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   1 /* -*-c-*-
   2  *
   3  * $Id: share.h,v 1.2 2000/06/24 18:29:05 mdw Exp $
   4  *
   5  * Shamir's secret sharing
   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 /*----- Revision history --------------------------------------------------*
  31  *
  32  * $Log: share.h,v $
  33  * Revision 1.2  2000/06/24 18:29:05  mdw
  34  * Interface change: allow shares to be extracted from a context on demand,
  35  * rather than building them all up-front.
  36  *
  37  * Revision 1.1  2000/06/17 12:09:38  mdw
  38  * Shamir's secret sharing system.
  39  *
  40  */
  41
  42 /*----- Notes on the sharing system ---------------------------------------*
  43  *
  44  * Shamir's secret-sharing system is based on polynomial interpolation modulo
  45  * a prime number.  It is `perfect' in that fewer participants than the
  46  * threshold can derive no information about the secret by pooling their
  47  * shares, and `ideal' in that the shares are the same size as the secret.
  48  *
  49  * This implementation stays close to the definition, in order to support
  50  * other schemes for (e.g.) threshold cryptography.  It is, however, rather
  51  * slow.
  52  */
  53
  54 #ifndef CATACOMB_SHARE_H
  55 #define CATACOMB_SHARE_H
  56
  57 #ifdef __cplusplus
  58   extern "C" {
  59 #endif
  60
  61 /*----- Header files ------------------------------------------------------*/
  62
  63 #ifndef CATACOMB_GRAND_H
  64 #  include "grand.h"
  65 #endif
  66
  67 #ifndef CATACOMB_MP_H
  68 #  include "mp.h"
  69 #endif
  70
  71 /*----- Data structures ---------------------------------------------------*/
  72
  73 /* --- A secret sharing context --- */
  74
  75 typedef struct share_pt {
  76   unsigned x;                           /* Index of this share */
  77   mp *y;                                /* Payload of this share */
  78 } share_pt;
  79
  80 typedef struct share {
  81   unsigned t;                           /* Threshold */
  82   unsigned i;                           /* Next free slot in the vector */
  83   mp *s;                                /* The secret */
  84   mp *p;                                /* Modulus for arithmetic */
  85   share_pt *v;                          /* Vector of share information */
  86 } share;
  87
  88 #define SHARE_INIT(t) { t, 0, 0, 0, 0 }
  89
  90 /*----- Functions provided ------------------------------------------------*/
  91
  92 /* --- @share_create@ --- *
  93  *
  94  * Arguments:   @share *s@ = pointer to share context to initialize
  95  *              @unsigned t@ = threshold for the system
  96  *
  97  * Returns:     ---
  98  *
  99  * Use:         Initializes a sharing context.
 100  */
 101
 102 extern void share_create(share */*s*/, unsigned /*t*/);
 103
 104 /* --- @share_destroy@ --- *
 105  *
 106  * Arguments:   @share *s@ = pointer to share context to destroy
 107  *
 108  * Returns:     ---
 109  *
 110  * Use:         Disposes of a sharing context.  All memory is freed, all
 111  *              integers are dropped.
 112  */
 113
 114 extern void share_destroy(share */*s*/);
 115
 116 /* --- @share_mkshares@ --- *
 117  *
 118  * Arguments:   @share *s@ = pointer to share context to fill in
 119  *              @grand *r@ = pointer to random number source
 120  *
 121  * Returns:     ---
 122  *
 123  * Use:         Initializes a sharing context to be able to create shares.
 124  *              The context structure is expected to be mostly filled in.  In
 125  *              particular, @t@ and @s@ must be initialized.  If @p@ is zero,
 126  *              a prime number of appropriate size is generated
 127  *              automatically.  If @v@ is zero, a vector of appropriate size
 128  *              is allocated.  You should use the macro @SHARE_INIT@ or
 129  *              @share_create@ to construct sharing contexts.
 130  */
 131
 132 extern void share_mkshares(share */*s*/, grand */*r*/);
 133
 134 /* --- @share_get@ --- *
 135  *
 136  * Arguments:   @share *s@ = pointer to share conext
 137  *              @mp *d@ = destination for the share
 138  *              @unsigned x@ = share index to fetch
 139  *
 140  * Returns:     The share, as requested.
 141  *
 142  * Use:         Extracts a share from the system.  You may extract @MPW_MAX@
 143  *              shares, or @s->p@ shares from the system, whichever is
 144  *              smaller.  Shares are indexed from 0.
 145  */
 146
 147 extern mp *share_get(share */*s*/, mp */*d*/, unsigned /*x*/);
 148
 149 /* --- @share_add@ --- *
 150  *
 151  * Arguments:   @share *s@ = pointer to sharing context
 152  *              @unsigned x@ = which share number this is
 153  *              @mp *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 prime @p@ and threshold @t@.
 159  */
 160
 161 extern unsigned share_add(share */*s*/, unsigned /*x*/, mp */*y*/);
 162
 163 /* --- @share_combine@ --- *
 164  *
 165  * Arguments:   @share *s@ = pointer to share context
 166  *
 167  * Returns:     The secret, as a multiprecision integer.
 168  *
 169  * Use:         Reconstructs a secret, given enough shares.
 170  */
 171
 172 extern mp *share_combine(share */*s*/);
 173
 174 /*----- That's all, folks -------------------------------------------------*/
 175
 176 #ifdef __cplusplus
 177   }
 178 #endif
 179
 180 #endif