git.distorted.org.uk Git - u/mdw/catacomb/blob - gfshare.h

   1 /* -*-c-*-
   2  *
   3  * $Id$
   4  *
   5  * Secret sharing over %$\gf{2^8}$%
   6  *
   7  * (c) 2000 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of Catacomb.
  13  *
  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.
  18  *
  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.
  23  *
  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,
  27  * MA 02111-1307, USA.
  28  */
  29
  30 /*----- Notes on the system -----------------------------------------------*
  31  *
  32  * This uses a variant of Shamir's secret sharing system.  Shamir's original
  33  * system used polynomials modulo a large prime.  This implementation instead
  34  * uses the field %$\gf{2^8}$%, represented by
  35  *
  36  *   %$\gf{2}[x]/(x^8 + x^4 + x^3 + x^2 + 1)$%
  37  *
  38  * and shares each byte of the secret independently.  It is therefore limited
  39  * to 255 players, although this probably isn't a serious limitation in
  40  * practice.
  41  *
  42  * Share creation and reconstruction is extremely efficient.  Contrast the
  43  * performance of the straightforward implementation based on multiprecision
  44  * arithmetic.
  45  */
  46
  47 #ifndef CATACOMB_GFSHARE_H
  48 #define CATACOMB_GFSHARE_H
  49
  50 #ifdef __cplusplus
  51   extern "C" {
  52 #endif
  53
  54 /*----- Header files ------------------------------------------------------*/
  55
  56 #include <mLib/bits.h>
  57
  58 #ifndef CATACOMB_GRAND_H
  59 #  include "grand.h"
  60 #endif
  61
  62 /*----- Data structures ---------------------------------------------------*/
  63
  64 /* --- A secret sharing context --- */
  65
  66 typedef struct gfshare {
  67   unsigned t;                           /* Threshold */
  68   unsigned i;                           /* Next free slot in vector */
  69   size_t sz;                            /* Size of the secret and shares */
  70   octet *v;                             /* Vector of share information */
  71 } gfshare;
  72
  73 #define GFSHARE_INIT(t, sz) { t, 0, sz, 0 }
  74
  75 #define GFSHARE_INDEX(s, i) ((s)->v[(i) * ((s)->sz + 1)])
  76
  77 /*----- Functions provided ------------------------------------------------*/
  78
  79 /* --- @gfshare_create@ --- *
  80  *
  81  * Arguments:   @gfshare *s@ = pointer to share context to initialize
  82  *              @unsigned t@ = threshold for the system
  83  *              @size_t sz@ = size of the secret
  84  *
  85  * Returns:     ---
  86  *
  87  * Use:         Initializes a sharing context.
  88  */
  89
  90 extern void gfshare_create(gfshare */*s*/, unsigned /*t*/, size_t /*sz*/);
  91
  92 /* --- @gfshare_destroy@ --- *
  93  *
  94  * Arguments:   @gfshare *s@ = pointer to share context to destroy
  95  *
  96  * Returns:     ---
  97  *
  98  * Use:         Disposes of a sharing context.  The allocations for the
  99  *              individual shares and the vector @v@ are freed; the secret is
 100  *              left alone.
 101  */
 102
 103 extern void gfshare_destroy(gfshare */*s*/);
 104
 105 /* --- @gfshare_mkshares@ --- *
 106  *
 107  * Arguments:   @gfshare *s@ = pointer to share context to fill in
 108  *              @grand *r@ = pointer to random number source
 109  *              @const void *buf@ = pointer to the secret to share
 110  *
 111  * Returns:     ---
 112  *
 113  * Use:         Initializes a sharing context to be able to create shares.
 114  *              The context structure is expected to be mostly filled in.  In
 115  *              particular, @t@ must be initialized.  If @v@ is zero, a
 116  *              vector of appropriate size is allocated.  You should use the
 117  *              macro @GFSHARE_INIT@ or @gfshare_create@ to construct sharing
 118  *              contexts.
 119  */
 120
 121 extern void gfshare_mkshares(gfshare */*s*/, grand */*r*/,
 122                              const void */*buf*/);
 123
 124 /* --- @gfshare_get@ --- *
 125  *
 126  * Arguments:   @gfshare *s@ = pointer to share conext
 127  *              @unsigned x@ = share index to fetch
 128  *              @void *buf@ = pointer to output buffer
 129  *
 130  * Returns:     ---
 131  *
 132  * Use:         Extracts a share from the system.  You may extract up to 255
 133  *              shares from the system.  Shares are indexed from 0.
 134  */
 135
 136 extern void gfshare_get(gfshare */*s*/, unsigned /*x*/, void */*buf*/);
 137
 138 /* --- @gfshare_addedp@ --- *
 139  *
 140  * Arguments:   @gfshare *s@ = pointer to sharing context
 141  *              @unsigned x@ = which share number to check
 142  *
 143  * Returns:     Nonzero if share @x@ has been added already, zero if it
 144  *              hasn't.
 145  */
 146
 147 extern int gfshare_addedp(gfshare */*s*/, unsigned /*x*/);
 148
 149 /* --- @gfshare_add@ --- *
 150  *
 151  * Arguments:   @gfshare *s@ = pointer to sharing context
 152  *              @unsigned x@ = which share number this is
 153  *              @const void *y@ = the share value
 154  *
 155  * Returns:     Number of shares required before recovery may be performed.
 156  *
 157  * Use:         Adds a share to the context.  The context must have been
 158  *              initialized with the correct threshold @t@.
 159  */
 160
 161 extern unsigned gfshare_add(gfshare */*s*/,
 162                             unsigned /*x*/, const void */*y*/);
 163
 164 /* --- @gfshare_combine@ --- *
 165  *
 166  * Arguments:   @gfshare *s@ = pointer to share context
 167  *              @void *buf@ = pointer to output buffer for the secret
 168  *
 169  * Returns:     ---
 170  *
 171  * Use:         Reconstructs a secret, given enough shares.
 172  */
 173
 174 extern void gfshare_combine(gfshare */*s*/, void */*buf*/);
 175
 176 /*----- That's all, folks -------------------------------------------------*/
 177
 178 #ifdef __cplusplus
 179   }
 180 #endif
 181
 182 #endif