80a6ef6f431d77b4f89afb0763bf2994a1f02bea
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 /*----- Header files ------------------------------------------------------*/
35 #include <mLib/alloc.h>
36 #include <mLib/bits.h>
42 /*----- Static variables --------------------------------------------------*/
44 extern const octet gfshare_log
[256], gfshare_exp
[510];
46 /*----- Main code ---------------------------------------------------------*/
48 /* --- @gfshare_create@ --- *
50 * Arguments: @gfshare *s@ = pointer to share context to initialize
51 * @unsigned t@ = threshold for the system
52 * @size_t sz@ = size of the secret
56 * Use: Initializes a sharing context.
59 void gfshare_create(gfshare
*s
, unsigned t
, size_t sz
)
67 /* --- @gfshare_destroy@ --- *
69 * Arguments: @gfshare *s@ = pointer to share context to destroy
73 * Use: Disposes of a sharing context. The allocations for the
74 * individual shares and the vector @v@ are freed; the secret is
78 void gfshare_destroy(gfshare
*s
)
84 /* --- @gfshare_mkshares@ --- *
86 * Arguments: @gfshare *s@ = pointer to share context to fill in
87 * @grand *r@ = pointer to random number source
88 * @const void *buf@ = pointer to the secret to share
92 * Use: Initializes a sharing context to be able to create shares.
93 * The context structure is expected to be mostly filled in. In
94 * particular, @t@ must be initialized. If @v@ is zero, a
95 * vector of appropriate size is allocated. You should use the
96 * macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
100 void gfshare_mkshares(gfshare
*s
, grand
*r
, const void *buf
)
102 s
->v
= XS_ALLOC(s
->sz
* s
->t
);
103 r
->ops
->fill(r
, s
->v
, s
->sz
* (s
->t
- 1));
104 memcpy(s
->v
+ s
->sz
* (s
->t
- 1), buf
, s
->sz
);
107 /* --- @gfshare_get@ --- *
109 * Arguments: @gfshare *s@ = pointer to share conext
110 * @unsigned x@ = share index to fetch
111 * @void *buf@ = pointer to output buffer
115 * Use: Extracts a share from the system. You may extract up to 255
116 * shares from the system. Shares are indexed from 0.
119 void gfshare_get(gfshare
*s
, unsigned x
, void *buf
)
122 const octet
*p
= s
->v
;
123 unsigned ilog
= gfshare_log
[x
+ 1];
125 /* --- Evaluate the polynomial at %$x = i + 1$% --- */
127 memcpy(buf
, p
, s
->sz
);
130 for (i
= 1; i
< s
->t
; i
++) {
133 for (k
= 0; k
< s
->sz
; k
++) {
136 qq
= gfshare_exp
[ilog
+ gfshare_log
[qq
]];
142 /* --- @gfshare_addedp@ --- *
144 * Arguments: @gfshare *s@ = pointer to sharing context
145 * @unsigned x@ = which share number to check
147 * Returns: Nonzero if share @x@ has been added already, zero if it
151 int gfshare_addedp(gfshare
*s
, unsigned x
)
155 for (i
= 0; i
< s
->i
; i
++) {
156 if (GFSHARE_INDEX(s
, i
) == x
+ 1)
162 /* --- @gfshare_add@ --- *
164 * Arguments: @gfshare *s@ = pointer to sharing context
165 * @unsigned x@ = which share number this is
166 * @const void *y@ = the share value
168 * Returns: Number of shares required before recovery may be performed.
170 * Use: Adds a share to the context. The context must have been
171 * initialized with the correct threshold @t@.
174 unsigned gfshare_add(gfshare
*s
, unsigned x
, const void *y
)
178 assert(((void)"Share context is full", s
->i
< s
->t
));
179 assert(((void)"Share already present", !gfshare_addedp(s
, x
)));
181 /* --- If no vector has been allocated, create one --- */
184 s
->v
= XS_ALLOC(s
->t
* (s
->sz
+ 1));
188 /* --- Store the share in the vector --- */
190 p
= &GFSHARE_INDEX(s
, s
->i
);
197 return (s
->t
- s
->i
);
200 /* --- @gfshare_combine@ --- *
202 * Arguments: @gfshare *s@ = pointer to share context
203 * @void *buf@ = pointer to output buffer for the secret
207 * Use: Reconstructs a secret, given enough shares.
210 void gfshare_combine(gfshare
*s
, void *buf
)
215 /* --- Sanity checking --- */
217 assert(((void)"Not enough shares yet", s
->i
== s
->t
));
219 /* --- Grind through the shares --- */
221 memset(buf
, 0, s
->sz
);
223 for (i
= 0; i
< s
->t
; i
++) {
225 octet
*q
= &GFSHARE_INDEX(s
, i
);
226 unsigned c
= 0, ci
= 0;
228 /* --- Compute the magic coefficient --- */
231 for (j
= 0; j
< s
->t
; j
++) {
234 xj
= GFSHARE_INDEX(s
, j
);
235 c
+= gfshare_log
[xj
];
238 ci
+= gfshare_log
[xi
^ xj
];
246 /* --- Work out another layer of the secret --- */
249 for (j
= 0; j
< s
->sz
; j
++) {
251 *p
^= gfshare_exp
[c
+ gfshare_log
[*q
]];
257 /*----- Test rig ----------------------------------------------------------*/
263 static int verify(grand
*r
)
265 unsigned n
= r
->ops
->range(r
, 16) + 8;
266 unsigned t
= r
->ops
->range(r
, n
- 1) + 1;
267 unsigned len
= r
->ops
->range(r
, 32) + 1;
269 octet
*v
= xmalloc(t
* len
);
270 unsigned *p
= xmalloc(n
* sizeof(unsigned));
271 octet
*sec
= xmalloc(len
), *sbuf
= xmalloc(len
);
277 for (i
= 0; i
< n
; i
++)
279 for (i
= 0; i
< t
; i
++) {
280 unsigned long j
= r
->ops
->range(r
, n
- i
);
286 r
->ops
->fill(r
, sec
, len
);
288 gfshare_create(&s
, t
, len
);
290 gfshare_mkshares(&s
, r
, sec
);
291 for (i
= 0; i
< t
; i
++)
292 gfshare_get(&s
, p
[i
], v
+ (i
* len
));
295 gfshare_create(&s
, t
, len
);
296 for (i
= 0; i
< t
; i
++)
297 gfshare_add(&s
, p
[i
], v
+ (i
* len
));
298 gfshare_combine(&s
, sbuf
);
301 if (memcmp(sec
, sbuf
, len
) != 0) {
303 fprintf(stderr
, "\nbad recombination of shares\n");
317 grand
*r
= fibrand_create(0);
321 fputs("gfshare: ", stdout
);
322 for (i
= 0; i
< 40; i
++) {
330 fputs(" ok\n", stdout
);
332 fputs(" failed\n", stdout
);
333 return (ok ? EXIT_SUCCESS
: EXIT_FAILURE
);
338 /*----- That's all, folks -------------------------------------------------*/