3 * $Id: gfshare.c,v 1.9 2004/04/08 01:36:15 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 /*----- Header files ------------------------------------------------------*/
37 #include <mLib/alloc.h>
38 #include <mLib/bits.h>
42 #include "gfshare-tab.h"
45 /*----- Static variables --------------------------------------------------*/
47 static const octet gflog
[] = GFSHARE_LOG
, gfexp
[] = GFSHARE_EXP
;
49 /*----- Main code ---------------------------------------------------------*/
51 /* --- @gfshare_create@ --- *
53 * Arguments: @gfshare *s@ = pointer to share context to initialize
54 * @unsigned t@ = threshold for the system
55 * @size_t sz@ = size of the secret
59 * Use: Initializes a sharing context.
62 void gfshare_create(gfshare
*s
, unsigned t
, size_t sz
)
70 /* --- @gfshare_destroy@ --- *
72 * Arguments: @gfshare *s@ = pointer to share context to destroy
76 * Use: Disposes of a sharing context. The allocations for the
77 * individual shares and the vector @v@ are freed; the secret is
81 void gfshare_destroy(gfshare
*s
)
87 /* --- @gfshare_mkshares@ --- *
89 * Arguments: @gfshare *s@ = pointer to share context to fill in
90 * @grand *r@ = pointer to random number source
91 * @const void *buf@ = pointer to the secret to share
95 * Use: Initializes a sharing context to be able to create shares.
96 * The context structure is expected to be mostly filled in. In
97 * particular, @t@ must be initialized. If @v@ is zero, a
98 * vector of appropriate size is allocated. You should use the
99 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
103 void gfshare_mkshares(gfshare
*s
, grand
*r
, const void *buf
)
105 s
->v
= XS_ALLOC(s
->sz
* s
->t
);
106 r
->ops
->fill(r
, s
->v
, s
->sz
* (s
->t
- 1));
107 memcpy(s
->v
+ s
->sz
* (s
->t
- 1), buf
, s
->sz
);
110 /* --- @gfshare_get@ --- *
112 * Arguments: @gfshare *s@ = pointer to share conext
113 * @unsigned x@ = share index to fetch
114 * @void *buf@ = pointer to output buffer
118 * Use: Extracts a share from the system. You may extract up to 255
119 * shares from the system. Shares are indexed from 0.
122 void gfshare_get(gfshare
*s
, unsigned x
, void *buf
)
125 const octet
*p
= s
->v
;
126 unsigned ilog
= gflog
[x
+ 1];
128 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
130 memcpy(buf
, p
, s
->sz
);
133 for (i
= 1; i
< s
->t
; i
++) {
136 for (k
= 0; k
< s
->sz
; k
++) {
139 qq
= gfexp
[ilog
+ gflog
[qq
]];
145 /* --- @gfshare_add@ --- *
147 * Arguments: @gfshare *s@ = pointer to sharing context
148 * @unsigned x@ = which share number this is
149 * @const void *y@ = the share value
151 * Returns: Number of shares required before recovery may be performed.
153 * Use: Adds a share to the context. The context must have been
154 * initialized with the correct threshold @t@.
157 unsigned gfshare_add(gfshare
*s
, unsigned x
, const void *y
)
161 /* --- If no vector has been allocated, create one --- */
164 s
->v
= XS_ALLOC(s
->t
* (s
->sz
+ 1));
168 assert(((void)"Share context is full", s
->i
< s
->t
));
170 /* --- Store the share in the vector --- */
172 p
= s
->v
+ s
->i
* (s
->sz
+ 1);
179 return (s
->t
- s
->i
);
182 /* --- @gfshare_combine@ --- *
184 * Arguments: @gfshare *s@ = pointer to share context
185 * @void *buf@ = pointer to output buffer for the secret
189 * Use: Reconstructs a secret, given enough shares.
192 void gfshare_combine(gfshare
*s
, void *buf
)
197 /* --- Sanity checking --- */
199 assert(((void)"Not enough shares yet", s
->i
== s
->t
));
201 /* --- Grind through the shares --- */
203 memset(buf
, 0, s
->sz
);
205 for (i
= 0; i
< s
->t
; i
++) {
207 octet
*q
= s
->v
+ i
* (s
->sz
+ 1);
208 unsigned c
= 0, ci
= 0;
210 /* --- Compute the magic coefficient --- */
213 for (j
= 0; j
< s
->t
; j
++) {
216 xj
= s
->v
[j
* (s
->sz
+ 1)];
220 ci
+= gflog
[xi
^ xj
];
228 /* --- Work out another layer of the secret --- */
231 for (j
= 0; j
< s
->sz
; j
++) {
233 *p
^= gfexp
[c
+ gflog
[*q
]];
239 /*----- Test rig ----------------------------------------------------------*/
245 static int verify(grand
*r
)
247 unsigned n
= r
->ops
->range(r
, 16) + 8;
248 unsigned t
= r
->ops
->range(r
, n
- 1) + 1;
249 unsigned len
= r
->ops
->range(r
, 32) + 1;
251 octet
*v
= xmalloc(t
* len
);
252 unsigned *p
= xmalloc(n
* sizeof(unsigned));
253 octet
*sec
= xmalloc(len
), *sbuf
= xmalloc(len
);
259 for (i
= 0; i
< n
; i
++)
261 for (i
= 0; i
< t
; i
++) {
262 unsigned long j
= r
->ops
->range(r
, n
- i
);
268 r
->ops
->fill(r
, sec
, len
);
270 gfshare_create(&s
, t
, len
);
272 gfshare_mkshares(&s
, r
, sec
);
273 for (i
= 0; i
< t
; i
++)
274 gfshare_get(&s
, p
[i
], v
+ (i
* len
));
277 gfshare_create(&s
, t
, len
);
278 for (i
= 0; i
< t
; i
++)
279 gfshare_add(&s
, p
[i
], v
+ (i
* len
));
280 gfshare_combine(&s
, sbuf
);
283 if (memcmp(sec
, sbuf
, len
) != 0) {
285 fprintf(stderr
, "\nbad recombination of shares\n");
299 grand
*r
= fibrand_create(0);
303 fputs("gfshare: ", stdout
);
304 for (i
= 0; i
< 40; i
++) {
312 fputs(" ok\n", stdout
);
314 fputs(" failed\n", stdout
);
315 return (ok ? EXIT_SUCCESS
: EXIT_FAILURE
);
320 /*----- That's all, folks -------------------------------------------------*/