3 * $Id: gfshare.c,v 1.1 2000/06/17 10:56:30 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 1.1 2000/06/17 10:56:30 mdw
34 * Fast but nonstandard secret sharing system.
38 /*----- Header files ------------------------------------------------------*/
44 #include <mLib/alloc.h>
45 #include <mLib/bits.h>
49 #include "gfshare-tab.h"
52 /*----- Static variables --------------------------------------------------*/
54 static octet gflog
[] = GFSHARE_LOG
, gfexp
[] = GFSHARE_EXP
;
56 /*----- Main code ---------------------------------------------------------*/
58 /* --- @gfshare_create@ --- *
60 * Arguments: @gfshare *s@ = pointer to share context to initialize
61 * @unsigned t, n@ = threshold parameters for the system
62 * @size_t sz@ = size of the secret
66 * Use: Initializes a sharing context.
69 void gfshare_create(gfshare
*s
, unsigned t
, unsigned n
, size_t sz
)
79 /* --- @gfshare_destroy@ --- *
81 * Arguments: @gfshare *s@ = pointer to share context to destroy
85 * Use: Disposes of a sharing context. The allocations for the
86 * individual shares and the vector @v@ are freed; the secret is
90 void gfshare_destroy(gfshare
*s
)
95 /* --- Dispose of the share vector --- */
104 for (i
= 0; i
< n
; i
++) {
112 /* --- @gfshare_mkshares@ --- *
114 * Arguments: @gfshare *s@ = pointer to share context to fill in
115 * @grand *r@ = pointer to random number source
119 * Use: Generates @c->n@ secret shares, such that any @c->t@ of them
120 * may be used to recover the secret.
122 * The context structure is expected to be mostly filled in. In
123 * particular, @t@, @n@, @ssz@ and @s@ must be initialized. If
124 * @v@ is zero, a vector of appropriate size is allocated. You
125 * should use the macro @GFSHARE_INIT@ or @gfshare_create@ to
126 * construct sharing contexts.
129 void gfshare_mkshares(gfshare
*s
, grand
*r
)
134 /* --- Construct the coefficients --- */
136 v
= XS_ALLOC(s
->sz
* s
->t
);
137 r
->ops
->fill(r
, v
, s
->sz
* (s
->t
- 1));
138 memcpy(v
+ s
->sz
* (s
->t
- 1), s
->s
, s
->sz
);
141 /* --- Construct the shares --- */
144 s
->v
= xmalloc(s
->n
* sizeof(gfshare_pt
));
146 for (i
= 0; i
< s
->n
; i
++) {
149 unsigned ilog
= gflog
[i
+ 1];
151 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
153 s
->v
[i
].y
= XS_ALLOC(s
->sz
);
154 memcpy(s
->v
[i
].y
, p
, s
->sz
);
156 for (j
= 1; j
< s
->t
; j
++) {
157 octet
*q
= s
->v
[i
].y
;
159 for (k
= 0; k
< s
->sz
; k
++) {
162 qq
= gfexp
[ilog
+ gflog
[qq
]];
169 /* --- Dispose of various bits of old rubbish --- */
174 /* --- @gfshare_add@ --- *
176 * Arguments: @gfshare *s@ = pointer to sharing context
177 * @unsigned x@ = which share number this is
178 * @const octet *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 octet
*y
)
188 /* --- If no vector has been allocated, create one --- */
191 s
->v
= xmalloc(s
->t
* sizeof(gfshare_pt
));
195 assert(((void)"Share context is full", s
->i
< s
->t
));
197 /* --- Store the share in the vector --- */
200 s
->v
[s
->i
].y
= XS_ALLOC(s
->sz
);
201 memcpy(s
->v
[s
->i
].y
, y
, s
->sz
);
206 return (s
->t
- s
->i
);
209 /* --- @gfshare_combine@ --- *
211 * Arguments: @gfshare *s@ = pointer to share context
212 * @octet *buf@ = pointer to output buffer for the secret
216 * Use: Reconstructs a secret, given enough shares.
219 void gfshare_combine(gfshare
*s
, octet
*buf
)
224 /* --- Sanity checking --- */
226 assert(((void)"Not enough shares yet", s
->i
== s
->t
));
228 /* --- Precomputation of coefficients --- */
232 for (i
= 0; i
< s
->t
; i
++) {
233 unsigned c
= 0, ci
= 0;
234 for (j
= 0; j
< s
->t
; j
++) {
237 c
+= gflog
[s
->v
[j
].x
+ 1];
240 ci
+= gflog
[(s
->v
[i
].x
+ 1) ^ (s
->v
[j
].x
+ 1)];
250 /* --- Grind through the shares --- */
252 for (i
= 0; i
< s
->sz
; i
++) {
254 for (j
= 0; j
< s
->t
; j
++) {
256 x
^= gfexp
[v
[j
] + gflog
[s
->v
[j
].y
[i
]]];
264 /*----- Test rig ----------------------------------------------------------*/
270 static int verify(grand
*r
)
272 unsigned n
= r
->ops
->range(r
, 16) + 8;
273 unsigned t
= r
->ops
->range(r
, n
- 1) + 1;
274 unsigned len
= r
->ops
->range(r
, 32) + 1;
276 gfshare_pt
*v
= xmalloc(t
* sizeof(gfshare_pt
));
277 unsigned *p
= xmalloc(n
* sizeof(unsigned));
278 octet
*sec
= xmalloc(len
), *sbuf
= xmalloc(len
);
284 for (i
= 0; i
< n
; i
++)
286 for (i
= 0; i
< t
; i
++) {
287 unsigned long j
= r
->ops
->range(r
, n
- i
);
293 r
->ops
->fill(r
, sec
, len
);
295 gfshare_create(&s
, t
, n
, len
);
298 gfshare_mkshares(&s
, r
);
299 for (i
= 0; i
< t
; i
++) {
300 v
[i
].x
= s
.v
[p
[i
]].x
;
301 v
[i
].y
= xmalloc(len
);
302 memcpy(v
[i
].y
, s
.v
[p
[i
]].y
, len
);
306 gfshare_create(&s
, t
, n
, len
);
307 for (i
= 0; i
< t
; i
++) {
308 gfshare_add(&s
, v
[i
].x
, v
[i
].y
);
310 gfshare_combine(&s
, sbuf
);
313 if (memcmp(sec
, sbuf
, len
) != 0) {
315 fprintf(stderr
, "\nbad recombination of shares\n");
321 for (i
= 0; i
< t
; i
++)
331 grand
*r
= fibrand_create(0);
335 fputs("gfshare: ", stdout
);
336 for (i
= 0; i
< 40; i
++) {
344 fputs(" ok\n", stdout
);
346 fputs(" failed\n", stdout
);
347 return (ok ? EXIT_SUCCESS
: EXIT_FAILURE
);
352 /*----- That's all, folks -------------------------------------------------*/