {
170 qq
= gfexp
[ilog
+ gflog
[qq
]];
177 /* --- Dispose of various bits of old rubbish --- */
182 /* --- @gfshare_add@ --- *
184 * Arguments: @gfshare *s@ = pointer to sharing context
185 * @unsigned x@ = which share number this is
186 * @const octet *y@ = the share value
188 * Returns: Number of shares required before recovery may be performed.
190 * Use: Adds a share to the context. The context must have been
191 * initialized with the correct threshold @t@.
194 unsigned gfshare_add(gfshare
*s
, unsigned x
, const octet
*y
)
196 /* --- If no vector has been allocated, create one --- */
199 s
->v
= xmalloc(s
->t
* sizeof(gfshare_pt
));
203 assert(((void)"Share context is full", s
->i
< s
->t
));
205 /* --- Store the share in the vector --- */
208 s
->v
[s
->i
].y
= XS_ALLOC(s
->sz
);
209 memcpy(s
->v
[s
->i
].y
, y
, s
->sz
);
214 return (s
->t
- s
->i
);
217 /* --- @gfshare_combine@ --- *
219 * Arguments: @gfshare *s@ = pointer to share context
220 * @octet *buf@ = pointer to output buffer for the secret
224 * Use: Reconstructs a secret, given enough shares.
227 void gfshare_combine(gfshare
*s
, octet
*buf
)
231 /* --- Sanity checking --- */
233 assert(((void)"Not enough shares yet", s
->i
== s
->t
));
235 /* --- Grind through the shares --- */
237 memset(buf
, 0, s
->sz
);
239 for (i
= 0; i
< s
->t
; i
++) {
240 unsigned c
= 0, ci
= 0;
242 /* --- Compute the magic coefficient --- */
244 for (j
= 0; j
< s
->t
; j
++) {
247 c
+= gflog
[s
->v
[j
].x
+ 1];
250 ci
+= gflog
[(s
->v
[i
].x
+ 1) ^ (s
->v
[j
].x
+ 1)];
258 /* --- Work out another layer of the secret --- */
260 for (j
= 0; j
< s
->sz
; j
++) {
262 buf
[j
] ^= gfexp
[c
+ gflog
[s
->v
[i
].y
[j
]]];
267 /*----- Test rig ----------------------------------------------------------*/
273 static int verify(grand
*r
)
275 unsigned n
= r
->ops
->range(r
, 16) + 8;
276 unsigned t
= r
->ops
->range(r
, n
- 1) + 1;
277 unsigned len
= r
->ops
->range(r
, 32) + 1;
279 gfshare_pt
*v
= xmalloc(t
* sizeof(gfshare_pt
));
280 unsigned *p
= xmalloc(n
* sizeof(unsigned));
281 octet
*sec
= xmalloc(len
), *sbuf
= xmalloc(len
);
287 for (i
= 0; i
< n
; i
++)
289 for (i
= 0; i
< t
; i
++) {
290 unsigned long j
= r
->ops
->range(r
, n
- i
);
296 r
->ops
->fill(r
, sec
, len
);
298 gfshare_create(&s
, t
, n
, len
);
301 gfshare_mkshares(&s
, r
);
302 for (i
= 0; i
< t
; i
++) {
303 v
[i
].x
= s
.v
[p
[i
]].x
;
304 v
[i
].y
= xmalloc(len
);
305 memcpy(v
[i
].y
, s
.v
[p
[i
]].y
, len
);
309 gfshare_create(&s
, t
, n
, len
);
310 for (i
= 0; i
< t
; i
++) {
311 gfshare_add(&s
, v
[i
].x
, v
[i
].y
);
313 gfshare_combine(&s
, sbuf
);
316 if (memcmp(sec
, sbuf
, len
) != 0) {
318 fprintf(stderr
, "\nbad recombination of shares\n");
324 for (i
= 0; i
< t
; i
++)
334 grand
*r
= fibrand_create(0);
338 fputs("gfshare: ", stdout
);
339 for (i
= 0; i
< 40; i
++) {
347 fputs(" ok\n", stdout
);
349 fputs(" failed\n", stdout
);
350 return (ok ? EXIT_SUCCESS
: EXIT_FAILURE
);
355 /*----- That's all, folks -------------------------------------------------*/